Development of an interatomic potential for the simulation of defects, plasticity, and phase transformations in titanium

Deformation mechanisms based on the multiscale molecular dynamics of a gradient TA1 titanium alloy

The heterogeneous gradient TA1 titanium alloy holds great potential for a wide range of industrial applications. Considering the influence of the gradient structure on the plastic deformation behavior of the material, the TA1 gradient polycrystalline model under uniaxial compression is established. The deformation behavior of TA1 gradient polycrystals under uniaxial compression is investigated by molecular dynamics simulation. The simulation shows that there is significant transmission during the plastic deformation of TA1 gradient polycrystals. The transmissibility of plastic deformation is specified by the alternating appearance of twinning and grain refinement. Besides, the uniaxial compression process is accompanied by active dislocation motions. Moreover, the movement of dislocations is a dynamic cyclic process. In the same uniaxial compression environment, the triggering of the plasticity mechanism in the gradient polycrystalline model is closely related to grain size. The smaller grain size crystals hardly produce plastic deformation. Grain boundary migration of medium grain size crystals dominates in plastic deformation. The proliferation of dislocations under compressive stress is the primary trigger mechanism in larger grain size crystals. In addition, the stress concentration phenomenon in regions with medium grain sizes is more significant than that in regions with larger and small grain sizes.

Atomic structure of intermetallic compound Nb5Si3by new cluster transformation analysis method

The structure of Nb₅Si₃at the atomic level is fundamental for identifying its complicated structure in atomic simulations and for further understanding the phase selection behaviors during the solidification of Nb-Si alloys. In this study, the structure of Nb₅Si₃was investigated using deep-learning molecular dynamic simulations. The idealβNb₅Si₃is characterized by Nb-centered Voronoi polyhedrons (VPs) <0,0,12,3>, <0,0,12,2>, and Si-centered VPs <0,2,8,2>, <0,2,8,0>. Most initial VPs are distorted at high temperatures due to intense thermal perturbation. A new cluster transformation analysis (CTA) method was proposed to evaluate the stability of ideal VPs against perturbation and predict the possible transformations of the initial VPs in atomic simulations. Most transformations of the initial VPs inβNb₅Si₃originate from distortions at the edges of the Nb-centered VPs and the faces/vertices of the Si-centered VPs. The distorted VPs inβNb₅Si₃at high temperatures are dominated by <0,1,10,4>, <0,1,10,5>, <0,2,8,1> and <1,2,5,3> VPs, which are predicted as the primary transformations by the CTA.

Ultrastrong nanotwinned titanium alloys through additive manufacturing

Titanium alloys, widely used in the aerospace, automotive and energy sectors, require complex casting and thermomechanical processing to achieve the high strengths required for load-bearing applications. Here we reveal that additive manufacturing can exploit thermal cycling and rapid solidification to create ultrastrong and thermally stable titanium alloys, which may be directly implemented in service. As demonstrated in a commercial titanium alloy, after simple post-heat treatment, adequate elongation and tensile strengths over 1,600 MPa are achieved. The excellent properties are attributed to the unusual formation of dense, stable and internally twinned nanoprecipitates, which are rarely observed in traditionally processed titanium alloys. These nanotwinned precipitates are shown to originate from a high density of dislocations with a dominant screw character and formed from the additive manufacturing process. The work here paves the way to fabricate structural materials with unique microstructures and excellent properties for broad applications.

Interatomic Potential to Predict the Favored Glass-Formation Compositions and Local Atomic Arrangements of Ternary Al-Ni-Ti Metallic Glasses

An empirical potential under the formalism of second-moment approximation of tight-binding potential is constructed for an Al-Ni-Ti ternary system and proven reliable in reproducing the physical properties of pure elements and their various compounds. Based on the constructed potential, molecular dynamic simulations are employed to study metallic glass formations and their local atomic arrangements. First, a glass-formation range is determined by comparing the stability of solid solutions and their corresponding counterparts, reflecting the possible composition region energetically favored for the formation of amorphous phases. Second, a favored glass-formation composition subregion around Al0.05Ni0.35Ti0.60 is determined by calculating the amorphous driving forces from crystalline-to-amorphous transition. Moreover, various structural analysis methods are used to characterize the local atomic arrangements of Al0.05NixTi0.95-x metallic glasses. We find that the amorphous driving force is positively correlated with glass-formation ability. It is worth noting that the addition of Ni significantly increases the amorphous driving force configurations of fivefold symmetry and structural disorder in Al0.05NixTi0.95-x metallic glasses until the content of Ni reaches approximately 35 at%.

