Plastic Deformation of Calcium Tungstate Single Crystals

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Abstract
Calcium tungstate deforms by slip on the (001) 〈100〉 system above 400°C. Simple shear over this plane is the only deformation mode observed under compression at temperatures up to 900°C and a strain rate of 10−3sec−1. The critical resolved shear stress is strongly temperature dependent. Crystals cannot be plastically deformed at room temperature, at 400°C they are still quite brittle but by 900°C they are soft (yield stress, 500 psi) and plastic. Slip traces at all temperatures are fine and straight but those produced at lower temperatures (600°–700°C) have a greater surface tilt and are more sharply defined than those produced above 800°C. This is due to fewer dislocation sources and restrictions on dislocation multiplication at lower temperatures. The role of bonding and crystal structure in the selection of slip parameters is discussed. It is considered that the stacking of the oxygen ions is the dominant factor dictating the slip plane and slip direction. Slip cannot occur without some distortion of the covalently bonded WO42− radical, which probably accounts for the strong temperature dependence of the critical resolved shear stress. It is shown that relatively easy slip in a given [100] or [010] direction can only occur between every other pair of (001) planes. Thus, a jogged dislocation which is mobile on one plane will become immobile on the next (or any odd number) plane. This is considered to restrict dislocation multiplication at low temperatures.