Martensitic transformation has attracted continual interest for a century, owing to its key role in hardening steels and in the functional properties of shape memory alloys. Martensitic transformation can be viewed as a long-range ordering of lattice strain (distortion of the parent lattice) below Ms, with strain being the order parameter. This is analogous to the ordering of magnetic moments in a ferromagnet or electric dipoles in a ferroelectric. The strain-ordering transition creates a low symmetry martensite phase characterized by a micron-sized hierarchical twin/domain microstructure.

The above “normal” martensite has been the research subject of our field to date. Here we show that, when doping point defect into a martensitic system beyond a critical value, there appears a hitherto unrecognized wide composition range in which an “abnormal martensitic state” comes into being. As the first example, Ni-rich Ti_50-xNi_50-x alloys for x>1 (here the excess Ni is regarded as point defect), which have been regarded as non-transforming so far, are shown to undergo a “strain glass transition” below a critical temperature T_g. Such a transition is characterized by the formation of nano-sized martensite domains instead of micron-sized hierarchical twins as the case of normal martensite. The strain glass transition is not accompanied by a change in the average structure, or a thermal peak in the DSC curves. It is a freezing of the nano-martensite domains. We show that the seemingly “non-martensitic” strain glass exhibits unexpected properties: shape memory effect and superelasticity, like a normal martensitic alloy, although there is no sign of a spontaneous martensitic transformation in such a system.

Being parallel to cluster-spin glass and relaxor, strain glass may provide a new horizon for martensite research.

1. S. Shampa, X. Ren, and K. Otsuka, Phys. Rev. Lett., 95, 205702 (2005)

2. Y. Wang, X. Ren, K. Otsuka, Phys. Rev. Lett.，97，225703 (2006)

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