The effects of microstructural stability on the compressive response of two cast aluminum alloys up to 300 °C

In this study, the high temperature compressive response of cast aluminum alloys 319 and RR350 are compared in light of their microstructures. The 319 alloy is widely used in thermally critical automotive applications and provides a baseline for comparison with the RR350 alloy, whose microstructural stability at high homologous temperatures was recently reported. Cylindrical compression samples from each alloy were tested at four temperatures up to 300 °C at a constant true strain rate varied over four orders of magnitude. Although both alloys are strengthened by metastable θ′ precipitates (nominally Al2Cu) in the as-aged condition, their mechanical response diverges at temperatures greater than 250 °C as the strengthening precipitates coarsen and evolve in the 319 alloy while they retain their as-aged morphology in the RR350 alloy. Deformation mechanisms of each alloy are examined using microstructural analysis and empirical activation energy calculations. The stability of the θ′ phase in the RR350 alloy leads to effective precipitation hardening at homologous temperatures up to 0.6 and an extensive regime of grain boundary controlled deformation.