Microscopic and Quantitative Investigations on PST Ti-Al / Ti Reaction Diffusion Couples
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Microscopic and Quantitative Investigations on PST Ti-Al / Ti Reaction Diffusion Couples Ling Pan, David E. Luzzi Department of Materials Science and Engineering, University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104, U.S.A. ABSTRACT Interdiffusion in multi-phase diffusion couples of polycrystalline Ti and polysynthetically twinned (PST) Ti-49.3 at.% Al, with the diffusion direction parallel to the lamellar planes, is investigated in the temperature range 973 – 1173 K. A reaction zone (RZ) of the α2-Ti3Al phase forms between the end materials and exhibits deeper penetration in the α2 lamellae than in the primary γ lamellae. The mass balance and the lamellar thickness across the RZ / PST interface are believed to be the major factors that lead to the different behaviors in the penetration depth of the RZ. Direct measurements of the RZ thickness reveal a parabolic growth of the RZ, indicating a diffusion-controlled growth macroscopically. Concentration profiles from the Ti, through the RZ, into the PST γ and α2 lamellae are measured by x-ray spectroscopy in a transmission electron microscope. Deviations from a diffusion-controlled composition profile indicate some extent of interface-controlled growth. Plateaus are seen in the concentration profiles in the RZ adjacent to the RZ/PST interface, extending through most of the deeply penetrated well region. The interfacial energy and strain energy are possible reasons for the plateaus. The interdiffusion coefficients are found to be largely independent of composition with a temperature dependence that obeys the Arrhenius relationship. INTRODUCTION Titanium aluminide based intermetallic alloys have great potential in high temperature structural applications in aerospace and automotive industries [1-2]. Mechanically superior alloys of this system usually have a two-phase lamellar structure of γ-TiAl and α2-Ti3Al. In the past decade, special interest has been paid to so-called polysynthetically twinned (PST) Ti-Al, composed of alternate lamellae of γ-TiAl and α2-Ti3Al with the orientation relationship {111}γ //(0001)α 2 and 1 1 0 // 11 20 [3]. Diffusion plays an important role in the formation γ
α2
and high-temperature stability of the lamellar structure. Furthermore, diffusion is an essential determinant of some mechanical properties, such as creep resistance. Therefore, a fundamental understanding of the diffusion mechanisms in PST Ti-Al alloys is of great importance for the development of titanium aluminide alloys. There has been a large amount of tracer diffusion data available in Ti-Al system [4-8]. However, tracer diffusion experiments measure the average layered concentration profile. The technique cannot distinguish the effects of each individual phase and of the various types of interfaces in a PST crystal on diffusion. Little has been known about the fundamental correlation between the microstructure and diffusion properties, especially in reactive interdiffusion cases. In the present paper, we present the microscopic and quantitative resu
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