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

Abstract

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 °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 σ phase rich (Cr, Mo) element. The as-annealed alloy precipitates a fine and disordered secondary σ phase from the FCC matrix around the primary σ phase. The average areal size of the secondary σ phase gradually increases from 1.848 μm2 to 2.307 μm2 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 σ 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 σ phase strengthening and σ/FCC interface strengthening coordination. Dislocations accumulate and strongly interact at the σ/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 σ phase, and primary σ phase to realize strength-ductility synergy.