Effects of composition on microstructures of rapidly solidified Ni-Al alloys

Microstructures of melt-spun NI-AI alloys with compositions from 61-85 at % Ni were studied by means of transmission electron microscopy, X-ray diffra ction analysis and optical microscopy. The microstructures of as-quenched r ibbons exposed to cooling rates of the order of 10(6) Ks(-1) reflect the tr ansition from primary beta-NiAl to gamma-Ni solidification with increasing nickel content. In 70 at % Ni alloy ribbons, martensitic NiAl grains were d etected near the wheel-side surface contrasting with anomalous and lamellar eutectic microstructure in the top part. Directly ordered Ni3Al grains wit h single (or large) antiphase domains (APDs) and a minor eutectic fraction were observed in 75 at % Ni alloy ribbons. Samples containing 80 at % Ni ex hibit mainly single-phase Ni3Al grains with 10-20 nm sized APDs indicating sequential ordering. Weak L1(2) ordering was even detected in 85 at% Ni rib bons which displayed ordered antiphase zones of 1 nm size. Disordered gamma -(Ni) films on grain boundaries can be discounted for 80 at % Ni ribbons, b ut occurred near the top of 85 at % Ni samples. The results are explained i n terms of the reassessed Ni-Al phase diagram employing recent corrections near to the Ni3Al composition and new results on phase formation in underco oled Ni-Al melts. (C) 1998 Kluwer Academic Publishers.