Body-Diagonal Diffusion Couples for High Entropy Alloys

Body-diagonal diffusion couples provide a systematic way to investigate the diffusivity of high entropy alloys, a.k.a. HEAs. By definition these alloys contain 5-elements or more. All independent concentration differences in a body-diagonal couple are equal, except for being positive or negative. They are similar to diagonal diffusion couples for 3-element alloys, but the term body-diagonal applies only to couples with 4-elements or more. Composition vectors for these couples are aligned with body-diagonals in cubes located in the multi-element composition space of HEAs. The body-diagonals can be represented with a simple vector notation. The notation and other properties of body-diagonals are given in this work, including the number of diagonals and the angles between them. An advantage for HEAs is the fact that there are more diagonal directions and, therefore, more potential diffusion couples than needed to measure or validate diffusivity databases. That advantage gives flexibility to avoid 2-phase regions and, when validating databases, to avoid eigenvector directions of the diffusivity matrix. When body-diagonal diffusion couples are compact, their diffusion paths cross at the same composition, making it possible to measure an HEA diffusivity using the Boltzmann-Matano-Kirkaldy method.