Comprehensive Investigation of the Mechanical Behavior of Nial-x (X=pt, Pd) Alloys: Effect of Composition, Orientation and Temperature

Abstract

Gas turbine engines in the turbine section operate at temperatures exceeding 1300°C, which can cause high-temperature oxidation and reduce the lifespan of turbine blades. To counter this, thermal barrier coating (TBC) systems are employed, which not only provide thermal insulation but should also possess mechanical durability, resist strain, and prevent phase transitions. The TBC system comprises a bond coat, thermally grown oxide, and ceramic topcoat. NiAl, an intermetallic compound, is the main constituent of the bond coat and is crucial in the aerospace industry. The study aims to investigate the mechanical behaviour of NiAl by studying the effect of ternary and quaternary additions of noble elements like Pt and Pd. The study involved preparing alloys with varying Pt and Pd concentrations. Site occupancy studies were carried out using XRD and DFT calculations. The results showed that Pt and Pd strongly prefer to occupy the Ni sublattice in both stoichiometric and Ni-rich NiAl due to their good bonding interaction with Al, as revealed by density of states curves. The effects of addition of Pt and Pd along with the stoichiometry and orientation on the mechanical properties of NiAl-X alloys were also investigated using high throughput nanoindentation techniques. The addition of Pt and Pd was found to decrease the modulus and increase the metallic character of bonding in NiAl, which was demonstrated through density states calculations. Elastic constants were obtained using the indentation modulus with the help of the theoretical model proposed by Vlassak and Nix and compared with those obtained from DFT calculations and the values were in reasonable agreement with each other. It was found that the hardness of the alloys was dependent on their orientation and stoichiometry, and the addition of Pt and Pd resulted in an increase in hardness due to solid solution strengthening. The effect of temperature on the nanoindentation hardness of the alloys was investigated up to 600°C. The study also investigates the effect of Pt and Pd on the brittle to ductile transition temperature (BDTT) of stoichiometric NiAl. To achieve this, binary, ternary (Ni-10Pt-50Al), and quaternary (Ni-5Pt-5Pd-50Al) alloys were tested using uniaxial compression tests performed up to 1000 °C. The results indicated that the addition of Pt and Pt-Pd significantly increased the BDTT of NiAl.