Hetero-deformation-induced strengthening behavior of as-annealed Co30Cr30Fe18Ni18Mo4 high entropy alloy by metastable σ phase

Abstract

Multiple deformation mechanisms have been utilized to overcome the strength-ductility trade-off in high entropy alloys (HEAs), especially at low temperatures and deformation conditions. To trigger the multiple deformation mechanisms at room temperature, the multiphase heterostructure Co30Cr30Fe18Ni18Mo4 HEA was developed at moderate temperatures (600 °C–1100 °C) and short time (1 h–8 h) annealing treatment. The phase formation, microstructure evolution, and tensile properties of as-annealed Co30Cr30Fe18Ni18Mo4 HEA were systematically investigated. At 600 °C for 2 h (A600-2), the alloy is consisted of the face-centered-cubic (FCC) phase and rich (Cr, Mo) primary σ phase. The fine needle-shaped secondary σ phase precipitates at 800 °C. With the increase of annealing temperatures and times, the volume fraction of secondary σ phase increases. FCC matrix, metastable secondary σ phase, and primary σ phase are gradient heterostructure. The thermodynamic metastable of the FCC matrix and the sluggish diffusion kinetics of elements induce the formation of fine and metastable secondary σ phase at the thermal driving force. The as-annealed alloy exhibits high yield strength and applicable plasticity with the increase of the secondary σ phase. Hetero-deformation-induced (HDI) strengthening is induced by piled-up dislocations at the σ/FCC interface. The dominant roles in the improvement of global yield strength are precipitation strengthening and back-stress strengthening. The back stress, stacking faults (SFs), and twins are generated in the soft FCC matrix, and a few SFs are observed in the metastable secondary σ phase, which induces high strain hardening ability. The cracks initiated in the hard primary σ phase are passivated by the FCC matrix and metastable secondary σ phase. TWIP, SFs, and HDI strengthening multiple deformation mechanisms contribute to improving strength.