The Quasi-Static and Dynamic Thermomechanical Behavior of Medium-Mn Steels with Different Stacking Fault Energies

Author(s):

Christopher S. Meredith, Daniel J. Magagnosc, Daniel M. Field, Timothy R. Walter, Jeffrey T. Lloyd, and Krista R. Limmer

Abstract:

We present the mechanical behavior of three different advanced high-strength steels that share an iron–manganese–aluminum–chromium–carbon base composition with stacking-fault energies (SFEs) tailored by adjusting alloy composition to obtain transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), or perfect glide of dislocations (referred to as Slip). The Slip alloy (SFE = 49.3 mJ/m²) was a duplex alloy consisting mostly of γ-austenite with some δ-ferrite; the TWIP alloy (SFE = 26.9 mJ/m²) was fully austenitic; and the TRIP steel (SFE = 3.9 mJ/m²) was a triplex alloy consisting of austenite, α-martensite, and ε-martensite. The mechanical behavior of the three alloys was measured as functions of strain rate, temperature, and load path (tension or compression) to determine how the plastic behavior changes. Results show the Slip alloy has the highest yield strength but lowest work-hardening rate; the TRIP has the lowest yield strength and highest work-hardening rate; and the TWIP is between the other two, at room temperature at both quasi-static and dynamic (~103 s^–1) strain rates and in tension and compression. At a 200 °C testing temperature, the Slip and TWIP alloys’ strengths drop as expected, but the work-hardening rates change little, while the TRIP alloy yield strength increases and hardening rate drastically decreases. Thus, increasing the temperature tends to homogenize the behavior of the three alloys to where there are only small differences in strength and hardening. The strain-rate sensitivity of the Slip and TWIP is a positive number for the strength while changing the hardening little, with the TRIP alloy being the opposite. These trends in behavior are discussed in terms of how the SFE changes under various experimental conditions, with the rise of the SFE with temperature being dominant in shifting the deformation mechanisms.