The Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe(β-CEZ)alloy is considered as a potential structural material in the aviation industry due to its outstanding strength and corrosion resistance.Electrochemical machining(ECM)is an effici...The Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe(β-CEZ)alloy is considered as a potential structural material in the aviation industry due to its outstanding strength and corrosion resistance.Electrochemical machining(ECM)is an efficient and low-cost technology for manufacturing theβ-CEZ alloy.In ECM,the machining parameter selection and tool design are based on the electrochemical dissolution behavior of the materials.In this study,the electrochemical dissolution behaviors of theβ-CEZ and Ti-6Al-4V(TC4)alloys in NaNO3solution are discussed.The open circuit potential(OCP),Tafel polarization,potentiodynamic polarization,electrochemical impedance spectroscopy(EIS),and current efficiency curves of theβ-CEZ and TC4 alloys are analyzed.The results show that,compared to the TC4 alloy,the passivation film structure is denser and the charge transfer resistance in the dissolution process is greater for theβ-CEZ alloy.Moreover,the dissolved surface morphology of the two titanium-based alloys under different current densities are analyzed.Under low current densities,theβ-CEZ alloy surface comprises dissolution pits and dissolved products,while the TC4 alloy surface comprises a porous honeycomb structure.Under high current densities,the surface waviness of both the alloys improves and the TC4 alloy surface is flatter and smoother than theβ-CEZ alloy surface.Finally,the electrochemical dissolution models ofβ-CEZ and TC4 alloys are proposed.展开更多
Titanium and titanium alloys have several advantages, but the cost of titanium alloys is very expensive compared with the traditional metal materials. This article introduces two new low-cost titanium alloys Ti-2.1Cr-...Titanium and titanium alloys have several advantages, but the cost of titanium alloys is very expensive compared with the traditional metal materials. This article introduces two new low-cost titanium alloys Ti-2.1Cr-1.3Fe (TCF alloy) and Ti-3Al-2.1Cr-1.3Fe (TACF alloy). In this study, we used Cr-Fe master alloy as one of the raw materials to develop the two new alloys. We introduce the microstructure and tensile properties of the two new alloys from β solution treated with different cooling methods. Optical microscopy (OM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM) were employed to analyze the phase constitution, and scanning electron microscopy (SEM) was used to observe the fracture surfaces. The results indicate that the microstructures consist of β grain boundary and α′ martensite after water quenching (WQ), β matrix and α phase after air cooling (AC) and furnace cooling (FC), respectively. Also, the microstructure is the typical basketweave structures after FC. Of course, athermal ω is also observed by TEM after WQ. The strength increases with decreasing cooling rates and the plasticity is reversed. Because of the athermal ω, the strength and ductility are highest and lowest when the cooling method is WQ. The strength of TACF alloy is higher than the TCF alloy, but the plasticity is lower. The fracture surfaces are almost entirely covered with dimples under the cooling methods of AC and FC. Also, we observe an intergranular fracture area that is generated by athermal ω, although some dimples are observed after WQ.展开更多
Complex thin-walled titanium alloy components play a key role in the aircraft,aerospace and marine industries,offering the advantages of reduced weight and increased thermal resistance.The geometrical complexity,dimen...Complex thin-walled titanium alloy components play a key role in the aircraft,aerospace and marine industries,offering the advantages of reduced weight and increased thermal resistance.The geometrical complexity,dimensional accuracy and in-service properties are essential to fulfill the high-performance standards required in new transportation systems,which brings new challenges to titanium alloy forming technologies.Traditional forming processes,such as superplastic forming or hot pressing,cannot meet all demands of modern applications due to their limited properties,low productivity and high cost.This has encouraged industry and research groups to develop novel high-efficiency forming processes.Hot gas pressure forming and hot stamping-quenching technologies have been developed for the manufacture of tubular and panel components,and are believed to be the cut-edge processes guaranteeing dimensional accuracy,microstructure and mechanical properties.This article intends to provide a critical review of high-efficiency titanium alloy forming processes,concentrating on latest investigations of controlling dimensional accuracy,microstructure and properties.The advantages and limitations of individual forming process are comprehensively analyzed,through which,future research trends of high-efficiency forming are identified including trends in process integration,processing window design,full cycle and multi-objective optimization.This review aims to provide a guide for researchers and process designers on the manufacture of thin-walled titanium alloy components whilst achieving high dimensional accuracy and satisfying performance properties with high efficiency and low cost.展开更多
Several detailed studies have comprehensively investigated the benefits and limitations of laser-assisted machining(LAM)of titanium alloys.These studies have highlighted the positive impact of the application of laser...Several detailed studies have comprehensively investigated the benefits and limitations of laser-assisted machining(LAM)of titanium alloys.These studies have highlighted the positive impact of the application of laser preheating on reducing cutting forces and improving productivity but have also identified the detrimental effect of LAM on tool life.This paper seeks to evaluate a series of the most common cutting tools with different coating types used in the machining of titanium alloys to identify whether coating type has a dramatic effect on the dominant tool wear mechanisms active during the process.The findings provide a clear illustration that the challenges facing the application of LAM are associated with the development of new types of cutting tools which are not subjected to the diffusion-controlled wear processes that dominate the performance of current cutting tools.展开更多
Based on the investigated microstructure of different zones in the annealed automatic gas tungsten arc weld joint of TA16 and TC4 titanium alloys,the mechanical property of them was assessed under fatigue crack growth...Based on the investigated microstructure of different zones in the annealed automatic gas tungsten arc weld joint of TA16 and TC4 titanium alloys,the mechanical property of them was assessed under fatigue crack growth rate tests.For evaluation of fatigue crack growth rate,three points bending specimens were used.The correlation between the range of stress intensity factor and crack growth rate was determined in different zones of the annealed weld joint.Fatigue crack growth rates were obviously different in different zones of weld joint of dissimilar titanium alloys,due to their different microstructures.Scanning electron microscope examinations were conducted on the fracture surface in order to determine the relevant fracture mechanisms and crack growth mechanisms with respect to the details of microstructure.展开更多
