Microstructure and creep behavior of an orthorhombic Ti-25Al-17Nb-1Mo alloy
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I.
INTRODUCTION
THE development of alloys based on the intermetallic Ti3Al has been driven by the need to bridge the gap in temperature capability between current conventional neara titanium alloys and nickel-based superalloys such as INCO* 718 or INCO 713. It is now established that plas*INCO is a trademark of INCO Alloys International, Inc., Huntington, WV.
ticity and toughness at room temperature can be improved by the addition of enough b stabilizers (such as Nb, V, Mo, etc.) to stabilize the high-temperature b/B2 phase (b and B2 are, respectively, the disordered and ordered bcc phases) and, possibly, to form the O phase (ordered orthorhombic structure based on Ti2AlNb) in the microstructure.[1,2] The alloys based on Ti3Al can be categorized into two distinct classes, according to their Nb contents. Alloys containing an amount of Nb lower than about 12 at. pct have a twophase microstructure of a2 (ordered hcp phase based on Ti3Al) and b/B2. The alloys in the other class contain 14 to 30 at. pct Nb, in which the O phase can exist as an equilibrium phase. Thus, the alloys in this class are called orthorhombic alloys, or O phase alloys. It has been preliminarily demonstrated that the mechanical properties of orthorhombic alloys can be superior to those of the alloys containing lower Nb contents.[1,2,3] However, it appears that there are complex phase equilibria and microstructural evolution in the orthorhombic alloys containing slightly lower than 22 at. pct Nb.[1,2,4] In these alloys, a2 phase can still exist, and a microstructure with three or two or single phases can be obtained, depending on the heat treatment. In addition, the volume fraction, size, and morphology of
J.W. ZHANG, on leave from the Department of Superalloy, Central Iron and Steel Research Institute, is Senior Engineer with the Department of Physics and Materials Science, City University of Hong Kong. C.S. LEE, Assistant Professor, and J.K.L. LAI, Chair Professor, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong. D.X. ZOU and S.Q. LI, Professors, are with the Department of Superalloy, Central Iron and Steel Research Institute, Beijing 100081, China. Manuscript submitted April 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
these constituent phases also sensitively depend on the thermal processing and the heat-treatment history.[2,5] Thus, further exploration of this topic is needed. The mechanical properties, especially creep properties, of the Ti3Al-based alloys can be effectively tailored by means of microstructural control.[6–10] It is expected that the more varied microstructures of the Ti3Al alloys containing 14 to 30 at. pct Nb can result in a wider range of mechanical properties. Although creep behavior of orthorhombic alloys in certain specific compositions has been reported,[3,11,12] relatively little work has been done to systematically study the creep behavior, and its relationship to microstructure, of orthorhombic alloys, especially those consisting of a2, B2, and O
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