Fe₅₅Co_17.5Ni₁₀Cr_12.5Mo₅ High-Entropy Alloy with Outstanding Cryogenic Mechanical Properties Driven by Deformation-Induced Phase Transformation Behavior

Understanding the deformation-induced transformation-induced plasticity (TRIP) in high-entropy alloys (HEAs) is critical for obtaining desired mechanical properties. In this study, the deformation-induced phase transformation behavior of Fe₅₅Co_17.5Ni₁₀Cr_12.5Mo₅ HEA was investigated by microstructural characterization using scanning electron microscopy-based techniques. The results showed the initiation of body-centered cubic (BCC) variants at the intersections of hexagonal close-packed (HCP) variants. The growth of BCC variants occurred through the HCP phase with a Burgers orientation relationship. Heterogeneous strain distributions at the phase boundaries also induced additional BCC variants with unusual crystallographic characteristics, although the growth of these BCC variants was inhibited. The TRIP behavior during a cryogenic-temperature tensile test of the Fe₅₅Co_17.5Ni₁₀Cr_12.5Mo₅ alloy provided an extra strain-hardening rate by absorbing plastic deformation energy to initiate BCC and HCP variants and additional strain-hardening capability of the transformed BCC phase. Because of the extra strain-hardening rate, the Fe₅₅Co_17.5Ni₁₀Cr_12.5Mo₅ alloy achieved an outstanding tensile strength (1.2 GPa) and ductility (0.81) combination at cryogenic temperatures.