Strengthening transformation toughening

When force is applied to a material, the effects of the stress will most likely appear at the tip of the cracks. These stresses, near the crack tips, will cause the particles to transform to the monoclinic symmetry by making the particles lose their coherency. Additional stress is then required for crack extension, i.e. application of additional stress can inhibit crack propagation thereby strengthening and toughening the PSZ. 

Honeyman-Colvin, P., Lange, F.F., Infiltration of porous alumina bodies with solution precursors strengthening via compositional grading, grain size control, and transformation toughening, J. Am. Ceram. Soc., 79(7), 1810-1814, 1996. 

Many other methods of strengthening are based on formation of composites by inclusion of fibers or whiskers or by crystallization to form glass-ceramics. Phase separation may also affect strength by altering crack propagation mechanisms. Transformation toughening has also been attained by formation of a small concentration of zirconia crystals in glasses. 

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