Stress Driven Phase Transformations and Recrystallization Processes in Two-Phase Titanium Aluminide Alloys
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Stress Driven Phase Transformations and Recrystallization Processes in Two-Phase Titanium Aluminide Alloys Fritz Appel and Michael Oehring Institute for Materials Research, GKSS Research Centre, Max-Planck-Str., D-21502 Geesthacht, GERMANY ABSTRACT The paper presents an electron microscope study of phase transformation and recrystallization in an intermetallic α2(Ti3Al) + γ(TiAl) titanium aluminide alloy, after long-term creep. The mechanisms are closely related to the atomic structure of the α2/γ phase boundaries and are probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α2 phase. The α2 → γ transformation is accompanied by the formation of precipitates, because the γ(TiAl) phase has a significantly lower solubility for interstitial impurities than the α2(Ti3Al) phase. 1. INTRODUCTION Titanium aluminides based on the intermetallic phases γ(TiAl) and α2(Ti3Al) are one of the few classes of emerging materials that have the potential for innovative applications in advanced energy conversion systems whenever low density and good high temperature strength are of major concern. Equally important for such application, almost by definition, is the need for good structural stability and creep resistance. As with other mechanical properties the creep behaviour of two-phase alloys is sensitive to the alloy composition and microstructure. Much emphasis has been placed on describing the temperature and stress dependence of the creep rate in terms of the Dorn equation [1, 2], however, our understanding of the micromechanisms behind the creep phenomena is still rudimentary. This holds particularly for stress induced structural changes due to phase transformations and recrystallization, which have been recognized in several studies [1, 3-5]. For an alloy design towards improved creep strength and structural stability it is necessary to understand the atomic details of these processes. The intention of the present paper is to provide such information by transmission electron microscope (TEM) examination of the defect character of lamellar interfaces in a two-phase alloy, which during long-term creep had undergone substantial structural changes. The alloy investigated had the composition (in at.%) Ti-48Al-2Cr and a duplex microstructure with a high volume content of lamellar colonies and, thus, might be considered as a model alloy for engineering alloys presently under consideration for technical applications. Tensile samples of this alloy were subjected to long-term creep loadings at T = 700 °C and applied stresses σa = 80 – 140 MPa, which lasted for creep times t = 6.000 to 14.000 hours [4, 5]. These conditions led to very low creep rates, which are characteristic of the intended service conditions of TiAl alloys and certainly conducive to the initiation of diffusion assisted structural changes. Among the various microstructures that can be established in two-phase alloys, the fullylamellar microstructure exhibits the best creep resistance, typically reducing strain rates at given test
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