Modelling of Deformation Behavior of Mesoscopic Scale of Materials and Its Application to Design of Functional Materia

TRIP(相变诱发塑性)钢由于应变诱发马氏体相变的作用，具有高强度、高塑性和高韧性等良好的力学性能。然而，由于相变强烈地依赖于温度、施加的应变和应变率以及它们的历史，因此只有在特定的限制条件下才能获得这样的性能。基于任何实验方法都很难完全理解与局部变形相反的相变机制。因此，更合适的本构方程的数值模拟是必不可少的。在本研究中，进行了几种环境温度下的单轴拉伸试验，以拟合本构方程中定义的材料参数。对本构方程进行了弹粘塑性有限元模拟，并利用显微硬度计测量了马氏体相的体积分数，验证了所提出的本构方程在分析非均匀变形行为中的有效性。阐明了缺口尖端附近应变诱发马氏体相变的变形行为和韧化机制。在室温附近，由于驱动力随温度升高而降低相变率，阻止了缺口尖端附近的过度相变，从而抑制了应力集中。结果表明，裂纹的发生大大延迟，从而提高了韧性。

A TRIP (transformation-induced plasticity) steel possesses such desirable mechanical properties as high strength, ductility and toughness due to the effect of strain-induced martensitic transformation. However such properties are achieved only under certain restricted conditions since the transformation strongly depends on temperature, applied strain and strain rate as well as their history. The transformation mechanism contrbiuting to the local deformation may be difficult to comprehend full based on any experimental methods. Numerical simulations with more appropriate constitutive equation are, therefore, indispensable.In this study, uniaxial tension tests at several environmental temperatures are carried out to fit the material parameters defined in the constitutive equation. Elastic-viscoplastic finite-element simulations nsing the constitutive equation are performed and the volume fraction of martensite phase measured by using a micro hardness testing is utilized to elucidate the validity of the proposed constitutive equation in the analysis of nonuniform deformation behavior. Then the deformation behavior and the toughuess improvememt mechanism associated with strain-induced martensitic trans-formations around notch tip has been clarified.Around the room temperature, the reduction of phase transformation rate due to decreasing of driving force with temperature rising prevents the excessive transformation around the notch tip which suppress the stress concentration as compared with that at the low teperature range. As a result, occurance of cracking is substantially delayd, which yields the improvement of the toughness.