EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Since their first mentioning in the early 2000s, the interest in high entropy alloys (HEA) as a new material class has been ever increasing. Starting with bulk materials, also thin films gained attention in the past years due to the wider flexibility in the available synthesis methods and attainable growth conditions. Among the different HEAs, alloys mainly comprising refractory metals, so-called refractory HEAs, showed a good thermal stability after annealing in vacuum which makes them a potential thin film material for high temperature applications. Within this study a series of refractory HEAs were deposited using high power impulse magnetron sputtering keeping the base alloy MoNbTaW constant and adding a fifth element: Ti, V, Cr, Mn, Zr and Hf. The targets used for the synthesis of each alloy contained all five elements in an equimolar concentration. As analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM), all films showed a bcc solid solution phase structure in as-deposited state. Subsequently, the thermal stability of the films was investigated using high temperature XRD up to 1200 °C in vacuum. While alloys like MoNbTaW+V and MoNbTaW+Cr showed minimum structural changes up to the maximum annealing temperature, others like MoNbTaW+Ti and MoNbTaW+Zr revealed several phase changes. The obtained high temperature XRD results are complemented by differential scanning calorimetry, temperature-stress measurements as well as by TEM analysis of selected annealed refractory HEA films. The performed work is intended to contribute to a comprehensive understanding about the thermal stability of refractory HEA thin films which in turn can enable their use for industrial high-temperature applications.