Phase stability of a ductile single-phase BCC Hf0.5Nb0.5Ta0.5Ti1.5Zr refractory high-entropy alloy

High-entropy alloys (HEAs) have attracted much attention since they can possess some unique properties. By adjusting their multi-principal elements, some systems with body centered cubic (BCC) structure could obtain an excellent trade-off of strength and ductility, e.g. recently reported single-phase Hf0.5Nb0.5Ta0.5Ti1.5Zr refractory high-entropy alloy (RHEA). However, researchers found that some HEAs are thermally metastable and could decompose within a certain temperature range. The thermal stability is especially important for RHEAS which are promising for high-temperature applications. Here we evaluated the phase stability of a BCC Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA after anneals for two weeks at 500-900 degrees C. Microstructural analyses, performed using X-ray diffraction, scanning electron microscopy, high-resolution transparent electron microscopy, show that the RHEA is a single-phase solid solution after recrystallization for 3 hat 1000 degrees C and remains in this state after a subsequent anneal at 900 degrees C for two weeks. However, it is unstable and forms second-phase precipitates at and below 800 degrees C. Tiny precipitates on the grain boundaries (GBs) were observed in the sample annealed at 800 degrees C. Annealed at 700 degrees C, precipitates with BCC structure (termed as BCC2) both on the GBs as chains and at intragranular regions as petaloid morphology are present. After the anneal at 500 degrees C, the RHEA decomposes into multi-phase microstructures with different morphologies: lamellar structure close to the GBs composed of BCC1 and BCC2 phases, and convoluted basket-like structure inside grains comprising a third hexagonal close-packed phase precipitates except for both BCC phases. Thus the phase stability of this RHEA should be carefully evaluated for elevated temperature applications.