High Entropy Alloys (HEAs) are made from nearly equal ratios of multiple elements. These novel materials are of considerable interest, as they have novel properties caused by combining the properties of the component elements.
One particularly interesting subset of HEAs comprises mainly refractory elements, a class of metals that are extraordinarily resistant to heat and wear. These alloys, denoted refractory HEAs, or RHEAs, have received a lot of attention due to their exceptional high-temperature properties that surpass even the most advanced nickel-based superalloys at temperatures above 800–1000°C.
Due to the combination of elements of different sizes in the same structure, the atoms are displaced from the ideal lattice sites. These displacements are known as local lattice distortions (LLDs). It's these LLDs that are responsible for the many of the novel properties of the material.
Being able to measure LLDs experimentally is challenging and often ambiguous. In this study, published in Physical Review Materials, the researchers used small-box analysis of pair distribution functions (PDFs) to characterise the LLDs in an RHEA.
To begin with, the researchers did an extensive study of the existing literature, confirming that the static displacements (such as the LLDs) were significant, in contrast to other types of HEAs.
They then studied the material HfNbTaTiZr using both real-space and reciprocal-space refinements of synchrotron X-ray data from DESY and compared real-space analysis of neutron total scattering data measured on GEM at ISIS. Their different analyses of their total scattering experiments were consistent with one another, and with the results of their literature review. This proves that total scattering is an accurate tool for measuring LLDs in these materials.
Having access to such methods allows investigations of how LLDs vary across the vast range of available RHEAs. This will help researchers to understand the origin of LLDs, and their effects on the properties of the alloys. Neutrons are critical for such measurements, as they provide sufficient penetration depth and gauge volume to obtain the necessary statistics for industrial relevant materials with large grains.
Large-box analysis of total scattering data, where the positions of several thousands of atoms are refined against experimental data allows calculation of LLDs in a way analogous to atomistic simulations. However, it takes a lot of time and computing power, and so the team looked into small-box analysis. This method, which is based on the refinement of the structure described by a single unit cell, is much quicker and therefore better suited to in situ experiments or others where large amounts of data are collected.
They found that their small-box modelling was successful, proving it to be a fast and reliable tool for measuring LLDs in RHEAs. This makes it ideal for analysis of large data sets from time-resolved in situ measurements.
The full paper can be found at DOI: 10.1103/PhysRevMaterials.8.083602
