Strengthening mechanisms in Al[sbnd]Ni[sbnd]Sc alloys containing Al3Ni microfibers and Al3Sc nanoprecipitates

Abstract

Dilute AlSc alloys (<0.4 wt% Sc) are creep-resistant up to ∼300 °C owing to coherent, nanosize Al3Sc particles created by solid-state precipitation. By contrast, eutectic AlNi alloys (Al-6 wt.% Ni) derive their high strength at elevated temperature from Al3Ni microfibers formed during solidification. Here, we investigate ternary Al6Ni-0.2Sc and Al6Ni-0.4Sc alloys with both types of strengthening precipitates (Al3Sc and Al3Ni) and compare them to binary Al6Ni, Al-0.2Sc and Al-0.4Sc alloys with a single population of precipitates. Kinetics of Al3Sc and Al3Ni precipitation and coarsening are studied via hardness measurements during isochronal and isothermal aging; the two phases resist coarsening up to 475 and 350 °C, respectively, upon short term exposures (1 h isochronal steps). No noticeable effect of Sc is observed on the Al3Ni micro-fiber composition and hardening in the alloy. Similarly, Ni does not significantly affect the hardening provided by the Al3Sc precipitates, despite the presence of 0.14–0.17 at.% Ni in the Al3Sc nano-precipitates. The strengthening contributions of the Al3Ni and Al3Sc phases at ambient temperature are cumulative in the ternary alloys. For creep deformation at 300 °C, all alloys show a creep threshold stress, indicating that both types of precipitates impede dislocation motion. The ternary AlNiSc alloys show higher creep threshold stresses than their binary counterparts, also consistent with cumulative strengthening effects of precipitation strengthening (from Al3Sc nano-precipitates) and load transfer (from Al3Ni micro-fibers).