Abstract
The unidirectionally solidified microstructure of the Ni-Nb eutectic alloy consists of alternate lamellae of Ni 3Nb (δ) and Ni-Nb solid solution (γ) phases. Nb has a solubility limit in γ phase that decreases with temperature, suggesting the possibility of obtaining additional δ phase and strength via precipitation processes. To this end. the microstructure, γ phase lattice parameter and microhardness of the aligned composite were examined in the as-grown, solution treated (ST). and ST + aged conditions. Little precipitation was found after a ten hour 1000°C ageing treatment in the as-grown ingots. However, approximately 12 v/o of Widmanstatten δ precipitates—{111}, habit planes—was produced after a 1220°C solution treatment followed by short time ageing at 1000°C. Considerable precipitation was found also after longer time ageing at SOO°C. Lattice parameter measurements indicate the γ phase to be in a metastable. supersaturated condition. Upon ST + ageing, the γ lattice parameter decreases slightly, reflecting the Nb loss associated with δ precipitation. The remaining solubility of Nb in γ is compared with previously reported results. The microhardness of the composite was determined as a function of heat treatment and found to increase with increasing v/o of δ (original lamellae + precipitates). A significant precipitation hardening effect was noted for the ST + 800°C ageing treatment with hardness measurements suggesting a substantial increase in yield strength.
Published Version
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