Abstract
In this study, the thermal stability of nanostructured Al-Mg alloy powders was investigated. Two alloy compositions, Al-5Mg-0.1Er and Al-5Mg-0.5Er (wt.%), were cryogenically milled for 30 h to produce nanostructured powders. The microstructure of the milled powders with increasing temperature was investigated by differential scanning calorimetry (DSC) with one-hour annealing performed at selected temperatures followed by X-ray diffraction (XRD) and electron microscopy analysis. Prolonged milling led to significant oxygen pick-up in the powders. The Al-5Mg-0.1Er powders experienced grain growth typical of cryomilled Al-Mg powders, while the Al-5Mg-0.5Er alloy showed improved thermal stability. An average grain size of ~20 nm was observed up to 400°C (~0.8Tm) in the Al-5Mg-0.5Er powders, and abnormal growth at 550°C resulted in a maximum observed grain size of 234 nm. Thermal stability in the Al-Mg-Er powders is attributed to the combined effects of solute/impurity drag and second-phase pinning (nanoscale oxides, nitrides, and oxynitrides) that impede grain boundary motion.
Highlights
Solutes/impurities and nanoscale inclusions, such as nitrides and oxides introduced during the milling process, interfere with boundary mobility and reduce grain growth in milled Al alloy powders [1,2,3,4]
This trend has been found in literature where the levels of Fe and Si impurity in a 5052 Al alloy were reduced by Er addition due to Er combining with Si and Fe in the melt [14]
Grain size distributions from annealing at selected temperatures are shown in Figure 8; the largest grain size observed at each temperature is included in the figure
Summary
Solutes/impurities and nanoscale inclusions, such as nitrides and oxides introduced during the milling process, interfere with boundary mobility and reduce grain growth in milled Al alloy powders [1,2,3,4]. In order to improve grain size stability in milled Al alloy powders, research into the addition of alloying elements such as Sc has been conducted [5]. Sc exhibits slow diffusion kinetics in Al and forms nanoscale, coherent L12 trialuminide Al3Sc precipitates that are able to pin grain boundaries and substructures [6]. Similar to Sc effects, Er addition retards the recrystallization of deformed structures in Al-Mg alloys. Limited studies at the nanocrystalline regime have been done to date. The thermal stability of nanocrystalline Al-Mg-Er system (
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