Microstructure and ordering of L1 2 titanium trialuminides
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I.
INTRODUCTION
THE trialuminide AI3Ti forms a tetragonal ordered crystal structure, D022, in its solid state, tl,21 This aluminide is of technological interest because of its relatively high melting point (1340 ~ oxidation resistance better than that of TiA1,t3~ low density (3.3 g/cm3[2]), and high elastic modulus (166 GPat4]). However, AI3Ti is extremely brittle, presumably due to its low-symmetry crystal structure and a lack of five independent slip systems required for homogeneous deformation of a polycrystal (von Mises criterion[S]). Recently, a considerable effort has been devoted to improving the mechanical properties of titanium trialuminides by changing their crystal structure from tetragonal to cubic L12 through macroalloying with Cu, Ni, Fe, Zn, Pd, Cr, and Mn. tl'2] The L12 crystal structure has the required five independent slip systems. The L12 titanium trialuminides obtained by macroalloying are usually referred to as "AlaTi-based." However, for the sake of clarity, it must be pointed out that such a designation as A13Ti-based may be misleading, as recently reported by Durlu e t al.[6] These authors have put forward a novel idea that the L12 trialuminide phases are in reality D023 Al~lTis-based rather than AlaTi-based. Despite the successful transformation into the cubic structure, the L12 trialuminides still remain brittle in tension although their apparent compressive ductility seems to be quite considerable. A substantial amount of research on their mechanical behavior has been done and is summarized in a recent review article by George e t al. tT~ For some time, the microstructural development and mechanical behavior of the Ll2 titanium trialuminides have been systematically studied in our laboratory. The present article is the first one in a series of articles devoted to the microstructural characterization and understanding of fundamental mechanisms responsible for the M.B. WINNICKA, Ph.D. Candidate, and R.A. VARIN, Professor of Materials Science and Engineering, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted April 17, 1992. METALLURGICAL TRANSACTIONS A
mechanical behavior of the L12 titanium trialuminides. The present article reports the results of the microstructural characterization of various modifications of L12 trialuminides containing various titanium contents, including the first ever report on their degree of ordering. II.
EXPERIMENTAL PROCEDURE
Cubic modifications (LI2) of A13Ti-based alloys with target compositions (at. IX:0, A1-8Cr-25Ti, A1-9Mn-25Ti, AI-11Mn-23Ti, A1-9Fe-25Ti (A122Fe3Tis), and AI-12.5Cu25Ti (A15CuTi2), referred to hereafter as 8Cr, Mn-high Ti, Mn-low Ti, 9Fe, and 12.5Cu, respectively, were prepared from pure elements, A1(99.99999 pct), Cu(99.9 pct), Fe(99.98 pct), Mn(99.9 pct), and Cr(99.99 pct). The ingots were fabricated by double induction melting at 1390 ~ + 10 ~ under a high-purity argon atmosphere (solidification in the crucible) and subsequently homogenized in vacuum (--~10-2
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