  Refractory high entropy alloys (R-HEAs) are having properties and uses as
high strength and high hardness materials for ambient and high temperature,
aerospace and nuclear radiation tolerance applications, orthopedic applications
etc. The mechanical properties like yield strength and ductility of TaNbHfZr
R-HEA depend on the local nanostructure and chemical ordering. In this study we
have computationally obtained various properties of the TaNbHfZr alloy like the
role of configurational entropy in the thermodynamic property, rate of
evolution of nanostructure morphology in thermally annealed systems,
dislocation simulation based quantitative prediction of yield strength, nature
of dislocation movement through short range clustering (SRC) and qualitative
prediction of ductile to brittle transition behavior. The simulation starts
with hybrid Monte Carlo/ Molecular Dynamics (MC/MD) based nanostructure
evolution of an initial random solid solution alloy structure with BCC lattice
structure created with principal axes along [1 1 1], [-1 1 0] and [-1 -1 2]
directions suitable for simulation of 1/2[1 1 1] edge dislocations.
Thermodynamic properties are calculated from the change in enthalpy and the
configurational entropy by next-neighbor bond counting statistics. The MC/MD
evolved structures mimic the annealing treatment at 1800{\deg}C and the output
structures are replicated in periodic directions to make larger 384000 atom
structures used for dislocation simulations. Edge dislocations were utilized to
obtain and explain for the extra strengthening observed because of the
formations of SRCs. Lastly the MC/MD evolved structures containing dislocations
are subjected to a high shear stress beyond CRSS to investigate the stability
of the dislocations and the lattice structures to explain the experimentally
observed transition from ductile to brittle behavior for the TaNbHfZr R-HEA.
