Effect of Mo and cold forging deformation on strength and ductility of cobalt-based alloy L605

Abstract

Mo element was added to cobalt-based alloy L605, and cold forging deformation was performed. The effects of the addition and cold forging deformation on the microstructure and mechanical properties of the alloy were studied by thermodynamic calculation, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The stacking fault energy (SFE) of the alloy decreased after the addition, and the formation of stacking faults and intersections were promoted to improve the strength and hardness. The tensile strength of the alloy with Mo increased from 1190 to 1702 MPa after 24% cold deformation, producing significant work hardening. The strengthening mechanism is strain-induced martensitic transformation (SIMT) and deformation twinning. The alloy, combined with Mo and after 24% deformation, had both high strength and ductility in comparison with the original cobalt-based alloy L605. This is attributed to the lower SFE which caused the increase in stacking fault density. During the tensile process, the ε-hcp phase was easily generated at the stacking fault to reduce the stress concentration and increase the ductility. Controlling SIMT by adjusting the density of stacking faults can improve the mechanical properties of cobalt-based alloys. The ε-hcp phase, the interaction between deformation twins and dislocations, and the interaction between ε-hcp phases during cold forging deformation caused local stress concentration, lowering ductility and toughness.