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1. Introduction A TRIP (transformation induced plasticity) steel possesses advantageous mechanical properties including high strength, ductility and toughness due to the effect of the strain-induced martensitic transformation, which strongly depends on temperature, stress, strain and strain rate. Therefore, computational simulations using appropriate constitutive equations [1 ]-[ 4] are indispensable for predicting the deformation processes, as well as the final strength, of the product. ). IUTAM Symposium on Mechanics of Martensitic Phase Transformation in Solids.

1 Equivalent plastic strain fP (a) equivalent stress-equivalent plastic (b) volume fraction of martensitic phasestrain relations equivalent plastic strain relations Figure 3 Deformation behavior of TRIP steels with tensile prestrain Figure 3 illustrates (a) equivalent stress- and (b) volume fraction of martensitic phase-plastic strain relationship with tensile prestrain at environmental temperature Tenv =77K. The same tendency, observed in the case, without prestrain is observed but is slightly suppressed, which is attributable to the reduction of the fraction of hardening due to martensitic transformation after prestrain.

Low magnification images of macrotwin boundaries of (a) crossing, and (b) step type. The angles a indicated are those between the microtwin interfaces in each plate. between them. However for the bulk and splat-cooled samples used in the experiments the transformation is too fast to observe the sequence of events leading to the final martensitic microstructure, and so indirect evidence is needed to confirm the above scenario. Some such evidence is provided by correlating details of the observed microstructures with predictions of the nonlinear elasticity model of martensitic transformations (see Ball & James [2], [3]), a model that incorporates the crystallographic theory of martensite (Wechsler, Lieberman & Read [13]) into a much more general framework.