Ab initio modeling of the energy landscape for screw dislocations in body-centered cubic high-entropy alloys

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.

Screw dislocations in BCC transition metals: from ab initio modeling to yield criterion

“…The understanding of alloying effects on BCC plasticity also benefits from the development of ab initio calculations. Modeling of dislocation interactions with solute atoms sheds new light on the mechanisms responsible for hardening or softening in dilute solid solutions [97][98][99] and in more concentrated solid solutions like high entropy alloys [100]. In addition, some substitutional solute atoms have been shown to induce a change of the core structure of the screw dislocation through the variation of the electronic density [52,[101][102][103], with the dislocation going from a symmetric compact to a degenerate polarized core.…”

Atomistic simulations of dislocation mobility in refractory high-entropy alloys and the effect of chemical short-range order

“…Specifically, to investigate the deformation of RHEAs, transmission electron microscopy (TEM) studies on RHEAs have shown a dominant role of screw dislocations with increasing plastic strain 18 , 19 , and slip activity on high-order-planes has been observed through in situ scanning electron microscopy experiments 16 ; indeed, strong intrinsic lattice resistance has been generally found in these concentrated solid-solution alloys 19 , 20 . Theoretical models for strengthening in RHEAs by screw dislocations have been developed 21 , 22 , and atomistic simulations of dislocation properties using molecular dynamics (MD) and density-functional theory (DFT) calculations have been undertaken 23 – 27 . Recently, mesoscale models, such as a kinetic Monte Carlo model, have also been developed to investigate the strengthening in RHEAs 28 .…”

“…Although HEAs possess crystalline order, but compositional disorder, they are not necessarily random solid solutions for every atomic species 29 – 37 . Specifically, a feature of HEAs is the possibility of the presence of local chemical short-range order (SRO), which can affect defect motion and hence may influence mechanical properties 27 , 30 , 38 , 39 . In previous studies, SRO effects in fcc or hcp alloys have been found to be able to change the dislocation morphology, i.e., from wavy to planar, and may impact twinning 40 , 41 .…”

“…Recent experimental results have provided direct evidence for the presence of SRO in certain fcc HEAs and its correlation with mechanical properties 39 , 42 . Moreover, for RHEAs, such as the MoNbTaW, numerical studies have revealed a significant energetic driving force for SRO 27 , 31 , 32 , 43 . The effect of such SRO is thus a relevant issue that requires further investigation for single-phase bcc -RHEA solid solutions.…”

Predicting ductility in quaternary B2 -like alloys

“…This would require both the consideration of the necessary and sufficient condition boundaries and calculations of new structure parameters and energies. There is evidence that it would be appropriate to maintain the same condition boundaries for study of disordered bcc structures: A recent DFT study of the bcc refractory MoNbTaW showed that while the degree of short range order affects fault energies and therefore is expected to influence mechanical properties, the average equilibrium energy of a screw dislocation is not significantly affected by the degree of short range order [35]. It would then be reasonable to assume that the value of the dislocation core radius, and with this the boundaries for the necessary conditions in the metric, could remain the same.…”