Microstructural analysis of the cast and melt-spun high entropy noble alloy Ag20Pd20Pt20Cu20Ni20

High entropy alloys (HEA) represent a relatively new class of materials with promising properties for various applications. In recent years, these alloys have received considerable attention as potential heterogeneous catalysts in chemical and electrochemical reactions. Their enhanced catalytic activity is controlled by chemical composition, surface atomic coordination, electronic configuration and degree of microstructural metastability. Single-phase HEAs are of particular importance, because they possess a uniform microstructure that is useful for designing and prediction of mechanical and potential functional properties. The cooling rate has a significant impact on the formation of the microstructure, affecting the size of grains, as well as distribution and composition of precipitates and phases that are being formed during solidification. The influence of different cooling rates on the microstructure of the alloy Ag20Pd20Pt20Cu20Ni20 was studied in our research work. The microstructure and phase constituents were characterised by Scanning Electron Microscopy and X-ray Diffraction. Liquid phase separation with a consequential monotectic reaction resulted in an anomalous multiphase cast microstructure. With an increase in the cooling rate using the melt spinning technique, the number of formed phases and the size of the grains decreased and the high entropy supersaturated solid solution was attained, as the atoms in the alloy did not have enough time to diffuse and to rearrange themselves into a stable, ordered structure. It was also confirmed that higher cooling rates cause severe lattice distortion and create coordinatively unsaturated sites at the surface which are essential for the bonding and activation of the reactants and therefore improve the potential catalytic properties of the Ag20Pd20Pt20Cu20Ni20 alloy significantly.