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Nano-domained Martensite: Strain glass
|Xiaobing Ren , Yu Wang , Kazuhiro Otsuka|
National Institute for Materials Science, Ibaraki 305-0047, Japan
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 Ti50-xNi50-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 Tg. 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)
Presentation: Invited at E-MRS Fall Meeting 2007, Symposium E, by Xiaobing Ren
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-16 04:57 Revised: 2009-06-07 00:44