Effects of deformation-induced martensitic transformation on cryogenic fracture toughness for metastable Si8V2Fe45Cr10Mn5Co30 high-entropy alloy

In this study, fracture toughness was evaluated at room and cryogenic temperatures for a metastable Si8V2Fe45Cr10Mn5Co30 (at.%) alloy, and roles of deformation-induced martensitic transformation (DIMT) on fracture behavior were investigated with respect to types of transformation mechanism and martensite morphology. The as-annealed alloy consisted mainly of face-centered-cubic (FCC) phase with athermal hexagonal-close-packed (HCP) martensite. The DIMT from FCC to body-centered-cubic (BCC) via intermediate HCP resulted in an excellent combination of strength and ductility by high strain-hardening effect. Considerably high fracture toughness and ductile-dimpled fracture mode appeared at room temperature, but the fracture toughness significantly deteriorated at cryogenic temperature. The BCC martensite exhibited a thick-plate morphology as a resultant product of stress-induced transformation from low-stability FCC, and the plates had an alternating variant characterized by a ladder martensite, based on a 60°-BCC-twin relationship with {112} twin planes. This plate-type BCC martensite had the fewer phase boundaries and geometrically necessary dislocations than the lath-type BCC martensite. In addition, cleavage {001}BCC-planes of two martensite blocks having the alternating variants were slightly deviated, thereby leading to the much lower toughness along with the brittle fracture mode of lamellar cleavage facets at cryogenic temperature. Thus, the present work suggests that the proper control of FCC stability is required to enhance the resistance to low-temperature embrittlement for securing cryogenic applications.