MICROSTRUCTURAL SUPERPLASTIC DEFORMATION IN MGO-CENTER-DOT-AL2O3 SPINEL

Polycrystals of hot-pressed and then hot-isostatically-pressed MgO . A l2O3 spinel, having large grains of about 10-20 mu m embedded in a fin e-grained matrix of 610 nm grain size, have been deformed in uniaxial compression. The results have been analysed by taking into account gra in growth. The variation in stress exponent n suggests a transition fr om interface-reaction (n = 2) at low stresses to diffusional mechanism (n = 1) at high stresses, but cavitational creep was responsible for the non-linear behaviour at stresses above 80 MPa. The grain size expo nent found at 60 MPa (p = 2.7), the absence of dislocations within fin e grains and the similarity of grain shapes before and after large flo w (55%), resemble the characteristics of ''micrograin superplastic flo w''. The values of the grain size exponent and of the activation energ y suggest that grain-boundary diffusion (p = 3) makes a significant co ntribution to the mixed deformation mechanism.