Characterization and Modeling of NbNiTaTiW and NbNiTaTiW-Al Refractory High-Entropy Alloys
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High-entropy alloys (HEAs) represent a new class of material that exhibit unique materials properties and complex microstructure. Little work to date has been done on refractory HEAs (RHEAs), which can loosely be defined as an HEA with the majority of the alloy being refractory metals. In this study, the authors examined two unique RHEA systems: (1) Nb20Ni20Ta20Ti20W20 and (2) Nb18Ni18Ta18Ti18W18Al10. The RHEAs were characterized in the as-cast and heat-treated condition using scanning electron microscopy equipped with energy dispersive spectroscopy to observe the microstructural evolution and elemental segregation and X-ray diffraction to determine the number of phases present and their crystal structure. The RHEA mechanical properties were tested via Vickers hardness measurements. Thermodynamic simulations of the solidification process were performed using a CALPHAD approach involving two basic models: equilibrium lever rule and non-equilibrium Scheil-Gulliver. The simulations were compared with the experimental data.
The authors thank the Forging Industry Association’s FIERF program for partial support of this research. Nathan Ley thanks the AFRL/RWMWS Munitions-Eglin program for partial support of this research. The authors especially thank Dr. Dan Miracle of AFRL AFRL/RX Materials-Dayton for providing his insight and extensive knowledge on the subject of HEAs/CCAs. The authors thank Richard Harris, Dr. Sean Gibbons, and Dr. Rachel Abrahams for their help with characterization as well as for access to facilities and equipment for Nathan Ley at AFRL/RWMWS Munitions-Eglin. The authors acknowledge UNT’s Materials Research Facility (MRF) for providing access to equipment. Stéphane Gorsse thanks Thermo-Calc Software AB for providing the TCHEA databases.
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