The influence of slip on the low-cycle fatigue behaviour was investigated in the Co-Ni system at room temperature. Three alloys of increasing stacking-fault energy were used: Co-31Ni (12mJ/m2) which deformed by slip and fcc → hcp strain-induced martensitic transformation, Co-33Ni (15mJ/m2) which exhibited slip and twinning and Co-45Ni (45mJ/m2) which deformed only by slip with easy cross-slip. Push-pull low-cycle fatigue tests were conducted under plastic strain control up to some 104cycles. The number of cycles to fracture increased with decreasing stacking-fault energy, which promoted planar deformation mechanisms. The life of Co-31Ni and Co-33Ni was about 6 and 3 times, respectively, higher than that of Co-45Ni. Measurements of striation spacings on the fracture surfaces showed that the influence of twinning upon fatigue life was due mainly to a large increase in the initiation period before stage-II crack propagation. This behaviour was associated with a difference in crack initiation sites along twin or hcp platelets, where there was strain localization in low stacking-fault energy alloys, or along grain boundaries in the high stacking-fault energy alloy.

The Slip Character and Low Cycle Fatigue Behaviour: the Influence of FCC Twinning and Strain-Induced FCC → HCP Martensitic Transformation. G.Chalant, L.Remy: Acta Metallurgica, 1980, 28[1], 75-88