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Ambient temperature a-to-b phase transition and nanocrystallization in the aged Ti-25Nb-3Mo-3Zr-2Sn titanium alloy was achieved by surface mechanical attrition treatment (SMAT). The phase transition occurs at a/b interfaces and extends to a phase interiors with increasing strain. It is irreversible and diffusion controlled. The stressinduced increase of Gibbs energy and enrichment of Nb may cause a high order of lattice instability of the a phase adjacent to the a/b interfaces and compel the a phase to b phase. The presence of fine a needles in the aged alloy and the phase transition from a to b with increasing strain are viewed to play a crucial role in the subsequent nanostructuring.

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