Mechanical properties of as-cast and wrought Mg-5Ni-xAl magnesium alloys

Microstructural development and mechanical properties of the newly developed Mg-5Ni-xAl (x = 2.5, 5, and 7.5 wt%) alloys were investigated. The microstructures of all alloys were composed of alpha-Mg, Mg17Al12, and Al3Ni2 phases. The increase in Al content (from 2.5% to 7.5%) extremely modified the as-cast grain size from 507 mu m to 62 mu m with the consequent enhancement of mechanical properties. While elevated temperature homogenization treatment led to the dissolution of the brittle Mg17Al12 eutectics, the Al3Ni2 intermetallics showed high thermal stability. Moreover, hot extrusion led to a remarkable structural refinement via dynamic recrystallization (DRX), as well as fragmentation and dispersion of particles during thermomechanical processing. Grain refinement via Al addition and hot extrusion (investigated by the Hall-Petch law), as well as the dissolution of unfavorable intergranular Mg17Al12 intermetallics and solid solution hardening (via increasing the Al content of the alpha-Mg matrix) during homogenization were found to be the main contributing factors to the enhancement of tensile properties. As a result, the best combination of the ultimate tensile strength (354 MPa) and total elongation (23%) was obtained for the hot extruded alloy containing 7.5 wt% Al with the average grain size of 7.9 mu m.