The creep behaviors of as-cast Mg-5Zn-2.5Er alloy(mass fraction,%) ,under various applied stresses(50-70 MPa) and creep temperatures(150-200℃) for 100 h,were investigated.The stress exponent n is in the range of 1.5-...The creep behaviors of as-cast Mg-5Zn-2.5Er alloy(mass fraction,%) ,under various applied stresses(50-70 MPa) and creep temperatures(150-200℃) for 100 h,were investigated.The stress exponent n is in the range of 1.5-5.8,and the activation energy Qc is in the range of 28.3-77.1 kJ/mol.With respect to the calculated n and Qc as well as the microstructures after creep,it is suggested that there is a transition region between grain boundary sliding(GBS) dominated creep to dislocation creep mechanism(from n<3 to n>3) ,arising in the steady-stage creep rate value of 2.89×10-9 s-1.展开更多
Mg-Gd-Zn based alloys have better creep resistance than other Mg alloys and attract more attention at elevated temperatures.However,the multiple alloying elements and various heat treatment conditions,combined with co...Mg-Gd-Zn based alloys have better creep resistance than other Mg alloys and attract more attention at elevated temperatures.However,the multiple alloying elements and various heat treatment conditions,combined with complex microstructural evolution during creep tests,bring great challenges in understanding and predicting creep behaviors.In this study,we proposed to predict the creep properties and reveal the creep mechanisms of Mg-Gd-Zn based alloys by machine learning.On the one hand,the minimum creep rates were effectively predicted by using a support vector regression model.The complex and nonmonotonic effects of test temperature,test stress,alloying elements,and heat treatment conditions on the creep properties were revealed.On the other hand,the creep stress exponents and creep activation energies were calculated by machine learning to analyze the variation of creep mechanisms,based on which the constitutive equations of Mg-Gd-Zn based alloys were obtained.This study introduces an efficient method to comprehend creep behaviors through machine learning,offering valuable insights for the future design and selection of Mg alloys.展开更多
High temperature performance of magnesium alloys can be tailored by either grain size or precipitates in the grain interior.In this study,exceptional creep resistance was successfully acquired in a RE-free cast Mg-Al-...High temperature performance of magnesium alloys can be tailored by either grain size or precipitates in the grain interior.In this study,exceptional creep resistance was successfully acquired in a RE-free cast Mg-Al-Ca-Ti(AC51Ti)alloy.Microalloying of Ti(0.01 wt.%)has been found to be beneficial to the improvement of the tensile creep resistance in a RE-free cast Mg-5Al-0.35Mn-(1Ca)(AC51)alloy,showing a low state creep rate(SCR)of 2.70×10^(−9)s^(−1)at 200℃/50 MPa,which is even better than that of many reported RE-containing Mg alloys.The presence of trace Ti contributes to the substantial refinement and more uniform distribution of Al_(2)Ca precipitates in the matrix.At the same time,the microalloying of Ti improves the solubility of Al and Ca in the matrix.It is reasonable to believe that the microalloying of Ti induced re-organization of Al_(2)Ca precipitates,dissolved a larger amount of Al and Ca atoms into magnesium lattice,and increased the possibility of interaction between GB/dislocations and precipitates,which strongly correlates with the high temperature properties.The creep strengthening mechanisms primarily attributed to both second phase strengthening and solid solution strengthening were separately proposed based on the experimental investigations.展开更多
In this research,the microstructure evolution,mechanical properties,and creep mechanisms of Mg-12 Gd-1 MM-0.6 Zr(wt%)alloy under different conditions were systematically studied using scanning electron microscopy(SEM)...In this research,the microstructure evolution,mechanical properties,and creep mechanisms of Mg-12 Gd-1 MM-0.6 Zr(wt%)alloy under different conditions were systematically studied using scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and tensile creep tests.Regarding the microstructure of the as-cast sample,the average grain size is about 42μm,and the eutectic compounds were determined to be Mg_(5)(Gd_(0.8)MM_(0.2)).During homogenization,these eutectic compounds gradually dissolve,and Mg_(12)MM particles are precipitated.During hot extrusion,complete dynamic recrystallization(DRX)occurs,resulting in equiaxial grains with an average grain size of about 12μm and the formation of streamlines consisting of Mg_(12)MM particles along the extrusion direction(ED).After T5 treatment(225℃for 7 h),a large number ofβ'(Mg_(7)Gd)phases are precipitated on the{11-20}αhabit plane and are interconnected,forming an interlaced network structure.The ultimate tensile strength(R_(m)=405 MPa)and yield strength(R_(P0.2)=288 MPa)of the T5 sample are significantly higher than those of the as-extruded sample(R_(m)=289 MPa,R_(P0.2)=185 MPa),but the elongation(A=4%)was remarkably lower than that of the as-extruded sample(A=18%).When crept at225℃under 100 MPa,the steady-state creep rates of the as-cast,as-extruded,and T5 samples are1.59×10^(-8),1.08×10^(-8),and 1.40×10^(-8)s^(-1),respectively,and their total strains within 100 h are respectively breaking,0.81%,and 0.92%,indicating that the as-extruded alloy exhibits the best creep resistance.TEM analysis reveals that,during the creep process of the T5 sample,theβ'particles coarsen and the precipitate-free zones(PFZs)widen,which increase the steady-state creep rate and the total strain within 100 h as compared with the as-extruded sample.展开更多
The study developed the three alloy systems with different precipitates and examined the role of precipitated phase particles in modifying the creep properties of as-cast Mg^(-1)0Bi-0.5Mn-0.5Ag(BMQ1000),Mg-5Bi-5Sn-0.5...The study developed the three alloy systems with different precipitates and examined the role of precipitated phase particles in modifying the creep properties of as-cast Mg^(-1)0Bi-0.5Mn-0.5Ag(BMQ1000),Mg-5Bi-5Sn-0.5Mn-0.5Ag(BTMQ5500)and Mg^(-1)0Sn-0.5Mn-0.5Ag(TMQ1000)alloys at temperatures ranging from 423 to 473 K and stresses of 45-85 MPa.The values of n for the BMQ1000,BTMQ5500,and TMQ1000 alloys were determined as 6.67,5.75,and 5.92,respectively.Moreover,the activation energy for these alloys was found to be 164.71,134.68,and 135.93 k J/mol,respectively.The results suggested that the creep properties followed the order of BTMQ5500>TMQ1000>BMQ1000.A coarse and uneven distribution of the Mg_(3)Bi_(2)precipitated phase in the BMQ1000 alloy leads to a feeble pining effect on dislocation slipping and generates substantial stress concentrations.For TMQ1000 alloy,while the nanoscale Mg_(2)Sn precipitates have a stronger barrier effect on dislocations than the Mg_(3)Bi_(2)in the BMQ1000 alloy,their ability to inhibit dislocation climbing is comparatively weak.Furthermore,it was found that Mg_(2)Sn precipitates andα-Mg exhibited a preferential orientation relationship of(020)Mg_(2)Sn//(-1010)Mg,and the morphology of the precipitated phase transformed in a way that hinders dislocation movement effectively.In addition,elastic interactions between the precipitated phases are identified.The above-mentioned factors are largely responsible for the notable enhancement in creep resistance.Further,it is ascertained that cross-slip and pyramidal(c+a)slip are primary creep mechanisms in the BMQ1000 alloy.In contrast,the dominant mechanisms in TMQ1000 and BTMQ5500 alloys are dislocation climb and pyramidal(c+a)slip.Moreover,stacking faults(SFs)and twinning assist in the creep deformation of the BTMQ5500 alloy.展开更多
