Effect of Strain Rate on Deformation Response of Metastable High Entropy Alloys Upon Friction Stir Processing
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nventional materials often exhibit strong strain rate-dependent mechanical properties such as ultimate tensile strength (UTS) and uniform elongation because of altered work hardening rate with strain rate. Extensive studies[1–7] have advanced the understanding of the effect of strain rates on room temperature tensile properties for various materials, including medium Mn steels, Al- and Mg-based alloys with distinct microstructures. These materials show a proportional increase in UTS with increased strain rate.[8] In 304 austenitic stainless steel, strain rate sensitivity depends on austenite stability and sensitivity to martensite formation during deformation.[7] In transformation-induced plasticity (TRIP)-assisted superaustenitic stainless steels, strain rate does not affect yield strength (YS) or UTS; however, it has beneficial impact on uniform elongation.[9] Das et al.[6] reported that increasing strain rate promotes the early formation of deformation-induced martensite while suppressing its saturation value at fracture. Silva et al.[10] observed low strain rate sensitivity in Fe50Mn30Co10Cr10 high entropy alloy (HEA)
K. LIU, S.S. NENE, M. FRANK, and R.S. MISHRA are with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of North Texas, Denton, TX 76203. Contact e-mail: [email protected] Manuscript submitted September 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
within the strain rate range of 1 9 104 s1 to 5 9 103 s1. They determined that the lack of strain rate sensitivity was due to the dominance of the TRIP mechanism. Increase in strain rate is reported to suppress the TRIP effect and leads to transition to twinning induced plasticity (TWIP) domain in steels because of localized adiabatic heating as well as decreased mean path for dislocation motion.[11] An important aspect of mechanical behavior that needs to be explored is the influence of strain rate on TRIP phenomena in friction-processed dual-phase high entropy alloys. Our recent work on metastability-based HEA design and subsequent friction stir processing exhibited high strength-ductility synergy due to engineered f.c.c. phase stability by Si addition in the Fe-Mn-Co-Cr matrix.[12] Addition of Si in this matrix is found to increase the driving force for h.c.p. phase formation under synergistic activation of strain and temperature. The TRIP effect can be altered dramatically by tuning the fraction of prior h.c.p. phase and grain size through either minor addition of Si or major alloying with Co.[13,14] Moreover, increase in the strain rate is reported to promote easy TRIP-to-TWIP transition during deformation in TRIP steels due to lower h.c.p. stability at higher strain rates.[15] As a result, TRIP-to-TWIP transition with increase in strain rate can be delayed by effective alloying of Co and Si in the TRIP-assisted alloys due to increased h.c.p. stability. Considering this background, two distinct HEA compositions were chosen, namely Fe42Mn30Cr15Co10Si3 and Fe40Mn20Co20Cr15Si5, which differs in
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