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.