We focus on the electrochemical dissolution characteristics of new titanium alloys such as near-αtitanium alloy Ti60,α+βtitanium alloy TC4andβtitanium alloy Ti40 which are often used for aerospace industry.The exp...We focus on the electrochemical dissolution characteristics of new titanium alloys such as near-αtitanium alloy Ti60,α+βtitanium alloy TC4andβtitanium alloy Ti40 which are often used for aerospace industry.The experiments are carried out by electrochemical machining tool,and the surface morphology of the specimens is observed by the scanning electron microscope(SEM)and three-dimensional video microscope(DVM).The appropriate electrolyte is selected and the relationships between surface roughness and current density are achieved.The results show that the single-phase titanium alloy Ti40 has a better surface roughness after ECM compared with theα+βtitanium alloy TC4 and the near-αtitanium alloy Ti60.The best surface roughness is Ra 0.28μm when the current density is 75A/cm2.Furthermore,the surface roughness of the near-αtitanium alloy Ti60 is the most sensitive with the current density because of the different electrochemical equivalents of substitutional elements and larger grains than TC4.Finally,the suitable current density for each titanium alloy is achieved.展开更多
In order to accelerate the research on the property optimization of titanium alloy based on high-throughput methods,it is necessary to reveal the relationship between hardness and other mechanical properties which is ...In order to accelerate the research on the property optimization of titanium alloy based on high-throughput methods,it is necessary to reveal the relationship between hardness and other mechanical properties which is still unclear.In this work,taking Ti20C alloy as research object,almost all the microstructure of dual-phase titanium alloys were covered by traversing over 100 heat treatment schemes.Then,massive experiments including microstructure characterization and performance test were conducted,obtaining 51,590 pieces of microstructure data and 3591 pieces of mechanical property data.Subsequently,based on large-scale data-driven technology,the quantitative mapping relationship between hardness and other mechanical properties was deeply discussed.The results of random forest models showed that the correlation between hardness(H)and Charpy impact energy(A_(k))(or elongation,A)was hardly dependent on the microstructure types,while the relationship between H and tensile strength(R_(m))(or yield strength,R_(p0.2))was highly dependent on microstructure types.Specifically,combined with statistical analysis,it was found that the relationship between H and Ak(or A)were negatively linear.Interestingly,the relationship between H and strength was positively linear for equiaxed microstructure,and strength was linked to d^(−1/2)(d,equivalent circle diameter)ofα-grains in the form of classical Hall–Petch formula;but for other microstructures,the relationships were quadratic.Furthermore,the above rules were nearly the same in the rolling direction and transverse direction.Finally,a"four-quadrant partition map"between H and R_(p0.2)/R_(m) was established as a versatile material-screening tool,which can provide guidance for on-demand selection of titanium alloys.展开更多
Commercially pure titanium and titanium alloys have been among the most commonly used materials for biomedical applications since the 1950 s.Due to the excellent mechanical tribological properties,corrosion resistance...Commercially pure titanium and titanium alloys have been among the most commonly used materials for biomedical applications since the 1950 s.Due to the excellent mechanical tribological properties,corrosion resistance,biocompatibility,and antibacterial properties of titanium,it is getting much attention as a biomaterial for implants.Furthermore,titanium promotes osseointegration without any additional adhesives by physically bonding with the living bone at the implant site.These properties are crucial for producing high-strength metallic alloys for biomedical applications.Titanium alloys are manufactured into the three types ofα,β,andα+β.The scientific and clinical understanding of titanium and its potential applications,especially in the biomedical field,are still in the early stages.This review aims to establish a credible platform for the current and future roles of titanium in biomedicine.We first explore the developmental history of titanium.Then,we review the recent advancement of the utility of titanium in diverse biomedical areas,its functional properties,mechanisms of biocompatibility,host tissue responses,and various relevant antimicrobial strategies.Future research will be directed toward advanced manufacturing technologies,such as powder-based additive manufacturing,electron beam melting and laser melting deposition,as well as analyzing the effects of alloying elements on the biocompatibility,corrosion resistance,and mechanical properties of titanium.Moreover,the role of titania nanotubes in regenerative medicine and nanomedicine applications,such as localized drug delivery system,immunomodulatory agents,antibacterial agents,and hemocompatibility,is investigated,and the paper concludes with the future outlook of titanium alloys as biomaterials.展开更多
The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat ...The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.展开更多
In this study, microstructure and texture evolution of TB8 titanium alloys during hot deformation were investigated by using electron back-scattered diffraction(EBSD) analysis. The results showed that dynamic recrysta...In this study, microstructure and texture evolution of TB8 titanium alloys during hot deformation were investigated by using electron back-scattered diffraction(EBSD) analysis. The results showed that dynamic recrystallization(DRX) behavior of TB8 titanium alloys was drastically sensitive to the strain. As the true strain raised from 0.2 to 0.8, the degree of DRX gradually increased. The nucleation mechanism of recrystallization was observed, including discontinuous dynamic recrystallization(DDRX) resulting from the bulging of original boundaries. Furthermore, continuous dynamic recrystallization(CDRX) occurred because of the transformation of low-angle grain boundaries(LAGBs) to high-angle grain boundaries(HAGBs) in the interior of the original deformed grains. The texture evolution of TB8 titanium alloy during hot deformation process was analyzed in detail, and five texture components were observed,including{001}h100 i,{011}h100 i,{112}h110 i,{111}h110 i, and {111}h112 i. As the true strain increased,deformation textures were gradually weakened due to an increase in the volume fraction of DRX grains. When the true strain was 0.8, the main texture components consisted of the recrystallization texture components of the{001}h100 i and {011}h100 i textures.展开更多
