Flow Stress of MgO Single Crystals Compressed Along [100] at High Hydrostatic Pressures

The compressive yield strength of the soft orientation of MgO single crystals (the [100] loading axis) was measured under hydrostatic pressures up to 1000 MPa in two devices. An initial set of monotonic loading experiments indicated that the yield strength under pressure was decreased by 2.1 to 3.1×10−4MPa−1 in units of fractional decrease (dσ/σ) per increase in chamber pressure. Individual crystals were deformed in increments at various pressures in a series of interrupted tests; the results indicated that pressure had no effect on yield strength. (The interrupted tests would be more likely than the monotonic experiments to demonstrate intrinsic material behavior since they avoided variations in strength among crystals and had more uniform platen friction conditions.) All specimens were tested without protective jackets, leading to slip‐band observations that contradict a previous theory of the effect of jackets on the flow stress of MgO single crystals during high‐pressure tests. It is concluded that hydrostatic pressure up to 1000 MPa has a negligible effect on the yield strength of the soft orientation of MgO single crystals, in accordance with predictions made from earlier models by Davis and Gordon.