The influence of temperatures on the stacking fault energies and deformation mechanism of a Re- containing single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calc...The influence of temperatures on the stacking fault energies and deformation mechanism of a Re- containing single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calculating the stacking fault energy of alloy, measuring creep properties and performing contrast analysis of dislocation configuration. The results show that the alloy at 760 ℃ possesses lower stacking fault energy, and the stacking fault of alloy increases with increasing temperature. The defor- mation mechanism of alloy during creep at 760 ℃ is 7＇ phase sheared by 〈110〉 super-dislocations, which may be decomposed to form the configuration of Shockley partials plus super-lattice intrinsic stacking fault, while the deformation mechanism of alloy during creep at 1070 ℃ is the screw or edge super- dislocations shearing into the rafted 7＇ phase. But during creep at 7（50 and 980 ℃, some super- dislocations shearing into 7＇ phase may cross-slip from the {111} to {100} planes to form the K-W locks with non-plane core structure, which may restrain the dislocations slipping to enhance the creep resis- tance of alloy at high temperature. The interaction between the Re and other elements may decrease the diffusion rate of atoms to improve the microstructure stability, which is thought to be the main reason why the K-W locks are to be kept in the Re-containing superalloy during creep at 980 ℃.展开更多
Catastrophic failure in engineering structures of island reefs would occur when the tertiary creep initiates in coral reef limestone with a transition from short-to long-term load.Due to the complexity of biological s...Catastrophic failure in engineering structures of island reefs would occur when the tertiary creep initiates in coral reef limestone with a transition from short-to long-term load.Due to the complexity of biological structures,the underlying micro-behaviors involving time-dependent deformation are poorly understood.For this,an abnormal phenomenon was observed where the axial and lateral creep deformations were mutually independent by a series of triaxial tests under constant stress and strain rate conditions.The significantly large lateral creep deformation implies that the creep process cannot be described in continuum mechanics regime.Herein,it is hypothesized that sliding mechanism of crystal cleavages dominates the lateral creep deformation in coral reef limestone.Then,approaches of polarizing microscope(PM)and scanning electronic microscope(SEM)are utilized to validate the hypothesis.It shows that the sliding behavior of crystal cleavages combats with conventional creep micro-mechanisms at certain condition.The former is sensitive to time and strain rate,and is merely activated in the creep regime.展开更多
The effects of Ca and Sr addition on the microstructure and creep properties of Mg-4Al-2Sn alloys were examined. Tensile tests at 25 ℃ and 200 ℃ and creep tests at 150 ℃ and 200 ℃ were carried out to estimate the ...The effects of Ca and Sr addition on the microstructure and creep properties of Mg-4Al-2Sn alloys were examined. Tensile tests at 25 ℃ and 200 ℃ and creep tests at 150 ℃ and 200 ℃ were carried out to estimate the room temperature and high temperature mechanical properties of these alloys. The microstrueture of the Mg-4Al-2Sn alloy showed dendritic a-Mg, Mg17Al12 and Mg2Sn phases. The latter two phases precipitated along the grain boundaries. The addition of Ca and Sr resulted in the formation of ternary CaMgSn and SrMgSn phases within the grain. The grain size was reduced slightly with the addition of Sr and Ca. The tensile strength was decreased by the addition of Ca and Sr at room temperature. However, the high temperature tensile strength was increased. The creep strength was improved by the addition of Ca and Sr.展开更多
The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4 Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range b...The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4 Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ/G(where σ is the stress;G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable Mg Sn2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg2 Ca at the grain boundaries which improve the creep resistance of Mg-4 Sn alloys. According to the stress exponents(6.04<n<6.89) and activation energies(101.37 k J/mol<Q<113.8 k J/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.展开更多
High-temperature creep properties of sintered uranium dioxide pellets with two grain sizes （9.0 μm and 23.8μm） were studied. The results indicate that the creep rate becomes a little faster with the reduction of t...High-temperature creep properties of sintered uranium dioxide pellets with two grain sizes （9.0 μm and 23.8μm） were studied. The results indicate that the creep rate becomes a little faster with the reduction of the uranium dioxide grain size at the same temperature and the same load. At the same temperature, the logarithmic value of the steady creep rate vs stress has linear relation, and with increasing load, the steady creep rate of the sintered uranium dioxide pellet increases. Under the same load, the steady creep rate of the sintered uranium dioxide pellet increases with increasing temperature; and the creep rates of sintered uranium dioxide pellet with the grain size of 9.0 μm and 23.8 μm under 10 MPa are almost the same. The creep process is controlled both by Nabarro--Herring creep and Hamper-Dorn creep for uranium dioxide pellet with grain size of 9.0 μm, while Hamper---Dora creep is the dominantmechanism for uranium dioxide with grain size of 23.8 μm.展开更多
Localized creep damage in a notched round specimen has been investigated in this paper based on the creep damage mechanics and the DCPD technique. Expressions of creep damage equivalent stress under multi-axial state ...Localized creep damage in a notched round specimen has been investigated in this paper based on the creep damage mechanics and the DCPD technique. Expressions of creep damage equivalent stress under multi-axial state are given to describe the validity for localized damage in ductile materials. A DCPD method is introduced into the measurement of local creep damage near the tip of V-type notch of round bar. The technique with instrument configuration, selection of probe position and measuring calibration is also presented in the paper. Some results of creep damage estimation are shown on 2.25Cr-1Mo at 550℃.展开更多
