FRACTURE CHEMISTRY OF 6-AL2O3 (SAFFIL) FIBERS IN AN MGLI MATRIX ENVIRONMENT

Fracture surfaces of delta-Al2O3 (Saffil) fibres embedded in an Mg-8 w t % Li matrix by the pressure infiltration process were investigated b y in situ Auger electron spectroscopy to study the chemistry of embrit tlement of the former, resulting from a cross-section attack by the mo lten Mg-8 wt % Li matrix. The unaffected fibres failed transgranularly without any indications of silica being the crack-controlling flaws. Displacement reductive reactions between fibre constituents (delta-Al2 O3, silica) and penetrating lithium and magnesium species produce the phases that disturb the structural coherency of fibres and provide the paths for the crack propagation. In the early fibre/matrix reaction s tage (slightly affected fibres) there are elemental silicon and alumin ium and, most likely, also Li2O that cause the intergranular fracture of fibres, whereas in the advanced reaction stage (strongly affected f ibres), MgO is predominantly formed and the fracture propagates throug hout the fibres via the MgO-enriched regions.