Revealing the role of loading orientation on the dynamic mechanical behavior of ZK60 magnesium alloy

Abstract

Magnesium alloy is a high-quality lightweight material but the poor plasticity at ambient temperature restricts its application potential. The combined effect of dynamic condition and loading orientation is believed to extend the ductility of magnesium alloy, while the mechanism underlying orientation-dependent dynamic deformation is still poorly understood. The dynamic mechanical behaviors as well as microstructure evolutions were evaluated on ZK60 Mg alloy via dynamic tension in both extrusion direction (ED) and transverse direction (TD) with strain rates of 2000 s⁻¹, 3200 s⁻¹ and 4300 s⁻¹ using the split Hopkinson tensile bar (SHTB), and quasi-static tension with 0.001 s⁻¹ strain rate. Subsequent electron back-scattered diffraction (EBSD) observation and scanning electron micrograph (SEM) of fracture surfaces, employing an interrupt strain test method, was conducted to elucidate the loading orientation correlations for dynamic deformation. The dynamic deformation mechanisms in the initial stage, are primarily governed by non-basal slip in ED and extension twinning in TD, as indicated by Schmid factor (SF) calculations and confirmed by misorientation angle distributions. This renders positive strain rate sensitivity (SRS) for the former, whereas rate-insensitive behavior for the latter. The non-basal slip allows rapid dislocation multiplication in the ED, while the phenomenon is delayed in the TD until about 0.05 strain, at which the SRS is highlighted. More intense interaction between first-formed twins and later-activated dislocations contributes to higher strain hardening and elongation due to the dynamic loading in TD. In the later stages of dynamic strain, the adiabatic temperature of ∼80 °C in TD is also regarded as the source of the decreasing strain hardening and increasing elongation, evidenced by the fracture mode transitions from quasi-brittle to ductile compared to quasi-static tension.