Twinning-Slip Transitions in Mg AZ31B

Abstract

Many metals, particularly ones with hexagonal close-packed (HCP) crystal structures, undergo deformation by combinations of twinning and slip, the proportion of which depends on variables such as the temperature and strain rate. The typical techniques to reveal such mechanisms rely on metallography, X-ray diffraction, or electron optics. Simpler, faster, less expensive mechanical tests were developed in the current work and applied to Mg AZ31B. The curvature of compressive stress-strain plots over a fixed strain range was found to be a consistent indicator of twinning magnitude, independent of the temperature and strain rate. The relationship between the curvature and areal fraction of twins was determined. The transition temperatures determined based on stress-strain curvature were consistent with the ones determined by metallographic analysis and flow stresses, and the results depended on the strain rate according to the Zener-Hollomon parameter, a critical value for which was measured. The transition temperature was found to depend significantly on the grain size, a relationship for which was established. Finally, it was shown that the transition temperature can be determined consistently, and much faster, using a single, novel "step-temperature" test.