The Character of Steps on Gamma/Alpha-2 Interfaces in a Low Misfit Lamellar TiAl-Based Alloy
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P. Shang', T.T. Cheng2 and M. Aindow ' 1. School of Metallurgy & Materials, 2. Interdisciplinary Research Centre in Materials for High Performance Applications, The University of Birmingham. Birmingham, B15 2TT, UK ABSTRACT The defect character of steps on lamellar y/a2 interfaces in a quinternary TiAl-based alloy has been studied using high resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces separated by interfacial steps across equal even numbers of { 11})yand (0002)%2 planes. Circuit mapping was used to identify the Burgers vectors of these steps from lattice images obtained at [0llyand [110]yzone axes. It was found that the Burgers vectors exhibited by the two-layer steps are different from those reported previously and are consistent with those expected for perfect disconnections as described by Pond's topological theory of interfacial defects, and not with the usual partial dislocation model. INTRODUCTION It has been shown that two-phase (yzTiAl + o2 -Ti3AI) alloys with the lamellar microstructure generally exhibit higher strengths and ductilities than single phase TiAl-based alloys [1,2]. It is thought that the structure of the Ya2 interfaces plays an important role in determining these mechanical properties. The structure and defect content of the -va,2 interfaces in the lamellar microstructure have been studied extensively using both conventional transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). In most studies of binary Ti-Al alloys, two types of intrinsic interfacial dislocations were observed; lattice dislocations with Burgers vectors 1/2 which exist to accommodate the lattice mismatch between the two phases, and interfacial steps two atomic layers high with Burgers vectors 1/6 [3-7]. In many cases the character and distribution of the defects in the r/a2 interfaces has been used to infer the mechanism(s) by which the lamellar structure forms. In this paper we present a HREM study of the two-layer steps on Y/a% interfaces in a TiAlbased alloy with minimal misfit between the two phases in the lamellar colonies. It is shown that Burgers vectors for these steps are different from those reported previously and that these are consistent with those expected for perfect interfacial disconnections as described by Pond's topological theory of interfacial defects [8] rather than a partial dislocation model. Possible reasons for these discrepancies are discussed. EXPERIMENTAL PROCEDURE The investigations were performed on an alloy with a nominal composition of Ti-44AI-4Zr4Ta-0.2Si (in atomic %) in which the misfit between the yand % 2 phases is < 0.1%. The alloy was heat treated at 1350°C for lh followed by furnace cooling at =10C/min. TEM specimens were prepared by electropolishing to perforation in a solution of 6% perchloric acid, 34% butan-l-ol, and 60% methanol at -20tC followed by Ar ion milling to clean the sample surfaces. HREM was performed in a JEOL 200CX operating at 200 kV. KK3.7.1 Mat. Res. Soc. Symp. Proc. Vol
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