Effect of Microstructure on Fracture Toughness of AZ91 Magnesium Alloy.

Abstract

The effect of grain size on mechanical properties of a T6-treated AZ91-magnesium alloy was studied. In order to obtain samples free from shrinkage, oxides and gas defects, the alloy was produced using a continuous casting process with various casting temperatures, and a constant cooling rate. In the T6-treated AZ91 alloy, discontinuous precipitates were observed clearly along small grain boundaries, while continuous precipitates were observed in large grains. It was found that hardness in a grain depended on the form of precipitation and the magnesium alloy having a smaller grain size showed larger values of fracture toughness, ultimate tensile strength, 0.2% proof stress and elongation at fracture. Fracture toughness of the alloy having a finer grain size increased from 18.3 MPa·m1/2 to 21.4 MPa·m1/2 after T6 treatment. The strengthening mechanisms leading to these results are concluded to be the grain-size dependence of stress concentration at the grain boundary, the density of grain boundaries and the grain boundary precipitates which act as obstacles when cracks propagate.