Microstructure Evolution of Twin-Roll Cast and Hot-Rolled WZ73 Alloy during the Finishing Heat-Treatment

The microstructural evolution of the WZ73 magnesium alloy (Mg-7.4Y-3.8Zn-0.4Zr) was systematically investigated during finishing heat-treatments following twin-roll casting and hot rolling at equivalent strain rates of 17 s⁻¹ and 50 s⁻¹. Hot rolling at 500 °C was performed to achieve a logarithmic strain of 0.7 (thickness reduction from 5.3 mm to 3 mm). Higher strain rates during hot rolling enhanced dynamic recrystallization (DRX), resulting in refined microstructures, whereas lower strain rates promoted the formation of lamellar long period stacking ordered (LPSO) phases. Subsequent heat-treatments at 200 °C to 550 °C for up to 24 hours revealed temperature-dependent microstructural transformations. At 500 °C, complete recrystallization occurred with minimal grain growth, while 550 °C caused grain coarsening, partial grain boundary melting, and morphological changes of the LPSO phase from lamellar to spherical and rod-like structures. Notably, at temperatures above 500 °C, prior hot rolling had limited influence on microstructure. The microstructure and phase evolution were characterized using optical and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).