Deformation behaviour of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying

Bulk Mg - 5Al alloys were consolidated from powders that had been mechanically alloyed over different milling durations. The microstructural evolution, and physical and mechanical properties of the alloys were investigated. Mechanical measurements revealed a change in deformation behaviour after certain milling durations. At short milling duration, high yield strength was obtained through dislocation strengthening mechanisms predominantly by grain refinement and to a lesser extent by solid solution strengthening and particle dispersion strengthening. However, at longer milling durations, low yield strength was observed and the strengthening mechanisms at work in short milling durations appeared to be no longer effective. Enhanced ductility with no work hardening behaviour was observed in samples with a mean grain size of 45 nm. It appeared that the significantly large increase in the grain boundary regions played an important role in the room temperature deformation of the alloys. The possibility of a diminishing effect of the dislocation strengthening mechanisms and the onset of grain boundary deformation modes for the softening phenomenon and the absence of work hardening at some nanoscale grain sizes are discussed.