Mo-5Ta targets were prepared by the spark plasma sintering(SPS)technology under the sintering temperatures of 1400-1600℃,the holding times of 0-20 min,and the axial pressure of 30 MPa.The microstructure,performance,a...Mo-5Ta targets were prepared by the spark plasma sintering(SPS)technology under the sintering temperatures of 1400-1600℃,the holding times of 0-20 min,and the axial pressure of 30 MPa.The microstructure,performance,and grain growth kinetics of Mo-5Ta sputtering targets were studied.With the increase of sintering temperatures and times,Ta can more dissolve in Mo and form a Mo(Ta)solid solution.The grain sizes of Mo-5Ta targets remain unchanged at low temperatures(1400-1500℃)while increasing significantly at high temperature(1600℃)with the extension of the holding time.In addition,the densities and Vickers hardness(HV_(0.5))first ascend and then decrease with sintering proceeding.The thermal conductivity is positively correlated with the grain size and density,as a result of their joint action.Based on the comprehensive analysis,the grain growth is dominated by the combination of boundary diffusion and volume diffusion.When n=2,the activation energies of grain growth under holding times of 5,10,20 min are calculated as 762.70,617.86,and 616.52 kJ/mol,respectively.展开更多
Critical properties of metallic materials,such as the yield stress,corrosion resistance and ductility depend on the microstructure and its grain size and size distribution.Solute atoms that favorably segregate to grai...Critical properties of metallic materials,such as the yield stress,corrosion resistance and ductility depend on the microstructure and its grain size and size distribution.Solute atoms that favorably segregate to grain boundaries produce a pinning atmosphere that exerts a drag pressure on the boundary motion,which strongly affects the grain growth behavior during annealing.In the current work,the characteristics of grain growth in an annealed Mg-1 wt.%Mn-1 wt.%Nd magnesium alloy were investigated by advanced experimental and modeling techniques.Systematic quasi in-situ orientation mappings with a scanning electron microscope were performed to track the evolution of local and global microstructural characteristics as a function of annealing time.Solute segregation at targeted grain boundaries was measured using three-dimensional atom probe tomography.Level-set computer simulations were carried with different setups of driving forces to explore their contribution to the microstructure development with and without solute drag.The results showed that the favorable growth advantage for some grains leading to a transient stage of abnormal grain growth is controlled by several drivers with varying importance at different stages of annealing.For longer annealing times,residual dislocation density gradients between large and smaller grains are no longer important,which leads to microstructure stability due to predominant solute drag.Local fluctuations in residual dislocation energy and solute concentration near grain boundaries cause different boundary segments to migrate at different rates,which affects the average growth rate of large grains and their evolved shape.展开更多
Microstructure evolution of Ti14 （α＋Ti2Cu） alloy during semi-solid isothermal process at different temperatures was investigated. The results reveal that both the temperature and holding time have effect on the gr...Microstructure evolution of Ti14 （α＋Ti2Cu） alloy during semi-solid isothermal process at different temperatures was investigated. The results reveal that both the temperature and holding time have effect on the grain growth behavior. The grains grow obviously and the degree of globularity increases with the increase of holding time. According to the statistic analysis of experimental data, the grain growth indices are 0.88 and 0.97 at 1 000 ℃ and 1 050 ℃, respectively, which indicates that increasing isothermal temperature would accelerate microstructural evolution.展开更多
The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5...The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5, the effects of some factors of MC algorithm, i.e. the lattice types, the methods of selecting lattice sites, and the neighbors selection for energy calculations, on the simulation results of grain growth are studied. Two methods of regression were compared, and the three-parameter nonlinear regression is much more suitable for fitting the grain growth kinetics. A better model with appropriate factors included triangular lattice, the attempted site randomly selected, and the first and second nearest neighbors for energy calculations is obtained.展开更多
Grain growth behavior of 5Cr21Mn9Ni4NRE steel was experimentally studied at various solid solution treatment temperatures and holding for different times. The experimental results show that the 5Cr21Mn9Ni4NRE steel ha...Grain growth behavior of 5Cr21Mn9Ni4NRE steel was experimentally studied at various solid solution treatment temperatures and holding for different times. The experimental results show that the 5Cr21Mn9Ni4NRE steel has the feature of sharp austenite grain coarsening after solid solution treatment at the temperature above 1150 ℃. RE added in the steel has the benefit to restrain grain growth and increase grain growth activation energy.展开更多
The austenitic stainless steels usually present an excellent combination of corrosion resistance and mechanical properties such as ductility in the annealed condition and high yield strength after cold deformation. So...The austenitic stainless steels usually present an excellent combination of corrosion resistance and mechanical properties such as ductility in the annealed condition and high yield strength after cold deformation. Solution annealing in the AISI 304H is recommended before deformation process in order to improve ductility. However, long annealing during solution annealing can cause GG （grain growth） or AGG （abnormal grain growth） in the AISI 304H. In these cases, ductility is strongly decreased. Therefore, GG or AGG must be avoided during solution annealing. In this article, grain growth during solution annealing of the AISI 304H samples was determined. Samples of the AISI 304H were annealed at 1,100 ℃ for solution-annealing times varying from 15 min to 180 min. The results show that AGG took place for samples annealed at 1,100 ℃ for 30 min. In this condition, grain size reached 70 ± 10 μm. After annealing solution at 1,100 ℃ for 180 min, grain size reached 120 ~ 20 μm. In summary, the results shown that solution annealing at 1,100 ℃ even for relatively short annealing promotes the prompt increase of the grain size.展开更多
The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then gre...The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.展开更多
In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at ...In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.展开更多
In order to deeply understand the grain growth behaviors of Ni80A superalloy,a series of grain growth experiments were conducted at holding temperatures ranging from 1223 to 1423 K and holding time ranging from 0 to 3...In order to deeply understand the grain growth behaviors of Ni80A superalloy,a series of grain growth experiments were conducted at holding temperatures ranging from 1223 to 1423 K and holding time ranging from 0 to 3600 s.A back-propagation artificial neural network(BP-ANN)model and a Sellars model were solved based on the experimental data.The prediction and generalization capabilities of these two models were evaluated and compared on the basis of four statistical indicators.The results show that the solved BP-ANN model has better performance as it has higher correlation coefficient(r),lower average absolute relative error(AARE),lower absolute values of mean value(μ)and standard deviation(ω).Eventually,a response surface of average grain size to holding temperature and holding time is constructed based on the data expanded by the solved BP-ANN model,and the grain growth behaviors are described.展开更多
The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyze...The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyzed.Hence,possible grain growth mechanisms in these magnets were qualitatively discussed.The Nd2Fe14B grain growth proceeded at quite a high rate in the initial 0~1 h of sintering and from then onwards the grain growth rate decreased.A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth.On the basis of experimental results,two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets,that is,dissolution and re-precipitation of Nd2Fe14B particles,and Nd2Fe14B particle growth by coalescence.It was believed that Nd2Fe14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution,but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.展开更多
For most commercial steels the prediction of the final properties depends on accurately calculating the room temperature ferrite grain size. A grain growth model is proposed for low carbon steels Q235B during hot roll...For most commercial steels the prediction of the final properties depends on accurately calculating the room temperature ferrite grain size. A grain growth model is proposed for low carbon steels Q235B during hot rolling. By using this model, the initial ferrite grain size after continuous cooling and ferrite grain growing in coiling procedure can be predicted. In-plant trials were performed in the hot strip mill of Ansteel. The calculated final ferrite grain sizes are in good agreement with the experimental ones. It is helpful both for simulation of microstructure evolution and prediction of mechanical properties.展开更多
