Chemically induced local lattice distortions versus structural phase transformations in compositionally complex alloys

Abstract

Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti–Zr–Hf–Ta and Ti–Zr–Nb–Hf–Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed.

Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials

“…In order to detect the structural ω-bcc phase transition we utilize the recently proposed projected displacement method [25] as also described in Sec. II F above.…”

“…1), which indicates the necessity of further retraining. Nevertheless, it is important to stress that, even though the MTP is initially trained on bcc configurations only, the AL approach successfully reveals regimes of bcc lattice instability and enables the prediction of the transition to ω-like structures [25].…”

“…IV), this is not straightforwardly possible, because the ideal crystallographic symmetry is broken due to lattice distortions caused by the chemical disorder. To overcome this difficulty, an approach utilizing projected atomic displacements was recently proposed [25]. This approach utilizes the fact that the ω structure can be obtained by a martensitic transformation from the bcc structure via atomic displacements along one of the 111 bcc directions.…”

“…The measure of proximity of an arbitrary structure with respect to the ω structure, i.e., the structural descriptor, is then characterized either by p 1 or by p 1 − p 2 [25].…”

“…For example, in chemically complex bcc alloys, being in the focus of the present work, the distinction between a distorted bcc lattice and an ω-type structure is not straightforward anymore. This issue has recently been discussed and analyzed by a projected displacement scheme that allows one to disentangle these delicate structural features [25]. However, this detection scheme has been mainly limited to ground state structural analysis and did not involve any analysis on its relation to elastic properties.…”