The slip character and low cycle fatigue behaviour: The influence of F.C.C. twinning and strain-induced F.C.C. → H.C.P. martensitic transformation

Abstract

The influence of slip character on the low cycle fatigue behaviour was investigated in the CoNi system at room temperature. Three alloys of increasing stacking fault energy (SFE) were used: Co31Ni ( SFE ∼- 12 mJ m −2) which deforms by slip and f.c.c. → h.c.p. strain-induced martensitic transformation, Co33Ni ( SFE = 15 mJ m −2) which exhibits slip and twinning and Co45Ni ( SFE = 45 mJm −2) which deforms only by slip with easy cross-slip. Push-pull low cycle fatigue tests were conducted under plastic strain control up to a few 10 4 cycles. The number of cycles to fracture was found to increase with decreasing SFE which promotes planar deformation mechanisms: the life obtained in Co31 Ni and Co33Ni alloys is respectively about 6 and 3 times higher than that of Co45Ni alloy. Measurements of striation spacings on the fracture surfaces have enabled to show that the influence of twinning on fatigue life is mainly due to a large increase of the initiation period before stage II crack propagation. This behaviour was associated with a difference in crack initiation sites along twin or h.c.p. platelets where there is a strain localization in low SFE alloys or along grain boundaries in the high SFE alloy. The increase of crack initiation period was explained on the basis of a reduced stage I crack propagation rate in the alloys exhibiting planar deformation mechanisms.