The microstructural evolution and underlying grain growth mechanism of a{1012}-twin-containing Mg alloy during annealing are investigated through quasi in situ electron backscatter diffraction measurements of a rolled...The microstructural evolution and underlying grain growth mechanism of a{1012}-twin-containing Mg alloy during annealing are investigated through quasi in situ electron backscatter diffraction measurements of a rolled AZ31 alloy subjected to precompression along the rolling direction(RD).The precompressed material shows a partially twinned structure consisting of a twinned region and a residual matrix region,and this structure changes to an almost twin-free structure consisting of grown grains with serrated grain boundaries in twinand matrix-originated regions after annealing at 250℃ for 1 h.In addition,the average grain size almost doubles and the internal strain energy decreases significantly after annealing.These microstructural variations are induced mainly by grain growth through the strain-induced migration of high-angle grain boundaries without the movement of twin boundaries.The twinned region of the precompressed material has higher stored strain energy than the residual matrix region because the crystallographic orientation of the former region is favorable for basal slip and because of the occurrence of the dislocation transmutation reaction in the twins.For reducing the total strain energy accumulated in the precompressed material,the residual matrix region—having lower stored strain energy—preferentially grows while consuming the twinned regions formed in the surrounding grains during annealing.As a result,the area fraction of grains with a matrix texture increases whereas that of grains with a twin texture decreases after annealing.The twin texture intensity increases significantly and this texture becomes more concentrated along the RD because the highly RD oriented twins tend to grow during annealing on account of their fairly low stored strain energy.展开更多
Recrystallization and grain growth in Mg-4.9Zn-0.7Zr and Mg-4.9Zn-0.9Y-0.7Zr alloys as a function of temperature on deformation were investigated with regards to hot rolling and annealing. The influence of yttrium add...Recrystallization and grain growth in Mg-4.9Zn-0.7Zr and Mg-4.9Zn-0.9Y-0.7Zr alloys as a function of temperature on deformation were investigated with regards to hot rolling and annealing. The influence of yttrium addition on the microstructure was examined by X-ray diffraction （XRD）, optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The results indicated that yttrium addition promoted nucleation of recrystallization during hot rolling process. The grain size of Mg-4.9Zn-0.7Zr alloy samples grew significantly with annealing temperature （300-400 ℃） and holding time （0-120 min）, while the microstructure of the alloy with yttrium addition remained unchanged and fine. The activation energy of grain boundary migration for Mg-4.9Zn-0.9Y-0.7Zr alloy samples （56.34 kJ/mol） was higher than that for Mg-4.9Zn-0.7Zr （42.66 kJ/mol） owing to the pinning effect of Y-containing particles. The proposed growth models of recrysta/lized grains for the two studied alloys conformed well to E. Robert＇s grain-growth equation. Besides, the ultimate strength and yield strength of the alloys with yttrium addition were improved with good plasticity.展开更多
Microstructure stability is essential to maintain a fine grain structure for an alloy throughout its processing.The effects of Er addition and its existing form on the static recrystallization and grain growth during ...Microstructure stability is essential to maintain a fine grain structure for an alloy throughout its processing.The effects of Er addition and its existing form on the static recrystallization and grain growth during annealing of an extruded Mg-1.5Zn-0.6Zr magnesium alloy were studied in this paper.The results showed that microstructure stability was much improved by Er addition and the best thermability was obtained in 2 wt.%Er-containing alloy.For the incomplete dynamic recrystallization(DRX)microstructures extruded at a lower temperature of 350℃,Er addition increased the resistance of static recrystallization;and for the complete DRX microstructures extruded at a relatively high temperature of 420℃,Er addition suppressed grain growth.The difference in microstructure stability was then correlated with the microstructure features.Both the intermetallic phase and the solute atoms of Er inα-Mg matrix contributed to the microstructure stability.Moreover,it is believed that the existing form of Er-Zn atom pairs in theα-Mg solid solution favored the most to improve the thermal stability of the alloy.展开更多
The grain growth behavior in reactive spray formed 7075+2.91 vol percent TiCAl alloy was studied and compared with that of spray formed 7075 Al alloy at semi-solid state. Theeffects of in-situ TiC particles on the mic...The grain growth behavior in reactive spray formed 7075+2.91 vol percent TiCAl alloy was studied and compared with that of spray formed 7075 Al alloy at semi-solid state. Theeffects of in-situ TiC particles on the microstructure of spray formed 7075 Al alloy were alsoinvestigated. The specimens were heat-treated isothermally at various temperatures between thesolidus and liquidus of 7075 Al alloy for times in the range of 10-60 min, then quenched in water.The microstructure of reheated specimens was characterized using scanning electron microscopy andoptical microscopy. The grain size was measured using a mean linear intercept method. Results showthat the in-situ TiC particles can effectively retard grain growth and refine the grain at a limitedsize. The grain growth exponent in Arrhenius equation increases from 2 to 3, which indicates thatthe in-situ TiC particles have the significant pinning effect on grain coarsening in the semi-solidstate.展开更多
