Microstructure in rapidly solidified Al-Ti-Ni alloys

Rapidly quenched Al-Ti-Ni alloys, with total solute concentrations of 5-20 wt%, have been produced in thin ribbon form (30-50 μm thickness) by controlled-atmosphere chill block melt spinning and the resulting microstructures have been characterized using analytical transmission electron microscopy. As-solidified microstructure of alloys, with a Ti:Ni weight ratio in the range 3:1-4:1, invariably contained a uniform, fine-scale dispersion of cuboidal particles of a novel metastable face-centred cubic phase (space group Fm3c, ac = 2.42 ± 0.01 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, particles of a metastable, primitive orthorhombic phase (ao = 1.80 ± 0.05 nm, bo = 1.40 ± 0.05 nm, co = 2.20 ± 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 increasing total solute concentration. As a result of the high quench rates and large undercooling achieved during solidification, up to 2.5 wt% Ti was retained in rapidly solidified α-Al solid solution. For a total solute concentration of 5 wt%, a decrease in Ti:Ni ratio led to the formation of rapidly solidified microstructures comprising an increasing fraction of spheroidal intermetallic particles of the equilibrium Al3Ni and metastable Al9Ni2 phases, which again appeared to form directly from the melt as a result of micro-cellular solidification.