Interphase boundary structures associated with diffusional phase transformations in Ti-base alloys
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INTRODUCTION
A L T H O U G H there seems to be no reason at the present time to consider that interphase boundary structures generated by diffusional phase transformations in Ti-base alloys differ significantly from their counterparts in A1base, Cu-base, and other substitutional alloy systems, particularly when the pair of crystal structures and orientation relationships between them are the same, there are, nonetheless, at least two major reasons for preparing a review of these structures. One is that they have proved to be considerably more amenable to detailed transmission electron microscopy (TEM) studies than equivalent interfaces in many other alloy systems. A second is that many commercially important alloys based upon titanium contain two or more phases, with the hcp a and the bcc fl phases forming the most frequently occurring interphase boundaries. There can be little doubt that these boundaries significantly influence the mechanical properties of Ti-base alloys. The present review will deal mainly with a:fl boundaries formed during the proeutectoid a reaction in Ti-X alloys. The interphase boundaries developed during the bainite reaction in Ti-X a l l o y s - - d e f i n e d by the generalized microstructural definition as a nonlamellar, T. FURUHARA, Graduate Student, and H.I. AARONSON, R.F. Mehl Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. H.J. LEE is with the Pressure and Vacuum Laboratory, Korea Standards Research Institute, P.O. Box 3, Taedok Science Town, Taejon, Chungnam 300-651, Republic of Korea. E.S.K. MENON is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. This paper is based on a presentation made in the symposium "Interfaces and Surfaces of Titanium Materials" presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25-29, 1988, under the auspices of the TMS Titanium Committee. METALLURGICAL TRANSACTIONS A
non-cooperative, and competitive mode of eutectoid decompositiont~]--will also be considered. Finally, the interphase boundary structures associated with 3' titanium hydride will be examined as an example of such structures in interstitial alloys. Inasmuch as this review is confined, by the authors' choice, to the products of diffusional phase transformations, consideration of interphase boundaries produced during martensitic and omega phase formation will be foregone. However, since the literature on the topics to be covered is still relatively sparse, results of parallel studies on the proeutectoid a and the 3, hydride reactions in Zr-base alloys will also be utilized in order to broaden the base of available information. This paper first defines the three principal types of interphase boundary structure and then examines the mechanisms through which each migrates during diffusional growth. Next, a brief general summary will be given of interphase boundary structures on intragranular precipitates, followed by a coun
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