Studies of thermally activated processes in gas-atomized Al alloy powders: in situ STEM heating experiments on FIB-cut cross sections

Abstract

Gas atomization is the most common approach used to produce powders of metallic alloys, and the high cooling rates involved frequently lead to the formation of non-equilibrium microstructures and phases. The transformations that occur in the powders upon heating are of great interest but are challenging to study experimentally. Here we use a novel focused ion beam-based specimen preparation protocol to obtain cross sections through individual gas-atomized powder particles of three different aluminum alloys: solid solution-strengthened Al5056, precipitation-hardenable Al6061, and an Al–Cr–Mn–Co–Zr alloy which contains icosahedral quasicrystal dispersoids. In situ scanning transmission electron microscopy heating experiments were performed on these cross-sectional specimens to investigate the changes that occur in the metastable phases and non-equilibrium microstructures upon heating. The experiments reveal the details of a wide variety of thermally activated processes occurring in the particles including: solute redistribution to eliminate micro-segregation; dissolution, coarsening, transformation and decomposition of secondary phases; and precipitation within the aluminum matrix.