This study aims to examine the effect of replacing vanadium by niobium and iron on the tribological behavior of hot-isostatic-pressed titanium alloy(Ti–6Al–4V)biomaterial,using a ball-on-disk-type oscillating tribom...This study aims to examine the effect of replacing vanadium by niobium and iron on the tribological behavior of hot-isostatic-pressed titanium alloy(Ti–6Al–4V)biomaterial,using a ball-on-disk-type oscillating tribometer,under wet conditions using physiological solution in accordance with the ISO7148 standards.The tests were carried out under a normal load of 6 N,with an AISI 52100 grade steel ball as a counter face.The morphological changes and structural evolution of the nanoparticle powders using different milling times(2,6,12 and 18 h)were studied.The morphological characterization indicated that the particle and crystallite size continuously decrease with increasing milling time to reach the lowest value of 4 nm at 18-h milling.The friction coefficient and wear rate were lower in the samples milled at 18 h(0.226,0.297 and 0.423;and 0.66 9 10^(-2),0.87 9 10^(-2)and 1.51 9 10^(-2)lm^3N^(-1)lm^(-1))for Ti–6Al–4Fe,Ti–6Al–7Nb and Ti–6Al–4V,respectively.This improvement in friction and wear resistance is attributed to the grain refinement at 18-h milling.The Ti–6Al–4Fe samples showed good tribological performance for all milling times.展开更多
The influence of titanium alloy(Ti–5 Al–2.5 Sn) and commercially pure titanium(cp Ti) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti–5 Al–2.5 Sn/cp Ti was investigated in terms of microstructur...The influence of titanium alloy(Ti–5 Al–2.5 Sn) and commercially pure titanium(cp Ti) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti–5 Al–2.5 Sn/cp Ti was investigated in terms of microstructure, mechanical/nano-mechanical properties, and residual stresses. A partial martensitic transformation was observed in the weldments for all the welding conditions due to high heat input. The microstructure evolved in the FZ/cp Ti interfacial region was observed to be the most sensitive to the proportion of α stabilizer in the filler alloy. Furthermore, the addition of filler alloy improved the tensile properties and nano-mechanical response of the weld joint owing to the increased volume of metal in the weld joint. As compared to the Ti–5 Al–2.5 Sn wire, the use of cp Ti filler wire proved to be better in terms of energy absorbed during tensile and impact tests, tensile strength and ductility of the dissimilar welds. An asymmetrical residual stresses profile was observed close to the weld centerline, with high compressive stresses on the Ti–5 Al–2.5 Sn side for both the weldments obtained with and without filler wires. This was attributed to mainly the low thermal conductivity of Ti–5 Al–2.5 Sn. The presence of residual stresses also influenced the nano-hardness profile across the weldments.展开更多
The design of alloys with simultaneous high strength and high ductility is still a difficult challenge.Here,we propose a new approach to designing multi-phase alloys with a synergistic combination of strength and duct...The design of alloys with simultaneous high strength and high ductility is still a difficult challenge.Here,we propose a new approach to designing multi-phase alloys with a synergistic combination of strength and ductility by engineering heterogeneous precipitate microstructures through the activation of different transformation mechanisms.Using a two-phase titanium alloy as an example,phase field simulations are carried out firstly to design heat treatment schedules that involve both conventional nucleation and growth and non-conventional pseudospinodal decomposition mechanisms,and the calculated microstructures have been evaluated by crystal plasticity finite element modeling.According to simulations,we then set a two-step heat treatment to produce bimodalα+βmicrostructure in Ti-10V-2Fe-3Al.Further mechanical testing shows that the ductility of the alloy is increased by~50%and the strength is increased by~10%as compared to its unimodal counterpart.Our work may provide a general way to improve the mechanical properties of alloys through multiscale microstructure design.展开更多
Porous titanium and its alloys have been considered as promising implants owing to their low elastic modulus and capability to provide channels for bone growth.Currently,additive manufacturing(3D printing)techniques h...Porous titanium and its alloys have been considered as promising implants owing to their low elastic modulus and capability to provide channels for bone growth.Currently,additive manufacturing(3D printing)techniques have been successfully applied to produce porous titanium alloys owing to the advantages of controllable and precise fabrication.Considering the safety aspect,an understanding of corrosion in porous titanium alloys and the corresponding mechanisms is important for their long-term application in the human body.In this paper,the recent progress in improving the corrosion properties of porous titanium alloys fabricated via 3D printing techniques is reviewed.The effects of pore type,porosity,electrolyte,and modification of the material on the corrosion properties of porous titanium alloys are introduced and discussed.In addition,the limitations of traditional methods for measuring the corrosion performance of porous titanium alloys were analysed.Perspectives for evaluating and improving the corrosion performance of porous titanium alloys using new methods are provided.展开更多
The microstructure and mechanical properties of the TB8 titanium alloy were controlled by a secondary processing technology of solution-equal channel angular pressing(ECAP)-aging treatment,which combined strong plasti...The microstructure and mechanical properties of the TB8 titanium alloy were controlled by a secondary processing technology of solution-equal channel angular pressing(ECAP)-aging treatment,which combined strong plastic deformation with heat treatment. The effects of ECAP and heat treatment on the microstructure and properties of the titanium alloy were systematically investigated by optical microscopy(OM), scanning electron microscopy(SEM), hardness tests, and tensile property analysis. The results indicate that the metallographic structure without ECAP treatment is mainly equiaxed β-phase, while that after ECAP treatment is equiaxed β-phase with grain fragmentation, slip bands, and new small grains. After 850 ℃ solutionECAP-520 ℃ aging treatment, the titanium alloy has the smallest grain size, while the directionality of tissue growth along the ECAP direction is the most apparent. Under the same solution-aging conditions, the hardness of the titanium alloy increases from 431.5 to 531.2 HV compared to that without ECAP treatment, i e, increases by 23.11%, and the tensile strength increases from 1 045.30 to 1 176.25 MPa, i e, increases by 12.5%.展开更多
