Phase stability and strengthening behavior of as-annealed Co30Cr30Fe17Ni17Mo6 high entropy alloy at room and low temperatures

To obtain fine and disordered precipitated phases and coordinate multiple strengthening mechanisms in face -centered cubic (FCC) high entropy alloys (HEAs), the Co30Cr30Fe17Ni17Mo6 HEA was prepared and annealed at various temperatures (800, 1000, and 1200 degrees C for 2 h, furnace cooling). The phase stability, microstructure evolution, and tensile properties were systematically investigated at room and low temperatures. The as-cast alloy is consisted of the FCC phase and primary sigma phase rich (Cr, Mo) element. The as-annealed alloy pre-cipitates a fine and disordered secondary sigma phase from the FCC matrix around the primary sigma phase. The average areal size of the secondary sigma phase gradually increases from 1.848 mu m2 to 2.307 mu m2 with the increase of as -annealed temperature. The highest Gibbs free energy of the FCC phase in thermodynamic and the sluggish diffusion kinetics of elements induce the fine and disordered secondary sigma phase precipitation. The as-annealed HEA has a high yield strength of 557 MPa at A800, increased by 21% compared with the as-cast alloy, and maintains applicable plasticity of 16.91% at 298 K. The as-annealed HEA at A1000 exhibits strength-ductility synergy at 203 K. The high yield strength is attributed to secondary sigma phase strengthening and sigma/FCC inter-face strengthening coordination. Dislocations accumulate and strongly interact at the sigma/FCC interfaces, which induces the formation of back stress in softer matrix phases. The back stress and dislocation cells provide the hardening of the FCC matrix at 298 K. The back stress, the net dislocations, and stack faults are activated in the FCC matrix phase to induce high strain hardening ability at 203 K. The FCC matrix with high strain hardening ability reduces stress concentration and delays crack initiation. The strain gradients are accumulated in the FCC matrix, secondary sigma phase, and primary sigma phase to realize strength-ductility synergy.