Compressive strain-rate sensitivity of magnesium-aluminum die casting alloys

Compressive properties of three die cast magnesium alloys (AM20, AM50, and AM60) have been investigated at a nominal strain rate range from 0.001 s-1 to approximately 1700 s-1. The high strain rate experiments were conducted using a Split Hopkinson Bar apparatus. The strain rate sensitivity decreased with increasing Al content at the low strain rates. At strain rates above 1500 s-1 there was substantially higher work hardening and strain rate sensitivity compared with that observed for the quasi-static tests at rates from 0.001 to 1 s-1. Furthermore, the ultimate compressive stress increased substantially at high strain rates. The failure strain was greatest at 1 s-1, with lower failure strains at 0.001 s-1 and above 1000 s-1. Fracture surface analysis indicated that at low strain rates, the alloys cleaved along shear planes. However, at the high strain rates there was some evidence of dimpling on the fracture surface, characteristic of a more ductile fracture, probably caused by increased temperature through deformation heating. Based on the experimental results, empirical strain-rate-dependent constitutive equations are recommended, which provide an improved estimate of the strain-rate-sensitive behavior of these materials