Calibre rolling of Mg-3Al-1Zn alloy at 300℃ led to development of fine grain size of 3μm.Subsequent annealing,from 5 to 6000 minutes at 300-450℃,revealed faster grain growth initially up to 60 minutes,which became ...Calibre rolling of Mg-3Al-1Zn alloy at 300℃ led to development of fine grain size of 3μm.Subsequent annealing,from 5 to 6000 minutes at 300-450℃,revealed faster grain growth initially up to 60 minutes,which became sluggish on prolonged annealing.The time exponent for grain growth kinetics(n)suggests bi-linear behaviour with n=0.11 and 0.008 over these time scales.The activation energy,based on various n values,varied over wide ranges that made the understanding of the mechanisms for grain growth difficult.This problem is explained by concurrent evolution of texture and grain boundary structure.The effect of grain growth on hardness at ambient temperature was found to follow the H-P type relationship.展开更多
An appropriate Monte Carlo method was developed to simulate the three-dimensional normal grain growth more completely. Comparative investigation on the three-dimensional and the cross-sectional characteristics of norm...An appropriate Monte Carlo method was developed to simulate the three-dimensional normal grain growth more completely. Comparative investigation on the three-dimensional and the cross-sectional characteristics of normal grain growth was done. It was found that the time exponent of grain growth determined from cross-section exhibits the same rule of increasing slowly with time and approaching the theoretical value n = 0.5 of steadygrain growth as the three-dimensional (3-D) system. From change of the number of grains per unit area with timemeasured in cross-section, the state of 3-D normal grain growth may be predicted. The gtain size distribution incross-section is different from that in 3-D system and can not express the evolution characteristic of the 3-D distribution. Furthermore, there exists statistical connection between the topological parameters in cross-section and thosein three-dimensions.展开更多
The grain growth morphology,kinetics and texture change after uniaxial compression at 430 ℃ of an extruded AZ31 alloy were studied.The samples were loaded following two routes insuring two initial textures of the sam...The grain growth morphology,kinetics and texture change after uniaxial compression at 430 ℃ of an extruded AZ31 alloy were studied.The samples were loaded following two routes insuring two initial textures of the samples with compression direction parallel and normal to the extrusion direction.For both initial textures,a stable grain size is attained upon isothermal annealing and the grain growth kinetics can be described by:dn= dRn+kt with an n value of around 15.The annealing texture with grown grains is a retained hot deformation texture without emerging or strengthening other components.Abnormal grain growth is not observed for annealing time up to 10 000 h at 450℃.展开更多
The contribution of recrystallization and grain growth to the texture evolution in AZ31 alloy and a modified version AZ31+0.5 wt.%Ca was investigated utilizing a multi-step annealing process.The results showed that th...The contribution of recrystallization and grain growth to the texture evolution in AZ31 alloy and a modified version AZ31+0.5 wt.%Ca was investigated utilizing a multi-step annealing process.The results showed that the addition of Ca triggered a considerable texture modification by increasing the texture spread and decreasing the overall texture intensity.This effect was found to be temperature dependent.When the annealing temperature remained lower than 450℃,a weak double peak texture with large basal pole tilt towards the RD was formed.This is correlated to microstructure observations of a large number of Ca-containing nano-sized particles that seemed to suppress grain growth below 450℃,which stabilized the weak recrystallization texture.This favorable texture was lost upon annealing at higher temperatures.In AZ31,recrystallization nuclei were found to preserve the orientation of their deformed parents,which offered limited potential to optimize the texture via annealing treatments.Grain growth of recrystallized grains resulted in a distinct sheet texture transition from a double-peak to a single-peak basal texture.Aspects of grain boundary energy and grain topology are discussed to explain the growth advantage of the sharp basal component over other orientations.展开更多
Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sinter...Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sintering process of nano-powders were reviewed, to which sintering of the traditional materials compared. The microstructural development, i.e., grain growth and densification during sintering as well as the mechanism of crystal surface diffusion and boundary migration were analyzed, and the dynamic models on sintering process were summarized by the relationship of grain growth and pores size, interface diffusion, densification rate, and sintering temperature. Finally, the research tendency of this major on the basis of above models was discussed.展开更多
基金“Solid Waste Recycling”Key Project of National Key R&D Plan of China(No.2018YFC1901700)the Fundamental Research Funds for the Central Universities of China(Nos.JZ2021HGTB0094 and PA2021GDGP0059)。
文摘Mo-5Ta targets were prepared by the spark plasma sintering(SPS)technology under the sintering temperatures of 1400-1600℃,the holding times of 0-20 min,and the axial pressure of 30 MPa.The microstructure,performance,and grain growth kinetics of Mo-5Ta sputtering targets were studied.With the increase of sintering temperatures and times,Ta can more dissolve in Mo and form a Mo(Ta)solid solution.The grain sizes of Mo-5Ta targets remain unchanged at low temperatures(1400-1500℃)while increasing significantly at high temperature(1600℃)with the extension of the holding time.In addition,the densities and Vickers hardness(HV_(0.5))first ascend and then decrease with sintering proceeding.The thermal conductivity is positively correlated with the grain size and density,as a result of their joint action.Based on the comprehensive analysis,the grain growth is dominated by the combination of boundary diffusion and volume diffusion.When n=2,the activation energies of grain growth under holding times of 5,10,20 min are calculated as 762.70,617.86,and 616.52 kJ/mol,respectively.
基金support of the Deutsche Forschungsgemeinschaft(DFG),Grant no.AL 1343/7–1,AL 1343/8–1,Yi 103/3–1。
文摘Critical properties of metallic materials,such as the yield stress,corrosion resistance and ductility depend on the microstructure and its grain size and size distribution.Solute atoms that favorably segregate to grain boundaries produce a pinning atmosphere that exerts a drag pressure on the boundary motion,which strongly affects the grain growth behavior during annealing.In the current work,the characteristics of grain growth in an annealed Mg-1 wt.%Mn-1 wt.%Nd magnesium alloy were investigated by advanced experimental and modeling techniques.Systematic quasi in-situ orientation mappings with a scanning electron microscope were performed to track the evolution of local and global microstructural characteristics as a function of annealing time.Solute segregation at targeted grain boundaries was measured using three-dimensional atom probe tomography.Level-set computer simulations were carried with different setups of driving forces to explore their contribution to the microstructure development with and without solute drag.The results showed that the favorable growth advantage for some grains leading to a transient stage of abnormal grain growth is controlled by several drivers with varying importance at different stages of annealing.For longer annealing times,residual dislocation density gradients between large and smaller grains are no longer important,which leads to microstructure stability due to predominant solute drag.Local fluctuations in residual dislocation energy and solute concentration near grain boundaries cause different boundary segments to migrate at different rates,which affects the average growth rate of large grains and their evolved shape.
基金Projects (2005CCA06400, 2007CB613807) supported by the National Basic Research Program of China Project (CHD2010JC115) supported by the Special Fund for Basic Scientific Research of Central Colleges,China
文摘Microstructure evolution of Ti14 （α＋Ti2Cu） alloy during semi-solid isothermal process at different temperatures was investigated. The results reveal that both the temperature and holding time have effect on the grain growth behavior. The grains grow obviously and the degree of globularity increases with the increase of holding time. According to the statistic analysis of experimental data, the grain growth indices are 0.88 and 0.97 at 1 000 ℃ and 1 050 ℃, respectively, which indicates that increasing isothermal temperature would accelerate microstructural evolution.
基金the International Science & Technology Cooperation Project of Shandong Province(2006)the Natural Science Foundation of Shandong Province(Y2007F06).
文摘The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5, the effects of some factors of MC algorithm, i.e. the lattice types, the methods of selecting lattice sites, and the neighbors selection for energy calculations, on the simulation results of grain growth are studied. Two methods of regression were compared, and the three-parameter nonlinear regression is much more suitable for fitting the grain growth kinetics. A better model with appropriate factors included triangular lattice, the attempted site randomly selected, and the first and second nearest neighbors for energy calculations is obtained.
