Incommensurate Structure in Al-rich TiAl Alloys
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Incommensurate Structure in Al-rich TiAl Alloys S. Wang, D. Fort, I. P. Jones1 and J. S. Abell School of Metallurgy and Materials, The University of Birmingham, Birmingham, B15 2TT, England 1 Also IRC in Materials for High Performance Applications, The University of Birmingham, B15 2TT, England
ABSTRACT The microstructures of Ti-Al alloys in the range 56-62 at.% Al have been studied. These compounds consist of a basic L10 structure on which is superimposed an irrational modulation (Ti3Al5 related). This is incommensurate by occupation. It has two-dimensional modulations in which the wave vectors are q1 = α(a* + b*) and q2 = α(-a* + b*) where a* and b* are the L10 reciprocal basis and α is an irrational fraction. α decreases from 0.282 to 0.26 as the Al concentration increases to 60 at.% Al, i.e., the positions of the satellites change continuously with varying Al concentration. A third phase, TiAl2, appears at a composition of 62 at.% Al which means that no Ti3Al5 (62.5 at.% Al) of stoichiometric composition can ever exist. INTRODUCTION TiAl single phase (γ phase, tetragonal L10 structure with cγ/aγ ≈ 1.02 where aγ and cγ are lattice constants) has been found to exist over a wide composition range (49-56%Al at 700°C in Fig. 1). Another phase, Ti3Al5, was first reported to coexist with the L10 structure by Miida et al. [1]. This is a superstructure based on L10 and has tetragonal symmetry with the space group, P4/mbm, and lattice constants, as=2 2 aγ and cs=cγ. According to Miida et al. [1], the Ti3Al5 structure is commensurate and forms in the range 55-63at.%Al, the intensities of the additional reflections increasing with Al content. This phase has been confirmed to form precipitates at an alloy composition Ti-58at.%Al [2,3]. After studies of TiAl alloys over the composition range 56-63 at.% Al, Loiseau and Lasalmonie [4] reported weak irrational diffraction spots besides the main L10 reflections. It was originally proposed that these weak diffraction spots were caused by two phases with tetragonal and orthorhombic crystal structures [4], but this interpretation was questionable as the diffraction patterns could not be indexed in term of two such phases. Furthermore, dark field images using these superstructure reflections failed to reveal discrete precipitates, which should have been the case if other phases had been present. Subsequently, Loiseau et al. [5] re-interpreted these effects as being the result of islands surrounded by a phase having an intermediate composition, assumed to be Ti7Al11 (a tetragonal structure with the lattice constants a=3aγ and c=cγ). However, the diffraction patterns do not support such an interpretation since no split satellite spots can be resolved, which would be the case if two intergrowth structures had formed. Indeed, high resolution electron microscopy failed to show such islands [6]. TiAl, and alloys based upon it, have received increasing attention in the recent two decades because of their low density and high strength up to 1000°C. On the other hand, their low
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