Abstract
In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. However, unlike face-centered cubic high-entropy alloys (HEAs), there have been far fewer investigations into bcc HEAs, specifically on the possible effects of chemical short-range order (SRO) in these multiple principal element alloys on dislocation mobility. Here, using density functional theory, we investigate the distribution of dislocation core properties in MoNbTaW RHEAs alloys, and how they are influenced by SRO. The average values of the core energies in the RHEA are found to be larger than those in the corresponding pure constituent bcc metals, and are relatively insensitive to the degree of SRO. However, the presence of SRO is shown to have a large effect on narrowing the distribution of dislocation core energies and decreasing the spatial heterogeneity of dislocation core energies in the RHEA. It is argued that the consequences of the mechanical behavior of HEAs is a change in the energy landscape of the dislocations, which would likely heterogeneously inhibit their motion.
Highlights
Previous investigation of the fundamentals of deformation in body-centered cubic transition metals have revealed that the core properties of the 1⁄2〈111〉 screw dislocations play an essential role in their plasticity[1], especially at low temperatures where the deformation is thermally activated through the kink-pair nucleation mechanism[2], and is expected to be strongly temperature dependent
For each configuration representing a different degree of screw dislocations in the refractory MoNbTaW high-entropy alloys (HEAs), we employ chemical short-range order (SRO), we calculated 231 different supercells with the density functional theory (DFT) calculations, making use of the Vienna ab initio simulation dislocation dipole, in which the position of the cores initialized in package (VASP)[57,58,59]; details of the DFT calculations are provided different local environments
If we take the highest Peierls barrier value in the pure bcc elements as the reference for the refractory high-entropy alloys (RHEAs), it is found that when the core energies follow the Gaussian distribution, the rugged energy landscape and variance in RHEA will inevitably lead to another scenario during the calculation of the dislocation Peierls potentials, in which the final configuration has a much higher potential energy than that of the initial configuration, as shown by the right-side histogram in Fig. 6, which is noted as a Type-2 barrier
Summary
Previous investigation of the fundamentals of deformation in body-centered cubic (bcc) transition metals have revealed that the core properties of the 1⁄2〈111〉 screw dislocations play an essential role in their plasticity[1], especially at low temperatures where the deformation is thermally activated through the kink-pair nucleation mechanism[2], and is expected to be strongly temperature dependent. There are still only very limited studies on the quadrupolar arrangement[16] with triclinic symmetry to minimize deformation behavior of this new class of bcc alloys, as compared to single-phase bcc transition metals Another important aspect of HEAs is the presence of local any effects of periodic boundary conditions and image stress. These alloys can be described as “topologically ordered yet chemically disordered”, the local chemical environments are unlikely to be characterized by a perfectly random distribution for every atomic species[47,48,49,50,51] Their disordered multiple-element compositions lead to a strong possibility of SRO, for example, the preference for certain types of bonds within the first few neighbor shells.
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