Combinatorial study on mechanical properties of Ni-Co-Cr thin film

Abstract

NiCoCr is a medium entropy alloy that has recently been of much interest due to its high strength as well as excellent ductility even at low temperatures, making it a promising material for extreme environment applications. Yet, relatively poor yield strength at room temperature in comparison to other conventional alloys motivated numerous studies to improve the mechanical properties by exploring non-equiatomic compositions. This study aims to efficiently determine the composition of non-equiatomic Ni-Co-Cr alloy with optimal strength by using nanoindentation of the combinatorial thin film library. Co-sputtering from Ni, Co, and Cr targets allowed for the synthesis of a thin film with a wide composition range in a single specimen as determined by Energy Dispersive X-ray Spectroscopy analysis. X-ray diffraction revealed that the thin film was mostly a single phase of FCC solid solution for Cr < ~43 at.% but a small region with secondary sigma phase formation (Cr ≥ ~43 at.%) was also reported. In this study, the single phase region was chosen for nanoindentation analysis to determine the sole effect of the composition variance in the absence of any secondary phase formations. Modulus and hardness mapping using nanoindentation revealed that the Ni-poor compositions near Ni$_{20}$Co$_{40}$Cr$_{40}$ showed the highest hardness of 7.45 GPa, possibly due to the increased non-linear interaction between Co and Cr that hinders dislocation motion. Hardness mapping was therefore successfully used to identify the composition leading to the highest strength, although there is a discrepancy in the absolute value of the strength when compared to the bulk values due to the generally known mechanisms for thin films being stronger than their bulk counterparts. Therefore, the combinatorial thin film approach presented in this work was demonstrated to be a high throughput, efficient methodology in identifying the non-equimolar NiCoCr alloy composition with optimal mechanical properties.