Determination of homogenizing conditions and aging behaviors of cast 62Be-37.6Al-0.4Sc alloy

In this work, the effects of the homogenizing and aging conditions on the microstructural evolution of Al3Sc precipitates and the mechanical properties of cast 62Be-37.6Al-0.4Sc alloy were investigated. Stereoscopic observations by selective phase leaching indicated that the dendritic Be matrix remained continuous with weakened secondary dendrite arms compared to those of other cast Be-Al alloys. The activation energy of diffusion Q was calculated to be 160.3 kJ/mol for Sc in the cast alloy though resistivity measurements, which indicated that the diffusion of Sc in the Be matrix could be neglected below 640 °C. The homogenization kinetics curves of the cast alloy were acquired, and the homogenizing conditions were optimized to be 620 °C for 2 h followed by water-quenching. The temporal evolution of the hardness and thermal analysis indicated that Al3Sc started precipitating at approximately 250 °C and underwent a coarsening stage when heated at temperatures greater than 300 °C for times longer than 1 h during isochronal aging. The Vickers hardness during isothermal aging revealed four distinct stages in the aging response of the cast alloys, and the aging treatment at 400 °C for 2 h followed by water-quenching was sufficient for considerable precipitation of Al3Sc in the as-homogenized alloys. The precipitating behaviors of Al3Sc during the aging processes were qualitatively obtained. The coarsening process of the Al3Sc precipitates due to the overall increase in the lattice mismatch between Al3Sc and Al was illustrated. Notably, it was observed for the first time that secondary Al3Sc phases with morphologies controlled by the elastic free energy grew preferentially along certain directions into elongated particles from the larger spherical primary Al3Sc particles in the Al matrix. Thermal analysis also revealed that the heating processes influenced the enthalpy needed by the Al matrix during melting and cooling processes owing to the effects of Al3Sc particle density and supersaturated solid solution of Sc.