Effect of Hot Extrusion, Other Constituents, and Temperature on the Strength and Fracture of Polycrystalline MgO

Abstract

Improved agreement was confirmed between the Petch intercept and single‐crystal yield stresses at 22°C. Hot‐extruded MgO crystal specimens (recrystallized with no obvious grain boundary phases or residual porosity), stressed parallel with the resultant (100) axial texture (1) gave the highest and least‐scattered strength–grain size results at 22°C, (2) showed direct fractographic evidence of microplastic initiated fracture at 22°C and showed macroscopic yield at 1315° and especially 1540°C, and (3) fractured entirely via transgranular cleavage, except for intergranular failure initiation from one or a few grain boundary surfaces exposed on the subsequent fracture surface, mainly at 1540°C. Hot‐extruded, hot‐pressed MgO billets gave comparable strength when fracture initiated transgranularly, but lower strength when fracture initiated from one or especially a few grain boundary surfaces exposed on the fracture (with residual pores). The extent and frequency of such boundary fracture increased with test temperature. While oxide additions of ≤5% or impurities in hot‐pressed or hot‐extruded MgO can make limited strength increases at larger grain sizes, those having limited solubility can limit strength at finer grain sizes, as can coarser surface finish (the latter especially at 22°C). Overall, MgO strength is seen as a balance between flaw and microplastic controlled failure, with several parameters shifting the balance.