Interface-specific reorientation of embedded tetragonal ZrO2 particles in Ni1−xO polycrystals

ZrO2/Ni1−xO (1:9 in molar ratio) composites were sintered and then annealed at 1650 °C for 24 and 100 h in air to study Ni1−xO surface-controlled reorientation of the tetragonal (t-) ZrO2 particles, which transformed into monoclinic (m-) twin variants upon cooling. Transmission electron microscopy indicated that the ZrO2 particles fell into three topotaxial relationships with respect to the host Ni1−xO grains: (1) parallel topotaxy, (2) “eutectic” topotaxy, i.e. [1 0 0]Z//[1 1 1]N, [0 1 0]Z//[ 0   1 ¯   1 ]N and (3) “occasional” topotaxy [1 0 0]Z//[1 1 1]N, [ 0   1   1 ¯ ]Z//[ 0   1 ¯   1 ]N. The parallel topotaxy has a beneficial low energy for the family of {1 0 0}Z,N and {1 1 1}Z,N interfaces. The change from the occasional topotaxy to an energetically more favorable eutectic topotaxy was likely achieved by a rotation of the ZrO2 particles over a specific (1 0 0)Z/(1 1 1)N interface. Brownian-type rotation is probable for the embedded t-ZrO2 particles in terms of anchorage release at the interphase interface with the Ni1−xO host. Detachment or bypassing of grain boundaries could also cause reorientation and shape change of intergranular ZrO2 particles.