Effect of grain orientation on small fatigue crack growth behaviour in magnesium alloy AZ31 rolled plate

Abstract

Plane bending fatigue test was performed using magnesium alloy AZ31 rolled plate whose mean grain size was 106 μm. A small hole was introduced in the centre of a specimen as a crack starter notch and a fatigue crack initiated from the notch was monitored by optical and atomic force microscopes. The crystallographic orientations of grains on fatigue crack growth path were measured by an Electron Backscatter Diffraction (EBSD) method in advance, and the effect of grain orientation on small fatigue crack growth behaviour was discussed. The fatigue crack growth behaviour was strongly affected by grain orientations. Transgranular fatigue crack tended to grow due to slip deformation along the basal slip plane whose Schmid factor was relatively large. When the Schmid factors of the basal slip system were very small, the crack grew with the activation of the cylindrical slip systems. Twining deformation exerted a little influence on the crack growth behaviour. The crack growth rates were slower when the crack growth was due to slip deformation along the cylindrical slip planes.