Superplastic Behavior of a Fine Grained AZ61 Alloy Processed by Large Strain Hot Rolling

A processing route based on conventional hot rolling has been recently developed to induce significant grain refinement in Mg alloys. The simplicity and rapidity of the processing route as well as the fact that conventional rolling is used, may allow it to be put into practice successfully in industry. This method consists of only two to three rolling passes, each producing a large thickness reduction, and intermediate annealings of 5 minutes duration. The resulting microstructure is mainly formed by very fine grains. Tensile tests at different temperatures and strain rates were performed in order to analyse the mechanical behaviour of the processed AZ61 alloy under different testing conditions. Microstructure and texture evolution during deformation were examined by optical microscopy (OM) and X-ray diffraction, respectively. It is shown that the microstructures developed by large strain hot rolling are capable of exhibiting significant superplastic elongations at moderate to low temperatures. Stress exponents close to 2 were measured during deformation under optimum superplastic conditions. Additionally, grains remained equiaxed and a significant decrease in the texture intensity is observed. This is consistent with the predominance of grain boundary sliding as the main deformation mechanism responsible for superplasticity.