文摘Grain growth behavior of 5Cr21Mn9Ni4NRE steel was experimentally studied at various solid solution treatment temperatures and holding for different times. The experimental results show that the 5Cr21Mn9Ni4NRE steel has the feature of sharp austenite grain coarsening after solid solution treatment at the temperature above 1150 ℃. RE added in the steel has the benefit to restrain grain growth and increase grain growth activation energy.
文摘The austenitic stainless steels usually present an excellent combination of corrosion resistance and mechanical properties such as ductility in the annealed condition and high yield strength after cold deformation. Solution annealing in the AISI 304H is recommended before deformation process in order to improve ductility. However, long annealing during solution annealing can cause GG （grain growth） or AGG （abnormal grain growth） in the AISI 304H. In these cases, ductility is strongly decreased. Therefore, GG or AGG must be avoided during solution annealing. In this article, grain growth during solution annealing of the AISI 304H samples was determined. Samples of the AISI 304H were annealed at 1,100 ℃ for solution-annealing times varying from 15 min to 180 min. The results show that AGG took place for samples annealed at 1,100 ℃ for 30 min. In this condition, grain size reached 70 ± 10 μm. After annealing solution at 1,100 ℃ for 180 min, grain size reached 120 ~ 20 μm. In summary, the results shown that solution annealing at 1,100 ℃ even for relatively short annealing promotes the prompt increase of the grain size.
基金financially supported by the Postdoctoral Science Foundation of China (No. 2014M550415)the National Natural Science Foundation of China (No. 50734004)
文摘The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.
基金the National Natural Science Foundation of China(No.51274154)the National High-Tech Research and Development Program of China(No.2012AA03A504)+1 种基金the State Key Laboratory of Development and Application Technology of Automo-tive Steels(Baosteel Group)the Key Project of Hubei Education Committee(No.D20121101)
文摘In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.
基金Project(cstc2018jcyjAX0459)supported by Chongqing Basic Research and Frontier Exploration Program,ChinaProjects(2019CDQYTM027,2019CDJGFCL003)supported by the Fundamental Research Funds for the Central Universities,China。
文摘In order to deeply understand the grain growth behaviors of Ni80A superalloy,a series of grain growth experiments were conducted at holding temperatures ranging from 1223 to 1423 K and holding time ranging from 0 to 3600 s.A back-propagation artificial neural network(BP-ANN)model and a Sellars model were solved based on the experimental data.The prediction and generalization capabilities of these two models were evaluated and compared on the basis of four statistical indicators.The results show that the solved BP-ANN model has better performance as it has higher correlation coefficient(r),lower average absolute relative error(AARE),lower absolute values of mean value(μ)and standard deviation(ω).Eventually,a response surface of average grain size to holding temperature and holding time is constructed based on the data expanded by the solved BP-ANN model,and the grain growth behaviors are described.
基金Project supported by China National Development Plan for Key Fundamental Research(973)(G2000-67201-3)
文摘The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyzed.Hence,possible grain growth mechanisms in these magnets were qualitatively discussed.The Nd2Fe14B grain growth proceeded at quite a high rate in the initial 0~1 h of sintering and from then onwards the grain growth rate decreased.A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth.On the basis of experimental results,two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets,that is,dissolution and re-precipitation of Nd2Fe14B particles,and Nd2Fe14B particle growth by coalescence.It was believed that Nd2Fe14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution,but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.
基金financially supported by the National Key Basic Research and Development Programme of China (Grant No. G1998061512).
文摘For most commercial steels the prediction of the final properties depends on accurately calculating the room temperature ferrite grain size. A grain growth model is proposed for low carbon steels Q235B during hot rolling. By using this model, the initial ferrite grain size after continuous cooling and ferrite grain growing in coiling procedure can be predicted. In-plant trials were performed in the hot strip mill of Ansteel. The calculated final ferrite grain sizes are in good agreement with the experimental ones. It is helpful both for simulation of microstructure evolution and prediction of mechanical properties.
基金This work was supported by a grant from the National Research Foundation of Korea(NRF)funded by the Korean government(MSIP,South Korea)(No.2019R1A2C1085272).
