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Two-dimensional monolayers derived from 3D bulk structures remain a relatively unexplored class of materials because of the challenge of stabilizing nonepitaxial interfaces. Here, we report an unusual reconstruction during the deposition of precursors when targeting the synthesis of heterostructures with an odd number of PbSe monolayers. Multilayer elemental precursors of Pb|Se + V|Se were deposited to have the correct number of atoms to form [(PbSe)1+δ]q(VSe2)1 where q is the number of PbSe monolayers in the heterostructure. Structural analysis of the self-assembled precursor via X-ray reflectivity, X-ray diffraction, and HAADF-STEM suggests three different behaviors upon deposition. Precursors with q ≥ 7 and even values of q have the targeted nanoarchitectures after deposition, which are maintained as the products are self-assembled through a near diffusionless process. Significant lateral surface diffusion occurred during the deposition of precursors with q = 1, 3, and 5, resulting in the precursor to have a different nanoarchitecture than targeted. Additional perpendicular long-range diffusion occurs during self-assembly of these precursors, resulting in different final products than targeted. Density functional theory (DFT) calculations of PbSe blocks show that the odd-numbered layers are less stable than the even-numbered layers, which suggests an energetic driving force for the observed rearrangement. This work highlights the importance of understanding the reaction mechanism when attempting to prepare 2D layers of constituents with bulk 3D structures.

Diffusion between Al and Mg was investigated comprehensively using both high-throughput experiments and density functional theory (DFT) calculations. Experimental diffusion coefficients in fcc Al, hcp Mg, β–Mg17Al12, γ–Mg2Al3 and e − Mg23Al30 phases were collected by combining diffusion multiples with forward-simulation analysis together with a critical review of the experimental studies in the literature. The best settings to compute the dilute (impurity) diffusion coefficients of Al and Mg using DFT were tested by comparing the computed data using various DFT settings with the critically assessed experimental diffusion coefficients in stable phases (fcc Al and hcp Mg). The optimal DFT settings were employed to calculate the dilute diffusion coefficients of Al and Mg in metastable (hypothetical) phases (hcp Al and fcc Mg) where experimental measurements were impossible. The atomic mobilities of Al and Mg in the Al–Mg binary system across the entire composition range were then reliably optimized for both the fcc and hcp phases based on the comprehensive diffusion data obtained from both experimental measurements for the stable phases and DFT calculations for the metastable phases. This study demonstrates an efficient and reliable way to develop fundamental mobility databases using integrated experimental and computational methods.

Twinning is an important mechanism during the plastic deformation of materials with hexagonal-close-packed crystal structure. Strength and formability are critical properties for this family of structural materials and can consequently be improved by controlling twin nucleation and growth. Here, we study the effect of alloying elements on strain rate and temperature dependence of twin growth. Without using any fitting parameters, we derive an analytical model which takes atomic scale calculated parameters as input and predicts the equilibrium concentration of segregated solutes and strengthening due to solute-twin interactions. This model predicts the strain-rate/temperature conditions of negative strain rate sensitivity in titanium, under loading conditions that favor twinning, with oxygen interstitial. The predictions are in agreement with available experiments. Our findings present a new mechanism for dynamic strain aging as a result of solute segregation to the twin boundaries.

Individually, increasing the concentration of either oxygen or aluminum has a deleterious effect on the ductility of titanium alloys. For example, extremely small amounts of interstitial oxygen can severely deteriorate the tensile ductility of titanium, particularly at cryogenic temperatures. Likewise, substitutional aluminum will decrease the ductility of titanium at low-oxygen concentrations. Here, we demonstrate that, counter-intuitively, significant additions of both Al and O substantially improves both strength and ductility, with a 6-fold increase in ductility for a Ti-6Al-0.3 O alloy as compared to a Ti-0.3 O alloy. The Al and O solutes act together to increase and sustain a high strain-hardening rate by modifying the planar slip that predominates into a delocalized, three-dimensional dislocation pattern. The mechanism can be attributed to decreasing stacking fault energy by Al, modification of the “shuffle” mechanism of oxygen-dislocation interaction by the repulsive Al-O interaction in Ti, and micro-segregation of Al and O by the same cause., Individual addition of O or Al is known to deteriorate ductility of Ti alloys, especially at cryogenic temperatures. Here the authors demonstrate a counter-intuitive effect where significant addition of both Al and O substantially improves both strength and ductility in a Ti-6Al-0.3 O ternary alloy.

