Multi-scale Investigation of Chemical Short-Range Order and Dislocation Glide in the MoNbTi and TaNbTi Refractory Multi-Principal Element Alloys

Abstract

Refractory multi-principal element alloys (RMPEAs) are promising materials for high-temperature structural applications. Here, we investigate the role of chemical short-range ordering (CSRO) on dislocation glide in two model RMPEAs -TaNbTi and MoNbTi -using a multi-scale modeling approach. A highly accurate machine learning interatomic potential was developed for the Mo-Ta-Nb-Ti system and used to demonstrate that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies (USFE). From mesoscale phase-field dislocation dynamics simulations, we find that increasing SRO leads to higher mean USFEs, thereby increasing the stress required for dislocation glide. The gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations, with higher SRO decreasing the degree of USFE dispersion and hence, amount of hardening. Finally, we show how the morphology of an expanding dislocation loop is affected by the applied stress, with higher SRO requiring higher applied stresses to achieve smooth screw dislocation glide.

Sluggish dislocation mobility and high propensity for cross-slip at elevated temperatures in the MoNbTi Refractory Multi-Principal Element Alloy

“…The local atomic composition at each grid point is defined by the atoms with 2b, only including atoms within the slip plane being sheared. The local atomic compositions are then converted to local USFE values using USFE composition maps that were calculated previously with molecular statics 24 . The local USFE values are used during PFDD simulations.…”

“…Unlike its pure metal constituent Nb, dislocation glide in an RMPEA such as MoNbTi will be influenced by both temperature and compositional fluctuations. For MoNbTi, screw dislocation motion was simulated for the same temperatures as Nb and under an applied shear stress that was half its 0K critical stress, i.e., 𝜎 𝑚𝑝𝑒𝑎 𝑎𝑝𝑝 = 0.069𝜇 𝑚𝑝𝑒𝑎 24 . Several similarities in glide motion are observed.…”

“…MoNbTi acts as an exemplar system to evaluate fundamental dislocation mechanisms because it is a single phase, equiatomic solid solution 18 . The PFDD formulation accurately captures key features of dislocations in bcc RMPEAs such as the nonplanar screw core, the planar edge core, the ability for the screws to cross-slip, and the unstable stacking fault energy (USFE) landscape for a random solid solution [21][22][23][24][25] . We aim to quantify and assess screw and edge dislocation mobility in MoNbTi versus its constituent Nb over a range of temperatures where slip is the dominant deformation mechanism.…”