Hydrostatic and triaxial compression experiments on unpoled PZT 95/5–2Nb ceramic: The effects of shear stress on the FR1 → AO polymorphic phase transformation

Abstract

We conducted a series of hydrostatic and constant shear stress experiments at room temperature on three different sintering runs of unpoled, niobium-doped lead-zirconate-titanate ceramic (PZT 95/5–2Nb) in order to quantify the influence of shear stress on the displacive (possibly martensitic), first-order, rhombohedral → orthorhombic phase transformation. Inter- and intra-batch variations were detected, but some generalizations can be made. In hydrostatic compression at room temperature, the transformation began at approximately 260 MPa, and was usually 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 constant shear stress tests were very different. First, the confining pressure and mean stress at which the transition begins decreased systematically with increasing shear stress. Second, we observed that the rate of transformation decreased with increasing shear stress and the associated elastic shear strain. This result contrasts with the typical observation that shear stresses increase reaction and transformation kinetics. Third, 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 effect does not appear to be an example of transformational plasticity: no additional unexpected strains accumulated during subsequent cycles through the transition under deviatoric loading. If subsequent hydrostatic cycles were performed on samples previously subjected to shear stress, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably resulted from a crystallographic preferred orientation that developed during the transformation under shear stress. Finally, in a few experiments on specimens from one particular sintering run, volume strain was often completely recovered and sporadic evidence for a shape memory effect was observed.