The kinetics of static globularization of Ti-6Al-4V
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THE lamellar microstructure associated with as-cast or beta-processed alpha/beta titanium alloys has been found to break up or globularize both dynamically during deformation and statically during postdeformation annealing in the alpha/beta phase field. The effects of various deformation parameters on dynamic globularization of the alpha phase have been investigated extensively.[1–5] For example, it has been shown that large strains of the order of 2 to 2.5 are required to obtain a fully globular-alpha structure during forging. Strain levels of this magnitude are usually not encountered in conventional fabrication processes, thus making static globularization during annealing an essential step in obtaining a lower-aspect-ratio alpha-grain structure. However, information relating processing parameters to the globularization process during postdeformation annealing is extremely fragmented and somewhat qualitative.[6–9] Despite the significance of the kinetics and mechanisms of the static globularization of the alpha phase, systematic or comprehensive studies relating temperature, time, and deformation to the overall globularization process are lacking. The objective of this work, therefore, was to provide quantitative relations to describe the microstructure changes that take place during annealing following deformation in the alpha/beta phase field. This information can be used to enhance current mill practices by providing accurate microstructure or morphology predictions when used concurrently with finite-element modeling of the deformation process.
N. STEFANSSON, formerly Visiting Scientist, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, is Research Engineer, Materials and Technology Group, Structurals Division, Precision Cast Parts, Portland, OR 97206-0898. Contact e-mail: [email protected] S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433-7817. D. EYLON, Professor, is with the Graduate Materials Engineering Department, University of Dayton, Dayton, OH 45431-0240. Manuscript submitted November 13, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. MATERIAL AND EXPERIMENTAL PROCEDURES A. Material The material used in this work was the alpha/beta titanium alloy Ti-6Al-4V; it was provided by TIMET Corp. (Henderson, NV). The nominal beta transus of this alloy was approximately 995 ⬚C, and its measured composition (in weight percent) was 6.51 aluminum, 4.21 vanadium, 0.18 oxygen, 0.012 carbon, 0.007 nitrogen, and 0.002 hydrogen. The starting ingot material was hot worked in the beta phase field followed by a degree of subtransus work and beta heat treatment to obtain a recystallized beta grain structure. After recrystallization, the material was subjected to a small amount of beta work prior to water quenching to obtain a transformed (colony-alpha) microstructure. The microstructure of the as-received material was
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