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Microstructure and Thermal Stability of Rapidly Quenched Al-Ti-Based Ternary Alloys
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The rapidly solidified microstructures in Al-3Ti-1Ce and Al-5Ti-5Ce alloys contained a fine-scale (- 0.1 μm) and uniform dispersion of intermetallic Al20Ti2Ce (diamond cubic, a= 1.445 ± 0.005 nm) and Al11Ce3 phases, which formed directly from the melt during rapid solidification. Although the dispersed intermetallic particles appeared to be quite resistant to coarsening for up to 240 hat 400°C, they were relatively coarse scale and were not complemented by significant solid state precipitation during post-solidification heat treatments. These alloys thus appear to have limited potential for practical application, unless the solute content were raised to very high levels (> 10 wt%).
In Al-Ti-Ni alloys, with a Ti:Ni weight ratio in the range 3:1 to 4:1, the rapidly quenched microstructures invariably contained a fine-scale (30-100 nm) and uniform dispersion of cuboidal particles of a novel metastable face-centred cubic phase (space group Fm3c, a = 2.40 ± 0.05 nm) with a composition of approximately 83.1 ± 1.0 at. %Al - 12.6 ± 0.5 at.% Ti - 4.3 ± 0.5 at %Ni. In addition to the presence of the ternary cubic phase, particles of a metastable, primitive orthorhombic phase (a = 1.80 ± 0.05 nm, b = 2.20 ± 0.05 nm, c = 1.40 ± 0.05 nm) were also detected in a rapidly quenched Al-4Ti-1Ni alloy. The ternary intermetallic particles appeared to form directly from the melt as primary phase during rapid solidification, and their volume fraction increased with an increased total solute concentration (up to 20 wt%) in the alloys. Due to the high rates of cooling and large undercooling achieved in rapid solidification, up to - 3.5 wt% of Ti was retained in solid solution in the a-Al matrix phase of the rapidly quenched alloys. Maintaining the total solute concentration at 5 wt%, a change in Ti:Ni ratio from 4: I to 1 :4 led to the formation of rapidly solidified microstructures comprising fine-scale dispersions of spheroidal intermetallic particles of the equilibrium AbNi and metastable Al9Ni2 phases, which again appeared to form directly from the melt as a result of microcellular solidification[...]