Alloying behavior of Co 3 Ti
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I.
INTRODUCTION
THE Co3Ti is an intermetallic compound having the L12 ordered structure in which Co atoms and Ti atoms occupy the face center sites and cube corner sites, respectively. The Co3Ti phase has a solid solution range between 20 and 25 at. pct Ti; it is formed at its lower Ti concentration region by a peritectic reaction between cobalt solid solution (fcc) and liquid at about 1473 K, and at its higher Ti concentration region by a eutectic reaction between Co2Ti (/3) and LlzCo3Ti (y') at about 1443 K. 1-4 It was demonstrated that in the Co-rich C03Ti compound excess Co atoms substitute for Ti atoms. 4 An interest has been shown in the C03Ti compound since the yield strength increases with increasing temperature as in other Llg compounds like Ni3A1 and Ni3Si, 5 and in addition C03Ti has an intrinsically good ductility at a wide range of temperature in contrast to other L12 compounds. 6'7 Several metallurgical, mechanical, and magnetic behaviors have been investigated by the present authors. 8-H The addition of ternary elements might improve or modify not only the mechanical properties but also a number of other properties of the C03Ti compound. The constructions of the ternary phase diagrams are therefore required to develop C03Ti-based high performance alloys. The alloying behavior of L12-type A3B compound, i.e., whether the addition substitutes exclusively for A or exclusively for B or for both sites, has been investigated and summarized in Ni3A1,12'13 Ni3Ga, Ni3Si, and Ni3Ge compounds. ~3 Suzuki et al. recently have advanced the thermodynamic argument involving the nearest neighbor interactions about these alloying behaviors.~3 However, in the alloying behavior of C03Ti compound, only the result of microprobe analyses on the extracted 3/' precipitates from cobalt-base alloys was available. 14 In the present paper, the substitution behavior of various kinds of third elements is first represented in the C03Ti compound and then evaluated with the existing thermodynamic concept.
Also, the purities of the alloying additions used in the present study are shown in Table I, together with the electronic nature, i.e., the group number in the periodic table. Alloy buttons of about 40 g were prepared by a nonconsumable arc melting technique under an argon gas. Alloy buttons were melted several times to achieve homogeneity. The total weight loss was in most cases small, and several alloys which were analyzed chemically showed good agreement within 0.2 pct between nominal and analyzed chemical compositions. Alloy buttons were homogenized in a vacuum of about 1.3 x 10-3 Pa at 1323 K for 172.8 ks and then rapidly cooled. The alloying behavior was examined by the preferred direction of solubility lobe of Co3Ti in the ternary diagram. Three series of alloys with a ternary element were studied; in the first series, which are denoted here as a-direction, a ternary element (C-component) was increased at the expense of cobalt content (A-component). In the second series, denoted as b-direction, a ternary element was increas
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