文摘The microstructural evolution and underlying grain growth mechanism of a{1012}-twin-containing Mg alloy during annealing are investigated through quasi in situ electron backscatter diffraction measurements of a rolled AZ31 alloy subjected to precompression along the rolling direction(RD).The precompressed material shows a partially twinned structure consisting of a twinned region and a residual matrix region,and this structure changes to an almost twin-free structure consisting of grown grains with serrated grain boundaries in twinand matrix-originated regions after annealing at 250℃ for 1 h.In addition,the average grain size almost doubles and the internal strain energy decreases significantly after annealing.These microstructural variations are induced mainly by grain growth through the strain-induced migration of high-angle grain boundaries without the movement of twin boundaries.The twinned region of the precompressed material has higher stored strain energy than the residual matrix region because the crystallographic orientation of the former region is favorable for basal slip and because of the occurrence of the dislocation transmutation reaction in the twins.For reducing the total strain energy accumulated in the precompressed material,the residual matrix region—having lower stored strain energy—preferentially grows while consuming the twinned regions formed in the surrounding grains during annealing.As a result,the area fraction of grains with a matrix texture increases whereas that of grains with a twin texture decreases after annealing.The twin texture intensity increases significantly and this texture becomes more concentrated along the RD because the highly RD oriented twins tend to grow during annealing on account of their fairly low stored strain energy.
基金the Science and Technology Program of Hunan Province (2004GK1008-2)
文摘Recrystallization and grain growth in Mg-4.9Zn-0.7Zr and Mg-4.9Zn-0.9Y-0.7Zr alloys as a function of temperature on deformation were investigated with regards to hot rolling and annealing. The influence of yttrium addition on the microstructure was examined by X-ray diffraction （XRD）, optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The results indicated that yttrium addition promoted nucleation of recrystallization during hot rolling process. The grain size of Mg-4.9Zn-0.7Zr alloy samples grew significantly with annealing temperature （300-400 ℃） and holding time （0-120 min）, while the microstructure of the alloy with yttrium addition remained unchanged and fine. The activation energy of grain boundary migration for Mg-4.9Zn-0.9Y-0.7Zr alloy samples （56.34 kJ/mol） was higher than that for Mg-4.9Zn-0.7Zr （42.66 kJ/mol） owing to the pinning effect of Y-containing particles. The proposed growth models of recrysta/lized grains for the two studied alloys conformed well to E. Robert＇s grain-growth equation. Besides, the ultimate strength and yield strength of the alloys with yttrium addition were improved with good plasticity.
基金The authors are grateful for the financial support of the National Natural Science Foundation of China(No.51271207)sharing fund of Chongqing University’s large-scale equipment.
文摘Microstructure stability is essential to maintain a fine grain structure for an alloy throughout its processing.The effects of Er addition and its existing form on the static recrystallization and grain growth during annealing of an extruded Mg-1.5Zn-0.6Zr magnesium alloy were studied in this paper.The results showed that microstructure stability was much improved by Er addition and the best thermability was obtained in 2 wt.%Er-containing alloy.For the incomplete dynamic recrystallization(DRX)microstructures extruded at a lower temperature of 350℃,Er addition increased the resistance of static recrystallization;and for the complete DRX microstructures extruded at a relatively high temperature of 420℃,Er addition suppressed grain growth.The difference in microstructure stability was then correlated with the microstructure features.Both the intermetallic phase and the solute atoms of Er inα-Mg matrix contributed to the microstructure stability.Moreover,it is believed that the existing form of Er-Zn atom pairs in theα-Mg solid solution favored the most to improve the thermal stability of the alloy.
基金This work was financially supported by the National Natural Science Foundation of China (No.50171010)
文摘The grain growth behavior in reactive spray formed 7075+2.91 vol percent TiCAl alloy was studied and compared with that of spray formed 7075 Al alloy at semi-solid state. Theeffects of in-situ TiC particles on the microstructure of spray formed 7075 Al alloy were alsoinvestigated. The specimens were heat-treated isothermally at various temperatures between thesolidus and liquidus of 7075 Al alloy for times in the range of 10-60 min, then quenched in water.The microstructure of reheated specimens was characterized using scanning electron microscopy andoptical microscopy. The grain size was measured using a mean linear intercept method. Results showthat the in-situ TiC particles can effectively retard grain growth and refine the grain at a limitedsize. The grain growth exponent in Arrhenius equation increases from 2 to 3, which indicates thatthe in-situ TiC particles have the significant pinning effect on grain coarsening in the semi-solidstate.
