P. M. Kelly, L.R. Francis Rose/Progress in Materials Science 47(2002 )463-557 1.3. Martensitic transformations in ceramics Although originally associated with the transformation in quenched steels that leads to extraordinary increases in strength and hardness, martensitic transforma- tions also occur in a number of minerals and ceramics and these have been studied for decades [12, 14-16]. However, the worldwide interest in martensitic transforma- tions in non-metallic materials exploded with the discovery of transformation toughening in zirconia ceramics [1]in 1975. The toughness of a traditionally brittle ceramic could be increased by a factor of 4 or more. This held out the prospect of developing engineering ceramics that could be safely used in structural applications, their other superior properties - wear resistance, low density, high melting point-would give them an advantage over their metallic rivals. Experimental and heoretical work on transformation toughening in ceramics blossomed. The litera- ture in the 1980s and early 1990s was inundated with publications on zirconia and nternational conferences specifically devoted to zirconia were held at least once a year. G This review is primarily concerned with the important connection between trans- ormation toughening and the martensitic transformation responsible for the toughening. As a result, attention will be concentrated almost exclusively on the transformation in zirconia - the main ceramic system that has, to date, exhibited any significant transformation toughening. Readers interested in the broader field of martensitic transformations in non-metallic materials -ceramics and minerals should consult one of the excellent review articles in the literature [12, 15] The dominant role of the shape strain in transformation toughening means that any credible model for the transformation toughening process must rely on having a sound knowledge of the shape strain associated with the martensitic transformation Where can this data be obtained for zirconia? In principle the shape strain can be determined experimentally. To date however, there have been relatively few quanti tative experimental measurements reported [17, 18] and these have only provided values for the overall magnitude of the shape strain, with no real indication of its crystallographic direction or the relative amounts of shear and dilatation. In view of the microscopic scale on which the transformation occurs, this dearth of experi mental data is not at all surprising An alternative source for the values of the shape strain is the theoretical predic tions of the crystallographic or phenomenological theory of martensitic transfor- mations(PTMT)[19-24]. Of course it is essential to be confident that the phenomenological theory does give reliable predictions for the shape strain. Section 3 provides a detailed comparison between the theoretical predictions and the experimental results for zirconia collected over the last 30 years. The absolutely excellent agreement between theory and experiment indicates that the transforma tion in zirconia obeys the predictions of the phenomenological theory better than many other martensitic transformations in metals and ceramics. Hence, any pre dicted values for the shape strain are likely to be extremely reliable. Before com mencing this comparison, the next section (Section 2) will cover the general formulation of the crystallographic or phenomenological theory, explore some<-)- !     !! *        '        /     +              (                        M ! 16 5N - '  ' '          (     (    +$  '               )      M N  827
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