Effect of Sintering Temperature on Microstructure and Mechanical Properties of AZ91 Magnesium Alloy via Spark Plasma Sintering

Abstract

Gas‐atomized AZ91 powder with superhigh compressive strength is sintered by spark plasma sintering (SPS) at the temperatures of 350, 400, and 450 ºC. The grain size, Mg17Al12 precipitate, and geometrically necessary dislocations (GNDs) density are investigated by transmission electron microscopy, X‐ray diffractometer, scanning electron microscopy, and electron backscattered diffraction. Microstructure characterization reveals that with the increase in sintering temperature, the grain size increases, while the Mg17Al12 precipitate content and GND density decrease. The eutectic intergranular Mg17Al12 precipitate partially transforms to lamella intragranular Mg17Al12 phase during SPS process. The compressive yield strength (CYS) decreases with the increases in sintering temperature, whereas the ultimate compressive strength (UCS) and strain show a contrary tendency. The theoretical strengthening mechanism of sintered AZ91 alloy is calculated, which indicates that the grain refinement, dislocation, and Orowan strengthening contribute 59.3–66.3, 28.8–34.5, and 4.9–6.2% to the CYS, respectively. It is found that the dislocation strengthening plays an important role in SPS AZ91 alloy. Compared with pure Mg, the sintered AZ91 alloy shows a higher σ 0 and a lower k. The enhanced UCS and fracture strain at higher sintering temperature are attributed to the lower content of MgO and Mg17Al12 phases and the coarser grain size.