Alpha/Beta Heat Treatment of a Titanium Alloy with a Nonuniform Microstructure
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TRODUCTION
THE conversion of large ingots of alpha/beta titanium alloys into wrought mill products is very challenging, typically comprising numerous hot working and heat-treatment steps.[1] Following casting, initial thermomechanical processing (TMP) is usually conducted in the high-temperature, beta phase field in order to produce a recrystallized beta-grain microstructure. The objective of subsequent TMP below the beta transus (temperature below which beta fi alpha + beta) is to break down the colony-alpha structure (developed during cooling following beta processing) into a fine and uniform equiaxed-alpha morphology. Such spheroidization (or globularization, as it is known in the industry) occurs during both deformation and intermediate or final heat treatment. Previous research has shown that dynamic spheroidization (i.e., that during deformation) requires strains in excess of those that can be readily imposed for commercial products.[2] Thus, static spheroidization (during subsequent heat treatment), in which remnant alpha lamellae undergo boundary splitting and termination migration,[3,4,5] is very important to obtain a uniform, wrought, final microstructure. S.L. SEMIATIN, Senior Scientist, and J.D. MILLER, Materials Research Engineer are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, WrightPatterson Air Force Base, Ohio, OH 45433, USA. Contact e-mail: [email protected] T.M. LEHNER, Undergraduate Student, is with the Department of Mechanical Engineering, University of Dayton, Dayton, OH 45409, USA. R.D. DOHERTY, A.W. Grosvenor Professor of Materials Engineering, is with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA. D.U. FURRER, Senior Staff Specialist, is with the Rolls-Royce Corporation, Indianapolis, IN 46206, USA. Manuscript submitted August 8, 2006. Article published online April 20, 2007. 910—VOLUME 38A, APRIL 2007
The time required to complete the static spheroidization of a remnant alpha platelet of diameter da and thickness ta has been shown to depend on its aspect ratio (da/ta), ta3 , and 1/Db, in which Db denotes the diffusivity through the beta matrix of the rate-limiting solute.[5] For a given platelet aspect ratio and thickness, therefore, it can be concluded that alloys with slow-diffusing solutes may require substantially longer (and commercially unfeasible) times to complete spheroidization during heat treatment. In such instances, remnant alpha platelets, known as spaghetti alpha, may be retained in mill products and persist during secondary processes such as part forging, plate rolling, and final alpha/beta heat treatment. Depending on the thickness of the lamellae and the scale of the remnant colonies, strength, ductility, and fatigue performance may be adversely affected.[6,7,8] The objective of the present work was twofold: first, to determine how partially spheroidized microstructures evolve during final alpha/beta heat treatment and thus to establish the impact of remnant lamellae or lamell
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