Influence of Grain Size on Mechanical Properties of a Refractory High Entropy Alloy under Uniaxial Tension

Abstract

There is a growing interest in High Entropy Alloys (HEAs) due to their outstanding mechanical properties. Most simulation studies have focused on face-centered cubic (fcc) HEAs; however, bcc HEAs can offer a larger elastic modulus and plastic yielding, thus, becoming possible candidates for the next generation of refractory materials. In this work, we focus on molecular dynamics (MD) simulations of bcc HfNbTaZr nanocrystalline samples, with a grain size (d) between 5 and 17 nm, deformed under tension at 300 K. The elastic modulus increases with the grain size and reaches a plateau near 10 nm. We find the typical inverse Hall–Petch (HP) behavior with yield strength, ultimate tensile stress (UTS), and flow stress increasing with d. Up to 12 nm, there are contributions from dislocations and twins; however, grain boundary (GB) activity dominates deformation. For the 5 nm grains, the GB disorder extends and leads to extensive amorphization and grain size reduction. For d>10 nm, there is a HP-type behavior with dislocations and twinning controlling deformation. For this regime, there is hardening at large strains. Compared to bcc single metal samples, the HP maximum of this HEA appears at a lower grain size, and this could be related to the chemical complexity facilitating dislocation nucleation. We use machine learning to help understand deformation regimes. We also compare our results to a single crystal (SC) HfNbTaZr HEA deformed along [001] and find that the single crystal is weaker than the nanocrystalline samples. The single crystal deforms initially by twinning and then rapidly by dislocation multiplication, leading to strong hardening. It has been proposed that edge dislocations play a major role in bcc HEA plasticity, and we also analyze the relative contributions of edge versus screw dislocations during deformation for both single crystal and nanocrystalline samples.

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 .…”

“…Such misidentification by CNA can occur also for atoms in cubic phases, within the naturally distorted HEA structures 76 . Amorphization, supported by the lack of structure in the pair correlation function, was found in nanocrystalline HfNbTaZr under tension, for 5 nm grain size 72 .…”

Simulated nanoindentation into single-phase fcc Fe$$_{x}$$Ni$$_{1-x}$$ alloys predicts maximum hardness for equiatomic stoichiometry

“…Finally, dislocation segments can be analyzed according to their screw- or edge-character. To this end, we determine the angle between the Burgers vector and the dislocation line vector of each dislocation segment; we denote a segment as ‘screw’ if the angle is , as ‘edge’ if it is , and as ‘mixed’ otherwise 46 . Figure 10 shows the fraction of screw and edge segments in the entire sample.…”

“…In order to extract the twinned regions, we use the orientation quaternions obtained from the Polyhedral Template Matching (PTM) filter 44 available in OVITO, and transform them to vectors in Rodrigues space 45 . We calculate the misorientation angle for each atom from the dot product between the Rodrigues vector of that atom at a given frame and the vector at the initial frame 46 . Plasticity, including twinning, dislocations and stacking faults, leads to rotations with greater than .…”

Improved Thermophysical and Mechanical Properties in LiNaSO4 Composites for Thermal Energy Storage

“…It is well known that some parameters, such as the incorporation of binders [ 40 ], porosity [ 41 ], grain size [ 42 , 43 ], or density [ 44 ], strongly affect the mechanical behavior of the final materials. For this reason, before the study of the mechanical properties of the composites, the bulk and real density, together with the total porosity, were measured for all the samples (shown in Table 3 ), as well as the morphology of the composites by means of SEM.…”