Thermal stability of friction stir processed ultrafine grained AlMgSc alloy

Two different aspects of the thermal stability of grains were studied — (i) thermal stability of grain structure evolution during friction stir processing (FSP), and (ii) thermal stability of grains after FSP. It was concluded that Al3(Sc,Zr) dispersoids are effective in limiting grain growth to ultrafine grained regime (UFG) by stabilizing the microstructure during FSP. The presence of these dispersoids before processing was more effective in refining the grain size than the case in which these dispersoids precipitated during FSP. The mean grain sizes for these cases were 0.40 ± 0.17 μm and 0.50 ± 0.25 μm, respectively. Isothermal grain growth study revealed that UFG grain structure was maintained up to 723 K even after 16 h of annealing whereas most of the UFG alloys reported in the literature show extensive grain growth above 473 K. To understand the role of Al3(Sc,Zr) dispersoids in this alloy, 5086AlH32 was friction stir processed and subjected to identical annealing conditions. Annealing of FSP 5086AlH32 at 623 K for 1 h resulted into abnormal grain growth (AGG). The onset of AGG was observed for UFG sample only after annealing at 823 K for 1 h. This was rationalized using Humphreys' model for AGG [F.J. Humphreys, Acta Mater., 45 (1997) 5031].