To study the static bending creep properties of glass fiber reinforced wood,glass fiber reinforced poplar(GFRP)specimens were obtained by pasting glass fiber on the upper and lower surfaces of Poplar(Populus euramevic...To study the static bending creep properties of glass fiber reinforced wood,glass fiber reinforced poplar(GFRP)specimens were obtained by pasting glass fiber on the upper and lower surfaces of Poplar(Populus euramevicana,P),the performance of Normal Creep(NC)and Mechanical Sorptive Creep(MSC)of GFRP and their influencing factors were tested and analyzed.The test results and analysis show that:(1)The MOE and MOR of Poplar were increased by 17.06%and 10.00%respectively by the glass fiber surface reinforced composite.(2)The surface reinforced P with glass fiber cloth only exhibits the NC pattern of wood and loses the MSC characteristics of wood,regardless of the constant or alternating changes in relative humidity.(3)The instantaneous elastic deformation,viscoelastic deformation,viscous deformation and total creep deflection of GFRP are positively correlated with the stress level of the external load applied to the specimen.Still,the specimen’s creep recovery rate is negatively correlated with the stress level of the external load applied to the specimen.The static creep deflection and viscous deformation of GFRP increase with the increase of the relative humidity of the environment.(4)The MSC maximum creep deflection of GFRP increased by only 7.41%over the NC maximum creep deflection,but the MSC maximum creep deflection of P increased by 199.25%over the NC maximum creep deflection.(5)The Burgers 4-factor model and the Weibull distribution equation can fit the NC and NC recovery processes of GFRP well.展开更多
Three kinds of polymeric materials are taken as example for the verification of linear ex-trapolation method from unified master lines with reduced universal equations on creep and stress relaxation tests. The theoret...Three kinds of polymeric materials are taken as example for the verification of linear ex-trapolation method from unified master lines with reduced universal equations on creep and stress relaxation tests. The theoretical values of long-term mechanical behavior and lifetime for a cured epoxide, polypropylene, poly（methyl-methacrylate）, and SBR rubber are directly evaluated with the universal equations on reduced creep compliance and reduced stress relax-ation modulus and are compared with their predicted values by the linear extrapolation from the unified master lines of creep and stress relaxation. The results show that the theoretical values of dimensional stability, bearing ability and lifetime are in an excellent agreement with the predicted values, it shows that the linear extrapolation method is more simple and reliable. The dependences of long-term mechanical behaviors and lifetime on the different aging times are discussed.展开更多
Through-thickness heterogeneity in creep properties of 7B50-T7451 aluminum alloy Friction Stir Welding(FSW)joints was investigated.Creep tests for three slices of the FSW joint were conducted at the temperature of 150...Through-thickness heterogeneity in creep properties of 7B50-T7451 aluminum alloy Friction Stir Welding(FSW)joints was investigated.Creep tests for three slices of the FSW joint were conducted at the temperature of 150-200℃ and applied stress of 60-225 MPa.The theta projection method was used to predict creep curves and minimum creep rate.The results show that the minimum creep rate increases and creep rupture life decreases with the increase of creep temperature and applied stress.Creep properties of the FSW joint deteriorate along the thickness direction from the top to the bottom.The threshold stress of all three slices of the FSW joint decreases with the increase of creep temperature and even disappears at 200℃ for the bottom slice.Creep activation energy approaches the activation energy of the lattice self-diffusion of aluminum.The value of true stress exponent for different slices is approximately equal to three.The predominant creep mechanism of the FSW joint is dislocation viscous glide by lattice self-diffusion.What is more,a constitutive model is established based on the theta method to accurately describe creep behavior ofdifferent slices of the FSW joint.展开更多
Here we consider our four-point flexure and compression creep results obtained under Ar protection at 1800℃ to predict the tensile creep behavior of a ZrB_(2)-20 vol%SiC ultra-high temperature ceramic.Assuming power ...Here we consider our four-point flexure and compression creep results obtained under Ar protection at 1800℃ to predict the tensile creep behavior of a ZrB_(2)-20 vol%SiC ultra-high temperature ceramic.Assuming power law creep,and based on four-point bend data,we estimated the uniaxial creep parameters using an analytical method present in the literature.Both predicted and experimental compressive stress exponents were found to be in excellent agreement,1.85 and 1.76 respectively,while observation of the microstructure suggested a combination of diffusion and grain boundary sliding creep mechanisms in compression.Along with the microstructural evidence associated with the tensile regions of the flexure specimens,the predicted tensile stress exponent of 2.61 exceeds the measured flexural value of 2.2.We assert an increasing role of cavitation to the creep strain in pure tension.This cavitation component adds to the dominant grain boundary sliding mechanism as described below and elsewhere for flexural creep.展开更多
Anode-free all-solid-state batteries(AF-ASSBs)have received significant attention as a next-generation battery system due to their high energy density and safety.However,this system still faces challenges,such as poor...Anode-free all-solid-state batteries(AF-ASSBs)have received significant attention as a next-generation battery system due to their high energy density and safety.However,this system still faces challenges,such as poor Coulombic efficiency and short-circuiting caused by Li dendrite growth.In this study,the AF-ASSBs are demonstrated with reliable and robust electrochemical properties by employing Cu-Sn nanotube(NT)thin layer(~1μm)on the Cu current collector for regulating Li electrodeposition.Li_(x)Sn phases with high Li-ion diffusivity in the lithiated Cu-Sn NT layer enable facile Li diffusion along with its one-dimensional hollow geometry.The unique structure,in which Li electrodeposition takes place between the Cu-Sn NT layer and the current collector by the Coble creep mechanism,improves cell durability by preventing solid electrolyte(SE)decomposition and Li dendrite growth.Furthermore,the large surface area of the Cu-Sn NT layer ensures close contact with the SE layer,leading to a reduced lithiation overpotential compared to that of a flat Cu-Sn layer.The Cu-Sn NT layer also maintains its structural integrity owing to its high mechanical properties and porous nature,which could further alleviate the mechanical stress.The LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)(NCM)|SE|Cu-Sn NT@Cu cell with a practical capacity of 2.9 mAh cm^(−2) exhibits 83.8%cycle retention after 150 cycles and an average Coulombic efficiency of 99.85%at room temperature.It also demonstrates a critical current density 4.5 times higher compared to the NCM|SE|Cu cell.展开更多
