Precipitation in Mg-Sm binary alloy during isothermal ageing: atomic-scale insights from scanning transmission electron microscopy

Abstract

The paper reports on an atomic-scale investigation into the precipitation hardening process in Mg-2.5Sm(wt%) alloy, using advanced Cs-corrected high angle annular dark field – scanning transmission electron microscopy (HAADF-STEM). Various novel precipitate structures existing in different ageing conditions are identified and discussed in depth. In the under-aged sample, three nucleation mechanisms are observed: βL, βH and β′, which are uniformly-scattered atomic-size rare earth clusters, displaying a significant strengthening effect in a very short ageing time of 30min. In the peak-aged sample, the main strengthening structures are needle-like precipitates composed of zigzag line repetitive units and hexagonal repetitive units. In the over-aged sample, the needle-like precipitates grow coarse and a subset of the precipitates transform into β1 precipitates, both of which are detrimental to the mechanical properties. From an engineering perspective, it is of great interest to design advanced Mg-RE materials with the uniformly-scattered atomic-size rare earth clusters, because they are effective strengthening structures requiring short ageing time. In the second place, the paper reveals three types of transformation processes occurring during the precipitation: the transformation between βL precipitates and needle-like precipitates, the transformation within needle-like precipitates along [0001]Mg and the transformation between needle-like precipitates and β1 precipitates. The RE atoms remain coherent with the α-Mg matrix before they transform into β1 precipitates.