EFFECT OF HOT EXTRUSION, OTHER CONSTITUENTS, AND TEMPERATURE ON THE STRENGTH AND FRACTURE OF POLYCRYSTALLINE MGO

Improved agreement was confirmed between the Petch intercept and singl e-crystal yield stresses at 22-degrees-C. Hot-extruded MgO crystal spe cimens (recrystallized with no obvious grain boundary phases or residu al porosity), stressed parallel with the resultant (100) axial texture (1) gave the highest and least-scattered strength-grain size results at 22-degrees-C, (2) showed direct fractographic evidence of microplas tic initiated fracture at 22-degrees-C and showed macroscopic yield at 1315-degrees and especially 1540-degrees-C, and (3) fractured entirel y via transgranular cleavage, except for intergranular failure initiat ion from one or a few grain boundary surfaces exposed on the subsequen t fracture surface, mainly at 1540-degrees-C. Hot-extruded, hot-presse d MgO billets gave comparable strength when fracture initiated transgr anularly, but lower strength when fracture initiated from one or espec ially a few grain boundary surfaces exposed on the fracture (with resi dual pores). The extent and frequency of such boundary fracture increa sed with test temperature. While oxide additions of less-than-or-equal -to 5% or impurities in hot-pressed or hot-extruded MgO can make limit ed strength increases at larger grain sizes, those having limited solu bility can limit strength at finer grain sizes, as can coarser surface finish (the latter especially at 22-degrees-C). Overall, MgO strength is seen as a balance between flaw and microplastic controlled failure , with several parameters shifting the balance.