Heat-Treatment Effects on the Microstructure and Tensile Properties of Powder Metallurgy Ti-6Al-4V Alloys Modified with
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TITANIUM (Ti) alloys continue to be vital for aerospace applications, including many engine and airframe components, due to their attractive mechanical and physical property combinations. These alloys are currently utilized to achieve goals of weight reduction, enhanced performance, and elevated service temperature. The drive for higher structural efficiency and affordability in aircraft technology has motivated the development of advanced Ti alloys that provide further improvements. A number of phase additions such as SiC, TiC, TiN, and TiB have been considered to improve the mechanical properties of conventional Ti alloys. The most attractive balance of thermal, physical, and mechanical characteristics is offered by the TiB phase that precipitates in situ due to the addition of boron.[1] Unlike other interstitial elements such as carbon and hydrogen, boron is essentially insoluble in Ti[2] and its alloys,[3] and therefore does not cause lattice embrittlement. The addition of boron to conventional Ti DALE J. MCELDOWNEY, formerly Graduate Student, with the Department of Chemical and Materials Engineering, University of Dayton, Dayton, OH 45469, is Lead Materials Engineer, with Emerson Technologies, Sydney, OH 45365. SESHACHARYULU TAMIRISAKANDALA, Senior Research Scientist, is with FMW Composite Systems, Inc., Bridgeport, WV 26330. Contact e-mail: [email protected] DANIEL B. MIRACLE, Senior Scientist, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, WrightPatterson Air Force Base (AFB), Dayton, OH 45433. Manuscript submitted October 2, 2008. Article published online January 23, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys such as Ti-6Al-4V (Ti-64; all compositions are given in weight percent) results in the precipitation of TiB whiskers via a eutectic reaction. Addition of boron below the eutectic concentration (Ti-2B for Ti[4] and Ti-64-1.55B for Ti-64[5]) avoids the formation of coarse, brittle primary TiB particles and generally produces whiskers 1 to 2 lm in diameter and an aspect ratio in the range 5 to 20. These TiB whiskers provide improvements in strength and stiffness and retain ductility roughly equivalent to the unmodified Ti-64.[6] A critical characteristic of Ti alloys is the ability to tailor microstructural features via heat treatment to obtain a variety of property combinations. Typical heat treatments[7] used for Ti-64 are listed in Table I. Various objectives, such as relieving the residual stresses developed during fabrication processes, producing optimum combinations of ductility-machinability and dimensional and structural stability, increasing strength, and optimizing special properties such as fracture toughness, fatigue strength, and creep strength, can be achieved using well-established heat treatments. Microstructural parameters that can be altered in a-b alloys such as Ti-64 by heat treatment include the a grain size and morphology (lamellar vs equiaxed), a-b colony width and lamellae thickness, prior-b grain size,
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