Effect of temperature of isothermal multidirectional forging on microstructure development in the Al-Mg alloy with nano-size aluminides of Sc and Zr

Abstract

The effect of deformation temperature on structural changes in the homogenized ingot of a complex-alloyed aluminum alloy 1570C (Al-5% Mg-0.2% Sc-0.1% Zr) during hot isothermal multidirectional forging (IMF) was investigated. The alloy with equiaxial grains of 25 μm and coherent precipitates of Al3(Sc, Zr) 5–10 nm in diameter was subjected to IMF at the temperatures of 325° and 450 °C (∼0.6 and 0.8 Tm, respectively) at a strain rate of 10−2 s−1 to a total strain of ε = 8.4 with a pass strain of 0.7. It has been shown that grain refinement takes place during IMF at both temperatures. The formation of new fine (sub)grains surrounded by low- and high-angle boundaries started in the vicinity of the original grain boundaries in the earlier stages of deformation. With increasing strain, the volume fraction and boundary misorientations of such crystallites increased; that results in almost complete replacement of the original structure with a new fine-grained one via continuous dynamic recrystallization. The alloy grain refinement was controlled by the interaction of lattice dislocations and/or grain boundaries with precipitates, effectively inhibiting migration of the boundaries as well as dynamic recovery, leading to rearrangement of lattice dislocations, and their annihilation. Herewith, the kinetics of the fine-grained structure formation was practically independent on the processing temperature, resulting in roughly similar microstructures after the same strains, which characterized by the closed mean misorientation angles and fractions of high-angle boundaries. However, the size of (sub)grains processed at 325 °C was significantly lower than at 450 °C, i.e., 1.2 and 2.5 μm, respectively. This indicated that parameters of the developed structure were thermally activated and controlled by precipitates. The features of microstructure formation and the nature of grain refinement of a complex-alloyed aluminum alloy under high-temperature severe plastic deformation conditions are discussed.