Metallic glasses are spatially heterogeneous at the nanometer scale.However,the effects of external excitation on their structural and mechanical heterogeneity and the correlation to their properties are still unresol...Metallic glasses are spatially heterogeneous at the nanometer scale.However,the effects of external excitation on their structural and mechanical heterogeneity and the correlation to their properties are still unresolved.Nanoindentation,atomic force microscopy(AFM) and high-resolution transmis sion elec tron micro scopy(HRTEM) were carried out to explore the effects of cryogenic thermal cycling(CTC) on mechanical/structural heterogeneity,nano sc ale creep deformation and optical properties of nano structured metallic glass thin films(MGTFs).The results indicate that CTC treatment alters the distribution fluctuations of hardness/modulus and energy dissipation and results in an increase-then-decrease variation in mechanical heterogeneity.By applying Maxwell-Voigt model,it can be shown that CTC treatment results in a remarkable activation of more defects with longer relaxation time in soft regions but has only a slight effect on defects in hard regions.In addition,CTC treatment increases the transition time from primary-state stage to steady-state stage during creep deformation.The enhanced optical reflectivity of the MGTFs after 15 thermal cycles can be attributed to increased aggregation of Cu and Ni elements.The results of this study shed new light on understanding mechanical/structural heterogeneity and its influence on nanoscale creep deformation and optical characteristics of nanostructured MGTFs,and facilitate the design of high-performance nanostructured MGTFs.展开更多
Materials in engineering applications are rarely uniaxially-loaded.In reality,failures under multiaxial loading has been widely observed in engineering structures.The life prediction of a component under multiaxial st...Materials in engineering applications are rarely uniaxially-loaded.In reality,failures under multiaxial loading has been widely observed in engineering structures.The life prediction of a component under multiaxial stresses has long been a challenging issue,particularly for high temperature applications.To distinguish the mode of failure ranging from a maximum principal stress intergranular damage to von Mises effective stress rupture mode a multiaxial stress rupture criterion(MSRC)was originally proposed by Sdobyrev and then Hayhurst and Leckie(SHL MSRC).A multiaxial-factor,α,was developed as a result which was intended to be a material constant and differentiates the bias of the MSRC between maxi-mum principal stress and effective stress.The success of the SHL MSRC relies on accurately calibrating the value ofαto quantify the multiaxial response of the material/geometry combination.To find a more suitable approach for determining MSRC,the applicability of different methods are evaluated.Given that the resulting analysis of the various approaches can be affected by the creep failure mechanism,princi-ples in the determination of MSRC with and without using continuum damage mechanics approaches are recommended.The viability of uniaxial material parameters in correlating withαthrough the analysis of available data in literature is also presented.It is found that the increase of the uniaxial creep dam-age tolerance parameterλis accompanied bythe decreaseof theα-value,whichimplies thatthe creep ductility plays an important role in affecting the multiaxial rupture behavior of materials.展开更多
Background:Following combat-related,extensive soft tissue injury from gunshot wounds or blasts,prolonged duration from injury to full wound closure is associated with infection,increased morbidity and mortality,failur...Background:Following combat-related,extensive soft tissue injury from gunshot wounds or blasts,prolonged duration from injury to full wound closure is associated with infection,increased morbidity and mortality,failure to mobilize,poor functional outcome and increased cost.The purpose of this study was to evaluate a novel treatment enabling early primary closure of combat wounds.Methods:This was a retrospective study of 10 soldiers and civilians with extensive combat-related soft tissue limb injuries(5 gunshot wounds,5 blasts)treated using the TopClosure^(■)Tension Relief System(TRS)with simultaneous administration of regulated oxygen-enriched and irrigation negative pressure-assisted wound therapy(ROINPT)via the Vcareα^(■)device.Results:Nine patients were treated during the acute phase of injury and one was treated following removal of a flap due to deep infection 20 years after injury and flap reconstruction.Two patients had upper limb injury and the rest lower limb injury.With the aid of the TRS and/or ROINPT,immediate primary closure during reconstruction was achieved in 6 patients and delayed primary closure in three.Only one patient required a skin graft to close a small area of the wound after most of the wound had been closed by delayed primary closure.Wound closure was achieved within 0–37 days(median:12.5 days,interquartile range:2.75–19.75)from injury.Conclusions:The TRS is a novel device for effective,early skin stretching and secure wound closure through the application of stress relaxation and mechanical creep,achieving primary closure of large defects using a simplified surgical technique and reducing the need for closure using skin grafts and flaps and the use of tissue expanders.Delivering supplemental oxygen to the wound by ROINPT reverses the reduced oxygen levels inherent in conventional negative pressure-assisted wound therapy,mitigating anaerobic contamination and reducing infection.Irrigation may accelerate the evacuation of infectious material from the wound and provide a novel method for antibiotic administration.The combination of TRS and ROINPT devices allow for early primary closure with improved functionality of combat-related limb injuries.展开更多
基金Project(2007CB613706) supported by the National Basic Research Program of China
文摘The creep behaviors of as-cast Mg-5Zn-2.5Er alloy(mass fraction,%) ,under various applied stresses(50-70 MPa) and creep temperatures(150-200℃) for 100 h,were investigated.The stress exponent n is in the range of 1.5-5.8,and the activation energy Qc is in the range of 28.3-77.1 kJ/mol.With respect to the calculated n and Qc as well as the microstructures after creep,it is suggested that there is a transition region between grain boundary sliding(GBS) dominated creep to dislocation creep mechanism(from n<3 to n>3) ,arising in the steady-stage creep rate value of 2.89×10-9 s-1.
