A new understanding of transformation induced plasticity (TRIP) effect in austenitic steels

Abstract

Zackay et alproposed the transformation induced plasticity (TRIP) effect in the study of austenitic steels in 1967, and they emphasized that the TRIP effect is caused by an increase in strain hardening rate (SHR) due to strain-induced martensitic transformation (SIMT), but they do not specify what kind of SIMT is. To do this, we modeled their experiments of A-1 steel and A-2 steels in this work, and used the microstructural characterization of X-ray diffraction and transmission electron microscopy to obtain a new understanding of the TRIP effect in austenitic steels. There are two types of martensitic transformations: FCC γ → HCP ε-martensite and FCC γ →BCC α-martensite in pre-deformation A-2 steel, and in the early stage of deformation during tension, the considerable amount of strain-induced HCP ε-martensite leads to a low SHR, but in the late stage of deformation, the almost single strain-induced BCC α-martensite leads to a high SHR. Further experiment gives an amazing result: considerable Dislocations Across Martensite/Austenite Interface (DAMAI) move from martensite into austenite during deformation, which results in the rapid increase of dislocation density in austenite accompanying with the 233 MPa increment of flow stress, being higher than the 167 MPa increment from SIMT, therefore, in two main deformation mechanisms for the strength and plasticity enhancement of austenitic steels the contribution of DAMAI effect on SHR is greater than that of SIMT on SHR proposed by Zackay et al. Besides, since dislocations cannot move from HCP ε-martensite to FCC γ-austenite because the generation condition of DAMAI are not met, there are no DAMAI phenomenon, which results in a low SHR.