Characterization of Microstructure, Texture, and Microtexture in Near-Alpha Titanium Mill Products
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TITANIUM and titanium alloys are used in a variety of aerospace, marine, and chemical industry applications.[1] Many of these applications require the fabrication of components via operations such as closed-die forging, shape extrusion, conventional or high-speed machining (‘‘hogout’’), and superplastic forming. In turn, the microstructure, texture, and hence properties of the finished parts rely heavily on the uniformity and quality of mill products such as billet, plate, and sheet from which they are made. Utilized for a substantial fraction of all aerospace applications, mill products of alpha/beta and near-alpha titanium alloys are usually synthesized via vacuum-arc or cold-hearth melting followed by casting in round (or rectangular) molds having cross-sectional dimensions of the order of 500 to 1000 mm.[2] In the as-cast condition, the macrostructure consists typically of a thin, surface layer (‘‘chill zone’’) of fine, equiaxed beta grains surrounding regions of coarse columnar and equiaxed beta grains, the exact extent of each depending on the size of the ingot and casting conditions.[3,4] The coarse,
A.L. PILCHAK and C.J. SZCZEPANSKI, Materials Research Engineers, and S.L. SEMIATIN, Senior Scientist, are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXCM, Wright Patterson Air Force Base, OH 45433-7817. Contact e-mail: [email protected] J.A. SHAFFER, Materials Research Engineer, and A.A. SALEM, CEO, are with the Materials Resources, LLC, Dayton, OH 45434. Manuscript submitted December 4, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
columnar beta grains are typically multiple millimeters in diameter and length. Following casting, the ingot structure is broken down by a series of thermomechanical (TMP) steps comprising hot working above the beta transus temperature (at which hcp alpha + bcc beta fi beta), initial alpha/ beta working, beta recrystallization, and secondary alpha/beta working. The objective of the first three steps is to generate a uniform, recrystallized structure of equiaxed beta grains, the average size of which is ~1 mm. After the beta recrystallization treatment, titanium billets are usually water quenched, and the metastable high-temperature beta phase decomposes to form a transformation product that may vary from lamellar, colony alpha (at slow-to-moderate cooling rates at the interior) or acicular, possibly martensitic alpha (at rapid cooling rates experienced at and near the surface). Measurable non-uniformity in the microstructure and the texture of the alpha and beta phases may persist following these steps, however, because of heterogeneity in the initial macrostructure, unavoidable radial and axial strain variations associated with the upsetting and cogging operations used during hot working of ingots, and cooling-rate variations during the beta-quench operation.[5] Secondary alpha/beta hot working is imposed to spheroidize the colony/martensitic alpha, thereby producing a microstructure of fine equiaxed (‘‘globular’’) alpha parti
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