High cycle fatigue of spinodally decomposed alloys

Abstract

High cycle fatigue of spinodally decomposed Cu-4 at.% Ti and Cu-30% Ni-3.2% Cr was studied in comparison with the solution treated state—mostly at constant stress amplitudes. Surfaces of fatigued specimens were investigated by scanning electron microscopy. An improvement of the highcycle fatigue strength by spinodal decomposition is only observed for CuNiCr, not for CuTi. The Wöhler (S-N) curve of CuNiCr scales with the yield stress, so that the resolved high cycle fatigue shear strength τ 10 7 is close to the critical resolved shear stress τ 0. In CuTi τ 10- is only about τ 0/2 after spinodal decomposition. At low amplitudes cyclic softening in persistent slip bands occurs locally and cannot be measured macroscopically. A model of cyclic softening by irreversible dislocation motion is suggested which considers the coherency strains to be the driving force for slip irreversibility. This model predicts a decrease of the fatigue resistance with increasing lattice misfit, which is in agreement with experimental results on different spinodal alloys.