We performed a series of hydrostatic and constant-stress-difference (CSD) experiments at room temperature on modified lead-zirconate-titanate (PZT 95/5-2Nb) ceramic in order to quantify the influence of shear stress on the displacive, and possibly martensitic, first-order, ferroelectric/rhombohedral→antiferroelectric/orthorhombic phase transformation. In hydrostatic compression, the transformation began at approximately 260 MPa and was incompletely reversed upon return to ambient conditions. Strains associated with the transformation were isotropic, both on the first and subsequent hydrostatic cycles. Results for the CSD tests were quite different. First, the confining pressure and mean stress at which the transition begins decreased approximately linearly with increasing stress difference. Second, we observed that the rate of transformation apparently decreased with increasing shear stress and the accompanying purely elastic shear strain. This result contrasts with the almost universal assertion that shear stresses accelerate reaction and transformation kinetics. Finally, strain was not isotropic during the transformation: axial strains were greater and lateral strains smaller than for the hydrostatic case, though volumetric strain behavior was comparable for the two types of tests.

However, this last effect does not appear to be an example of transformational plasticity but, rather, a ''one-time'' occurrence: no additional unexpected strains accumulated during subsequent cycles through the transition under nonhydrostatic loading. If subsequent hydrostatic cycles were performed on samples previously run under CSD conditions, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably results from a crystallographic preferred orientation that developed during the transformation under shear stress. ¿ American Geophysical Union 1993