In order to improve the oxidation and wear resistance of blades tip of titanium alloys as well as the sealing performance of the gas turbine engine,a Ni/cBN abrasive coating was prepared on titanium alloys through com...In order to improve the oxidation and wear resistance of blades tip of titanium alloys as well as the sealing performance of the gas turbine engine,a Ni/cBN abrasive coating was prepared on titanium alloys through composite electroplating.Oxidation,mechanical and tribological properties of the abrasive coating were investigated.Furthermore,the effect of the oxidation on the mechanical and tribological properties was also evaluated.Oxidation results revealed that the abrasive coating underwent slight oxidation within 700℃.Meanwhile,some intermetallic compounds,Ni3Ti,NiTi and NiTi2,were formed at the coating/substrate interface during oxidation.Due to the pinning effect of cBN particles and the different thermal expansion coefficients of the coating and substrate,the coating/substrate interface was cracked after oxidation at 700℃.Tensile results showed that the presence of coating reduced the strength of the alloy significantly at room temperature,while only marginal variations of the strength of the coated and uncoated specimens at elevated temperatures.Besides,the wear tests indicated that the coating had the excellent cutting ability and wear resistance,which can effectively protect the blades tip of titanium alloys.As the temperature increased,the wear resistance decreased due to the decrease in the mechanical properties of the Ni/cBN coating.展开更多
In this study,a novel strategy for developingα+βdual-phase titanium alloys with low Young's modulus and high yield strength was proposed,and a Ti-15Nb-5Zr-4Sn-1 Fe alloy was developed through theoretical composi...In this study,a novel strategy for developingα+βdual-phase titanium alloys with low Young's modulus and high yield strength was proposed,and a Ti-15Nb-5Zr-4Sn-1 Fe alloy was developed through theoretical composition design and microstructure manipulation.After hot-rolling and subsequent annealing,a high volume fraction of ultrafine grainedαphase embedded in metastableβ-matrix was formed in the microstructure as intended.Consequently,this alloy exhibits both low Young's modulus(61 GPa)and high yield strength(912 MPa).The experimental results prove that the proposed strategy is appropriate for developing titanium alloys with superior yield strength-to-modulus ratio than those of conventional metallic biomedical materials.Present study might shed light on the research and development of advanced biomedical titanium alloys with low Young's modulus and high yield strength.展开更多
Cold crucible directional solidification(CCDS)is a newly developed technique,which combines the advantages of the cold crucible and continuous melting.It can be applied to directionally solidify reactive,high purity a...Cold crucible directional solidification(CCDS)is a newly developed technique,which combines the advantages of the cold crucible and continuous melting.It can be applied to directionally solidify reactive,high purity and refractory materials.This paper describes the principle of CCDS and its characteristics;development of the measurement and numerical calculation of the magnetic field,flow field and temperature field in CCDS;and the CCDS of Ti based alloys.The paper also reviews original data obtained by some scholars,including the present authors,reported in separate publications in recent years.In Ti based alloys,Ti6Al4V,TiAl alloys and high Nb-containing TiAl alloys,have been directionally solidified in different cold crucibles.The crosssections of the cold crucibles include round,near rectangular and square with different sizes.Tensile testing results show that the elongation of directionally solidified Ti6Al4V can be improved to 12.7%from as cast5.4%.The strength and the elongation of the directionally solidified Ti47Al2Cr2Nb and Ti44Al6Nb1.0Cr2.0V are 650 MPa/3%and 602.5MPa/1.20%,respectively.The ingots after CCDS can be used to prepare turbine or engine blades,and are candidates to replace Ni super-alloy at temperatures of 700 to 900°C.展开更多
The microstructural evolution,oxidation resistance and mechanical properties of Ti-6.5Al-2Sn-4Hf-2 Nbbased alloys with different contents of tungsten(W)additions ranging from 0 to 4.0 wt.%have been investigated in thi...The microstructural evolution,oxidation resistance and mechanical properties of Ti-6.5Al-2Sn-4Hf-2 Nbbased alloys with different contents of tungsten(W)additions ranging from 0 to 4.0 wt.%have been investigated in this study.The addition of W changed the microstructure from Widmanstatten colony of the W-free alloy to basketweave microstructure.After thermal exposure at 650℃ for 1000 h,the retainedβphase became less continuous,and secondaryβnano-particles with high W concentration were precipitated fromαlamellas.Withinαlamellas,the W was found to mainly partitioned into the secondaryβphase and refined the ordered α_(2)-Ti_(3)Al precipitates due to increased solubility of Nb in α_(2) phase.High W addition increased activation energy for oxidation,promoting the formation of more uniform and compact compound oxides,therefore substantially enhanced the oxidation resistance of the alloy.Besides,the W addition also improved the room and high-temperature yield strength without obviously losing plasticity after long-time thermal exposure.The improved mechanical performance was mainly attributed to the introduction of moreα/βinterfaces,the precipitation of secondaryβphase and the refinedα_(2) phase with Nb segregation.展开更多
Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed...Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.展开更多
基金supported by the National Natural Science Foundation of China(No.92160301)the Industrial Technology Development Program,China(No.JCKY2021605 B026)。
文摘The Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe(β-CEZ)alloy is considered as a potential structural material in the aviation industry due to its outstanding strength and corrosion resistance.Electrochemical machining(ECM)is an efficient and low-cost technology for manufacturing theβ-CEZ alloy.In ECM,the machining parameter selection and tool design are based on the electrochemical dissolution behavior of the materials.In this study,the electrochemical dissolution behaviors of theβ-CEZ and Ti-6Al-4V(TC4)alloys in NaNO3solution are discussed.The open circuit potential(OCP),Tafel polarization,potentiodynamic polarization,electrochemical impedance spectroscopy(EIS),and current efficiency curves of theβ-CEZ and TC4 alloys are analyzed.The results show that,compared to the TC4 alloy,the passivation film structure is denser and the charge transfer resistance in the dissolution process is greater for theβ-CEZ alloy.Moreover,the dissolved surface morphology of the two titanium-based alloys under different current densities are analyzed.Under low current densities,theβ-CEZ alloy surface comprises dissolution pits and dissolved products,while the TC4 alloy surface comprises a porous honeycomb structure.Under high current densities,the surface waviness of both the alloys improves and the TC4 alloy surface is flatter and smoother than theβ-CEZ alloy surface.Finally,the electrochemical dissolution models ofβ-CEZ and TC4 alloys are proposed.
