Micro-hardness and strain-rate-dependent compressive response of an ultra-light-weight Mg-Li-Al alloy

Abstract

A study on the microstructure and composition, micro-hardness and strain-rate-dependent compressive behaviors, and the associated failure mechanisms of an ultra-light-weight Mg-Li-Al alloy were conducted. X-ray diffraction and X-ray photoelectron spectroscopy showed a multi-phase material with ~35 wt% Li and ~20 wt% Al, and a dendritic “fishbone” microstructure resulted from the high percentage of both Li and Al. Micro-indentation measurements showed a superior hardness (1.63 ± 0.08 GPa) that is> 1.5x higher than other Mg-Li-Al alloys reported in the literature, with a low density (~1.68 g/cm3) comparable to Mg alloys. Strain-rate-dependent uniaxial compression experiments demonstrated no strain-rate-sensitivity in the peak strength (699.4 ± 74.0 MPa) at strain rates between 10−5 and 103 s−1. High-speed imaging revealed a shear-mode brittle fracture under both quasi-static and dynamic conditions, with an additional splitting crack mechanism observed under dynamic loading. Crack propagation speeds demonstrated a positive correlation with strain rate from ~480 m/s at ~100 s−1 to ~1000 m/s at ~2000 s−1. Post-mortem analysis showed that the “fishbone” structure with a peeling fracture mechanism appears to be the dominant site promoting shear failure across all strain rates.