Abstract
Ternary additions of metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc to tetragonal (DO22) Al3Ti are known to lead to stabilization of the cubic (L12) structure. This DO22 to L12 transition has been studied in the Al-Ti-Ni system using X-ray diffraction and scanning electron microscopy. The results show that nickel substitution has no significant effect on the lattice parameter (and therefore on the tetragonality) of the DO22 phase and that the solid solubility of nickel in the DO22 phase is very limited. The L12 phase precipitates out on addition of nickel to Al3Ti, its amount increasing with increasing nickel content of the alloy. The compositions of the DO22 and L12 phases do not change significantly with the alloy composition. These results are discussed in terms of theories of structural transitions in ordered alloys. Similar transitions have been reported in transition metal-based systems. An analysis of the transition in intermetallic systems is presented.
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