Abstract
The present article reports and discusses the results of the microstructural characterization of various modifications of Ll2 trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll2 trialuminide alloys Al3Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al3Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al3Ti + Cu alloy. The homogenized Al3Ti + Cr and Mn alloys have greater lattice parameters than the Al3Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll2 Al3Ti + X alloys dramatically decreases with increasing titanium content.
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