Constitutive analysis, twinning, recrystallization, and crack in fine-grained ZK61 Mg alloy during high strain rate compression over a wide range of temperatures

Abstract

This study aimed to explain the uniaxial compression of T5 heat-treated extruded ZK61 magnesium alloy at the temperatures of 298−673 K and the strain rates of 0.001−10 s−1. Flow stress data were analyzed to establish a constitutive equation by means of numerical simulation and activation energy was estimated to be 141 ± 1 kJ/mol. Microstructure analysis revealed a low fraction of primary and secondary twinning at temperature 623 K in the early stages of deformation and dense twinning at low temperatures ≤ 473 K. Pole figure analysis suggested virgin fiber texture where c−axis of HCP is tilted at angle ~25° with respect to extrusion direction. During early stages of deformation at a strain 0.1 and at high-temperature c-axis of the crystal reorients and changes to ~40° along transverse direction, while at strain 0.45 highly aligned typical extruded texture was developed, which proposed that {0002} basal planes are perfectly parallel to the extrusion direction. Moreover, the results indicated that at high temperature> 548 K the dominant discontinuous dynamic recrystallization was changed into continuous dynamic recrystallization with the increase in strain; this phenomenon is attributed to initial processing history; fine grain size and activation energy 141 ± 1 kJ/mol. The twin dynamic recrystallization was dominant at temperatures ≤548 K. Thus, hot deformation in the strain rate range of 0.001−1 s−1 at temperature 548 K or in the strain rate range of 0.001−10 s−1 and in the temperature range of 573−623 K is optimum for the ZK61 Mg alloy (T5). Fracture behavior revealed that principal and secondary cracks were nucleated and propagated owing to the low fraction of dynamic recrystallization, twin−twin, twin−dislocation interaction and voids at triple junctions of grain boundaries.