Role of surrounding phases on deformation-induced martensitic transformation of retained austenite in multi-phase TRIP steel

Abstract

Deformation-induced martensitic transformation is a key phenomenon to manage both high strength and large ductility in low alloy multi-phase steels. Significant enhancement of strain hardening ability could be achieved by the phase transformation from soft austenite to hard martensite during deformation, which is known as transformation induced plasticity (TRIP) effect. The occurrence of TRIP effect can be controlled by austenite characteristics like carbon content, grain size and morphology. Additionally, the mechanical interaction with other phases surrounding each austenite grain during deformation would affect the stability of austenite, which has not been clarified. The current study has clarified how surrounding phases affect the mechanical stability of austenite against deformation-induced martensitic transformation. Two types of multi-phase microstructures composed of three different phases, i.e., ferrite (α) + austenite (γ) + martensite (M) and two phases of ferrite (α) + austenite (γ) were fabricated in Fe-1.6Mn-1.4Si-1.0Ni-0.5Al-0.2C, maintaining the characteristics of retained austenite nearly the same. It was found that the α+γ+M specimen exhibited slower rate of deformation-induced martensitic transformation during tensile deformation than the α+γ specimen, indicating higher austenite stability in the α+γ+M specimen. The in-situ synchrotron XRD measurements during tensile deformation revealed that there was significant stress partitioning between austenite and adjacent phases (ferrite, martensite and deformation-induced martensite), which influenced the phase transformation rate of austenite into deformation-induced martensite. Furthermore, it was also clarified by the in-situ synchrotron XRD that the austenite in the α+γ+M specimen always had higher stress than that in the α+γ specimen due to higher dislocation density in austenite introduced by martensitic transformation to form pre-existing martensite, so that higher stress was required for deformation-induced martensitic transformation in the α+γ+M specimen during tensile deformation than the α+γ specimen.