Effects of titanium addition on the microstructure and mechanical properties of quaternary CoCrFeNi high entropy alloy

The effects of minor Ti addition on the microstructure and mechanical properties of the as-cast quaternary equiatomic CoCrFeNi high entropy alloy were studied experimentally and supported by thermodynamic modeling. The phase evolution, microstructure, and shear properties were investigated using X-ray diffraction, optical and scanning electron microscopy, and the shear punch test. It was shown that during the solidification process, the interdendritic segregation of Ti resulted in discontinuous precipitation of the R-phase and serrated grain boundaries. The mechanism of grain refinement was attributed to some additional constitutional undercooling and intersection of tortuous grain boundaries; this, in turn, led to the formation of a bimodal grain structure. The shear elongation and shear strength of 1 and 2 at.% Ti-containing samples simultaneously increased, in comparison with the base alloy. This interesting improvement of the mechanical properties was related to the simultaneous grain refinement, precipitation of the R-phase, and the formation of tortuous grain boundaries.