Mechanisms and Kinetics of Static Spheroidization of Hot-Worked Ti-6Al-2Sn-4Zr-2Mo-0.1Si with a Lamellar Microstructure
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TWO-phase (alpha/beta) titanium alloys are used widely in the aerospace industry because of their high specific strength, good corrosion resistance, and excellent high-temperature properties.[1,2] The most common method of producing semifinished titanium mill products comprises ingot melting and solidification followed by a series of hot-working and heat-treatment steps, each of which has a specific microstructural target. Usually, ingot breakdown is conducted above the beta transus temperature (at which alpha + beta fi beta) to produce a homogeneous, recrystallized beta-grain structure. On cooling, a transformed microstructure comprising alpha laths/platelets with a high aspect ratio (AR) is formed within each beta grain. The thickness of the transformation product is influenced by the cooling rate[3]; high cooling rates yield fine martensitic alpha, CHAN HEE PARK, Senior Researcher, is with the Advanced Materials Research & Implementation Center, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea. JONG WOO WON, Graduate Student, and CHONG SOO LEE, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] JIN-WOO PARK, Professor, is with the Department of Periodontology, School of Dentistry, Kyungpook National University, Daegu 700-412, Republic of Korea. S.L. SEMIATIN, Senior Scientist, is with the Air Force Research Laboratory, AFRL/RXLM, Wright-Patterson AFB, OH 45433-7817. Manuscript submitted March 15, 2011. Article published online November 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
whereas slow rates lead to coarse lamellar (colony) alpha. Subsequent thermomechanical processing steps in the two-phase field are then used to spheroidize the alpha in whole or in part during deformation[4–6] or during subsequent annealing.[7,8] The equiaxed (‘‘globular’’) alpha microstructure so produced provides excellent room-temperature strength and ductility[9] and elevated-temperature superplasticity.[10,11] For production-scale billets with a large cross section, water quenching after hot working and heat treatment in the beta phase field leads to relatively slow cooling rates at the center, thereby giving rise to a coarse colonyalpha microstructure. In this case, strains much greater than those typically imposed commercially (i.e., ~2.5 to 3.0) are required for dynamic spheroidization, thereby necessitating subsequent static heat treatment to obtain a fully equiaxed microstructure.[4,12,13] Thus, the determination of the optimum annealing temperature and annealing time to complete static spheroidization while minimizing static coarsening is important especially with regard to the superplastic-forming characteristics and mechanical properties of the final product. Previous work has shown that spheroidization during annealing after hot working of the common alpha/beta titanium alloy Ti-6Al-4V (Ti64) proceeds by two main mechanisms: boundary splitting at short times
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