Strengthening and fracture of deformation-processed dual fcc-phase CoCrFeCuNi and CoCrFeCu1.71Ni high entropy alloys

Abstract In this study, the microstructural evolution and strengthening mechanism of deformation-processed CoCrFeCuNi and CoCrFeCu1.71Ni were studied. Segregation and phase separation into dual fcc phases was found to be thermodynamically stable even after homogenizing annealing at 1273K. Dual fcc phase structure with elongated Cu-rich filaments developed by deformation processing and elongated dual-phase structure was found to remain stable even during annealing at1173 K. The high temperature stability of Cu-rich filaments is attributed to the low internal strain energy of filaments due to continuous recrystallization during deformation processing. One interesting consequence of stability of Cu-rich filaments is that the phase boundaries act as barriers to grain growth and the grain of Cr–Fe–Co–Ni matrix and Cu-rich filaments are controlled by the distribution and thickness of Cu-rich phase. The yield strengths of CoCrFeCuNi (506 MPa) and CoCrFeCu1.71Ni (477Mpa) were observed to be greater than that of the original Cantor alloy (220 MPa) with minor reduction of elongation less than 3–7%. Phase boundaries in CoCrFeCuNi or CoCrFeCu1.71Ni HEAs act as effective barriers to grain growth during annealing and to dislocation transmission during deformation. The strengthening with excellent ductility in HEAs of this study is attributed to the grain size refinement and the development of the enhanced back stress by the phase boundaries between Cu-rich filaments and Cu-lean matrix with higher modulus and strength. The lower strength of annealed CoCrFeCu1.71Ni than that of CoCrFeCuNi can be attributed to the presence of higher fraction of soft Cu-rich filaments.