基金supported by the National Science and Technology Major Project(Grant number J2019-VI-0004-0118)the National Natural Science Foundation of China(Grant number 51771152)+2 种基金the National Key R&D Program of China(Grant number 2018YFB1106800)supported by the Brain Pool Program through the National Research Foundation of Korea(NRF)(Grant No.RS-2023-00304296)supported by the Brain Pool Program through National Research Foundation of Korea(NRF)(Grant No.RS-2023-00222130).
文摘Mg-Gd-Zn based alloys have better creep resistance than other Mg alloys and attract more attention at elevated temperatures.However,the multiple alloying elements and various heat treatment conditions,combined with complex microstructural evolution during creep tests,bring great challenges in understanding and predicting creep behaviors.In this study,we proposed to predict the creep properties and reveal the creep mechanisms of Mg-Gd-Zn based alloys by machine learning.On the one hand,the minimum creep rates were effectively predicted by using a support vector regression model.The complex and nonmonotonic effects of test temperature,test stress,alloying elements,and heat treatment conditions on the creep properties were revealed.On the other hand,the creep stress exponents and creep activation energies were calculated by machine learning to analyze the variation of creep mechanisms,based on which the constitutive equations of Mg-Gd-Zn based alloys were obtained.This study introduces an efficient method to comprehend creep behaviors through machine learning,offering valuable insights for the future design and selection of Mg alloys.
基金funded by the National Natural Science Foundation of China(Grant numbers U22A20187,52374405,52371177).
文摘High temperature performance of magnesium alloys can be tailored by either grain size or precipitates in the grain interior.In this study,exceptional creep resistance was successfully acquired in a RE-free cast Mg-Al-Ca-Ti(AC51Ti)alloy.Microalloying of Ti(0.01 wt.%)has been found to be beneficial to the improvement of the tensile creep resistance in a RE-free cast Mg-5Al-0.35Mn-(1Ca)(AC51)alloy,showing a low state creep rate(SCR)of 2.70×10^(−9)s^(−1)at 200℃/50 MPa,which is even better than that of many reported RE-containing Mg alloys.The presence of trace Ti contributes to the substantial refinement and more uniform distribution of Al_(2)Ca precipitates in the matrix.At the same time,the microalloying of Ti improves the solubility of Al and Ca in the matrix.It is reasonable to believe that the microalloying of Ti induced re-organization of Al_(2)Ca precipitates,dissolved a larger amount of Al and Ca atoms into magnesium lattice,and increased the possibility of interaction between GB/dislocations and precipitates,which strongly correlates with the high temperature properties.The creep strengthening mechanisms primarily attributed to both second phase strengthening and solid solution strengthening were separately proposed based on the experimental investigations.
基金Project supported by National Key Research and Development Program of China(2013CB632205)。
文摘In this research,the microstructure evolution,mechanical properties,and creep mechanisms of Mg-12 Gd-1 MM-0.6 Zr(wt%)alloy under different conditions were systematically studied using scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and tensile creep tests.Regarding the microstructure of the as-cast sample,the average grain size is about 42μm,and the eutectic compounds were determined to be Mg_(5)(Gd_(0.8)MM_(0.2)).During homogenization,these eutectic compounds gradually dissolve,and Mg_(12)MM particles are precipitated.During hot extrusion,complete dynamic recrystallization(DRX)occurs,resulting in equiaxial grains with an average grain size of about 12μm and the formation of streamlines consisting of Mg_(12)MM particles along the extrusion direction(ED).After T5 treatment(225℃for 7 h),a large number ofβ'(Mg_(7)Gd)phases are precipitated on the{11-20}αhabit plane and are interconnected,forming an interlaced network structure.The ultimate tensile strength(R_(m)=405 MPa)and yield strength(R_(P0.2)=288 MPa)of the T5 sample are significantly higher than those of the as-extruded sample(R_(m)=289 MPa,R_(P0.2)=185 MPa),but the elongation(A=4%)was remarkably lower than that of the as-extruded sample(A=18%).When crept at225℃under 100 MPa,the steady-state creep rates of the as-cast,as-extruded,and T5 samples are1.59×10^(-8),1.08×10^(-8),and 1.40×10^(-8)s^(-1),respectively,and their total strains within 100 h are respectively breaking,0.81%,and 0.92%,indicating that the as-extruded alloy exhibits the best creep resistance.TEM analysis reveals that,during the creep process of the T5 sample,theβ'particles coarsen and the precipitate-free zones(PFZs)widen,which increase the steady-state creep rate and the total strain within 100 h as compared with the as-extruded sample.
