Coarsening behavior of Al3Sc interface precipitates and related impact on mechanical properties of AlFeNi-Sc alloy at 400 °C

In the eutectic AlFeNi-0.2Sc alloy, microstructural features for two types of Al3Sc precipitates (i.e., matrix precipitates (Al3Sc(m)) and interface precipitates (Al3Sc(i)) were identified, and their effects on mechanical properties were analyzed and discussed. At 300 °C peak-aged state, both Al3Sc(m) and Al3Sc(i) precipitates exhibited isotropic strengthening behavior on Al/Al9FeNi eutectic structure. Through 400 °C over-aged treatment, Al3Sc(m) precipitates transformed to Al3Sc(i), making the excessive coarsening of Al3Sc(i) precipitates. Coupling with the morphological feature of Al9FeNi phase, the anisotropic strengthening behavior was induced on eutectic structure, as confirmed by hardness test. Based on these findings, the strengthening effects from both precipitates were modelled quantitatively, showing the transition from Orowan strengthening mechanism by Al3Sc(m) to load transfer effect by Al3Sc(i). As a result, AlFeNi-0.2Sc alloy exhibited superior 400 °C creep properties among available reports for Al alloys. Analyzing the creep constitution equation, the threshold stress from Al3Sc(m) was accounted for minor strengthening effect, and the load transfer effect from Al3Sc(i) worked as major strengthening effect to 400 °C creep of alloy. This study revealed that Al3Sc precipitates can effectively strengthen the eutectic structure in different forms. The excessive coarsening of Al3Sc(i) precipitates at higher temperatures was benefited to load transfer effect, which can be considered as an ideal strategy to improve the high temperature performance of Al alloys with multiphase structures.