The monoclinic-tetragonal phase transformation of ZrOn was examined with a vacuum hot-stage microscope.Polished sections of vacuumsintered 99.7y0 pure ZrOp were observed from room temperature to 13OO0C. The rapid formation of platelet substructure within ZrOB grains in the temperature range 1050" to 115OOC was associated with the heating transformation. Photomicrographs and motion pictures were taken of the specimen as the transformation progressed. The surface deformation was irreversible, preventing observation of the phase inversion on cooling. From these hot-stage studies, supplementary DTA, X-ray, and thermal expansion data, and other existing information, it is concluded that the transformation is of the diffusionless, athermal type, characteristic of Fe-Ni martensitic transformations.
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