Improvement of low-cycle fatigue resistance of Co-Ni alloys by silicon addition

The effect of silicon alloying on the low-cycle fatigue (LCF) resistance and fatigued microstructure of the Co-31Ni, Co-31Ni-3Si, Co-33Ni, and Co-33Ni-3Si (in wt%) alloys was studied under an axial strain control mode at the total strain amplitude, Δεt/2, of 0.01. The addition of 3 wt% of Si increases the fatigue life, Nf, of the examined Co alloys by more than 2.5 times. For instance, Nf of ~ 4000 and 10,000 cycles were achieved in the Co-31Ni and Co-31Ni-3Si alloys, respectively. It was observed that improved LCF resistance of the Si added Co alloys results from the moderate kinetics of the deformation-induced ε-martensitic transformation (ε-MT) and inhibited cross-slip of perfect dislocations provided by solid solution hardening of the austenite. The similarity in the fatigue behavior and fatigued microstructure of the Si added Co alloys and Fe-based high-Mn alloys is discussed.