基金financially sponsored by the International Science and Technology Cooperation Project (No.2010DFA52280)
文摘Titanium and titanium alloys have several advantages, but the cost of titanium alloys is very expensive compared with the traditional metal materials. This article introduces two new low-cost titanium alloys Ti-2.1Cr-1.3Fe (TCF alloy) and Ti-3Al-2.1Cr-1.3Fe (TACF alloy). In this study, we used Cr-Fe master alloy as one of the raw materials to develop the two new alloys. We introduce the microstructure and tensile properties of the two new alloys from β solution treated with different cooling methods. Optical microscopy (OM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM) were employed to analyze the phase constitution, and scanning electron microscopy (SEM) was used to observe the fracture surfaces. The results indicate that the microstructures consist of β grain boundary and α′ martensite after water quenching (WQ), β matrix and α phase after air cooling (AC) and furnace cooling (FC), respectively. Also, the microstructure is the typical basketweave structures after FC. Of course, athermal ω is also observed by TEM after WQ. The strength increases with decreasing cooling rates and the plasticity is reversed. Because of the athermal ω, the strength and ductility are highest and lowest when the cooling method is WQ. The strength of TACF alloy is higher than the TCF alloy, but the plasticity is lower. The fracture surfaces are almost entirely covered with dimples under the cooling methods of AC and FC. Also, we observe an intergranular fracture area that is generated by athermal ω, although some dimples are observed after WQ.
基金This work was financially supported by the Program of National Natural Science Foundation of China(Nos.U1937204 and 51905124)China Postdoctoral Science Foundation(2019M661278).
文摘Complex thin-walled titanium alloy components play a key role in the aircraft,aerospace and marine industries,offering the advantages of reduced weight and increased thermal resistance.The geometrical complexity,dimensional accuracy and in-service properties are essential to fulfill the high-performance standards required in new transportation systems,which brings new challenges to titanium alloy forming technologies.Traditional forming processes,such as superplastic forming or hot pressing,cannot meet all demands of modern applications due to their limited properties,low productivity and high cost.This has encouraged industry and research groups to develop novel high-efficiency forming processes.Hot gas pressure forming and hot stamping-quenching technologies have been developed for the manufacture of tubular and panel components,and are believed to be the cut-edge processes guaranteeing dimensional accuracy,microstructure and mechanical properties.This article intends to provide a critical review of high-efficiency titanium alloy forming processes,concentrating on latest investigations of controlling dimensional accuracy,microstructure and properties.The advantages and limitations of individual forming process are comprehensively analyzed,through which,future research trends of high-efficiency forming are identified including trends in process integration,processing window design,full cycle and multi-objective optimization.This review aims to provide a guide for researchers and process designers on the manufacture of thin-walled titanium alloy components whilst achieving high dimensional accuracy and satisfying performance properties with high efficiency and low cost.
文摘Several detailed studies have comprehensively investigated the benefits and limitations of laser-assisted machining(LAM)of titanium alloys.These studies have highlighted the positive impact of the application of laser preheating on reducing cutting forces and improving productivity but have also identified the detrimental effect of LAM on tool life.This paper seeks to evaluate a series of the most common cutting tools with different coating types used in the machining of titanium alloys to identify whether coating type has a dramatic effect on the dominant tool wear mechanisms active during the process.The findings provide a clear illustration that the challenges facing the application of LAM are associated with the development of new types of cutting tools which are not subjected to the diffusion-controlled wear processes that dominate the performance of current cutting tools.
基金Funded in Part by the Grant from Technology and Industry for National Defense,China(No.AXXD1818)。
文摘Based on the investigated microstructure of different zones in the annealed automatic gas tungsten arc weld joint of TA16 and TC4 titanium alloys,the mechanical property of them was assessed under fatigue crack growth rate tests.For evaluation of fatigue crack growth rate,three points bending specimens were used.The correlation between the range of stress intensity factor and crack growth rate was determined in different zones of the annealed weld joint.Fatigue crack growth rates were obviously different in different zones of weld joint of dissimilar titanium alloys,due to their different microstructures.Scanning electron microscope examinations were conducted on the fracture surface in order to determine the relevant fracture mechanisms and crack growth mechanisms with respect to the details of microstructure.
