Microstructural evolution of rapidly solidified hypoeutectic Al-10Cu alloy during non-isothermal annealing transients induced by nano-second laser pulses

The evolution of characteristic nonequilibrium features presenting in morphologically distinct regions of rapid solidification (RS) microstructures in a hypoeutectic Al-10Cu (atomic %) in response to non-isothermal annealing transients has been studied by transmission electron microscopy (TEM). The capabilities of the Movie-Mode Dynamic TEM (MM-DTEM) instrument were used to expose select regions of the RS microstructure to sequences of rapid heating and cooling transients induced by nanosecond laser pulses while permitting in-situ observation. Partial melting, microstructural scale coarsening, morphological changes of the nonequilibrium features in the multi-phase RS microstructure, and solid-state phase transformation were observed. Heterogeneous nucleation of nanoscale 0-Al2Cu phase involved metastable supersaturated alpha-Al and the 0'-Al2Cu phases, establishing different sets of orientation relationships for the stable 0-Al2Cu and alpha-Al phases. Replacement of banded morphology grains that formed under conditions driven farthest from equilibrium by an equiaxed nanocrystalline structure comprised of alpha-Al phase, the primary solidification product, and an intergranular network of Al2Cu crystals has been attributed to local remelting. The experimental approach explored here permitted discovery of mechanistic details of location-specific transformation pathways activated in the multiphase RS microstructure of hypoeutectic Al-Cu during subsequent nonisothermal transients.