Effect of Microstructure and Zr Addition on the Crystallographic Orientation Relationships among Phases related to the E
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Effect of Microstructure and Zr Addition on the Crystallographic Orientation Relationships among Phases related to the Eutectoid Decomposition of Nb3Si in near Eutectic Nb-Si alloy Seiji Miura†, Kenji Ohkubo and Tetsuo Mohri Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan †
) corresponding author. Tel. & Fax : +81-11-706-6347
E-mail address: [email protected] (S. Miura)
ABSTRACT The authors have reported in the previous study that the sluggish decomposition of Nb3Si phase is effectively accelerated by Zr addition [1]. This is obvious at lower temperature range than the nose temperature of the TTT curve. In the present study a eutectic alloy containing 1.5 % of Zr was investigated. The crystallographic orientation relationships among phases, such as eutectic Nb and product phases formed by eutectoid decomposition of Nb3Si (eutectoid Nb and Nb5Si3 phases) in the Zr-containing sample which was heat treated at 1300oC were investigated by FESEM/EBSD for further understanding of the decomposition process in alloy with a different microstructure.
INTRODUCTION Recently many efforts have been dedicated to the investigation of alloys based on Nb-silicides because they exhibit superior high temperature strength than the commercial nickel base superalloys [2-4]. Since the lack of room temperature ductility and high temperature oxidation resistance have been the major drawbacks for further development, various additives and controlled microstructures have been introduced to overcome these problems. For further improvement of the toughness of these alloys, advanced microstructure control is needed by which the alloys based on brittle intermetallic compounds are endowed with ductile phase toughening. The Nb/Nb5Si3 two-phase structure forms a fine microstructure through the eutectoid decomposition from Nb3Si, but the kinetics of the reaction in the binary alloy was reported to be so sluggish that it takes more than 100 hr to finish the decomposition even at the
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nose temperature of about 1500 oC [2]. This slow kinetics may be caused by its high melting point, and is, in turn, an advantage for high temperature applications in a practical viewpoint. Sekido et al. found that addition of the 10 at.% of Ti increases the decomposition rate of the Nb3Si [5-9]. Then, the effect of Zr on the eutectoid decomposition reaction was investigated by present authors [1, 10], and it was revealed that the kinetics of the eutectoid decomposition is enhanced by Zr additions. The time-temperature transformation (TTT) diagram for the decomposition was also experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at.% was confirmed by comparing with the reported TTT curves of binary and ternary alloys containing Ti. Although the role of the ternary elements on the decomposition kinetics is still not fully understood, it was found both crystallographic orientation relationships among phases a
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