Bimodal grain structure effect on the static and dynamic mechanical properties of transparent polycrystalline magnesium aluminate (spinel)

Transparent polycrystalline magnesium aluminate (spinel) with bimodal and unimodal grain structures were prepared. The influence of grain size distribution on static and dynamic mechanical properties were systematically investigated. The results showed that bimodal grain structure spinel has larger flexural strength (236.31 MPa) compared to unimodal grain structure spinel (221.38 MPa). Whereas, their values of hardness are very similar (15.1 vs 14.7 GPa) and fracture toughness remains unchanged (1.1 MPa∙m^1/2 for both spinel). Although static compression strength of bimodal grain structure spinel (1236 MPa) is higher than that of unimodal one (1078 MPa) due to a smaller average grain size in the former, the negative effect of bimodal grain structure reduced the spinel strength compared to theoretically predicted value. Bimodal grain structure spinel shows slightly lower increment (49%) in compression strength from static to dynamic loading compared to that of unimodal one (57%) due to a decreased strain-rate sensitivity ascribed to bimodal grain structure. A brittle mode in inelastic deformation at Hugoniot elastic limit was demonstrated in both bimodal and unimodal grain structures. Bimodal grain structure has an influence on the Hall-Petch-like relation of yield strength under planar impact loading.