Microstructural Evolution during Superplastic Deformation of Titanium Aluminides
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MICROSTRUCTURAL EVOLUTION DURING SUPERPLASTIC DEFORMATION OF TITANIUM ALUMINIDES H.S. YANG, M.G. ZELIN AND A.K. MUKHERJEE Department of Mechanical, Aeronautical and Materials Engineering University of California, Davis, CA 95616, U.S.A. ABSTRACT Microstructural studies have been performed on two superplastically deformed Ti3 Al-based alloys, regular aX2 and super a 2 . Dynamic grain growth, formation of phase stringers, and changes in the apparent volume fraction of the phases occurred in both alloys. The lath-like a2 phase in the super a2 alloy broke up during deformation. All these features are examined from the viewpoint of cooperative grain boundary sliding (CGBS). Cooperative grain boundary migration (CGBM), which is coupled with CGBS, is proposed to be the mechanism for strain induced grain growth and changes in the phase volume fraction. INTRODUCTION The current interest in superplasticity of titanium aluminides based on Ti3 AI stems from their potential for application in advanced aerospace industries due to their attractive high-temperature strength, low density, and good oxidation resistance [1, 21. While the mechanical behavior of two Ti3 Al-based titanium aluminides, Ti-25AI-10Nb3V-1Mo (super a2 alloy) and Ti-24AI-1 1Nb (regular (X2 alloy) during superplastic deformation has been relatively well-understood [3-6], microstructural evolution due to superplastic strain has not been thoroughly studied [6]. Ti 3Al alloys usually have a duplex a2 (ordered DO19 structure) + 03microstructure within the temperature regime of potential engineering applications [7, 8]. Upon cooling after superplastic deformation ( a2+ 0 region), the [3 phase transforms to (X2 and the untransformed part of 1phase in the case of intermediate to fast cooling rates will revert to the ordered structure, B2 [8]. Such basic knowledge of both microstructures and transformation kinetics is helpful in understanding the microstructural evolution of the Ti3AI alloys during superplastic deformation. Typical features of microstructural changes during superplastic deformation of common metallic materials are: (1) strain enhanced grain growth; (2) equiaxed grain structure even after very high superplastic strain; and (3) breaking-up of a lath-like phase [9, 10]. The focus of this paper is to address whether these microstructural features are also common for superplastic intermetallics such as Ti3AI. EXPERIMENTAL DETAILS Two Ti3 Al alloys, regular a2 (Ti-24Al-11 Nb) and super a2 (Ti-25AI-0ONb-3VIMo) were used in this study. The regular a2 and super a2 alloys were supplied by Rockwell International Science Center in the form of sheets 1.25 mm and 1 mm thick, respectively. The details of high temperature tensile testing and a special etching technique for microstructural studies of Ti3 AI are given in a previous paper [6]. The tested specimens generally exhibited significant necking. The local strains, 2ln(to/t) (where to and t are the initial and final local thickness, respectively), along the gauge were measured and related to the corresponding mic
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