Development of microstructure and properties during the multiple extrusion and consolidation of Al-4Mg-1Zr

Multiple extrusion of Al-4Mg-1Zr (wt.%) is presented as a severe plastic deformation technique to develop an ultra-fine grained material, using rapidly solidified needles as a feedstock. After each extrusion pass the microstructure is refined, the primary Al3Zr precipitates are broken down and <50-nm-scale cubic Al3Zr are precipitated from solution. Recovery dominates the initial passes with recrystallisation becoming dominant during later passes, but the grains remain at the sub-μm scale. Interestingly, the yield stress and the ultimate strength increased significantly after the first pass, and then decreased - any strength increment gained from the grain refinement was offset by coarsening of the metastable nano-scale Al3Zr dispersoids.