Effects of lattice distortion and chemical short-range ordering on the incipient behavior of Ti-based multi-principal element alloys: MD simulations and DFT calculations

One of key factors in designing high-performance alloys is to understand their incipient behavior. To get detailed real-time atomic scale evolutions of multi-principal element alloys (MPEAs) during nanoindentation incipient yielding, molecular dynamics (MD) simulations were carried out. Emphases are given to the effects of lattice distortion (LD) and chemical short-range ordering (CSRO) on the initiation of plasticity in body-centered cubic (BCC) MPEAs. The initial plastic mechanism is embryo nucleation assisted and the incipient behavior is strongly affected by LD and CSRO. Specifically, the embryo nucleation load Pe is reduced by LD and CSRO and the incipient drop P0 is lowered by LDI that only comes from atomic size difference and further declined by CSRO. This is because LD and CSRO could boost embryo nucleation, which can act as precursors for dislocation nucleation. This is consistent with the free-end nudged elastic band observations. Although Pe is weakened by LDII generated by introducing extra elements, P0 is intensified because severe LD could dominate the following process. The dislocation starvation mechanism is found in an average-atom (AA) model. However, it is replaced by dislocation interactions due to LD and CSRO. Density functional theory (DFT) calculations suggest that charge density distribution and the amount of directional Al-Al bonding would play a critical role in the onset of plastic deformation. When a larger indenter radius was used in MD simulations, the dislocation starvation mechanism is absent in AA model due to the reduced maximum shear stress. Instead, the dislocation propagation prevails. With respect to the orientation effect, three investigated orientations demonstrate a distinct yielding behavior.