Analysis of damage-tolerance of TRIP-assisted V10Cr10Fe45Co30Ni5 high-entropy alloy at room and cryogenic temperatures

Abstract

A single-phase face-centered-cubic (FCC) high- or medium-entropy alloys (HEAs or MEAs) have attracted great attentions due to their novel damage-tolerance properties (strength, ductility, and fracture toughness) by generating nano-twins at cryogenic temperature. The fracture toughness assessment is essential for evaluating the reliability of high-performance materials for cryogenic applications; however, fracture studies on single-phase FCC HEAs showing transformation-induced plasticity (TRIP) have been hardly conducted. In this study, thus, damage-tolerance mechanisms of a V10Cr10Fe45Co30Ni5 HEA showing the FCC to body-centered-cubic (BCC) TRIP were investigated at room and cryogenic temperatures. At room temperature (298 K), the alloy shows the tensile strength of 731 MPa, elongation of 40%, and fracture toughness (KJIc) of 230 MPa m1/2. At cryogenic temperature (77 K), the strength and elongation improve to 1.2 GPa and 66%, respectively, while the KJIc remains almost constant at 237 MPa m1/2. Dislocation-mediated plasticity prevails at 298 K; however, the TRIP from FCC to BCC occurs at 77 K. Deformation and fracture mechanisms are analyzed by stacking fault energies and differences in Gibbs free energies between phases calculated by ab-initio methods, and are compared to those of CrMnFeCoNi, CrCoNi, Fe50Mn30Co10Cr10, and V10Cr10Fe45Co20Ni15 alloys. Despite the presence of a considerable amount of BCC which is intrinsically brittle at low temperature, the transformed BCC martensite shows ductile fracture after the fracture toughness test even in cryogenic environments. These results demonstrate that the FCC to BCC TRIP can be an attractive route in a field of HEA design to overcome the strength and toughness trade-off at cryogenic temperature.