SEM/EBSD Analysis on Globularization Behavior of Lamellar Microstructure in Ti-6Al-4V During Hot Deformation and Anneali
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TRODUCTION
TITANIUM and titanium alloys are used extensively in the aerospace industry because of their light-weight and high-strength properties. a/b titanium alloy Ti-6Al-4V is one of the most popular titanium alloys for use in such applications.[1] Mechanical properties of the alloy depend strongly on its microstructure. Thermomechanical treatment is one of the most important processes to control the a-phase morphology to achieve favorable mechanical properties. In general, forging on solidified ingots is conducted in the b-phase region to refine coarse and inhomogeneous b grains that are
YOSHINORI ITO and SHOGO MURAKAMI are with the Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan. Contact e-mail: [email protected] NOBUHIRO TSUJI is with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted December 5, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
derived from solidification. During cooling after forging or after heat treatment in the b-phase region, an a phase forms. The morphology of the a phase depends on the cooling rate. A slow cooling rate in the case of billets with large dimensions leads to the formation of colony-type microstructures where each colony consists of a lamellae of the same crystallographic orientation that are aligned in the same direction. The a lamellar plates are typically broken in subsequent deformation and heat treatment processes in the a + b region to form microstructures that comprise globular a to achieve a better strength and ductility balance.[2] Because of its industrial importance, considerable studies have been conducted on the globularization behavior of lamellar microstructures of a during hot deformation and subsequent annealing in the Ti-6Al-4V alloy.[3–12] Weiss et al.[3,4] investigated the influence of the initial lamellar thickness on the globularization behavior during hot deformation and subsequent annealing and showed that equiaxed a grains were formed more rapidly in material with thinner a plates. A mechanism of globularization of a lamellae was also
proposed. a/a boundaries across the a lamellae evolve through localized shear banding, recovery, and/or recrystallization during hot deformation followed by annealing. Penetration of the b phase along the a/a boundaries, which is termed ‘‘boundary splitting’’, occurs and results in the segmentation of a lamellae. Semiatin et al.[5] and Shell and Semiatin[6] investigated dynamic globularization by hot compression and demonstrated that an equivalent strain of ~2.5 was necessary to obtain a fully globularized a structure. Stefansson et al.[7] and Stefansson and Semiatin[8] studied static globularization behavior during annealing after hot deformation and demonstrated that the kinetics of static globularization depended on the amount of applied strain and the annealing temperature but was independent of deformation temperature from 1173 K (900 C) to 1223 K (950 C). It has been shown that globularization was characterized by a s
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