Correlation between microstructure and shape memory properties in an Fe-9.5Ni-6.5Mn dual phase steel developed by intercritical annealing and subsequent ageing

Abstract

Research was conducted to investigate the shape memory properties related to the microstructure in an Fe-9.5Ni-6.5Mn (wt.%) dual phase steel developed by intercritical annealing and subsequent ageing. Electron back scattering diffraction (EBSD) analysis and high resolution transmission electron microscope (HRTEM) images revealed that the microstructure of the initial steel in the solution-annealed condition was a typical lath α′-martensite with a high density of dislocations. During the intercritical annealing treatment of the solution-annealed steel at 590 °C in the ferritic-austenitic (α+γ) two-phase region for various annealing times, a part of α′-martensite transformed to the reversed γ-austenite with a difusionless shear nature, resulting in the development of a dual-phase (α′+γ) steel. Also, subsequent ageing treatment of the intercritically annealed steel at 480 °C for 3.6 ks led to formation of the nanoscale θ-NiMn precipitates in the dual-phase microstructure. The experimental results indicated a maximum shape recovery of ∼55% at 1% pre-strain for the intercritically annealed steel for 1.8 ks, followed by subsequent ageing. The presence of austenite phase reinforced by precipitates in the dual-phase steel was the main reason for appearance the shape recovery. In addition, the existence of isolated stacking faults in the reversed austenite region after straining confirmed that the occurrence of the austenite to ε-martensite transformation (γ→ε) under loading and the reverse transformation (ε→γ) upon heating are the main causes of the shape memory effect in the studied steel.