Tensile Properties of B2-Type CoTi Intermetallic Compound
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Tensile Properties of B2-Type CoTi Intermetallic Compound Y. Kaneno and T. Takasugi Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan ABSTRACT B2-type CoTi intermetallic compound that was hot-rolled and recrystallized was tensile-tested as functions of temperature and testing atmosphere. The tensile strength showed a peak at intermediate temperature (~800K). The brittle-ductile transition (BDT) defined by tensile elongation took place at about 800K, above which large tensile elongation was observed. Corresponding to this transition, SEM fractography showed a change from cleavage-like pattern mixed with intergranular fracture pattern to large cross-sectional reduction, i.e. necking of the tensile specimen. Also, the observed mechanical properties were independent of heat-treatment procedures, indicating that retained vacancies did not affect the mechanical properties of CoTi intermetallic compound. However, the tensile elongation and UTS at room temperature were dependent on testing atmosphere, indicating that moisture-induced embrittlement occurred in CoTi intermetallic compound. INTRODUCTION B2-type CoTi intermetallic compound, which has its solid solution range from 50at.% to 55at.% Co, and is stable up to its melting point (-1600K) [1, 2], is one of intermetallic compounds showing strength anomaly [3]. Deformation behavior of CoTi has been studied by compression tests in both forms of polycrystal and single crystal [3-5]. Using CoTi polycrystals, it was shown that an increase in yield strength with increasing temperature took place at an intermediate temperature, and its behavior was significantly influenced by deviation from stoichiometric composition [3]. Using CoTi single crystals, the strength anomaly has been studied on the basis of deformed microstructures by a transmission electron microscope (TEM) [5]. However, tensile property of CoTi has not been reported so far. Also, it is interesting to know whether CoTi is susceptible to moisture-induced embrittlement because most intermetallic compounds suffer from this phenomenon. Many B2-type intermetallic compounds including CoTi have very low ductility at room temperature but can be plastically deformed at elevated temperatures. To observe reliable tensile properties of such a brittle material, specimens that consist of recrystallized microstructure and have not solidified defects should be prepared, e.g. by adopting a thermomechanical processing. In the present study, a stoichiometric CoTi was hot-rolled and then annealed. The tensile tests were conducted as functions of temperature and testing atmosphere. SEM fractography was also observed for tensile deformed specimens. Based on these results, the temperature and atmosphere dependence of tensile properties and fracture mode of polycrystalline CoTi was discussed. EXPERIMENTAL PROCEDURES A stoichiometric Co-50at.%Ti was prepared by arc melting in argon gas atmosphere on a copper hearth using a non-consumable
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