Strengthening mechanisms in Al–Ni–Sc alloys containing Al₃Ni microfibers and Al₃Sc nanoprecipitates

Dilute Al–Sc 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 Al–Ni alloys (Al-6 wt.% Ni) derive their high strength at elevated temperature from Al₃Ni microfibers formed during solidification. Here, we investigate ternary Al–6Ni-0.2Sc and Al–6Ni-0.4Sc alloys with both types of strengthening precipitates (Al₃Sc and Al₃Ni) and compare them to binary Al–6Ni, 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 Al–Ni–Sc 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).