Speaker
Description
Unlike conventional alloys, high entropy alloys (HEAs) contain up to six chemical elements in
nearly equiatomic proportion. Such a composition allows to stabilize solid solutions, exhibiting
in particular impressive mechanical properties and a ‘sluggish’ diffusion. The ternary bcc Zr-Ti-
Nb system is an interesting model HEA; it has a high yield strength and good ductility at room
temperature [1] and its three components are present in a family of refractory bcc HEAs,
exhibiting remarkable properties [1].
A deep understanding of the diffusion mechanisms in HEAs is needed. Since point
defects, such as vacancies, drive diffusion processes, it is important to study their fundamental
properties. Thus, we perform density functional theory (DFT) calculations on the bcc Zr-Ti-Nb
system to characterize their structural and electronic properties.
We first study vacancies for the three elements in the bcc phase, which is stable for Nb and
metastable for Ti and Zr. Surprising results are found for Ti and Zr: they have negative
formation energies, meaning that vacancies can be considered as constitutive defects. A detailed
analysis of the local structure around the vacancy indicates that a single vacancy allows the
metastable bcc structure to locally deforms towards the stable hcp structure. A signature of this
instability exists in the local electronic density of states of the vacancy neighbouring atoms. This
solves a question raised in the literature [2,3], and shows that the vacancy acts as a precursor for
phase transition. Preliminary results for finite-concentration bcc Nb-Zr-Ti alloys are finally
presented.
[1] O.N. Senkov et al., J. Alloys Compd. 783, 729 (2019),
[2] D. Connetable et al., J. Phys. Condens. Mater. 23, 405401 (2011)
[3] R.G. Hennig et al., PRB 78, 054121 (2008)
| Speaker Country | France |
|---|