Effect of Texture and Slip Mode on the Anisotropy of Plastic Flow and Flow Softening during Hot Working of Ti-6Al-4V S.L. SEMIATIN and T.R. BIELER The effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was determined by conducting isothermal, constant-strain- rate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transverse direction (TD), 45 deg, and normal) from hot-rolled plate. Testing was performed using a fixed strain rate (0.1 s -1 ) and various temperatures below the beta transus. The flow curves from all of the experiments exhibited a peak flow stress followed by a large and a small amount of flow softening for the colony and globular microstructures, respectively. Although the flow softening response did not depend noticeably on test direction for a given microstructure and test temperature, the peak flow stress and development of sample ovality did. This orientation dependence was interpreted using both lower-bound (isostress-type) and upper-bound (isostrain, Taylor/Bishop–Hill) models to deduce the operative slip systems in the alpha phase. These analyses suggested that prism a and basal a slip are considerably easier than pyramidal c + a or a slip at hot-working temperatures. A comparison of the flow curves for the colony and globular alpha microstructures suggested that slip transfer across alpha/beta interfaces and loss of Hall–Petch boundary strengthening can account for a substantial portion of the flow softening observed during hot working. I. INTRODUCTION inhomogeneities and lattice rotations. Although most crystal- plasticity FEM analyses have been conducted for single- THE modeling of metal deformation processes has phase metals, a limited amount of work has also been per- undergone remarkable advances during the last two decades. formed for the two-dimensional simulation of the cold defor- For example, during the 1980s, finite element method (FEM) mation of lamellar-colony microstructures of near-gamma codes were developed and validated for the prediction of titanium aluminide alloys. [4] Unfortunately, the application metal flow and defect formation during forging, extrusion, of crystal-plasticity FEM analyses to other two-phase sys- tems has been limited because of a lack of understanding etc. [1] Input properties for these analyses usually included of the physical details controlling deformation. flow-stress data generated from workability tests such as the One two-phase system for which a crystal-plasticity FEM uniaxial compression of cylinders. The influence of crystal- analysis would be particularly useful is that based on the lographic texture/texture evolution and flow inhomogeneit- alpha and beta phases of titanium and includes industrially ies on a microscopic (grain-size) scale were typically important alloys such as Ti-6Al-4V. These materials are overlooked in the interpretation and application of such flow typically produced by an ingot-metallurgy route, which stress measurements in FEM process models. Moreover, the includes a hot-working step below the beta transus (tempera- application of classical FEM techniques for the prediction ture below which beta → alpha + beta) to break down the of microstructure evolution was and has been limited essen- colony-alpha microstructure produced by prior beta working tially to spatial “averages” deduced by relating predicted and heat treatment. [5] During this breakdown step, alpha and strains, strain rates, and temperatures in the complex defor- beta lamellae undergo deformation which is nonuniform mation process to similar quantities (and thus resultant from one colony to another as well as within individual microstructures) from simulative workability tests. colonies. At large deformations (or during heat treatment Significant additional progress in the modeling of defor- following deformation), the colonies globularize. [6] The tex- mation processes is now being pursued through the incorpo- ture and microtexture that evolve are thus dependent on the ration of grain-scale plasticity effects in FEM analyses. [2,3] In deformation nonuniformity, the mechanism of globulariza- these approaches, a crystallographic-orientation dependent tion itself, and the rotations of alpha grains following globu- constitutive relation is used to describe local flow behavior. larization. In turn, each of these processes is affected by Each grain within a simulation aggregate is divided into crystallographic slip processes, an understanding of which elements, thus enabling the modeling of local deformation is necessary for crystal-plasticity FEM, regardless of whether the material undergoes a morphology change (such as the lamellar colony to equiaxed one that occurs during thermo- mechanical processing of alpha/beta titanium alloys). S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Sci- ence, is with Air Force Research Laboratory, Materials and Manufacturing The present article describes results from a long-term Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433- program whose ultimate goal is to develop a crystal-plastic- 7817. T.R. BIELER, Associate Professor, is with the Department of Materi- ity FEM analysis for the hot working and breakdown of als Science and Mechanics, Michigan State University, East Lansing, MI alpha/beta titanium alloys with colony microstructures. The 48824-1226. Manuscript submitted August 16, 2000. specific objective of the work reported herein was to assess METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 32A, JULY 2001—1787 NOT PROTECTED BY U.S. COPYRIGHT