基金National Natural Science Foundation of China(Grant Nos.51901153)Shanxi Scholarship Council of China(Grant No.2019032)Natural Science Foundation of Shanxi(Grant No.202103021224049)。
文摘The study developed the three alloy systems with different precipitates and examined the role of precipitated phase particles in modifying the creep properties of as-cast Mg^(-1)0Bi-0.5Mn-0.5Ag(BMQ1000),Mg-5Bi-5Sn-0.5Mn-0.5Ag(BTMQ5500)and Mg^(-1)0Sn-0.5Mn-0.5Ag(TMQ1000)alloys at temperatures ranging from 423 to 473 K and stresses of 45-85 MPa.The values of n for the BMQ1000,BTMQ5500,and TMQ1000 alloys were determined as 6.67,5.75,and 5.92,respectively.Moreover,the activation energy for these alloys was found to be 164.71,134.68,and 135.93 k J/mol,respectively.The results suggested that the creep properties followed the order of BTMQ5500>TMQ1000>BMQ1000.A coarse and uneven distribution of the Mg_(3)Bi_(2)precipitated phase in the BMQ1000 alloy leads to a feeble pining effect on dislocation slipping and generates substantial stress concentrations.For TMQ1000 alloy,while the nanoscale Mg_(2)Sn precipitates have a stronger barrier effect on dislocations than the Mg_(3)Bi_(2)in the BMQ1000 alloy,their ability to inhibit dislocation climbing is comparatively weak.Furthermore,it was found that Mg_(2)Sn precipitates andα-Mg exhibited a preferential orientation relationship of(020)Mg_(2)Sn//(-1010)Mg,and the morphology of the precipitated phase transformed in a way that hinders dislocation movement effectively.In addition,elastic interactions between the precipitated phases are identified.The above-mentioned factors are largely responsible for the notable enhancement in creep resistance.Further,it is ascertained that cross-slip and pyramidal(c+a)slip are primary creep mechanisms in the BMQ1000 alloy.In contrast,the dominant mechanisms in TMQ1000 and BTMQ5500 alloys are dislocation climb and pyramidal(c+a)slip.Moreover,stacking faults(SFs)and twinning assist in the creep deformation of the BTMQ5500 alloy.
基金supported by the National Natural Science Foundation of China (Grant No. 51271125)
文摘The influence of temperatures on the stacking fault energies and deformation mechanism of a Re- containing single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calculating the stacking fault energy of alloy, measuring creep properties and performing contrast analysis of dislocation configuration. The results show that the alloy at 760 ℃ possesses lower stacking fault energy, and the stacking fault of alloy increases with increasing temperature. The defor- mation mechanism of alloy during creep at 760 ℃ is 7＇ phase sheared by 〈110〉 super-dislocations, which may be decomposed to form the configuration of Shockley partials plus super-lattice intrinsic stacking fault, while the deformation mechanism of alloy during creep at 1070 ℃ is the screw or edge super- dislocations shearing into the rafted 7＇ phase. But during creep at 7（50 and 980 ℃, some super- dislocations shearing into 7＇ phase may cross-slip from the {111} to {100} planes to form the K-W locks with non-plane core structure, which may restrain the dislocations slipping to enhance the creep resis- tance of alloy at high temperature. The interaction between the Re and other elements may decrease the diffusion rate of atoms to improve the microstructure stability, which is thought to be the main reason why the K-W locks are to be kept in the Re-containing superalloy during creep at 980 ℃.
基金supported by the National Natural Science Foundation of China(Grant Nos.41877267,41877260)the Priority Research Program of the Chinese Academy of Science(Grant No.XDA13010201).
文摘Catastrophic failure in engineering structures of island reefs would occur when the tertiary creep initiates in coral reef limestone with a transition from short-to long-term load.Due to the complexity of biological structures,the underlying micro-behaviors involving time-dependent deformation are poorly understood.For this,an abnormal phenomenon was observed where the axial and lateral creep deformations were mutually independent by a series of triaxial tests under constant stress and strain rate conditions.The significantly large lateral creep deformation implies that the creep process cannot be described in continuum mechanics regime.Herein,it is hypothesized that sliding mechanism of crystal cleavages dominates the lateral creep deformation in coral reef limestone.Then,approaches of polarizing microscope(PM)and scanning electronic microscope(SEM)are utilized to validate the hypothesis.It shows that the sliding behavior of crystal cleavages combats with conventional creep micro-mechanisms at certain condition.The former is sensitive to time and strain rate,and is merely activated in the creep regime.
基金supported by a grant from the Metals Bank by the Ministry of Knowledge Economy and a grant-in-aid for the National Core Research Center Program (No.R15-2006-022-02001-0)
文摘The effects of Ca and Sr addition on the microstructure and creep properties of Mg-4Al-2Sn alloys were examined. Tensile tests at 25 ℃ and 200 ℃ and creep tests at 150 ℃ and 200 ℃ were carried out to estimate the room temperature and high temperature mechanical properties of these alloys. The microstrueture of the Mg-4Al-2Sn alloy showed dendritic a-Mg, Mg17Al12 and Mg2Sn phases. The latter two phases precipitated along the grain boundaries. The addition of Ca and Sr resulted in the formation of ternary CaMgSn and SrMgSn phases within the grain. The grain size was reduced slightly with the addition of Sr and Ca. The tensile strength was decreased by the addition of Ca and Sr at room temperature. However, the high temperature tensile strength was increased. The creep strength was improved by the addition of Ca and Sr.
文摘The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4 Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ/G(where σ is the stress;G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable Mg Sn2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg2 Ca at the grain boundaries which improve the creep resistance of Mg-4 Sn alloys. According to the stress exponents(6.04<n<6.89) and activation energies(101.37 k J/mol<Q<113.8 k J/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.
