The Effect of Scandium on Thermal Stability and Fatigue Behavior of Ultra Fine Grained Al-Mg-Sc Alloy Produced by Severe Plastic Deformation

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The fatigue properties and thermal stability of fine grained Al-Mg-Sc-Zr alloy produced by equal-channel Angular Pressing (ECAP) are explored. The grain size are reduced to about 0.3 μm by ECAP processing. Comparing with ordinary Al-Mg alloy (5056 Aluminum alloy), in high cycle fatigue regime Al-Mg-Sc-Zr exhibits higher fatigue limit. On the other hand, low cycle fatigue life of Al-Mg-Sc-Zr alloy is shorter than that of 5056 aluminum alloy. Structural changes of these alloys during fatigue tests were investigated by transmission electron microscopy. It is shown that the fine structure of Al-Mg alloy achieved during processing is unstable and tend to relax with cycling, resulting in local recovery. In contrast there is no substantial microstructure changes in Al-Mg-Sc-Zr alloy after fatigue. Furthermore -result of microhardness measurements after annealing reveals that structure of Al-Mg-Sc-Zr alloy is stable up to 773K. These observations prove that presence of Al3(Sc, Zr) precipitates improves alloy quality in terms of both mechanical and thermal stability.