By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ mode...By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ model, where the volume average of the yield functions of all BCC crystallites in Ω is taken as the macroscopic yield function of the BCC polycrystal. In constructing the formula, we try to find the relationship among the macroscopic yield function, the orientation distribution function （ODF）, and the single BCC crystal＇s plasticity. An expression for the yield stress of a uniaxial tensile problem is derived under Taylor＇s model in order to compare the expression with that of the macroscopic yield function.展开更多
The microstructure of crystal defects,e.g.,dislocation patterns,are not arbitrary,and it is possible that some of them may be related to the microstructure of crystals itself,i.e.,the lattice structure.We call those d...The microstructure of crystal defects,e.g.,dislocation patterns,are not arbitrary,and it is possible that some of them may be related to the microstructure of crystals itself,i.e.,the lattice structure.We call those dislocation patterns or substructures that are related to the corresponding crystal microstructure as the Geometrically Compatible Dislocation Patterns(GCDP).Based on this notion,we have developed a Multiscale Crystal Defect Dynamics(MCDD)to model crystal plasticity without or with minimum empiricism.In this work,we employ the multiscale dislocation pattern dynamics,i.e.,MCDD,to simulate crystal plasticity in body-centered cubic(BCC)single crystals,mainlyα-phase Tantalum(α-Ta)single crystals.The main novelties of the work are:(1)We have successfully simulated crystal plasticity at micron scale without any empirical parameter inputs;(2)We have successfully employed MCDD to perform direct numerical simulation of inelastic hysteresis of the BCC crystal;(3)We have used MCDD crystal plasticity model to demonstrate the size-effect of crystal plasticity and(4)We have captured cross-slip which may lead to size-effect.展开更多
Ti-24Nb-4Zr-8Sn, abbreviated as Ti2448 from its chemical composition in weight percent, is a multifunctional /3 type titanium alloy with body centered cubic （bcc） crystal structure, and its highly localized plastic ...Ti-24Nb-4Zr-8Sn, abbreviated as Ti2448 from its chemical composition in weight percent, is a multifunctional /3 type titanium alloy with body centered cubic （bcc） crystal structure, and its highly localized plastic deformation behavior contributes significantly to grain refinement during conventional cold processing. In the paper, the nanostructured （NS） alloy with grain size less than 50 nm produced by cold rolling has been used to investigate its superplastic deformation behavior by uniaxial tensile tests at initial strain rates of 1.5 ×10-2, 1.5×10^-3 and 1.6×10-4 s-1 and temperatures of 600,650 and 700℃. The results show that, in comparison with the coarse-grained alloy with size of 50 μm, the NS alloy has better superplasticity with elongation up to ～275% and ultimate strength of 50-100 MPa. Strain rate sensitivity （m） of the NS alloy is 0.21, 0.30 and 0.29 for 600,650 and 700℃, respectively. These results demonstrate that grain refinement is a valid way to enhance the superplasticity of Ti2448 alloy.展开更多
晶体塑性理论是将晶体微观尺度的位错运动与宏观尺度的塑性形变相结合的重要理论,提供了在细观尺度内研究材料力学行为的有效方法。位错的密度变化对金属晶体的硬化行为有着重要的影响。该文在晶体塑性理论的基础上引入位错运动理论,建...晶体塑性理论是将晶体微观尺度的位错运动与宏观尺度的塑性形变相结合的重要理论,提供了在细观尺度内研究材料力学行为的有效方法。位错的密度变化对金属晶体的硬化行为有着重要的影响。该文在晶体塑性理论的基础上引入位错运动理论,建立基于位错密度的体心立方晶体(body center cubic,BCC)塑性本构模型,研究BCC的力学行为;并借助ABAQUS有限元软件,编写UMAT子程序,实现对BCC结构的铁单晶及多晶单轴拉伸试验的数值模拟。结果表明:该本构模型能有效地模拟铁单晶及多晶单轴拉伸的力学行为。展开更多
基金Project supported by the National Natural Science Foundation of China (No. 10562004) the Natural Science Foundation of Jiangxi (Nos.0450035 and 0512021) the Science Foundation of Jiangxi Educational Department (No.[2006]3) the Oversea Returned Scholars Grant of China.
文摘By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ model, where the volume average of the yield functions of all BCC crystallites in Ω is taken as the macroscopic yield function of the BCC polycrystal. In constructing the formula, we try to find the relationship among the macroscopic yield function, the orientation distribution function （ODF）, and the single BCC crystal＇s plasticity. An expression for the yield stress of a uniaxial tensile problem is derived under Taylor＇s model in order to compare the expression with that of the macroscopic yield function.
文摘The microstructure of crystal defects,e.g.,dislocation patterns,are not arbitrary,and it is possible that some of them may be related to the microstructure of crystals itself,i.e.,the lattice structure.We call those dislocation patterns or substructures that are related to the corresponding crystal microstructure as the Geometrically Compatible Dislocation Patterns(GCDP).Based on this notion,we have developed a Multiscale Crystal Defect Dynamics(MCDD)to model crystal plasticity without or with minimum empiricism.In this work,we employ the multiscale dislocation pattern dynamics,i.e.,MCDD,to simulate crystal plasticity in body-centered cubic(BCC)single crystals,mainlyα-phase Tantalum(α-Ta)single crystals.The main novelties of the work are:(1)We have successfully simulated crystal plasticity at micron scale without any empirical parameter inputs;(2)We have successfully employed MCDD to perform direct numerical simulation of inelastic hysteresis of the BCC crystal;(3)We have used MCDD crystal plasticity model to demonstrate the size-effect of crystal plasticity and(4)We have captured cross-slip which may lead to size-effect.
基金supported by the National Natural Science Foundation of China (Nos. 51071152and 50901080)the National High Technology Research and Development Program of China (No.2011AA030106)
文摘Ti-24Nb-4Zr-8Sn, abbreviated as Ti2448 from its chemical composition in weight percent, is a multifunctional /3 type titanium alloy with body centered cubic （bcc） crystal structure, and its highly localized plastic deformation behavior contributes significantly to grain refinement during conventional cold processing. In the paper, the nanostructured （NS） alloy with grain size less than 50 nm produced by cold rolling has been used to investigate its superplastic deformation behavior by uniaxial tensile tests at initial strain rates of 1.5 ×10-2, 1.5×10^-3 and 1.6×10-4 s-1 and temperatures of 600,650 and 700℃. The results show that, in comparison with the coarse-grained alloy with size of 50 μm, the NS alloy has better superplasticity with elongation up to ～275% and ultimate strength of 50-100 MPa. Strain rate sensitivity （m） of the NS alloy is 0.21, 0.30 and 0.29 for 600,650 and 700℃, respectively. These results demonstrate that grain refinement is a valid way to enhance the superplasticity of Ti2448 alloy.
文摘晶体塑性理论是将晶体微观尺度的位错运动与宏观尺度的塑性形变相结合的重要理论,提供了在细观尺度内研究材料力学行为的有效方法。位错的密度变化对金属晶体的硬化行为有着重要的影响。该文在晶体塑性理论的基础上引入位错运动理论,建立基于位错密度的体心立方晶体(body center cubic,BCC)塑性本构模型,研究BCC的力学行为;并借助ABAQUS有限元软件,编写UMAT子程序,实现对BCC结构的铁单晶及多晶单轴拉伸试验的数值模拟。结果表明:该本构模型能有效地模拟铁单晶及多晶单轴拉伸的力学行为。
