The Influence of Second-Phase Dispersion on Environmental Embrittlement of Ni 3 (Si, Ti) Alloys
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Metallographic and structural observations for all the Ni 3(Si,Ti) alloys containing second-phase dispersions were carried out on recrystallized microstructures by an optical microscope (OM), x-ray diffraction (XRD), a scanning electron microscope (SEM) attached with a wave-length dispersive spectroscope (WDS). Tensile tests were conducted at room temperature in air and vacuum. Degree of moisture-induced embrittlement can be evaluated from difference in the tensile elongation (and the fracture mode) between vacuum and air. For some alloys, tensile tests were carried out as a function of strain rate, and thereby ductile-brittle transition (DBT) was evaluated. Fracture surfaces of deformed specimens were examined by a SEM.
Figure 1. SEM back scattering images of the quaternary Ni3(Si,Ti) alloys with second phase dispersions.
RESULTS Figure 1 shows SEM back scattering microstructures of quaternary Ni 3(Si,Ti) alloys with second-phase dispersions. Table 1 also summarizes crystal structures and alloy compositions of the constituent phases. It was found that most quaternary transition elements were less soluble in Ni3(Si,Ti) [9]. Among the added transition elements, Nb had the largest solid solubility (-2.3 at%). Concerning the second-phase dispersion, V-added Ni3(Si,Ti) alloy contained ductile fcc-type Ni solid solution in a form of eutectic microstructure within L12 phase matrix, while Zr-, Nb- and Hf-added Ni 3(Si,Ti) alloys contained hard dispersion compounds in forms of isolated particles. Here, it is noted that the secondphase dispersions precipitated in 2at%Zr-, 4at%Nb- and 2at%Hf-added Ni3(Si,Ti) alloys were very similar in terms of the morphology, size and volume fraction. Figure 2 plots tensile elongation of the quaternary Ni3(Si,Ti) alloys that were deformed at a strain rate of 1.2x10-3 s-1in air and vacuum, respectively. Here, data for the unalloyed Ni3(Si,Ti) (which was fabricated in a similar way to the present alloys) were referred from a previous work [4]. Zr- and Hf-added Ni 3(Si,Ti) alloys showed lower tensile elongation than the unalloyed Ni3(Si,Ti) when deformed in vacuum as well as in air. Consequently, Zr- and Hf-containing second-phase dispersions have little effect of improving the moisture-induced embrittlement, rather, enhancing the moistureinduced embrittlement of the Ni 3(Si,Ti) alloy. Very striking result was observed for quaternary Ni 3(Si,Ti) alloys with Ni solid solution and Nb-containing second-phase dispersion. In the V-added KK6.5.2
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35 30 Figure 2. Tensile elongation of the quaternary Ni3(Si,Ti) alloys that were deformed at a strain rate of 1.2x10 3s 1 in air and vacuum.
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Alloy Ni 3(Si,Ti) alloy with Ni solid solution, tensile elongation of the alloy deformed in air was little reduced, i.e. almost identical to that of the V-added Ni 3(Si,Ti) alloy deformed in vacuum and also to the unalloyed Ni3(SiTi) deformed in vacuum. In the Ni3(Si,Ti) alloy with Nb-containing second-phase dispersion, the alloy deformed in vacuum showed a lower tensi
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