Control of precipitation kinetics in equiatomic CoCrFeMnNi high-entropy alloy by microstructural design: On the role of grain boundaries and interfacesWednesday (01.01.2020) 00:15 - 00:30 Part of:
Multi-principal-element CoCrFeMnNi high-entropy alloys solidifying as a single-phase solid solution have attracted great attention as a promising material for various engineering applications due to their exceptional mechanical properties and thermal stability in a wide range of application temperatures. It has been shown that especially CoCrFeMnNi alloys with equiatomic compositions remain a single-phase solid solution and do not undergo any significant structural changes (except for grain coarsening) upon annealing of several hundreds of days at temperatures above 900°C. On the other hand, the alloy decomposes into multiple phases at intermediate temperatures ranging from about 500 to 800°C. A complex phase decomposition and formation of second-phase precipitates have been observed preferentially at grain boundaries or at intragranular defects such as inclusions and pores . In this study, equiatomic CrCoFeMnNi alloy was synthesized by physical vapor deposition as an epitaxial single-crystalline film at 650°C, which corresponds to a temperature at which the single-phase solid solution was found to be unstable and at which Cr-rich phase and traces of NiMn and FeCo precipitates have been observed . For comparison, a polycrystalline CoCrFeMnNi film was synthesized at the identical process conditions and characterized by transmission electron microscopy and X-ray diffraction. The results revealed the dominant role of grain boundaries on the precipitation kinetics. While the epitaxial single-crystalline CoCrFeMnNi alloy remained a single-phase solid solution, the microstructure of the polycrystalline alloy consisted of Cr-, Co- and Ni-rich precipitates formed preferentially at the grain boundaries. These findings were further validated by annealing of the single-crystalline alloy at various temperatures to induce its decomposition. The grain boundary-free single-crystalline alloy was, however, found to preserve its single-phase microstructure irrespective of the thermal loading. The fact that the activation barrier for nucleation is much higher in the grain interior than at the grain boundaries suggests a great potential of the microstructural design to supress phase decomposition of the alloy by controlling the grain size and distribution.