High Entropy Alloys (HEAs) are multicomponent alloys defined by being comprised of at least five elements in equiatomic or near-equiatomic ratios (that is usually interpreted as a range of 5-35 at.% per element). High configurational entropy of these alloys may enhance the formation of one equilibrium phase as solid solution with randomly distributed atoms occupying lattice points of usually BCC or FCC structure.
The special nature of these alloys allows them to have remarkable properties like high hardness, stability of phase composition, creep resistance, radiation resistance, low diffusion coefficient, biocompatibility and others.
The Hf-Nb-Ta-Ti-Zr alloy (HfNbTaTiZr) has been developed as a refractory alloy by Senkov in 2011. [1] It is also considered as a potential biocompatible material. [2] It forms a single phase BCC solid solution at RT.
Several ingots of this alloy have been prepared using the arc melting method in protective argon atmosphere by UJP s.r.o. company and they are being analysed using various methods, XRD being one of them. During preparation the alloy was melted and mixed in a copper crucible which was water-cooled at the bottom. Different cooling conditions in different places of the crucible resulted in nonhomogenous microstructure in the as-cast ingot. Samples were cut out transversely from the ingot bar and were analysed by XRD, SEM and optical microscopy.
Rietveld refinement was used (Fundamental Parameters Approach in TOPAS V5 software [3,4]) to determine the lattice parameter, size of coherently diffracting domains and microstrains from the XRD measurements .
The cross section of the ingot revealed both dendritic microstructure and big grains – the first mentioned was located near the free surface and the second near the bottom of cooled crucible. This heterogeneous microstructure caused large differences between XRD patterns obtained from different regions of the sample.
Special attention was dedicated to the investigation of thermal stability of the alloy up to 1400°C during in situ annealing in XRD high temperature chamber. So far the decomposition of the BCC solid solution into two BCC solutions with similar lattice parameter at cca 900°C has been observed at high-temperature XRD measurement. More measurements are to be carried out at HTXRD and also at DSC to understand better the behaviour of the alloy.
4. A. A. Coelho, TOPAS version 5 (Computer Software), Coelho Software, Brisbane, (2016).