Influence of strain rate on grain refinement in the Al-Mg-Sc-Zr alloy during high-temperature multidirectional isothermal forging

Abstract

The strain rate effect on ultrafine-grain structure development in the cast Al-Mg-Sc-Zr based alloy during multidirectional isothermal forging was studied. The alloy with the average grain size of 25 μm and coherent Al3(Sc,Zr) precipitates of 25–50 nm in diameter was deformed to a total strain of about 8.4 at 325 °C with strain rates of 10−4 and 10−2 s−1. In both cases development of new crystallites started at the initial grain boundaries via the formation and interaction of deformation/microshear bands. The volume fraction of these crystallites and their boundary misorientation rose with strain increasing; that resulted in formation of fine-grained microstructure through continuous dynamic recrystallization. The kinetics of grain refinement was practically independent of strain rate, leading to similar microstructures, characterized by nearly same misorientation parameters. This indicated that the fine-grain structure formation had an athermal origin. On the other hand, the size of (sub)grains after forging at 10−2 s−1 was smaller than that at 10−4 s−1, suggesting the importance of thermally activated processes in grain refinement being controlled by the rate of dynamic recovery. It was concluded that grain refinement of the studied alloy via continuous dynamic recrystallization was conditioned by both a series of athermal processes and a thermal activated process owing to dynamic recovery.