Effect of Multidirectional Forging with Decreasing Temperature on the Microstructure and Microhardness of the Aluminum-Magnesium-Scandium-Zirconium Alloy

Evolution of the microstructure and microhardness of the cast and homogenized aluminum alloy 1570C (aluminum [Al]-5 magnesium [Mg]-0.18 manganese [Mn]-0.2 scandium [Sc]-0.08 zirconium [Zr], wt. %) were studied under multidirectional forging (MDF) with decreasing the temperature. Sequential compression passes were performed at a strain rate of 10−2 s−1 with 90° changing of the loading axes from pass to pass; with the strain per pass of Δe = 0.7 and the 25°C decrease of temperature in each pass, starting from 450°C (T = 0.8 Tm). It has been shown that the alloy ductility was sufficient for straining the samples without cracking up to the total strain of Σe = 10.5 at 100°C (about 0.4 Tm). In the initial state, the alloy possessed a coarse-grained structure with a grain size of about 25 μm and a uniform distribution of the Al3(Sc,Zr) nanoscale aluminides. MDF led to a continuous grain refinement. At relatively low strains, Σe ≤ 4.2 (and high temperatures, T ≥ 325°C), new grains were evolved mainly in the mantle areas of initial grains, resulting in the formation of a bimodal (sub)grain structure, which persisted up to Σe = 8.4 (T = 175°C). During further MDF, the alloy structure became more homogeneous and fine-grained, and at Σe = 10.5 (T = 100°C), it almost completely transformed into a nanocrystalline grain structure with a crystallite size of 100–150 nm, stabilized by nanodispersed Al3(Sc,Zr) precipitates. The average size of the deformation-induced (sub)grains in the whole range of strains (temperatures) studied obeyed a power-law function of the flow stresses with the exponent close to −0.75. The microhardness testing showed that MDF did not lead to the notable alloy hardening in the samples deformed at relatively high temperatures (T > 325°C) and low strains (Σe < 4.2). With further processing, in contrast, a significant (about 1.5 times) hardness increase took place in accordance with the Hall-Petch relationship.