基金Project(50874126)supported by the National Natural Science Foundation of China
文摘High-temperature creep properties of sintered uranium dioxide pellets with two grain sizes （9.0 μm and 23.8μm） were studied. The results indicate that the creep rate becomes a little faster with the reduction of the uranium dioxide grain size at the same temperature and the same load. At the same temperature, the logarithmic value of the steady creep rate vs stress has linear relation, and with increasing load, the steady creep rate of the sintered uranium dioxide pellet increases. Under the same load, the steady creep rate of the sintered uranium dioxide pellet increases with increasing temperature; and the creep rates of sintered uranium dioxide pellet with the grain size of 9.0 μm and 23.8 μm under 10 MPa are almost the same. The creep process is controlled both by Nabarro--Herring creep and Hamper-Dorn creep for uranium dioxide pellet with grain size of 9.0 μm, while Hamper---Dora creep is the dominantmechanism for uranium dioxide with grain size of 23.8 μm.
文摘Localized creep damage in a notched round specimen has been investigated in this paper based on the creep damage mechanics and the DCPD technique. Expressions of creep damage equivalent stress under multi-axial state are given to describe the validity for localized damage in ductile materials. A DCPD method is introduced into the measurement of local creep damage near the tip of V-type notch of round bar. The technique with instrument configuration, selection of probe position and measuring calibration is also presented in the paper. Some results of creep damage estimation are shown on 2.25Cr-1Mo at 550℃.
基金The present work was financially sponsored by the National Natural Science Foundation of China(Grant No.31960291).
文摘To study the static bending creep properties of glass fiber reinforced wood,glass fiber reinforced poplar(GFRP)specimens were obtained by pasting glass fiber on the upper and lower surfaces of Poplar(Populus euramevicana,P),the performance of Normal Creep(NC)and Mechanical Sorptive Creep(MSC)of GFRP and their influencing factors were tested and analyzed.The test results and analysis show that:(1)The MOE and MOR of Poplar were increased by 17.06%and 10.00%respectively by the glass fiber surface reinforced composite.(2)The surface reinforced P with glass fiber cloth only exhibits the NC pattern of wood and loses the MSC characteristics of wood,regardless of the constant or alternating changes in relative humidity.(3)The instantaneous elastic deformation,viscoelastic deformation,viscous deformation and total creep deflection of GFRP are positively correlated with the stress level of the external load applied to the specimen.Still,the specimen’s creep recovery rate is negatively correlated with the stress level of the external load applied to the specimen.The static creep deflection and viscous deformation of GFRP increase with the increase of the relative humidity of the environment.(4)The MSC maximum creep deflection of GFRP increased by only 7.41%over the NC maximum creep deflection,but the MSC maximum creep deflection of P increased by 199.25%over the NC maximum creep deflection.(5)The Burgers 4-factor model and the Weibull distribution equation can fit the NC and NC recovery processes of GFRP well.
基金This work was supported by the National Natural Science Foundation of China （No.50973007）.
文摘Three kinds of polymeric materials are taken as example for the verification of linear ex-trapolation method from unified master lines with reduced universal equations on creep and stress relaxation tests. The theoretical values of long-term mechanical behavior and lifetime for a cured epoxide, polypropylene, poly（methyl-methacrylate）, and SBR rubber are directly evaluated with the universal equations on reduced creep compliance and reduced stress relax-ation modulus and are compared with their predicted values by the linear extrapolation from the unified master lines of creep and stress relaxation. The results show that the theoretical values of dimensional stability, bearing ability and lifetime are in an excellent agreement with the predicted values, it shows that the linear extrapolation method is more simple and reliable. The dependences of long-term mechanical behaviors and lifetime on the different aging times are discussed.
基金financially supported by the National Natural Science Foundation of China(No.52075450)the Fundamental Research Funds for the Central Universities,China(No.D5000220503).
文摘Through-thickness heterogeneity in creep properties of 7B50-T7451 aluminum alloy Friction Stir Welding(FSW)joints was investigated.Creep tests for three slices of the FSW joint were conducted at the temperature of 150-200℃ and applied stress of 60-225 MPa.The theta projection method was used to predict creep curves and minimum creep rate.The results show that the minimum creep rate increases and creep rupture life decreases with the increase of creep temperature and applied stress.Creep properties of the FSW joint deteriorate along the thickness direction from the top to the bottom.The threshold stress of all three slices of the FSW joint decreases with the increase of creep temperature and even disappears at 200℃ for the bottom slice.Creep activation energy approaches the activation energy of the lattice self-diffusion of aluminum.The value of true stress exponent for different slices is approximately equal to three.The predominant creep mechanism of the FSW joint is dislocation viscous glide by lattice self-diffusion.What is more,a constitutive model is established based on the theta method to accurately describe creep behavior ofdifferent slices of the FSW joint.
基金AFOSR program monitor Ali Sayir,under grant#FA9550-09-1-0200,for partial support of this work.
文摘Here we consider our four-point flexure and compression creep results obtained under Ar protection at 1800℃ to predict the tensile creep behavior of a ZrB_(2)-20 vol%SiC ultra-high temperature ceramic.Assuming power law creep,and based on four-point bend data,we estimated the uniaxial creep parameters using an analytical method present in the literature.Both predicted and experimental compressive stress exponents were found to be in excellent agreement,1.85 and 1.76 respectively,while observation of the microstructure suggested a combination of diffusion and grain boundary sliding creep mechanisms in compression.Along with the microstructural evidence associated with the tensile regions of the flexure specimens,the predicted tensile stress exponent of 2.61 exceeds the measured flexural value of 2.2.We assert an increasing role of cavitation to the creep strain in pure tension.This cavitation component adds to the dominant grain boundary sliding mechanism as described below and elsewhere for flexural creep.
