The Kinetics of Primary Alpha Plate Growth in Titanium Alloys
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TITANIUM alloys are widely used in aerospace gas turbines owing to their unrivaled specific fatigue-allowable strengths.[1,2] In the cold sections of the engine, e.g. the fan, Ti-6Al-4V is used in large forgings (disks) and plate and bar (blades), primarily owing to its relatively low density and ease of forging and machinability.[3] In the highest temperature sections of the compressor, near-a alloys are used due to their favorable creep performance, as the solute diffusivity is the lowest in the hexagonal close-packed alpha phase.[4,5] Often, strongly b stabilized a þ b alloys such as Ti-17 and
ABIGAIL K. ACKERMAN, ALEXANDER J. KNOWLES, IOANNIS BANTOUNAS, and DAVID DYE are with the Department of Materials, Royal School of Mines, Imperial College London, Prince Consort Road, London, SW7 2BP, UK. Contact e-mail: [email protected] HAZEL M. GARDNER, ANDRE´ A.N. NE´METH, MICHAEL P. MOODY, PAUL A.J. BAGOT, and ROGER C. REED are with the Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK. ANNA RADECKA and DAVID RUGG are with the Rolls-Royce plc., Elton Road, Derby, DE24 8BJ, UK. Manuscript submitted January 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Ti-6Al-2Sn-4Zr-6Mo are also used at intermediate temperatures in the compressor, since they retain their strengths to higher temperatures than Ti-6Al-4V.[2] However, their elevated Mo content reduces forgeability and machinability and increases the density. Nevertheless, Mo provides the opportunity to slow down the kinetics of formation of the a phase,[6] enabling basketweave multivariant primary a microstructures to be attained with a high fraction of secondary as, which result in a very strong, fatigue resistant alloy.[7] In the scheme of Ti-6246 processing, the cooling rate from the b can be controlled in order to ensure that the primary a, which nucleates at the prior b grain boundaries, fills the prior b grains. This is then followed by an aging heat treatment to precipitate out the secondary a from solution around the finest possible plate thickness. These three parameters—the cooling rate from the b, the secondary aging temperature, and aging time—enable the desired strength–toughness balance to be chosen according to the application. However, a process model of the primary a growth has not been presented, and therefore such optimization has historically been performed empirically.[1] Classical quasi-analytic models for a precipitation have been developed by Semiatin et al.[8] They used the
Ivantsov solution[9] for parabolic growth in an infinite medium, which was developed for the cases of 2D paraboloids by Horvay and Cahn,[10] taking the spherical solution to examine globular precipitate growth during the globularization heat treatment step in Ti-6Al-4V.[11,12] They found that the growth of globular primary a is well defined by a diffusion-controlled process, based on diffusion of aluminum and vanadium. However, the constant radius solution as outlined in their study underestimates the growth rate of primary a. The model c
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