UC Santa Barbara

Refractory metal alloys are candidates for the next generation of materials for extreme conditions, but challenges associated with their ductility, processability, and environmental resistance limit their application. Computational modeling and simulation can help understand the mechanisms underpinning these alloys' mechanical properties so they can be controlled in the future. In this dissertation, a mesoscale model, phase-field dislocation dynamics (PFDD), is extended to model the fundamental aspects of refractory alloys and used to simulate dislocation behavior in several refractory alloy systems.

First, the PFDD formulation is adjusted to simulate a newer class of materials, multi-principal element alloys (MPEAs). The behavior of Frank-Read sources is simulated in MoNbTi, revealing highly statistical behavior that is inherent to these random alloys. Simulations of long dislocations in the same material show start-stop dislocation glide, with the random nature of the MPEA providing both favorable kink-pair nucleation sites and local pinning points. A direct connection to atomistic short-range order is made, showing that the increase in strength with short-range order is caused by an increase in the local unstable stacking fault energy.

Then, a local concentration parameter is added to PFDD to simulate the effects of interstitial atoms such as oxygen and hydrogen. Both short-range and long-range interactions between interstitial atoms and dislocations are accounted for. Interstitial-dislocation interactions are simulated in two systems: Nb with O interstitials and W with H interstitials. The effect on dislocation core structures, critical glide stresses, and mobility are simulated and discussed. This work provides both new insights into dislocation behavior in refractory materials and a new mesoscale framework for simulating other alloy systems of interest.