Hot Deformation Behavior and Processing Map Development for Biodegradable Mg–1Zn–0.2Ca Alloy

The hot deformation behavior and workability of an extruded biodegradable Mg–1Zn–0.2Ca alloy (initial grain size 25.7 µm) were investigated by isothermal compression at temperatures of 200–400 °C and strain rates of 0.001–1 s-1. The flow stress decreased with increasing temperature and increased with increasing strain rate, and pronounced late-stage stress losses at low temperatures were associated, at least partly, with specimen cracking. Processing maps were developed at true strains of 0.3, 0.5, and 0.7 using the dynamic materials model, with the unstable domain defined as the union of the regions predicted by two flow-instability criteria for a conservative assessment. At a true strain of 0.7, the power dissipation efficiency reached a maximum of about 0.46 at approximately 360–370 °C and 0.005–0.01 s-1, and a broader stable domain was identified at approximately 340–380 °C and 0.001–0.1 s-1. The maps evolved non-monotonically with strain. Cracking occurred at all strain rates at 200 °C, at 0.1–1 s-1 at 240 °C, and at 1 s-1 at 280 °C, but not at 320–400 °C. Optical microscopy showed that the crack at 200 °C followed an inclined path of localized deformation, that a fine, uniform microstructure formed at the efficiency maximum, and that localized deformation bands developed without cracking at 360 °C and 1 s-1, where instability was predicted. The results establish a hot-working window for the extruded alloy, based on single tests per condition, and provide guidance for optimizing its thermomechanical processing.