Influence of microstructure refinement on co-plasticity in an Mg-4.7Al-2.9Ca alloy

Mg-Al-Ca alloys, composed of a soft Mg matrix and hard and brittle Laves phases, are suitable for applications at elevated temperatures. Unfortunately, the reinforcing skeleton is brittle and leads to premature failure due to cracking and decohesion from the matrix. In the present study, we achieve a largely self-similar reduction in microstructural length scale by increasing the cooling rate of an as-cast Mg-4.7Al-2.9Ca alloy. We observe this refinement mostly in terms of intermetallic strut spacing and width reduction, while the present intermetallic phase is determined to be C36. Upon refinement, the microstructures exhibit increasing compressive ultimate stress and yield stress from 96 ± 4 MPa and 223 ± 4 MPa at the coarsest microstructure to 117 ± 2 MPa and 244 ± 2 MPa at the finest microstructure. In addition, microstructure refinement leads to a change in governing damage mechanisms, which we quantified by means of both automated and manual analysis of large area scanning electron micrographs. Brittle cracking of the intermetallic skeleton dominates by a factor of 10 over the occurrence of decohesion at the Mg/intermetallic interface and both are reduced with decreasing microstructural length scale. This is accompanied by an increase of plastic slip transmission in the Laves phase from 50 to 250 slip events per 0.2 mm 2 , which goes alongside a reduction of the brittle phase from 290 to 33 per 0.2 mm 2 . We therefore propose that refinement of the microstructure may be used as an additional design parameter to manipulate the deformability of skeleton-reinforced Mg-Al-Ca alloys. • Microstructure of Mg-Al-Ca can be successfully refined by faster cooling. • Refinement leads to less cracking in the brittle Laves phase. • Refinement leads to enhanced co-plasticity by slip transmission into the Laves phase.