Structural changes during severe hot forging of the aluminum alloy 1570C

Abstract

Microstructure evolution in the cast Al-Mg-Sc-Zr alloy 1570C during multidirectional forging (MDF) to the strain 10.5 at 450оС (~0.77Тm) and 10-2 s-1, was investigated. The alloy belongs to advanced structural materials exhibiting high strength at ambient temperature. It can be easily hot worked, exhibits superior superplasticity in a fine-grained state, while its cold deformation causes difficulties due to high yield-stress and relatively low ductility. Consequently, an evaluation of grain refinement potentiality in this alloy at high temperatures may be important for industrial application. Before MDF, the alloy had the equiaxed grains ~25 µm in size with near uniformly distributed nanoscale coherent dispersoids Al3(Sc,Zr). In the early MDF stages, new fine (sub)grains with low- and high-angle boundaries were formed near original grain boundaries. With increasing strain, the fraction of these crystallites increased, finally resulting in almost fully recrystallized structure with grain size ~3 µm and fraction of high-angle boundaries (HABs) ~0.9. The strain dependency of microstructural parameters showed that grain refinement occurred mainly in accordance with continuous dynamic recrystallization. There was a strong interaction between lattice dislocations, (sub)grain boundaries and coherent dispersoids Al3(Sc,Zr). This implies that the latter effectively hindered (sub)grain boundary migration and provided dislocation accumulation, resulting in formation of high-density subboundaries and their transformation to HABs, even at high temperature. Moreover, hot MDF did not result to any degradation of the alloy high strength. The data suggested that there was a great potential for extensive grain refinement in this alloy at elevated temperatures alleviating difficulties of its thermo-mechanical processing.