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

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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.