Slip Transfer across Hetero-Interfaces in two-phase Titanium Aluminum Intermetallics
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Slip Transfer across Hetero-Interfaces in two-phase Titanium Aluminum Intermetallics Jörg M.K. Wiezorek1, Xiao-Dong Zhang2, Hamish. L. Fraser3 Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA; 2 Center for Materials Science, Bose Corporation, Framingham, MA; 3 Department of Materials Science & Engineering, The Ohio State University, Columbus, OH; 1
ABSTRACT The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations. INTRODUCTION Two-phase TiAl based intermetallics with slightly Ti-rich composition, e.g. Ti-48at.%Al, with microstructures containing large volume fractions of morphologically lamellar grains are promising candidate materials for applications in advanced gas-turbine-engines [1]. However, the widespread use of these alloys is hindered by the limited tensile elongation to failure and problems with cracking during metallurgical processing by forging for instance. Hence, it is important to develop a detailed understanding of the deformation behavior of the constituent phases, γ-TiAl and α2-Ti3Al, and the interplay between them during plastic straining. The basic deformation accommodating processes in the two phases, i.e. relevant dislocation glide and mechanical twinning, are understood reasonably well [1, 2-8]. However, the synergistic interplay between the deformation processes active in the γ- and α2-phases in lamellar TiAl requires further attention, since for general plasticity of the technologically important polycrystalline materials both phases must deform compatibly across the heterophase interfaces. The lamellar grains consist mainly of lamellae of the ordered tetragonal γ-TiAl phase with a small volume fraction of lamellae of the ordered hexagonal α2-Ti3Al phase, which obey a crystallographic orientation relationship, such that [1-10]γ//[11-20]α2 and (111)γ//(0001)α2 [2]. For the γ-lamellae six orientation variants are related by multiple 60˚-rotations about the common lamellar interface normal [111] and coexist with the α2-lamellae, which form crystallographically equivalent heterophase interfaces with each of the γ-phase variant lamellae [2]. The anisotropic mechanical properties associated with the lamellar grains are very well documented in the literature and have been s
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