基金Korea Institute of Energy Technology Evaluation and Planning,Grant/Award Number:20214000000520Ministry of Trade,Industry and Energy,Grant/Award Number:20009985。
文摘Anode-free all-solid-state batteries(AF-ASSBs)have received significant attention as a next-generation battery system due to their high energy density and safety.However,this system still faces challenges,such as poor Coulombic efficiency and short-circuiting caused by Li dendrite growth.In this study,the AF-ASSBs are demonstrated with reliable and robust electrochemical properties by employing Cu-Sn nanotube(NT)thin layer(~1μm)on the Cu current collector for regulating Li electrodeposition.Li_(x)Sn phases with high Li-ion diffusivity in the lithiated Cu-Sn NT layer enable facile Li diffusion along with its one-dimensional hollow geometry.The unique structure,in which Li electrodeposition takes place between the Cu-Sn NT layer and the current collector by the Coble creep mechanism,improves cell durability by preventing solid electrolyte(SE)decomposition and Li dendrite growth.Furthermore,the large surface area of the Cu-Sn NT layer ensures close contact with the SE layer,leading to a reduced lithiation overpotential compared to that of a flat Cu-Sn layer.The Cu-Sn NT layer also maintains its structural integrity owing to its high mechanical properties and porous nature,which could further alleviate the mechanical stress.The LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)(NCM)|SE|Cu-Sn NT@Cu cell with a practical capacity of 2.9 mAh cm^(−2) exhibits 83.8%cycle retention after 150 cycles and an average Coulombic efficiency of 99.85%at room temperature.It also demonstrates a critical current density 4.5 times higher compared to the NCM|SE|Cu cell.
基金financially supported by the National Natural Science Foundation of China (Nos. 51971061 and 52231005)the Natural Science Foundation of Jiangsu Province (No. BK20221474)。
文摘Metallic glasses are spatially heterogeneous at the nanometer scale.However,the effects of external excitation on their structural and mechanical heterogeneity and the correlation to their properties are still unresolved.Nanoindentation,atomic force microscopy(AFM) and high-resolution transmis sion elec tron micro scopy(HRTEM) were carried out to explore the effects of cryogenic thermal cycling(CTC) on mechanical/structural heterogeneity,nano sc ale creep deformation and optical properties of nano structured metallic glass thin films(MGTFs).The results indicate that CTC treatment alters the distribution fluctuations of hardness/modulus and energy dissipation and results in an increase-then-decrease variation in mechanical heterogeneity.By applying Maxwell-Voigt model,it can be shown that CTC treatment results in a remarkable activation of more defects with longer relaxation time in soft regions but has only a slight effect on defects in hard regions.In addition,CTC treatment increases the transition time from primary-state stage to steady-state stage during creep deformation.The enhanced optical reflectivity of the MGTFs after 15 thermal cycles can be attributed to increased aggregation of Cu and Ni elements.The results of this study shed new light on understanding mechanical/structural heterogeneity and its influence on nanoscale creep deformation and optical characteristics of nanostructured MGTFs,and facilitate the design of high-performance nanostructured MGTFs.
基金This work was financially supported by Projects of the National Natural Science Foundation of China(Nos.521130511,11502082,52075174)the Higher Education Discipline Innovation Project("111 Project")(No.B13020).Helpful discussions with Jian-Feng Wen and Guo-Zhen Wang at the East China University of Science and Technology and Qiang Xu at the University of Huddersfield are gratefully acknowledged.
文摘Materials in engineering applications are rarely uniaxially-loaded.In reality,failures under multiaxial loading has been widely observed in engineering structures.The life prediction of a component under multiaxial stresses has long been a challenging issue,particularly for high temperature applications.To distinguish the mode of failure ranging from a maximum principal stress intergranular damage to von Mises effective stress rupture mode a multiaxial stress rupture criterion(MSRC)was originally proposed by Sdobyrev and then Hayhurst and Leckie(SHL MSRC).A multiaxial-factor,α,was developed as a result which was intended to be a material constant and differentiates the bias of the MSRC between maxi-mum principal stress and effective stress.The success of the SHL MSRC relies on accurately calibrating the value ofαto quantify the multiaxial response of the material/geometry combination.To find a more suitable approach for determining MSRC,the applicability of different methods are evaluated.Given that the resulting analysis of the various approaches can be affected by the creep failure mechanism,princi-ples in the determination of MSRC with and without using continuum damage mechanics approaches are recommended.The viability of uniaxial material parameters in correlating withαthrough the analysis of available data in literature is also presented.It is found that the increase of the uniaxial creep dam-age tolerance parameterλis accompanied bythe decreaseof theα-value,whichimplies thatthe creep ductility plays an important role in affecting the multiaxial rupture behavior of materials.
文摘Background:Following combat-related,extensive soft tissue injury from gunshot wounds or blasts,prolonged duration from injury to full wound closure is associated with infection,increased morbidity and mortality,failure to mobilize,poor functional outcome and increased cost.The purpose of this study was to evaluate a novel treatment enabling early primary closure of combat wounds.Methods:This was a retrospective study of 10 soldiers and civilians with extensive combat-related soft tissue limb injuries(5 gunshot wounds,5 blasts)treated using the TopClosure^(■)Tension Relief System(TRS)with simultaneous administration of regulated oxygen-enriched and irrigation negative pressure-assisted wound therapy(ROINPT)via the Vcareα^(■)device.Results:Nine patients were treated during the acute phase of injury and one was treated following removal of a flap due to deep infection 20 years after injury and flap reconstruction.Two patients had upper limb injury and the rest lower limb injury.With the aid of the TRS and/or ROINPT,immediate primary closure during reconstruction was achieved in 6 patients and delayed primary closure in three.Only one patient required a skin graft to close a small area of the wound after most of the wound had been closed by delayed primary closure.Wound closure was achieved within 0–37 days(median:12.5 days,interquartile range:2.75–19.75)from injury.Conclusions:The TRS is a novel device for effective,early skin stretching and secure wound closure through the application of stress relaxation and mechanical creep,achieving primary closure of large defects using a simplified surgical technique and reducing the need for closure using skin grafts and flaps and the use of tissue expanders.Delivering supplemental oxygen to the wound by ROINPT reverses the reduced oxygen levels inherent in conventional negative pressure-assisted wound therapy,mitigating anaerobic contamination and reducing infection.Irrigation may accelerate the evacuation of infectious material from the wound and provide a novel method for antibiotic administration.The combination of TRS and ROINPT devices allow for early primary closure with improved functionality of combat-related limb injuries.
