Sample preparation and structure characterizations. The NiCoCrO alloy was produced by spark plasma sintering (SPS). The elemental powders of Ni, Co, Cr and NiO with purity greater than 99.95% and the initial particle size of approximately 44 μm (~325 mesh) were used as the starting materials. The stoichiometric ratio of Ni, Co, Cr and NiO was 3:3:3:1 (in molar proportion) for the preparation of the powder mixtures. Planetary ball milling (3SP2, MITR) was employed to mix the raw elemental powders in a tungsten-carbide vial at a speed of 300 rpm for up to 10 hours. Tungsten-carbide balls were added in a 10:1 ball-powder ratio under high-purity Ar as the protecting atmosphere. The as-milled powders were consolidated as bulk alloy by spark plasma sintering (SPS, 3T-3-MIN, Chenhua) in a carbon felt covered graphite mold. The samples were heated from room temperature to 1300 °C at a heating rate of 100 °C/min and then sintered at 1300 °C for 10 min under constant pressure of 30 MPa. The vacuum pressure was maintained at ≤ 5 Pa, followed by furnace cooling.
The TEM specimens were polished and then thinned by ion bombardment thinning in a Gatan PIPS Ⅱ 695. Structural characterization using atomic-resolution high angle annular dark-field (HAADF) and bright-field (BF) images were conducted in spherical-aberration-corrected TEM instruments (Titan G2 80-200 ChemiSTEM, FEI; and ARM200F, JEOL). The atomic-resolution EDS mapping was performed by aberration-corrected STEM (Spectra 300 X-CFEG, FEI). The characterization of oxygen atoms using the advanced integrated differential phase contrast (iDPC) technique was carried out by Spectra 300 X-CFEG (FEI).
In situ TEM and SEM experiments. In situ TEM tensile experiments were conducted at room temperature using PicoFemto in situ tensile TEM single tilt holder-FST-ST in a FEI Tecnai G2 F20 TEM operating at 200 kV. The samples for in situ compression tests were prepared by a dual-beam SEM/focused ion beam (FIB) system (Quanta 3D FEG, FEI). The FIB-milled micropillars were prepared with various diameters ranging from 0.5 μm to 3 μm, while the aspect ratio was ~2.2. The in situ compression experiments were performed using a Hysitron Pi87 SEM nanoindenter in a FIB (Helios 600, FEI).
The calculation of solid solution strengthening effect. The introduction of interstitial atoms leads to lattice distortion. The interaction between dislocation and lattice stress can improve the strength of alloys. Based on Labush model39-41, the yield stress increment due to interstitial solid solution strengthening is given by:
Where M is the Taylor factor, ν is Poisson’s ratio, w is a material parameter, b is the magnitude of the Burgers vector, μ is the shear modulus, is the misfit strains due to tetrahedral-interstitial and octahedral-interstitial solute atoms respectively, is the tetrahedral-interstitial solute content, and is the octahedral-interstitial solute content. For NiCoCrO, taking M = 3.06, ν = 0.30, w =5b42, μ = 88.5 GPa. Based on the first-principles calculation, The calculation result is about 185.91MPa.
DFT calculation of O interstitials formation energy. The equi-molar NiCoCr samples, containing 108 atoms, were generated as special quasi-random structure (SQS)44, which was used as the initial starting point of the simulations. For the sake of calculating the formation energies of interstitial oxygen in NiCoCr, one oxygen was randomly inserted into different octahedral and tetrahedral sites of NiCoCr. Energy calculations were performed using the Vienna ab initio simulation package45, 46 with a plane wave cutoff energy of 520 eV. Brillouin zone integrations were performed using 2×2×2 Monkhorst–Pack meshes47 and spin polarization was considered. Projector augmented wave potentials48 were adopted with the Perdew–Burke–Ernzerhof generalized-gradient approximation for the exchange-correlation functional49. The total energy tolerances were set to be 1.0×10-5 eV/atom.
AIMD simulation of diffusivity of O interstitials. The diffusivity of interstitial oxygen in NiCoCr was implemented via ab initio molecular dynamics simulations (AIMDs). Seven oxygen atoms (~6% concentration) are randomly inserted into the octahedral and tetrahedral sites of NiCoCr. The plane wave cutoff energy was set to 450 eV. Brillouin zone integrations were performed using a single k-point (Γ)47. The time step of 2 fs was used for AIMD simulation. The volume of initial configurations at each temperature was derived by fitting the pressure-volume curve and then, the supercell volume was fixed in the following AIMD simulations. After 20 ps of equilibration, a total of 1 ns of AIMD simulations is sufficient to acquire interstitial oxygen diffusion trajectories.
DFT calculation of dislocation-O interaction. DFT calculation was implemented to study the interaction between dislocations and O interstitials. At first, an edge dislocation dipole with Burgers vector b= 1/2<1 1 0> is introduced in different independent orthogonal periodic supercells (448 atoms) where the x, y, z axis along with [1 1 0], [1`1 1] and [1`1`2] directions, respectively. The dislocations are separated by a distance of about 2 nm away from each other in the supercells to avoid core-core interaction. Then, oxygen atoms (4 or 12) were randomly inserted into different tetrahedral or octahedral sites at both ends of the dislocation cores on the [1`1 1] slip surface. After fully relaxed the atoms of different initial O-containing configurations, we performed athermal quasi-static shear simulations: i.e. an increment of the shear strain, , was gradually applied to the supercell along the direction of Burgers vector b (i.e., x axis), followed by energy minimization. Repeating this process until the dislocation core starts to move from its strain-free position and interacts with the neighboring oxygen atoms. Energy calculations were performed using the Vienna ab initio simulation package45, 46 without considering magnetic effects. Projector augmented wave potentials were employed with the Perdew–Burke–Ernzerhof generalized-gradient approximation for the exchange-correlation functional48, 49. The plane wave cutoff energy was set to 400 eV with a maximum energy threshold of 2*10-4 meV/atom. Brillouin zone integrations were performed using a single k-point (Γ). We identify the dislocation cores based on the coordination neighbor analysis for each atom, which is visualized in the software of Ovito50.
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