Microstructural characteristics of the eutectoid mixture Zr2Cu and Zr7Cu10

Abstract

During their investigation into the phase transformations of an equiatomic ZrCu alloy, Carvalho et al. [1] found that two kinds of solid-state transformation occur: a martensitic transformation with Ms = 440± 5 K and a eutectoid reaction with TE = 985± 5 K. The equiatomic alloy has a bcc-based ordered structure (B2) which exists as a line phase above TE [2]. Below TE, the B2 structure undergoes a eutectoid decomposition to Zr7Cu10 and Zr2Cu which have orthorhombic and tetragonal structures, respectively [2]. By rapid cooling the B2 structure to below 413 K, however, the eutectoid decomposition process is suppressed and the B2 structure transforms martensitically into at least two monoclinic structures [3, 4]. The microstructural features associated with the martensitic transformation have been studied in detail by Soe et al. [5]. As for the microstructure after eutectoid decomposition, Nicholls et al. showed it possesses a fine lamellar morphology [6]. No further information is available in the literature with respect to this lamellar structure. The present paper reports the microstructural characteristics of the eutectoid mixture Zr7Cu10 and Zr2Cu examined by transmission electron microscopy (TEM). The orientation relationship between Zr7Cu10 and Zr2Cu is also reported. The equiatomic ZrCu alloy ingots were prepared from Zr (99.5 at%) and Cu (99.9%) by melting in a water-cooled copper hearth of a high-frequency induction furnace. In order to avoid oxidation of Zr and thus obtain high quality ingots, the furnace system was initially evacuated to a pressure of approximately 2× 10−3 Pa, and then refilled with high purity argon gettered by hot Ti granules prior to melting. The ingots were remelted several times to make them homogeneous. To produce a eutectoid-type microstructure at room temperature, the ingots were sealed in a silica tube, annealed at 550 ◦C for 24 h and quenched into liquid nitrogen. Afterwards, the structure of the ingots was characterized by X-ray diffractometry (XRD) at room temperature. Samples for the electron microscopy observations were electropolished in a Tenupol 2 twin-jet polisher using a solution of 15% nitric acid in methanol at−30 ◦C and a voltage of 30 V. The samples were examined in a Jeol FX4000 transmission electron microscope operating at 200 kV. Samples annealed at 550 ◦C for 24 h and subsequently liquid nitrogen quenched, consist of two phases Zr7Cu10 and Zr2Cu which are shown in the XRD pattern of Fig. 1. The ratio of Zr7Cu10 to Zr2Cu is about 2 as estimated by TEM and is consistent with the phase diagram of the Zr-Cu system [2]. These two phases adopt a lamellar morphology as illustrated in Figs 2 and 3. No martensitic phase was detected or observed by either XRD or TEM. These results indicate that the samples had undergone a complete eutectoid transformation during annealing, which is in good agreement with the reported results [1, 2, 6]. Figs 2 and 3 show two distinct microstructural features: i) regular lamellae (Fig. 2), and ii) irregular lamellae or isolated islands (Fig. 3). The regular lamellae constitute a well-developed structure with a high degree of parallelism. It is usually continuous over a large distance. The dark lamellae with an average thickness of