Plastic Deformation of Ceramic‐Oxide Single Crystals, II

Sapphire single crystals exhibit the same qualitative creep properties over the temperature range 900° to 1400°C. as do comparable metal single crystals at room temperature. The creep of sapphire under constant load has four portions: ( a ) a period of increasing creep rate, ( b ) a period of decreasing creep rate, ( c ) a period of constant creep rate, and ( d ) a final period of increasing creep rate. The stress required to initiate creep falls smoothly from about 780 kg. per sq. cm. at 900°C. to about 130 kg. per sq. cm. at 1400°C. After creep is initiated, it will continue at a lower stress (the addition of chromia increases the stress required to initiate creep). The electrical resistivity of sapphire is apparently increased by plastic deformation and decreased by subsequent heating near 1800°C. Slip lines in periclase and rutile were studied and slip systems were identified.