A Mechanism of Polycrystallization in Fully Lamellar Ti-48Al-8Nb Single Crystal Alloy Aged at Elevated Temperatures
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A Mechanism of Polycrystallization in Fully Lamellar Ti-48Al-8Nb Single Crystal Alloy Aged at Elevated Temperatures Yukinori Yamamoto, Masao Takeyama and Takashi Matsuo Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-2 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT Polycrystallization mechanism of a fully lamellar microstructure during aging at 1473 and 1273 K has been examined using Ti-48Al-8Nb fully lamellar single crystal, which consists mostly of γ/γ interfaces (variant, perfect-twin and pseudo-twin boundaries). After a certain period of aging, a few γ grains are formed within the lamellae and the lamellar microstructure collapses rapidly to become a γ grained microstructure at both temperatures. An EBSP analysis for aged sample revealed that most of the grains follow the orientation of variant domains in the lamellar microstructure. A kinetic analysis of the grain growth during aging revealed that the activation enthalpy of the growth rate is estimated to be 390 kJ/mol, which is very close to that for volume diffusion coefficient of Al and Nb in γ-TiAl. Based on the results, it is concluded that the formation of the grains is attributed to coarsening of variant domains within the lamellar plates and coalescence of the same variant domains across the lamellae, leading to a γ grained microstructure following the orientation of variant domains. These reactions also make the number of the variant domains decrease during aging, which remains only two variant domains with perfect-twin relationship.
INTRODUCTION Gamma-TiAl based alloys with an α2+γ two-phase lamellar microstructure have been developed for high temperature structural applications [1], so the thermal stability of the lamellar microstructure is a key for the alloys to be used at elevated temperatures. The α2/γ lamellar microstructure contains not only just α2/γ interfaces but also many γ/γ interfaces those can be classified crystallographically into three types, i.e. variant interface, perfect-twin and pseudo-twin boundaries [2-5]. Thus, in order to understand the thermal stability of the lamellar microstructure correctly, we have to evaluate the thermal stability of each interface separately. Based on our phase diagram study of Ti-Al-M ternary system [6-8], we successfully made a fully lamellar single crystal (PST crystal) consisting of almost γ/γ interfaces, so called “γ lamellar microstructure”, by unidirectional solidification of Ti-48Al-8Nb alloy. Using the PST crystal we have evaluated the thermal stability of the γ lamellar microstructure and each γ/γ interface [9]. At elevated temperatures, the γ lamellar microstructure is not stable with respect to the α2/γ lamellar microstructure of Ti-48Al PST crystal [10] and it collapses rapidly to become a γ grained microstructure after a certain incubation period of aging. Detailed observations revealed that energetically most unstable variant interface becomes extinct through the interface reaction with neighboring γ/γ interfaces, resulting in the formatio
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