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Autumn 2009 Seminar Series
October 30, 2009 at 3:30 p.m.
Room 264 MacQuigg Labs
Xiaobing Ren
Group Leader, Ferroic Physics Group
National Institute for Materials Science (NIMS), Japan
Strain Glass--A New Frontier of Martensite Research
Abstract
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 moment in a ferromagnet or electric dipole 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 regime 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 in the case of normal martensite [1]. The strain glass transition is not accompanied by a change in the average structure, or a thermal peak in the DSC measurement. 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 [2], like a normal martensitic alloy. Strain glass bears a striking similarity with cluster-spin glass, and ferroelectric relaxor, and they can be considered as a more general class of glass-ferroic glass [3]. Strain glass may become 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. C97 C225703 (2006)
3. Y. Wang, X. Ren, K. Otsuka, and A. Saxena, Phys. Rev. B, 76, 132201(2007)
Bio
Dr. Xiaobing Ren received his higher education at Xi'an Jiaotong University, where he was awarded a PhD in materials science in 1994. After working as a postdoc at Department of Physic, Nanjing University, and a JSPS fellow at Institute of Materials Science, University of Tsukuba, he joined the faculty of University of Tsukuba in 1997. In 2000 he moved to National Institute for Materials Science (NIMS) as a senior scientist, and from 2006 he became the leader of Ferroic Physics Group, NIMS. He is also an adjunct professor of Xi'an Jiaotong University.
Dr. Ren is an expert of ferroic materials with a broad interest. He discovered a universal symmetry property of point defects in all crystalline materials (Nature, 1997; PRL 2000). This principle successfully explained a 65-year old puzzle of martensite: the rubber-like behavior of martensite. This principle also led him discover a huge electrostrain effect in ferroelectric crystals (Nature Materials, 2004), which arose much interest in both academia and industry. Very recently, he discovered a "strain glass" in martensitic system (PRL 2005), which is a ferroelastic analog of spin glass in magnetic systems or relaxor in ferroelectric systems. This discovery makes a new horizon for martensite research and may bring about novel effects in ferroelastic systems. Over the past 15 years, he has authored and coauthored more than 100 papers.
Please join our speaker for light refreshments in 479 Watts Hall following the talk.
