Effect of Alloying Elements on Nb-Rich Portion of Nb-Si- X Ternary Systems and In Situ Crack Observation of Nb-Si-Based
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UCTION
RECENTLY, Nb-Si alloys have attracted attention as a candidate of high-temperature materials for replacing Ni-based superalloys because they exhibit superior high-temperature strength to the superalloys.[1–6] As shown in Figure 1, the Nb-Si binary system is composed of several intermetallic compounds, which have suitable properties for high-temperature use, such as high melting points and high strength. Among them, a Nb5Si3 is the most promising candidate as a phase dispersed in Nbss matrix for strengthening. However, the lack of room-temperature ductility and high-temperature oxidation resistance are major drawbacks of Nbss/a Nb5Si3 two-phase alloys for practical applications. For improving room-temperature toughness of the Nbss/a Nb5Si3 two-phase alloys, a microstructurecontrol technique has been proposed, which includes the formation and decomposition of Nb3Si phase, i.e., a eutectic reaction (L fi Nbss+Nb3Si) and eutectoid reaction (Nb3Si fi Nbss+a Nb5Si3),[7–9] which is schematically shown in Figure 2 as ‘‘Route 1.’’ Fine a Nb5Si3 lamellar plates are formed through the eutectoid decomposition reaction of Nb3Si, and then spheroidized by extended heat treatment. It is well known that cast iron with spherical graphite phase has many excellent material properties such as tensile strength, toughness, SEIJI MIURA, Associate Professor, and TETSUO MOHRI, Professor, are with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan. Contact e-mail: [email protected] TORU HATABATA and TAKUYA OKAWA, Graduate Students, are with the Graduate School of Engineering, Hokkaido University. Manuscript submitted December 31, 2012. Article published online January 3, 2014 1136—VOLUME 45A, MARCH 2014
fatigue resistance, wear resistance, and others, which are much better than that with flake-like graphite. With a similar microstructure composed of spheroidized a Nb5Si3 strengthening phase embedded in Nbss matrix, i.e., Nbss network, an improvement of toughness is expected. Therefore, the intermetallic phase Nb3Si is the key phase in this microstructure control (Route 1) that was proposed for an alloy with a composition of Nb18.1 at. pct Si-1.5 at. pct Zr-100 ppm Mg previously[7] and applied for other several alloys. For further improvement of mechanical properties, an alternative route for microstructure evolution is pursued. The addition of Mo and W were reported to form typical eutectic microstructures composed of Nbss/a Nb5Si3 or Nbss/b Nb5Si3.[3,10–12] This implies that these elements stabilize Nbss and Nb5Si3 phase (a or b) relative to Nb3Si phase. Nbss/a Nb5Si3 fine eutectic microstructure may turn into spheroidized microstructure because of a larger area of interphase boundaries in eutectic lamellar structure than in spheroidized microstructure. This microstructure evolution may be alternative technique, called ‘‘Route 2’’ shown in Figure 2, for obtaining a similar microstructure with the result of Route 1, which is composed of spheroidized a Nb5Si3 phase
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