The incidence of martensic transformations in systems containing ZrO2 particles (or precipitates) depends upon the size of the particles. The origin of this size effect is demonstrated to reside in the twinned or variant structure of the transformed phase, by virtue of a surface area related strain energy term. Predicted critical particle sizes correlate quite well with observations on two ZrO2 systems. The particle size effect can also be incorporated into toughening analyses, in order to predict both optimum toughening conditions and toughening trends. The predictions again correlate favorably with toughness data for ZrO2 containing materials.
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