Anomalous Strain Rate Dependence of Tensile Elongation in Moisture-Embrittled L1 2 Alloys
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Anomalous Strain Rate Dependence of Tensile Elongation in Moisture-Embrittled L12 Alloys T. Takasugi, Y. Kaneno and H. Inoue Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan ABSTRACT The effect of strain rate on tensile ductility of moisture-embrittled L12-type Co3Ti and Ni3(Si,Ti) ordered alloys was investigated at ambient temperatures (298K~423K) by tensile test and SEM fractography. The anomalous increase of tensile elongation and ultimate tensile stress was observed in a low strain rate region and also at high temperatures, accompanied with an increased area fraction in ductile transgranular fracture pattern. The anomalous strain rate dependence of tensile ductility was shown to become more evident with decreasing grain size. As a process counteracting to the hydrogen decomposition from moisture in air, oxidation process on the alloy surface was suggested. INTRODUCTION L12-type Co3Ti and Ni3(Si,Ti) ordered alloys have some attractive mechanical and chemical properties as structural materials, e.g. a positive temperature dependence [1,2] of flow strength and strong corrosion resistance at ambient temperature. However, many L12-type ordered alloys including Co3Ti [3] and Ni3(Si,Ti) [4] have been shown to be embrittled at ambient temperature by hydrogen decomposed from moisture in air. Therefore, the prevention of a so-called environmental embrittlement is a crucial issue to develop them as structural materials. It has been shown that the environmental embrittlement of L12-type ordered alloys substantially depends on material conditions as well as testing conditions. As the testing conditions, atmosphere, temperature and strain rate have been investigated, and found to greatly influence tensile ductility and fracture mode of L12-type ordered alloys in air [5-7]. For example, the tensile elongation of L12-type ordered alloys in air decreases in a manner of ductile-brittle transition (DBT) as strain rate decreases. Corresponding to this behavior, fracture mode changes from ductile transgranular fracture to brittle intergranular fracture. In the previous study [8], the tensile elongation of Co3Ti alloy in the embrittled condition (i.e. in a slow strain rate range) was low and insensitive to the strain rate, or, slightly increased with further decreasing strain rate, depending on the grain size. This behavior appears to be somehow distinct from the results observed in conventional metals and alloys; the hydrogen embrittlement becomes more severe as deformation rate decreases, i.e. approaches toward the so-called delayed fracture region. In this study, the effect of strain rate on the moisture-induced embrittlement of L12-type Co3Ti and Ni3(Si,Ti) ordered alloys is investigated at ambient temperatures by tensile test and fractography. An emphasis is placed on the anomalous increase of tensile ductility in a low strain rate region, i.e. 10-1 s-1), and then rapidly decreased with decreasing strain rate in an intermedia
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