Evidence for new glassy state- strain glass

in ferroelastic/martensitic system

“Glass” is a generic term for a frozen state of an essentially disordered system with local order only. It characterizes a wide range of complex systems from physics to biology. In general, a system tends to transform into a glassy state rather than into a long-range ordered configuration when there exist random defects and frustrations. The most common glass phenomenon is the window glass, which is a mixture of major composition of quartz with some impurities like soda and lime. In window glass, the impurity molecules prevent the long-range ordering of quartz molecules during cooling, thus a long-range ordered state (crystalline state) cannot form; instead the system freezes in a locally ordered glass state, short-range ordered molecular alignment is kept. Analogous glassy phenomena have also been known in magnetic systems (i.e., spin glass) and electric dipolar systems (i.e., relaxor), where the concept of “freezing of local atomic order” is replaced by the freezing of magnetic order and dipolar order, respectively.

Very recently, we discovered a new glass phenomenon termed strain glass in Ni-rich intermetallic Ti_50-xNi_50+x (x>=1.5), which is caused by a freezing of local “strain order”. The strain glass is derived from a martensitic system, TiNi shape memory alloy. A normal martensitic system undergoes a long-range strain-ordering transition called martensitic transformation, in which below the transformation temperature, the cubic unit cells of the parent phase spontaneously “distort” in the same way over very long range like a domino, and hence form a long-range ordered strain (distorted lattice) state. Strain glass is formed through doping point defects (excess solute atoms or alloying elements) into a martensitic alloy. Because of the strong hindrance of point defects, the system can not form a long-range strain ordering during cooling but transforms into a strain glass state with randomly distributed local strain order.

The frequency dispersion of AC dynamic property is a characteristic of glass transition with dynamic freezing. The strain glass transition of stain glass alloy Ti_48.5Ni_51.5 was manifested by the appearance of a frequency dependent dip in the AC elastic modulus(Fig. 1(a)) at the freezing temperature Tg and a corresponding peak in the internal friction tanδ(mechanical loss)(Fig. 1(b)) at a lower temperature. The locally ordered strain domains(Fig. 2(a)) of strain glass Ti₄₈Ni₅₂ was imaged with high-resolution transmission electron microscopy, and they have been found to yield diffuse scattering in electron diffraction(Fig. 2(b)).

See Shampa Sarkar, Xiaobing Ren and Kazuhiro Otsuka, PHYSICAL REVIEW LETTERS 95, 205702 (2005) for details.

Figure 1(a) and (b) show temperature dependence of ac storage modulus S(w,T) and internal friction tanδ(w,T) for the strain glass Ti_48.5Ni_51.5, respectively, at various frequencies (0.1 Hz – 10 Hz), which is the important feature for a glass transition.

Figure 2: Electron diffraction pattern for Ti₄₈Ni₅₂ is shown as a function of temperature in (a). The diffuse superlattice reflections at incommensurate 1/3 position (marked by yellow arrow) and fail to reach commensurate 1/3 value (which is characteristic of a long-range ordered R phase). The corresponding HREM dark-field images (b), reveal random distribution of tiny nano-domains(locally ordered strain). The nano-domains grow to some extent in size but are finally frozen in that configuration.