A New Understanding of Transformation Induced Plasticity (Trip) Effect

Zackay et al. proposed the transformation induced plasticity (TRIP) effect in the study of austenitic- stainless steels in 1967, and they emphasized that the origin of TRIP effect is from the increase of strain hardening rate (SHR) by the strain-induced martensitic transformation (SIMT) during plastic straining. However, they do not clarify which type of SIMTs. To do this, we modeled their experiments of A-1 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. 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 plenty of strain-induced HCP ε-martensite leads to a low SHR, but in the late stage, 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, the contribution of DAMAI effect on SHR is greater than that of SIMT. Besides, since dislocations cannot move from HCP ε-martensite to FCC γ-austenite, there are no DAMAI phenomenon, which results in a low SHR.