基金supported by the National Natural Science Foundation of China(No.51205199)the Program for New Century Excellent Talents in University (No.NCET12-0627)+1 种基金the Funding of Jiangsu Innovation Program for Graduate Education (No.CXLX13_141)the Fundamental Research Funds for the Central Universities
文摘We focus on the electrochemical dissolution characteristics of new titanium alloys such as near-αtitanium alloy Ti60,α+βtitanium alloy TC4andβtitanium alloy Ti40 which are often used for aerospace industry.The experiments are carried out by electrochemical machining tool,and the surface morphology of the specimens is observed by the scanning electron microscope(SEM)and three-dimensional video microscope(DVM).The appropriate electrolyte is selected and the relationships between surface roughness and current density are achieved.The results show that the single-phase titanium alloy Ti40 has a better surface roughness after ECM compared with theα+βtitanium alloy TC4 and the near-αtitanium alloy Ti60.The best surface roughness is Ra 0.28μm when the current density is 75A/cm2.Furthermore,the surface roughness of the near-αtitanium alloy Ti60 is the most sensitive with the current density because of the different electrochemical equivalents of substitutional elements and larger grains than TC4.Finally,the suitable current density for each titanium alloy is achieved.
基金This work was financially supported by the National Natural Science Foundation of China(Nos.51901102 and 52101005).
文摘In order to accelerate the research on the property optimization of titanium alloy based on high-throughput methods,it is necessary to reveal the relationship between hardness and other mechanical properties which is still unclear.In this work,taking Ti20C alloy as research object,almost all the microstructure of dual-phase titanium alloys were covered by traversing over 100 heat treatment schemes.Then,massive experiments including microstructure characterization and performance test were conducted,obtaining 51,590 pieces of microstructure data and 3591 pieces of mechanical property data.Subsequently,based on large-scale data-driven technology,the quantitative mapping relationship between hardness and other mechanical properties was deeply discussed.The results of random forest models showed that the correlation between hardness(H)and Charpy impact energy(A_(k))(or elongation,A)was hardly dependent on the microstructure types,while the relationship between H and tensile strength(R_(m))(or yield strength,R_(p0.2))was highly dependent on microstructure types.Specifically,combined with statistical analysis,it was found that the relationship between H and Ak(or A)were negatively linear.Interestingly,the relationship between H and strength was positively linear for equiaxed microstructure,and strength was linked to d^(−1/2)(d,equivalent circle diameter)ofα-grains in the form of classical Hall–Petch formula;but for other microstructures,the relationships were quadratic.Furthermore,the above rules were nearly the same in the rolling direction and transverse direction.Finally,a"four-quadrant partition map"between H and R_(p0.2)/R_(m) was established as a versatile material-screening tool,which can provide guidance for on-demand selection of titanium alloys.
基金supported by the University of Malaya(UM)Research Grant:(FRGS/1/2020/TK0/UM/02/40)。
文摘Commercially pure titanium and titanium alloys have been among the most commonly used materials for biomedical applications since the 1950 s.Due to the excellent mechanical tribological properties,corrosion resistance,biocompatibility,and antibacterial properties of titanium,it is getting much attention as a biomaterial for implants.Furthermore,titanium promotes osseointegration without any additional adhesives by physically bonding with the living bone at the implant site.These properties are crucial for producing high-strength metallic alloys for biomedical applications.Titanium alloys are manufactured into the three types ofα,β,andα+β.The scientific and clinical understanding of titanium and its potential applications,especially in the biomedical field,are still in the early stages.This review aims to establish a credible platform for the current and future roles of titanium in biomedicine.We first explore the developmental history of titanium.Then,we review the recent advancement of the utility of titanium in diverse biomedical areas,its functional properties,mechanisms of biocompatibility,host tissue responses,and various relevant antimicrobial strategies.Future research will be directed toward advanced manufacturing technologies,such as powder-based additive manufacturing,electron beam melting and laser melting deposition,as well as analyzing the effects of alloying elements on the biocompatibility,corrosion resistance,and mechanical properties of titanium.Moreover,the role of titania nanotubes in regenerative medicine and nanomedicine applications,such as localized drug delivery system,immunomodulatory agents,antibacterial agents,and hemocompatibility,is investigated,and the paper concludes with the future outlook of titanium alloys as biomaterials.
基金the financial support from Baosteel Australia Joint Research Centre(BA16003)ARC Research Hub for Computational Particle Technology(IH140100035)funded by Australian Research Council grant LE0882821。
文摘The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.
基金This study was financially supported by the National Natural Science Foundation of China(No.51804087)the Science and Technology Cooperative Foundation of Guizhou province(Nos.[2017]7240 and[2017]5788)+1 种基金the Basic Research Program of Guizhou Province(No.[2019]1091)the Youth Science and Technology Talent Growth Project of Guizhou Education Bureau(No.[2018]107)。
文摘In this study, microstructure and texture evolution of TB8 titanium alloys during hot deformation were investigated by using electron back-scattered diffraction(EBSD) analysis. The results showed that dynamic recrystallization(DRX) behavior of TB8 titanium alloys was drastically sensitive to the strain. As the true strain raised from 0.2 to 0.8, the degree of DRX gradually increased. The nucleation mechanism of recrystallization was observed, including discontinuous dynamic recrystallization(DDRX) resulting from the bulging of original boundaries. Furthermore, continuous dynamic recrystallization(CDRX) occurred because of the transformation of low-angle grain boundaries(LAGBs) to high-angle grain boundaries(HAGBs) in the interior of the original deformed grains. The texture evolution of TB8 titanium alloy during hot deformation process was analyzed in detail, and five texture components were observed,including{001}h100 i,{011}h100 i,{112}h110 i,{111}h110 i, and {111}h112 i. As the true strain increased,deformation textures were gradually weakened due to an increase in the volume fraction of DRX grains. When the true strain was 0.8, the main texture components consisted of the recrystallization texture components of the{001}h100 i and {011}h100 i textures.
