Behaviour of AZ31 magnesium alloy during compression at elevated temperatures

The hot deformation of AZ31 magnesium alloy has been studied by compression testing using a Gleeble 1500 machine at temperatures between 250 and 450 degrees C and at strain rates ranging from 0.005 to 5 s(-1). Optical microscopy and transmission electron microscopy (TEM) have been used to observe microstructures of the alloy. The experimental results show that the flow stress behaviour can be described by an exponential law at temperatures below 350 degrees C. At higher temperatures a power law of deformation is valid. The hot deformation activation energy 0 derived from the experimental data is 112 U mol(-1) with a stress exponent n=7. Optical microscopy and TEM observations show that dynamic recrystallisation (DRX) takes place during the deformation process and the formation of new grains occurs by conventional DRX nucleation by bulging. The average DRX grain size D-rex is sensitively dependent on deformation temperature T and strain rate (epsilon) over dot and is also a function of the Zener-Hollomon parameter Z. The relationship between Z and D-rex has been experimentally constructed using linear regression. In the present work, DRX grains nucleate by bulging of some portions of serrated grain boundaries, accompanied by the formation of twinning.