Atomistic Simulations of Ductile Failure in a b.c.c. High-Entropy Alloy

Abstract

Ductile failure is studied in a bcc HfNbTaZr High-Entropy Alloy (HEA) with a pre-existing void. Using molecular dynamics simulations of uniaxial tensile tests, we explore the effect of void radius on the elastic modulus and yield stress. The elastic modulus scales with porosity as in closed-cell foams. The critical stress for dislocation nucleation as a function of the void radius is very well described by a model designed after pure bcc metals, taking into account a larger core radius for the HEA. Twinning takes place as a complementary deformation mechanism, and some detwinning occurs at large strain. No solid-solid phase transitions are identified. The concurrent effects of element size mismatch and plasticity lead to significant lattice disorder. By comparing our HEA results to pure tantalum simulations, we show that the critical stress for dislocation nucleation and the resulting dislocation densities are much lower than for pure Ta, as expected from lower energy barriers due to chemical complexity.

Nanoindentation into a bcc high-entropy HfNbTaTiZr alloy—an atomistic study of the effect of short-range order

“…In experiment, 8 , 74 twinning was detected in deformation of HfNbTaTiZr for temperatures lower than 873 K; twinning at large shear strain has also been reported for the same EAM potential used here 9 , and twins under tension were found for simulations of HfNbTaZr when a void was present 67 , and for nanocrystalline samples 72 . However, Hu et al do not observe twinning under the high applied stress through direct impact Hopkinson bar test and compression strain 75 .…”

“…with the lasso mechanism 31 , and leads to the complete absence of prismatic loops for the SRO sample. Something similar was observed under tension for a bulk sample containing a void, where Ta exhibited long dislocation loops while HfNbTaZr showed only short dislocations 67 . We note that experiments, including TEM observations, suggest that the plasticity in HfNbTaTiZr was due to screw dislocation glide 34 , 68 , 69 .…”

“…For hcp, the enthalpy differences are similar, and hcp becomes preferred at around GPa. Something similar 67 happened for a different interatomic potential for HfNbTaZr, with both fcc and hcp becoming preferred over bcc under large tensile stress, but with smaller energy differences, near 0.03 eV. Ref.…”