Influence of strain rate on the work hardening, strain induced martensite formation, strain partitioning, and variant selection in a medium-Mn steel

Abstract

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′-martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant.