Understanding Precursor Phenomena for the R-Phase Transformation in Ti-Ni-Based Alloys
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Understanding
Precursor Phenomena for the R-Phase Transformation in Ti-Ni-Based Alloys
Daisuke Shindo, Yasukazu Murakami, and Takuya Ohba Abstract Precursor phenomena are critical issues for martensitic transformations. In this article, we show recent progress in understanding precursor phenomena to the R-phase transformation, which is important for both fundamentals and applications. Structural modulation in the parent phase was intensively studied by means of detailed analyses of the weak diffuse scattering of electrons with the aid of recently developed energyfiltered transmission electron microscopy coupled with x-ray diffraction. A peculiar domain-like structure, which originates from static transverse atomic displacements in the parent phase, was discovered by virtue of these advanced methods. The characteristics of this structure (e.g., size, shape, and temperature-dependence), as well as its role in the subsequent R-phase transformation, are discussed.
Introduction The cubic (parent phase) to trigonal transformation (called the R-phase transformation) in Ti-Ni-based alloys is important for both fundamental understanding and applications.1 In fact, most of the practical applications of Ti-Ni shape-memory alloys use the R-phase transformation, since it exhibits a small temperature hysteresis, as small as 1–1.5 K (Figure 1). The R-phase transformation was first found by Dautovich and Purdy2 and then was well characterized by Sandrock et al.3 by the sharp increase of resistivity, as shown in Figure 1. Since the R-phase transformation occurs prior to another martensitic transformation to a monoclinic phase, it was previously called a premartensitic transformation or believed to be a distinct second-order transformation. However, it is now established that the R-phase trans-
MRS BULLETIN/FEBRUARY 2002
formation and the subsequent transformation to the monoclinic phase are two competing martensitic transformations of the first order.4–7 Practically speaking, the R-phase transformation is realized if Ti-Ni alloys are subjected to work-hardening, precipitation-hardening, or the addition of a third element such as Fe or Al.1 The last method seems most effective; with its use, the subsequent martensitic transformation to the monoclinic phase is markedly suppressed,8 and hence the R phase is observed in a much wider temperature range near ambient temperature, as shown in Figure 1. From a physical point of view, the presence of diffuse incommensurate reflections5–7,9,10 and their correlation with the subsequent R-phase transformation are interesting and have been controversial for many years. (The temperature range in
which the diffuse incommensurate reflections are observed will be discussed later.) Understanding these precursor phenomena (i.e., those above the transformation temperature Rs) is of vital importance not only for establishing the transformation mechanism, but also for developing further smart and stable shape-memory alloys in the future. This article presents recent findings on the precursor phenome
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