文摘This study aims to examine the effect of replacing vanadium by niobium and iron on the tribological behavior of hot-isostatic-pressed titanium alloy(Ti–6Al–4V)biomaterial,using a ball-on-disk-type oscillating tribometer,under wet conditions using physiological solution in accordance with the ISO7148 standards.The tests were carried out under a normal load of 6 N,with an AISI 52100 grade steel ball as a counter face.The morphological changes and structural evolution of the nanoparticle powders using different milling times(2,6,12 and 18 h)were studied.The morphological characterization indicated that the particle and crystallite size continuously decrease with increasing milling time to reach the lowest value of 4 nm at 18-h milling.The friction coefficient and wear rate were lower in the samples milled at 18 h(0.226,0.297 and 0.423;and 0.66 9 10^(-2),0.87 9 10^(-2)and 1.51 9 10^(-2)lm^3N^(-1)lm^(-1))for Ti–6Al–4Fe,Ti–6Al–7Nb and Ti–6Al–4V,respectively.This improvement in friction and wear resistance is attributed to the grain refinement at 18-h milling.The Ti–6Al–4Fe samples showed good tribological performance for all milling times.
文摘The influence of titanium alloy(Ti–5 Al–2.5 Sn) and commercially pure titanium(cp Ti) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti–5 Al–2.5 Sn/cp Ti was investigated in terms of microstructure, mechanical/nano-mechanical properties, and residual stresses. A partial martensitic transformation was observed in the weldments for all the welding conditions due to high heat input. The microstructure evolved in the FZ/cp Ti interfacial region was observed to be the most sensitive to the proportion of α stabilizer in the filler alloy. Furthermore, the addition of filler alloy improved the tensile properties and nano-mechanical response of the weld joint owing to the increased volume of metal in the weld joint. As compared to the Ti–5 Al–2.5 Sn wire, the use of cp Ti filler wire proved to be better in terms of energy absorbed during tensile and impact tests, tensile strength and ductility of the dissimilar welds. An asymmetrical residual stresses profile was observed close to the weld centerline, with high compressive stresses on the Ti–5 Al–2.5 Sn side for both the weldments obtained with and without filler wires. This was attributed to mainly the low thermal conductivity of Ti–5 Al–2.5 Sn. The presence of residual stresses also influenced the nano-hardness profile across the weldments.
基金the National Key Research and Development Program of China(No.2016YFB0701302)the National Natural Science Foundation of China(Nos.52171012 and 51931004)“H2”High-Performance Cluster,the internal City University of Hong Kong under the Programs 7004894 and 9380060。
文摘The design of alloys with simultaneous high strength and high ductility is still a difficult challenge.Here,we propose a new approach to designing multi-phase alloys with a synergistic combination of strength and ductility by engineering heterogeneous precipitate microstructures through the activation of different transformation mechanisms.Using a two-phase titanium alloy as an example,phase field simulations are carried out firstly to design heat treatment schedules that involve both conventional nucleation and growth and non-conventional pseudospinodal decomposition mechanisms,and the calculated microstructures have been evaluated by crystal plasticity finite element modeling.According to simulations,we then set a two-step heat treatment to produce bimodalα+βmicrostructure in Ti-10V-2Fe-3Al.Further mechanical testing shows that the ductility of the alloy is increased by~50%and the strength is increased by~10%as compared to its unimodal counterpart.Our work may provide a general way to improve the mechanical properties of alloys through multiscale microstructure design.
基金the National Key Research and Development Program of China(Nos.2016YFC1100502,2017YFC1104903,and 2016YFC1102601)the National Natural Science Foundation of China(Nos.51631007,51501200,and 51871220)the Key Research Program of Frontier Sciences,CAS(No.QYZDJ-SSW-JSC031-02)。
文摘Porous titanium and its alloys have been considered as promising implants owing to their low elastic modulus and capability to provide channels for bone growth.Currently,additive manufacturing(3D printing)techniques have been successfully applied to produce porous titanium alloys owing to the advantages of controllable and precise fabrication.Considering the safety aspect,an understanding of corrosion in porous titanium alloys and the corresponding mechanisms is important for their long-term application in the human body.In this paper,the recent progress in improving the corrosion properties of porous titanium alloys fabricated via 3D printing techniques is reviewed.The effects of pore type,porosity,electrolyte,and modification of the material on the corrosion properties of porous titanium alloys are introduced and discussed.In addition,the limitations of traditional methods for measuring the corrosion performance of porous titanium alloys were analysed.Perspectives for evaluating and improving the corrosion performance of porous titanium alloys using new methods are provided.