文摘Calibre rolling of Mg-3Al-1Zn alloy at 300℃ led to development of fine grain size of 3μm.Subsequent annealing,from 5 to 6000 minutes at 300-450℃,revealed faster grain growth initially up to 60 minutes,which became sluggish on prolonged annealing.The time exponent for grain growth kinetics(n)suggests bi-linear behaviour with n=0.11 and 0.008 over these time scales.The activation energy,based on various n values,varied over wide ranges that made the understanding of the mechanisms for grain growth difficult.This problem is explained by concurrent evolution of texture and grain boundary structure.The effect of grain growth on hardness at ambient temperature was found to follow the H-P type relationship.
文摘An appropriate Monte Carlo method was developed to simulate the three-dimensional normal grain growth more completely. Comparative investigation on the three-dimensional and the cross-sectional characteristics of normal grain growth was done. It was found that the time exponent of grain growth determined from cross-section exhibits the same rule of increasing slowly with time and approaching the theoretical value n = 0.5 of steadygrain growth as the three-dimensional (3-D) system. From change of the number of grains per unit area with timemeasured in cross-section, the state of 3-D normal grain growth may be predicted. The gtain size distribution incross-section is different from that in 3-D system and can not express the evolution characteristic of the 3-D distribution. Furthermore, there exists statistical connection between the topological parameters in cross-section and thosein three-dimensions.
文摘The grain growth morphology,kinetics and texture change after uniaxial compression at 430 ℃ of an extruded AZ31 alloy were studied.The samples were loaded following two routes insuring two initial textures of the samples with compression direction parallel and normal to the extrusion direction.For both initial textures,a stable grain size is attained upon isothermal annealing and the grain growth kinetics can be described by:dn= dRn+kt with an n value of around 15.The annealing texture with grown grains is a retained hot deformation texture without emerging or strengthening other components.Abnormal grain growth is not observed for annealing time up to 10 000 h at 450℃.
基金F.Guo and R.S.Pei are grateful for financial support from the Chinese Scholarship Council(CSC)F.Guo also thanks for the support of Science and Technology Research Program of Chongqing Municipal Education Commission(Grant No.KJQN201801114)+2 种基金Scientific Research Foundation of Chongqing University of Technology(2017ZD35)L.Y.Jiang is sponsored by Chongqing Research Program of Basic Research and Frontier Technology(No.cstc2018jcyjAX0107)China Postdoctoral Science Foundation(2018M643407).
文摘The contribution of recrystallization and grain growth to the texture evolution in AZ31 alloy and a modified version AZ31+0.5 wt.%Ca was investigated utilizing a multi-step annealing process.The results showed that the addition of Ca triggered a considerable texture modification by increasing the texture spread and decreasing the overall texture intensity.This effect was found to be temperature dependent.When the annealing temperature remained lower than 450℃,a weak double peak texture with large basal pole tilt towards the RD was formed.This is correlated to microstructure observations of a large number of Ca-containing nano-sized particles that seemed to suppress grain growth below 450℃,which stabilized the weak recrystallization texture.This favorable texture was lost upon annealing at higher temperatures.In AZ31,recrystallization nuclei were found to preserve the orientation of their deformed parents,which offered limited potential to optimize the texture via annealing treatments.Grain growth of recrystallized grains resulted in a distinct sheet texture transition from a double-peak to a single-peak basal texture.Aspects of grain boundary energy and grain topology are discussed to explain the growth advantage of the sharp basal component over other orientations.
基金This project was financially supported by the Natural Science Foundation of Hebei Province (No.E2005000027) and Natural Science Foundation of Tianjin (No.06YFJMJC02400).
文摘Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sintering process of nano-powders were reviewed, to which sintering of the traditional materials compared. The microstructural development, i.e., grain growth and densification during sintering as well as the mechanism of crystal surface diffusion and boundary migration were analyzed, and the dynamic models on sintering process were summarized by the relationship of grain growth and pores size, interface diffusion, densification rate, and sintering temperature. Finally, the research tendency of this major on the basis of above models was discussed.
