Abstract
• The coarse-grained overaged WE54 alloy was refined to 1 µm by severe FSP. • Thus, deformation mechanism at high temperature changed from solute-drag creep to GBS. • Comparison of Mg alloys showed that heavier solutes give less superplastic response. • The accommodation process for GBS is hindered by low solute diffusion rates. The coarse-grained WE54 magnesium alloy was heat treated in order to have minimum hardness minimizing the effects of precipitates and solid solution. Friction stir processing (FSP) was applied in severe conditions to obtain fine, equiaxed and highly misoriented grains, with grain sizes even less than 1 µm. The high severity of processing demonstrated to have a strong impact in the microstructure. Consequently, the processed materials exhibited excellent superplasticity at the high strain rate 10 −2 s −1 , and temperatures between 300 and 400 ºC. The maximum tensile superplastic elongation of 756% was achieved at 400 ºC thanks to the operation of grain boundary sliding mechanism (GBS). Besides the new data obtained through tensile testing, the paper deals with a transcendental question regarding the large differences in strain rate values at a given stress in the superplastic regime at maximum elongation compared to other magnesium-based alloys. With this is mind, 19 magnesium alloys from 22 different investigations were analyzed to give some light to this behavior that never was treated before. It is proposed that this behavior has to be attributed to the accommodation process, necessary for GBS to occur, which is hindered by reinforcing solutes.
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