Stress-Induced Martensitic Transformation of Boundary gamma-Fe Particles in Cu

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Large coherent gamma-Fe particles were introduced on a small-angle boundary in bicrystals of Cu-Fe alloys. They were stressed by grain-boundary sliding at 548K or directly by external stress at 77K to study the stress-induced martensitic transformation from fcc to bcc. No plastic deformation due to crystal dislocation activity occurred during these loading processes. Larger particles transformed to bcc when they were cooled to 77K or were stressed by the external stress at 77K.Some particular variants of martensite were preferentially formed under the internal stress caused by boundary sliding or depending on the sense (tension or compression) and direction ([416]_f or [001]_f) of the applied stress. The preferential formation of the variants could be explained well by postulating that the stress helps the shear movement of atoms to change the lattice from fcc to bcc. No accelerating effect of a large pre-existing hydrostatic stress, caused by the lattice parameter difference between gamma-Fe and Cu, on the transformation is also consistent with this contention. Any effect of the stress on the overall shape strain has been excluded.