Influence of orientation on the low cycle fatigue of MAR-M 200 single crystals at 650 °C II: Cyclic stress-strain behaviour

Abstract

The cyclic stress-strain behaviour of MAR-M 200 single crystals was studied at 650 °C (923 K) under total axial strain control, using either low cycle fatigue tests under constant-strain-range or cyclic tests conducted under stepwise-increasing strain. Specimens having crystallographic orientation near [001], [111], [213] and [101] were tested under fully reversed strain. The cyclic stress-strain curves were found to be strongly orientation dependent and plastic strains were accommodated mainly by slip bands above a critical plastic strain threshold. Cyclic shear stress-strain curves for [111]〈1 1 0〉 slip exhibit a significant orientation dependence. Mean stresses were found to occur, the sign of which depends upon the crystallographic orientation. The nature of slip systems was analysed using slip band markings on the gauge length as well as accurate measurements of cross-section of tested specimens. The cyclic behaviour was then discussed using these observations and previous experimental data on γ-γ′ alloys.