The opposite transformation-induced plasticity effects of Fe61.5Cr17.5Ni13Al8 high-entropy alloy at ambient and cryogenic temperatures

Phase transformation-induced plasticity (TRIP) is recently reported in high-entropy alloys (HEAs). Here we reported opposite TRIP effects in Fe 61.5 Cr 17.5 Ni 13 Al 8 HEAs observed at room and cryogenic temperatures. Specifically, FCC + BCC dual-phase HEAs were prepared by cold rolling (CR) and subsequent isochronal non-isothermal heat treatments. The sample after CR has the FCC phase fraction of 56.6 %, and a transition from FCC-to-BCC to BCC-to-FCC transformation occurs with increasing annealing temperatures from 973 K to 1273 K, causing the FCC phase fraction increases from 30.4 % to 66.4 %. Tensile tests at room temperature show that with decreasing annealing temperatures from 1273 K to 973 K, the yield strength increases from 640 ± 7 MPa to 900 ± 13 MPa, while the ductility remains at ∼27.2 %. Microstructural analyses reveal that the sample with lower FCC phase fraction undergoes more martensitic phase transformation which provides extra ductility, i.e., the traditional TRIP effect. However, with decreasing annealing temperatures from 1273 K to 973 K, the yield strength at 77 K increases from 850 ± 7 MPa to 1300 ± 13 MPa, and the ductility decreases from 37.6 ± 0.9 % to 13.3 ± 3.4 %, following the strength-ductility trade-off relationship. Nevertheless, post-mortem microstructural analyses revealed a similar martensitic phase transformation trend as at room temperature, suggesting that martensitic phase transformation at 77 K did not provide the traditional TRIP effect. Fracture surface analysis revealed that the BCC phase undergoes a ductile-to-brittle transition as the tensile temperature decreases from 293 K to 77 K, causing the null weak TRIP effect at 77 K.