The effect of stacking fault energy on the formation of nano twin/HCP during deformation in FCC concentrated solid solution (CSS) alloys

Abstract

The exceptional mechanical properties of face-centered cubic (FCC) concentrated solid solution (CSS) alloys generally correlate various deformation mechanisms, such as deformation induced nano twins and/or hexagonal close packed (HCP) phases, which is normally determined by stacking fault energy (SFE). To systemically change the SFE but keep the influence of atomic size and modulus mismatches minimum, a series of NiCo-based alloys are designed by adjusting the Ni/Co ratio. All alloys are fabricated to be single phase FCC with grain size carefully controlled to be ∼60 μm. Deformation substructures after tensile tests over temperature ranging from 77 K to 1073 K are carried out to reveal the effect of SFE on mechanical properties and deformation mechanism. SFE has little effect on yield strength (YS) at all test temperature, but do affect the ultimate tensile strength (UTS) and elongation to fracture (EF) of the alloys. At temperature lower than 873 K, it seems that the lower the SFE, the higher the UTS. At 1073 K, there is no obvious trend for UTS and EF, indicating that SFE has little effect on deformation behavior of alloys at high temperature. Formation of deformation nano twin/HCP depends on not only the SFE of the alloys but also the temperature. As testing temperature and/or SFE of the alloys decreases, deformation induced nano twin/HCP are favorable and become dominant deformation modes for the FCC CSS alloys, resulting in a combination of high strength and good plasticity due to the great work hardening capacity.