HAADF-STEM studies of athermal and isothermal ω-phases in β-Zr alloy
Highlights
► We directly observed dumbbell structure in athermal and isothermal ω-phases. ► Dumbbell structure along is fairly visualized. ► Atomic displacement along 〈0 0 0 1〉ω cannot be determined quantitatively at present. ► The splitting of isothermal ω-phase takes place along {1 1 2}β.
Introduction
It has been recognized that there are three kinds of ω-phase, i.e. athermal, isothermal and stress induced ones, in β (bcc)-Ti and Zr alloys [1]. It is generally accepted that ω-structure is obtained by collapsing one pair of {1 1 1}β planes to the intermediate position leaving the next plane fixed, collapsing the next pair simultaneously as shown in Fig. 1a and b. Therefore, there are four variants of the ω-phase in the β-matrix [1]. The crystal structure of ω-phase is established to be trigonal with the space group of Pm1 [2], [3]. However, the trigonal structure is conventionally converted to the hexagonal system [4], and thus the atomic positions of three atoms in hexagonal unit cell are described to be 0 0 0, 1/3 2/3 u and 2/3 1/3 1-u as indicated in Fig. 1c. If the u is equal to 1/2, the resultant structure is considered to be hexagonal AlB2 type with the space group of P6/mmm. The value of u depends on alloying element and its concentration. For instance, the lattice constant of isothermal ω-phase in Ti–5 at.% Cr alloy has been reported to be a = 0.4607 and c = 0.2821 nm with u = 0.520 [5]. Consequently, there is the dumbbell structure along and 〈0 0 0 1〉 with inter atomic distance of and c(2u–1), respectively, in the image viewed from as indicated in Fig. 1d. The former and latter inter atomic distances of the isothermal ω-phase in Ti–5 at.% Cr alloy are estimated to be 0.1330 and 0.011 nm, respectively. Therefore, it has been very difficult to observe the dumbbell structure in ω-phase with high resolution transmission electron microscopy, i.e. conventional lattice imaging, so far. On the other hand, it has been well recognized recently that the HAADF-STEM is very powerful and useful technique in providing direct information of atomic arrangement in crystalline materials [6], [7], [8].
In the present study, we have applied HAADF-STEM technique to atomic scale characterization of athermal and isothermal ω-phase in β-Zr alloy. The orientation relationship between ω- and β-phases is also discussed.
Section snippets
Experimental procedure
The alloy used was Zr–7.5 at.% Nb alloy prepared by arc melting in argon atmosphere. It was confirmed from the preliminary experiments that both athermal and isothermal ω-phases can be observed in the present alloy. The ingot was homogenized at 1273 K for 36 ks and then quenched into ice water. Disks about 3 mm in diameter and 0.3 mm in thickness were fabricated from the ingot. These disks were solution-treated at 1273 K for 3.6 ks and then quenched into ice water. Several disks were aged at 673 K for
Athermal ω-phase
Fig. 2a shows a CTEM dark field image of athermal ω-phase in the quenched specimen by using reflection as indicated by the arrow in the corresponding electron diffraction pattern in Fig. 2b. The size and shape of the athermal ω-phase are random and indefinite. However, some of those have an elongate shape along the direction. It is well known that there are apparently four variants of ω-phase in β-phase as indexed in the key diagram of electron diffraction pattern in Fig. 2
Conclusion
The direct observation of the atomic arrangement in athermal and isothermal ω-phases in Zr–7.5 at.% Nb alloy has been performed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The results are summarized as follows:
- 1.
The dumbbell structure along is fairly visualized in both athermal and isothermal ω-phases, although the atomic displacement along 〈0 0 0 1〉ω, i.e. absolute value of u, cannot be determined quantitatively at present.
- 2.
The splitting of
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific Research (B), No. 23360281, from the Japanese Society for the Promotion of Science.
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