A systematic study of Ti-O alloys reveals the mechanism behind acute oxygen sensitivity in titanium., One of the most potent examples of interstitial solute strengthening in metal alloys is the extreme sensitivity of titanium to small amounts of oxygen. Unfortunately, these small amounts of oxygen also lead to a markedly decreased ductility, which in turn drives the increased cost to purify titanium to avoid this oxygen poisoning effect. Here, we report a systematic study on the oxygen sensitivity of titanium that provides a clear mechanistic view of how oxygen impurities affect the mechanical properties of titanium. The increased slip planarity of Ti-O alloys is caused by an interstitial shuffling mechanism, which is sensitive to temperature, strain rate, and oxygen content and leads to the subsequent alteration of deformation twinning behavior. The insights from our experimental and computational work provide a rationale for the design of titanium alloys with increased tolerance to variations in interstitial content, with notable implications for more widespread use of titanium alloys.

First-principles calculation of diffusion coefficients between Mg and Al is investigated comprehensively using density functional theory (DFT). The effect of different uncertainty sources arising from first principles calculations has been investigated systematically. These sources include the diffusion model, energetic, entropic and attempt frequency calculations. Variation in self and impurity diffusion coefficients of Mg and Al in stable phases are quantified using different DFT settings and compared with the experiments. Using the optimal DFT settings, diffusion coefficients in metastable phases of Al and Mg are predicted. The dataset refers to "An integrated experimental and computational study of diffusion and atomic mobility of the aluminum-magnesium system" [1].

We study the diffusivity of oxygen interstitials around a ( 10 1 ¯ 2 ) twin boundary in Titanium. First, we identify all possible stable interstitial sites around the twin boundary and compute the corresponding site energies and transition energy barriers for jumps between these sites, using density functional theory. We show that the site energies and the barriers are consistently lower than in bulk, suggesting the higher tendency of oxygen to segregate to the twin boundary region. Using the site and transition energies and an exact solution to the master equation, we then compute the diffusivity of oxygen in the presence of the twin boundary and find enhanced diffusivity around the boundary in all directions. Enhanced diffusivity towards the boundary determines the feasibility of oxygen segregation to favorable sites at the boundary, while increased diffusivity in the boundary plane provides a path for fast diffusion of oxygen. This result reveals the underlying mechanism governing the slow growth of ( 10 1 ¯ 2 ) twin by pinning at the segregated oxygen interstitials.

The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2 -type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a -type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2 dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing.

High-entropy alloys (HEAs) are an exciting new class of multi-component alloys some of which have unusual and remarkable properties. As of yet, little is understood about dislocation core structure and stacking fault energies in these alloys. For this study, a five-component, equiatomic alloy (CrMnFeCoNi) was deformed to 5% plastic strain at room temperature. Post-test observations using diffraction contrast scanning transmission electron microscopy (DC-STEM) analysis provide evidence for numerous planar slip bands composed of ½ dislocations. More detailed analyses of dislocation separation distances were performed using high-order diffraction vector DC-STEM and atomic resolution high angle annular dark field (HAADF) STEM on ½ dislocations in 60° orientation. Large variations in dissociation distances are found, leading to the concept of a local stacking fault energy (SFE). This finding is supported through embedded-atom-method (EAM) calculations of a model, concentrated, three-element solid solution. For the first time, the Nye tensor and center of symmetry analysis were used collectively to accurately determine dissociation distance. Lastly, using high-resolution energy dispersive X-ray spectroscopy, no ordering or segregation was observed, indicating that this alloys is a true solid solution down to the atomic scale in the recrystallized and lightly deformed state.

Dislocation/boundary interactions play a prominent role in mechanical properties and plastic deformation of materials. We study the interaction between prismatic screw , prismatic edge and pyramidal mixed dislocations with and twin boundaries in titanium using atomistic simulations. Details of the dislocation reactions depend on the slip system, atomic structure of boundary and stress/strain states. All interactions lead to nucleation of twinning dislocations on both twin boundaries, confirming that the interaction with incoming dislocations is a twin growth mechanism. In addition, dissociation of and dislocations on the results in nucleation of a (tension twin) embryo in the second grain-a new twin nucleation mechanism for twins as a result of and slip.