TRANSFORMATION PLASTICITY IN SINGLE AND 2-COMPONENT IONIC POLYCRYSTALS IN WHICH ONLY ONE-COMPONENT TRANSFORMS

Creep tests were performed on cylindrical cold-pressed polycrystalline specimens of pure NH,NO, and KNO3 while the specimens were simultaneo usly cycled through the known phase transformations. The observed defo rmation behaviour and the transformation strains were both qualitative ly and quantitatively consistent with the Greenwood and Johnson (1965) theory of transformational plasticity, which applies to external stre sses less than the yield stress of the weaker phase. A new and simpler development of this theory is given in the Appendix. The transformati on strains observed in composite specimens, in which only one componen t transformed, were all significantly less than those observed in the pure materials. This behaviour is observed even in specimens where the volume fraction of the marginally-stronger non-transforming phase is too small to form a continuous framework in the specimen. It is sugges ted that the stress associated with the difference in thermal expansio n of the two components, especially during a phase transformation, lea ds to the nucleation of a high density of tangled, essentially immobil e, dislocations in the transforming component, similar to work-hardeni ng. The relevance of these observations to the possibility of transfor mation plasticity as an important deformation mechanism in the Earth i s discussed.