In-situ TEM study on the precipitation behavior of an Al-Mn-Mg-Sc-Zr alloy formed by SLM

Abstract

In this study, an Al-Mn-Mg-Sc-Zr alloy was prepared using the selective laser melting (SLM) technique, and the precipitation behavior during heating was characterized by In-situ transmission electronic microscopy (TEM). The results show that the microstructure of the deposited alloy is mainly composed of columnar crystals in the molten pool and equiaxed crystals at the bottom edge of the molten pool. Al₆Mn was identified as the major phase within the deposited alloy, in which Al₆Mn showed rod-shaped and particulate behavior distributed along the grain boundary of columnar crystal and equiaxed crystal. A detailed characterization of the precipitation behavior was conducted using in-situ TEM at various temperatures. At 250 °C, it was observed that the dislocation density decreased with heating time, showing no precipitation behavior, indicating good thermal stability of the alloy at this temperature. After the temperature was raised from 250 °C to 425 °C, a large number of fine and dispersed needle-rod secondary Al₆Mn phases were precipitated in the inner and bottom of the molten pool. With the increase in heating time, the size of the needle-rod secondary Al₆Mn phase increased, and no Al₃Sc precipitates were observed. However, the precipitation of Al₃Sc was found by ex-situ TEM. Thermodynamic properties of Al₃Sc and Al₆Mn were calculated by first principles. It is found that the free energy of Al₆Mn is lower than that of Al₃Sc in the temperature range of 85 to 800 K, which indicates that Al₃Sc has better thermodynamic stability than Al₆Mn.