Structure of the aluminum alloy 1570C subjected to multidirectional forging with decreasing temperature

Abstract

The structure of cast and homogenized aluminium alloy 1570C (Al5Mg0.18Mn0.2Sc0.08Zr, wt. %) was studied after multidirectional isothermal forging (MIF). Repetitive upsetting was carried out with a 25°C decrease of temperature at each pass starting from 450°C, a pass strain of 0.7 and strain rate 10-2 s-1. The alloy ductility was sufficient for a successful (without cracking) straining of the sample up to е ~ 10.5 at 100°C. In the initial state, the alloy had a coarse-grained structure with a grain size of 25 μm and uniform distribution of the nanoscale aluminides Al3(Sc,Zr). MIF resulted in continuously refined (sub) grains and material hardening. At low strains (е ≤ 4.2) and high temperatures (Т > 325°С), new grains were developed mainly in the mantle regions of the initial grains. As a result, a bimodal structure was formed, which persisted up to е ~ 6.3 (250°C). With further MIF, the evolved grain structure became more homogeneous and occupied practically the whole material volume. Electron microscopy showed that a nanocrystalline structure with a crystallite size of 100 – 170 nm and a uniform distribution of dispersoids was developed. The deformation structure was characterized by a high dislocation density that is attributed to a decelerating rate of dynamic recovery under decreasing temperature. Note that the structure developed was close in its characteristics to that observed in some fcc or hcp alloys after high pressure torsion at room temperature. However, in the last case, the sample dimensions were significantly smaller than after MIF.