Thermal Stability of Cryomilled Al-Mg-Er Powders

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 (∼0.8 ) 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.

Thermal Stability of Aluminum Alloys

“…Thermal analysis methods are sometimes combined to gather complimentary data, e.g., DSC with TGA for studying the oxidation of aluminum [11] or Al−Mg−Er alloy thermal stability [47]. Most often, however, they are used with other experimental techniques, such as metallography, X-ray, or mechanical testing.…”

“…Although very useful, thermal analysis techniques have limitations, as shown in Figure 11 for the cryomilled Al−Mg−Er powders [47]. A combination of DSC and TGA measurements of the nanocrystalline system during heating successfully detected phenomena of melting at~635 • C (liquidus) and recovery at~160 • C. However, the solidus temperature, expected at~575 • C and recrystallization, expected at~330 • C, as verified by TEM/X ray, were not detected by the DSC/TGA techniques.…”

“…Although very useful, thermal analysis techniques have limitations, as shown in Figure 11 for the cryomilled Al−Mg−Er powders [47]. A combination of DSC and TGA measurements of the nanocrystalline system during heating successfully detected phenomena of melting at ~635 °C (liquidus) and recovery at ~160 °C.…”

“…Additions of Er to Al−Mg alloy powder improved its thermal stability through the combined effects of the solute/impurity drag and second-phase pinning, involving nanosize oxides, nitrides, and oxynitrides that impeded the grain boundary motion [47]. Small additions of the order of 0.1 wt.% Er were not effective and Al−Mg powders showed the abnormal grain growth at 180 °C.…”

“…Additions of Er to Al−Mg alloy powder improved its thermal stability through the combined effects of the solute/impurity drag and second-phase pinning, involving nanosize oxides, nitrides, and oxynitrides that impeded the grain boundary motion [47]. Small additions of the order of 0.1 wt.% Er were not effective and Al−Mg powders showed the abnormal grain growth at 180 • C. In contrast, the 0.5 wt.% Er addition improved thermal stability, maintaining the grain size of approximately 20 nm up to 400 • C. The controlled grain growth at higher temperatures resulted in an average grain size of 55 nm and a maximum observed grain size of~200 nm after one hour of annealing at 500 • C.…”