Anomalous strain rate dependence of tensile ductility in moisture-embrittled Co 3 Ti alloys
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The effect of strain rate on tensile ductility of moisture-induced embrittlement of Co3Ti alloys was investigated at ambient temperatures (298–353 K) by tensile test and scanning electron microscope fractography. The anomalous increase of tensile elongation and ultimate tensile stress was observed in a very low strain rate region and also at high temperatures, accompanied by an increase of 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 and also with deviation from alloy stoichiometry. Oxidation on the alloy surface was suggested as a process counteractive to the hydrogen decomposition process from moisture in air.
I. INTRODUCTION
Co3Ti alloy has some attractive mechanical properties as structural materials, e.g., a positive temperature dependence of flow strength,1 an apparent tensile ductility,2 and a strong corrosion resistance at ambient temperature. However, Co3Ti alloy3 as well as other L12-type ordered alloys such as Ni3Si4,5 and Ni3Al6 has been shown to be embrittled at ambient temperature by hydrogen decomposed from moisture in air. Therefore, the prevention of the so-called environmental embrittlement of Co3Ti alloy is a crucial issue to develop it for structural materials. It has been shown that the environmental embrittlement of Co3Ti alloy substantially depends on material condition as well as testing condition. For the material condition, alloying effects (such as stoichiometry,3 ternary alloying,7 and doping8) and also microstructural effects (such as grain size9) have been investigated and shown to strongly affect tensile ductility and fracture mode in air. On the other hand, for the testing condition, the effects of atmosphere, temperature, and strain rate have been investigated3,7 and found to greatly influence tensile ductility and fracture mode in air. For example, the tensile elongation of Co3Ti alloy in air decreases in a manner of ductile–brittle transition (DBT) as strain rate decreases.3,9 Corresponding to this behavior, the fracture mode changes from ductile transgranular fracture to brittle intergranular fracture.3,9 In the previous study, the tensile elongation of Co3Ti alloy in the embrittled condition (i.e., in the slow strain rate range) was insensitive to the strain rate or slightly increased with further decreasing strain rate, depending on the grain size.9 This behavior appears to be somehow different from the results observed in conventional metJ. Mater. Res., Vol. 15, No. 9, Sep 2000
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als and alloys. It was believed that 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 moistureinduced embrittlement of L12-type Co3Ti alloys is investigated at ambient temperatures by tensile test and fractography. An emphasis is placed on experimental confirmation and interpretation
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