Mechanical properties of ultra-fine grained AZ91 magnesium alloy processed by friction stir processing

The relationship of microstructure and mechanical properties in a friction stir processed (FSP) AZ91 magnesium alloy has been analyzed and compared with numerous investigations in the literature. Since the heat generation and sink during FSP drastically influences the final microstructure, several backing devices were used for controlling the stir zone temperature, producing a grain size refinement down to values close to 0.5 mu m. This microstructure often determines excellent mechanical properties at room temperature and superplastic behavior at high temperatures. The yield stress at room temperature shows a sharp decrease in the Hall-Petch slope related to a favorable orientation for slip of the basal planes. Noticeable changes in ductility are explained in terms of grain size and texture effects on work hardening behavior which joins both contributions. Finally, the analysis of the tensile tests performed at high temperature, together with the data reported by other authors, have been used to obtain an unitary description of the Grain Boundary Sliding (GBS) mechanism in the AZ91 magnesium alloys.