Work hardening behavior in a steel with multiple TRIP mechanisms

TL;DR

This study investigates the complex two-stage TRIP behavior in a multi-mechanism steel, revealing how alloy composition and microstructure influence strain hardening and strength.

Contribution

It provides new insights into the two-stage TRIP mechanisms in steel with multiple transformation pathways and the effects of alloying elements on stacking fault energies.

Findings

Two-stage TRIP behavior observed with initial austenite to {}-martensite transformation

Alloy segregation influences TRIP just before necking

High strain hardening exponent of 1.4 achieved, with tensile strength 1165 MPa and 35% elongation

Abstract

Transformation induced plasticity (TRIP) behavior was studied in steel with composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP mechanisms. The initial microstructure consisted of both {\epsilon}- and {\alpha}-martensites with 27% retained austenite. TRIP behavior in the first 5% strain was predominately austenite transforming to {\epsilon}-martensite (Stage I), but upon saturation of Stage I, the {\epsilon}-martensite transformed to {\alpha}-martensite (Stage II). Alloy segregation also affected the TRIP behavior with alloy rich regions producing TRIP just prior to necking. This behavior was explained by first principle calculations that revealed aluminum significantly affected the stacking fault energy in Fe-Mn-Al-C steels by decreasing the unstable stacking fault energy and promoting easy nucleation of {\epsilon}-martensite. The addition of aluminum also raised the intrinsic stacking fault energy and caused the {\epsilon}-martensite to be unstable and transform to {\alpha}-martensite under further deformation. The two stage TRIP behavior produced a high strain hardening exponent of 1.4 and led to ultimate tensile strength of 1165 MPa and elongation to failure of 35%.