Acta Materialia 51 (2003) 5613–5626 www.actamat-journals.com Constitutive analysis of the high-temperature deformation of Ti–6Al–4V with a transformed microstructure Jeoung Han Kim a , S.L. Semiatin b , Chong Soo Lee a,* a Center for Advanced Aerospace Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea b Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433-7817, USA Received 23 April 2003; received in revised form 22 July 2003; accepted 22 July 2003 Abstract The high-temperature deformation mechanisms of Ti–6Al–4V with a transformed microstructure were determined within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on samples with a lamellar structure containing different alpha-platelet thicknesses at temperatures of 715–900 °C. The flow stress- versus-strain rate curves for all the microstructures were well fit with an inelastic-deformation equation describing grain-matrix deformation (GMD) (dislocation glide + dislocation climb). However, for heavily pre-deformed specimens, grain-boundary sliding (GBS) as well as GMD was evident. The GBS rate was found to be most rapid for the microstruc- ture with the thinnest alpha laths/platelets. Softening of heavily deformed material was attributed to a decrease in the internal-strength variable s * associated with reduced alpha–beta interface strength and to the occurrence of GBS.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ti–6Al–4V; Internal-variable theory; Deformation mechanisms; Dynamic globularization 1. Introduction High-temperature forging and extrusion are widely applied in the aerospace industry. To pro- duce defect-free and homogeneous structural parts, it is essential not only to find optimum processing conditions, but also to have a clear understanding of material behavior at high temperature. During primary hot working of Ti–6Al–4V, for example, * Corresponding author. Tel.: +82-54-2792141; fax: +82- 54-2792399. E-mail address: cslee@postech.ac.kr (C.S. Lee). 1359-6454/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6454(03)00426-9 the conversion of the as-cast coarse-grain (“transformed”) microstructure to a fine, equiaxed two-phase microstructure (known in the titanium industry as globularization) is an important techno- logical process that leads to a highly formable material suitable for secondary hot-working pro- cesses such as forging and superplastic forming [1]. The process of breaking down the coarse-grain microstructure has been the focus of much research [2–6]. A number of efforts have also focused on consti- tutive modeling and the analysis of plastic flow at high temperature for various metals [7–10] as well as Ti–6Al–4V [11–14]. Recently, Semiatin and