Effect of applied stress on precipitation behavior in AZ91D magnesium alloy

The effect of applied stress on growth behavior of Mg17Al12 precipitate was investigated in an AZ91D magnesium alloy in the present study. The ingots of an AZ91D alloy were cast and subsequently extruded. Cylindrical tensile specimens were prepared from the extruded alloy and solution heat treatments were carried out in order to redissolve Mg17Al12 phase formed during solidification. The specimens were aged at 220degreesC under various applied tensile stresses from 0 to 80MPa and the holding time was changed from 10 to 40 minutes. It was found that the precipitation was accelerated along the tensile direction but decelerated along the transverse direction. In both directions, the growth distance was found to increase linearly with aging time. The growth rate of the precipitates, however, increased almost linearly along tensile direction, but decreased along the transverse direction, with increasing the applied stress. The changes in the growth rates of the Mg17Al12 precipitates could be interpreted by the changes in elastic strain energy and the grain boundary diffusivity caused by the applied stress.