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The Tetragonal to Monoclinic Martensitic Transformation in Zirconia

This paper reviews current knowledge of the martensitic tetragonal to monoclinic transformation that is the key to transformation toughening in zirconia ceramics. The most critical feature of this transformation is the shape strain, which not only governs the degree of toughening but, more important, determines whether or not stress-induced transformation will occur at the tip of a potentially dangerous crack. Determination of the shape strain by experiment is difficult, if not almost impossible, in zirconia but it is easily calculated by using the phenomenological theory of martensitic transformations. Therefore a major theme of the present paper is to demonstrate unequivocally that the phenomenological theory of martensitic transformations is capable of explaining all the reported microstructural and crystallographic features in these ceramics, and so can be used with confidence to provide the shape strain values, which are essential in developing a complete understanding of transformation toughening in zirconia.