Abstract
Refractory high entropy alloys (R-HEAs) are becoming prominent in recent years because of their 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, which in term depend on the annealing treatment. In this study we have computationally obtained various properties of the equimolar 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 ½[1 1 1] edge dislocations. Thermodynamic properties are calculated from the change in enthalpy and configurational entropy, which in term is calculated by next-neighbor bond counting statistics. The MC/MD evolved structures mimic the annealing treatment at 1800 °C and the output structures are replicated in periodic directions to make larger 384,000 atom structures used for dislocation simulations. Edge dislocations were utilized to obtain and explain for the critically resolved shear stress (CRSS) for the structures with various degrees of nanostructure evolution by annealing, where extra strengthening was 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.
Highlights
Refractory high entropy alloys (R-High entropy alloys (HEAs)) are becoming prominent in recent years because of their properties and uses as high strength and high hardness materials for ambient and high temperature, aerospace and nuclear radiation tolerance applications, orthopedic applications etc
It can be inferred from the whole computational study of TaNbHfZr Refractory high entropy alloys (R-HEAs) that many of the thermodynamic, microstructural, strengthening, ductility and dislocation related properties can be obtained from the simulations alone
The thermodynamic changes from the initial random solid solution alloy by hybrid Monte Carlo/ Molecular Dynamics (MC/MD) structure evolution technique indicates that the entropic contribution to the free energy can be almost 40% of the change of enthalpy due to the presence of local chemical clustering
Summary
Refractory high entropy alloys (R-HEAs) are becoming prominent in recent years because of their properties and uses as high strength and high hardness materials for ambient and high temperature, aerospace and nuclear radiation tolerance applications, orthopedic applications etc. In the literature experimental studies on R-HEAs and other complex concentrated alloys (CCAs) have established that the mechanical properties are largely correlated to the change in short-range ordering/clustering (SRO/SRC) of various constituent elements, nanostructure phase-instabilities and nanostructure morphologies arising due to the various processing r outes[12,15,16,17] All these processing parameters and related microstructure becomes more relevant in this era of new manufacturing technologies like additive manufacturing (AM) and power metallurgy, where it is observed that an increase in laser power of AM techniques like directed energy deposition or selective laser melting can stabilize single phase average structures of HEA compositions, which is difficult to stabilize otherwise[18]. This type of physics-based simulations of microstructure, thermodynamics and resulting properties are becoming important in the emerging field of Integrated Computational Materials Engineering (ICME) in which a large number of experimental trials are reduced and replaced by computational structure–property relationships depending on the processing routes, aimed at greatly reducing the materials development time and c ost[20]
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