Mechanical properties and microscopic deformation mechanism of polycrystalline magnesium under high-strain-rate compressive loadings

Polycrystalline magnesium was compressed under different strain rates (0.001, 800, 1000, 2000, and 3600 s−1) to investigate its dynamic mechanical properties, and microstructural characterization was performed to uncover the deformation mechanism. The results show that yield strength is insensitive to strain rate, while ultimate strength, fracture strain, and work hardening rate are highly sensitive to strain rate. Three deformation regimes (I, II, and III) were observed on the quasi-static and dynamic stress–strain curves. These regimes show respectively increasing work hardening rate in the early stage of plastic deformation, constant work hardening rate in the intermediate plastic deformation region, and decreasing work hardening rate in the end region right before fracture. Different deformation mechanisms operate for the quasi-static and dynamic loading conditions. Microscopically, twinning/detwinning is the dominating mechanism for quasi-static testing, while dynamic recrystallization and twinning/detwinning are the dominating mechanisms for dynamic testing. Analytic constitutive models were derived for predicting the dynamic stress–strain relations. The analysis indicated that different factors were in effect for different loading strain rates. The stress–strain relations were primarily affected by strain hardening for quasi-static testing; by strain hardening, strain rate hardening, and thermal softening for dynamic testing with ε ˙ ≤ 2000  s − 1 ; and by strain hardening, damping, and thermal softening for dynamic testing with ε ˙ > 2000   s − 1 , respectively.