Development of the Precursor and Subsequent R-Phase Transformation
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Development of the Precursor and Subsequent R-Phase Transformation Daisuke Shindo, Yasukazu Murakami, and Takuya Ohba
The following is a Web Extra expanding upon the article “Understanding Precursor Phenomena for the R-Phase Transformation in Ti-Ni-Based Alloys” by Daisuke Shindo, Yasukazu Murakami, and Takuya Ohba, published in MRS Bulletin 27 (2002) pp. 121–127. It provides a closer examination of the domain structure evolution over a temperature range extended beyond that shown in Figure 5 in the article, along with intensity profiles of the electron diffraction.
What is Formed Above the R-Phase Transformation Temperature Rs? A dark-field image (Figure 1a) verified the presence of tiny domains (less than 5 nm in diameter) in the parent phase of Ti50Ni48Fe2. To characterize these domains, we first clarified the extinction rule of diffuse scattering: it is originally absent along qhh0 vectors in reciprocal space across the
origin, depressing the dynamical diffraction effect (Figures 1d–1f); the diffuse scattering in Figure 1c is visible by means of multiple scattering. We also demonstrated that selection of a diffuse spot along another wave vector (e.g., q101 or q011) showed the domains at different positions within the field of view (Figure 1b). These characteristics are missing in the R phase, where both the features of satellites and the in-
ternal structure are distinct (Figure 2a). The results explicitly indicate that the tiny domains observed above the transformation temperature Rs are not in the R phase, but are still in the parent phase and are being subjected to 110 110-type transverse lattice displacements. Since there are several equivalent lattice displacements in this mode, owing to the cubic symmetry of the parent phase, the domains are classified into groups based on the wave vectors of the displacements, as shown in Figure 2b.
How Does the Precursor Develop as Rs Approaches? In situ dark-field images revealed the temperature-dependence of the domainlike structure in the parent phase. Figure 3a shows a typical dark-field image at 290 K, where the tiny domains are visible. If the specimen is cooled down to 288 K, new domains form, as indicated by arrowheads (Figure 3b), while the previous domains remain stable (see the invariance of the reference domain, marked R.D.). Subsequent cooling to 285 K induces the further formation of domains (Figure 3c). The size of domains also enlarges to some extent (3–4 nm) with cooling. These findings explain an essential character of the diffuse scattering pattern, that the intensity increases with cooling, as shown in Figure 3. ■
Figure 1. (a) Domain-like structure in the parent phase of Ti50 Ni48 Fe2 visualized by the dark-field method, where the encircled reflection was used. (b) Distinct distribution of domains observed by selecting other reflections (e.g., q101 or q011). (c)–(f) Electron diffraction patterns of the parent phase with [11 1] incidence (c) and with systematic excitation conditions (d)–(f). Observations were made at 296 K. Dashed lines in
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