基金the Key R&D Plan of Zhenjiang in 2018 (No.GY2018021)。
文摘The microstructure and mechanical properties of the TB8 titanium alloy were controlled by a secondary processing technology of solution-equal channel angular pressing(ECAP)-aging treatment,which combined strong plastic deformation with heat treatment. The effects of ECAP and heat treatment on the microstructure and properties of the titanium alloy were systematically investigated by optical microscopy(OM), scanning electron microscopy(SEM), hardness tests, and tensile property analysis. The results indicate that the metallographic structure without ECAP treatment is mainly equiaxed β-phase, while that after ECAP treatment is equiaxed β-phase with grain fragmentation, slip bands, and new small grains. After 850 ℃ solutionECAP-520 ℃ aging treatment, the titanium alloy has the smallest grain size, while the directionality of tissue growth along the ECAP direction is the most apparent. Under the same solution-aging conditions, the hardness of the titanium alloy increases from 431.5 to 531.2 HV compared to that without ECAP treatment, i e, increases by 23.11%, and the tensile strength increases from 1 045.30 to 1 176.25 MPa, i e, increases by 12.5%.
基金supported by the National Science and Technology Major Project(No.2017-Ⅶ-0012-0108)the Natural Science Foundation of China(No.51701157)。
文摘In order to improve the oxidation and wear resistance of blades tip of titanium alloys as well as the sealing performance of the gas turbine engine,a Ni/cBN abrasive coating was prepared on titanium alloys through composite electroplating.Oxidation,mechanical and tribological properties of the abrasive coating were investigated.Furthermore,the effect of the oxidation on the mechanical and tribological properties was also evaluated.Oxidation results revealed that the abrasive coating underwent slight oxidation within 700℃.Meanwhile,some intermetallic compounds,Ni3Ti,NiTi and NiTi2,were formed at the coating/substrate interface during oxidation.Due to the pinning effect of cBN particles and the different thermal expansion coefficients of the coating and substrate,the coating/substrate interface was cracked after oxidation at 700℃.Tensile results showed that the presence of coating reduced the strength of the alloy significantly at room temperature,while only marginal variations of the strength of the coated and uncoated specimens at elevated temperatures.Besides,the wear tests indicated that the coating had the excellent cutting ability and wear resistance,which can effectively protect the blades tip of titanium alloys.As the temperature increased,the wear resistance decreased due to the decrease in the mechanical properties of the Ni/cBN coating.
基金the National Natural Science Foundation of China(Nos.51671012,51831006 and 51971009)the International Science and Technology Cooperation Program of China(No.2015DFA51430)the Fundamental Research Funds for the Central Universities。
文摘In this study,a novel strategy for developingα+βdual-phase titanium alloys with low Young's modulus and high yield strength was proposed,and a Ti-15Nb-5Zr-4Sn-1 Fe alloy was developed through theoretical composition design and microstructure manipulation.After hot-rolling and subsequent annealing,a high volume fraction of ultrafine grainedαphase embedded in metastableβ-matrix was formed in the microstructure as intended.Consequently,this alloy exhibits both low Young's modulus(61 GPa)and high yield strength(912 MPa).The experimental results prove that the proposed strategy is appropriate for developing titanium alloys with superior yield strength-to-modulus ratio than those of conventional metallic biomedical materials.Present study might shed light on the research and development of advanced biomedical titanium alloys with low Young's modulus and high yield strength.
基金supported by the National Natural Science Foundation of China(51274076)the Program of New Century Excellent Talents in University(NCET-12-0153)the National Basic Research Program of China(2011CB605504)
文摘Cold crucible directional solidification(CCDS)is a newly developed technique,which combines the advantages of the cold crucible and continuous melting.It can be applied to directionally solidify reactive,high purity and refractory materials.This paper describes the principle of CCDS and its characteristics;development of the measurement and numerical calculation of the magnetic field,flow field and temperature field in CCDS;and the CCDS of Ti based alloys.The paper also reviews original data obtained by some scholars,including the present authors,reported in separate publications in recent years.In Ti based alloys,Ti6Al4V,TiAl alloys and high Nb-containing TiAl alloys,have been directionally solidified in different cold crucibles.The crosssections of the cold crucibles include round,near rectangular and square with different sizes.Tensile testing results show that the elongation of directionally solidified Ti6Al4V can be improved to 12.7%from as cast5.4%.The strength and the elongation of the directionally solidified Ti47Al2Cr2Nb and Ti44Al6Nb1.0Cr2.0V are 650 MPa/3%and 602.5MPa/1.20%,respectively.The ingots after CCDS can be used to prepare turbine or engine blades,and are candidates to replace Ni super-alloy at temperatures of 700 to 900°C.
基金supported by National Key Research and Development Program of China(No.2017YFB0306205)the National Natural Science Foundation of China(Grant No.51671012,51831006,51971009)the Fundamental Research Funds for the Central Universities。
文摘The microstructural evolution,oxidation resistance and mechanical properties of Ti-6.5Al-2Sn-4Hf-2 Nbbased alloys with different contents of tungsten(W)additions ranging from 0 to 4.0 wt.%have been investigated in this study.The addition of W changed the microstructure from Widmanstatten colony of the W-free alloy to basketweave microstructure.After thermal exposure at 650℃ for 1000 h,the retainedβphase became less continuous,and secondaryβnano-particles with high W concentration were precipitated fromαlamellas.Withinαlamellas,the W was found to mainly partitioned into the secondaryβphase and refined the ordered α_(2)-Ti_(3)Al precipitates due to increased solubility of Nb in α_(2) phase.High W addition increased activation energy for oxidation,promoting the formation of more uniform and compact compound oxides,therefore substantially enhanced the oxidation resistance of the alloy.Besides,the W addition also improved the room and high-temperature yield strength without obviously losing plasticity after long-time thermal exposure.The improved mechanical performance was mainly attributed to the introduction of moreα/βinterfaces,the precipitation of secondaryβphase and the refinedα_(2) phase with Nb segregation.
基金The authors acknowledge the financial supports of National Key Research and Development Program of China(2016YFB0301201)。
文摘Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.
