Variation of Material Properties in Tilt-Cast Cu-Al-Ni Alloy

Alloys based on CuAl have been a promising option for high-temperature SMAs (HTSMA) because of their procedural and cost advantages over NiTi-based high-temperature SMAs. Despite their excellent shape memory as well as phase stability at temperatures up to 250°C, their brittle behavior and the degradation of the shape memory effect under cyclic stress have provided obstacles to widespread application. While multiple remelting processes are often applied to avoid inhomogeneities in the production of these alloys, single step inductive melting is preferable in terms of productivity, especially for small alloy batches. The goal of this and consecutive work is to characterize, and reduce, the variation of material properties and microstructure in materials prepared by vacuum-induction melting of pure elements and tilt-casting. To this end, castings from pure metals with 5 target chemical compositions from Cu12.5wt.%Al4wt.%Ni to Cu13.2wt.%Al4wt.%Ni were prepared and characterized in terms of transformation temperatures and occurrence of martensitic phases. Samples taken from different positions in the casting were compared. Changes in microstructure with increased aluminum content of the alloy could be assessed in both metallographic and calorimetric analysis. Considerable consistency of transformation temperatures and phase composition in each individual casting, as well as between castings with identical parameters, could be achieved. This points to the high degree of homogenization that can be achieved, even with a single melting cycling and subsequent casting, using suitable induction melting parameters. The absence of oxide and carbide inclusions, despite potential reactions between nickel and the graphite of the crucible, is promising for future casting processes.