Piezoelectric and electrostrictive actuators composed of lead titanate zirconate (PZT) and lead magnesium niobate-lead titanate (PMN-PT) are important in a variety of engineering applications. A review of the literature indicated that published phase diagrams for the PMN-PT system are based primarily on temperatures of dielectric transitions. rather than direct measurements of structural changes. Structural transformations in the PMN-PT ceramic system have not been investigated at non-ambient temperatures for compositions with less than 0.29 PT. The review also revealed that crack growth in cyclic electric fields has been observed in only two compositions of PZT, while no crack initiation or growth is observed in a relaxor composition of PMN-PT.

The X-ray diffraction results showed that the PMN-PT samples undergo multiple structural transformations on cooling through the temperature regions of their dielectric transitions. In compositions containing 0.0, 0.08, 0.10 and 0.20, an upper transformation temperature Ta, derived from a sharp change in coefficient of thermal expansion, lies well above the temperature Tm, of the observed maximum in dielectric constant. Lower structural transformation temperatures observed in these c compositions lie close to the freezing temperature Tf, below which ceramics exhibit remanent polarization. The temperatures of an upper structural transformation in PMN-PT ceramics with the higher PT mole fractions 0.30 and 0.35, agree closely with the Curie Temperature Tc.

Cracks emanating from the corners of Vickers diamond indents in the soft piezoelectric ceramics BM532 PZT (Navy-Type VI) and PMN-0.30PT were increased in length when subjected to low frequency cyclic electric fields that were applied normal to the crack. Crack growth was observed in fields of 2-5 x Ec, but no new cracks were initiated when these higher fields were applied. Similar induced cracks in hard piezoelectric PZT ceramics, did not exhibit visible crack growth when subjected to low frequency fields with amplitudes up to +3.15 MV/m (1.6 x Ec), which is considered to be a safe maximum to avoid dielectric breakdown in PZT ceramics. No cracks were observed to emanate from the corners of either Vickers, or Knoop, diamond indents in samples of PMN-0.08PT. The intergranular cracks which emanate from the sides of the indents did not grow, and no new cracks were initiated, when the specimens were subjected to similar electric fields.