Two-step nucleation of the Earth's inner core

The Earth's inner core started forming when molten iron cooled below the melting point. However, the nucleation mechanism, which is a necessary step of crystallization, has not been well understood. Recent studies have found that it requires an unrealistic degree of undercooling to nucleate the stable, hexagonal, close-packed (hcp) phase of iron that is unlikely to be reached under core conditions and age. This contradiction is referred to as the inner core nucleation paradox. Using a persistent embryo method and molecular dynamics simulations, we demonstrate that the metastable, body-centered, cubic (bcc) phase of iron has a much higher nucleation rate than does the hcp phase under inner core conditions. Thus, the bcc nucleation is likely to be the first step of inner core formation, instead of direct nucleation of the hcp phase. This mechanism reduces the required undercooling of iron nucleation, which provides a key factor in solving the inner core nucleation paradox. The two-step nucleation scenario of the inner core also opens an avenue for understanding the structure and anisotropy of the present inner core.

Survey of Grain Boundary Energies in Tungsten and Beta-Titanium at High Temperature

Heat treatment is a necessary means to obtain desired properties for most of the materials. Thus, the grain boundary (GB) phenomena observed in experiments actually reflect the GB behaviors at relatively high temperature to some extent. In this work, 405 different GBs were systematically constructed for body-centered cubic (BCC) metals and the grain boundary energies (GBEs) of these GBs were calculated with molecular dynamics for W at 2400 K and β-Ti at 1300 K and by means of molecular statics for Mo and W at 0 K. It was found that high temperature may result in the GB complexion transitions for some GBs, such as the Σ11{332}{332} of W. Moreover, the relationships between GBEs and sin(θ) can be described by the functions of the same type for different GB sets having the same misorientation axis, where θ is the angle between the misorientation axis and the GB plane. Generally, the GBs tend to have lower GBE when sin(θ) is equal to 0. However, the GB sets with the <110> misorientation axis have the lowest GBE when sin(θ) is close to 1. Another discovery is that the local hexagonal-close packed α phase is more likely to form at the GBs with the lattice misorientations of 38.9°/<110>, 50.5°/<110>, 59.0°/<110> and 60.0°/<111> for β-Ti at 1300 K.

Femtosecond laser-induced nanoporous layer for enhanced osteogenesis of titanium implants

The osteogenic activity of medical metal can be improved by lowering its surface stiffness and elastic modulus. However, it is very difficult to directly reduce the elastic modulus of medical metal surfaces. In this paper, with selected parameters, the titanium surface was treated via femtosecond laser irradiation. Micro indentation revealed that the femtosecond laser ablation can effectively reduce the surface Young's modulus and Vickers hardness of titanium. Besides, In order to explain the mechanical properties of degradation of titanium surface, Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used to simulate the process of laser ablation process of titanium surface, and it was found that after the ablation of titanium surface, voids were produced in the subsurface layer. The simulation showed that the voids are formed by the cavitation of metastable liquid induced by high tensile stress and high temperature during femtosecond laser irradiation. Subsurface voids with a thickness of about 40 nm were observed under the oxide layer in the experiment. Cell experiments showed that the surface with low Young's modulus was more conducive to cell proliferation and osteogenic differentiation.

Transformations of the Microstructure and Phase Compositions of Titanium Alloys during Ultrasonic Impact Treatment. Part I. Commercially Pure Titanium

Experimental and theoretical studies helped to reveal patterns of surface roughening and the microstructure refinement in the surface layer of commercial pure titanium during ultrasonic impact treatment. Applying transmission electron microscopy technique, a gradient microstructure in the surface layer of the ultrasonically treated sample, where the grain size is varied from nano- to micrometers was revealed. It was shown that the surface plastic strains of the titanium sample proceeded according to the plastic ploughing mechanism, which was accompanied by dislocation sliding, twinning, and the transformations of the microstructure and phase composition. The molecular dynamics method was applied to demonstrate the mechanism of the phase transformations associated with the formation of stacking faults, as well as the reversible displacement of atoms from their sites in the hcp lattice, causing a change in coordination numbers. The role of the electronic subsystem in the development of the strain-induced phase transformations during ultrasonic impact treatment was discussed.

Direct atomistic simulations of metastable state destruction in titanium (β-α martensitic transition) caused by external influences

Large-scale classical molecular dynamics (CMD) is utilized to simulate the β→α phase transition in pure titanium. Samples with a metastable polycrystalline bcc structure are prepared using crystallization from liquid state and subsequent recrystallization at elevated temperatures. Controlling the heating-cooling regimes we prepared two different kinds of samples with coarse and fine grain structures. The metastable bcc samples were relaxed at temperatures noticeably lower than the equilibrium β-α transition temperature. During the following cooling of the samples down to room temperature, transitions to the α phase start. With the prepared metastable bcc samples of two kinds we perform the CMD study of the β→α transition under plain shock wave loading and imposed shear deformations. From the CMD simulations we obtain information about the transformation barriers, mechanisms, and kinetics. Results of CMD simulations suggest that grain boundaries hamper the hcp phase growth.

Recovery of Scratch Grooves in Ti-6Al-4V Alloy Caused by Reversible Phase Transformations

The deformation behaviors of Ti-6Al-4V alloy samples with lamellar and bimodal microstructures under scratch testing were studied experimentally and using molecular dynamics simulation. It was found that the scratch depth in the sample with a bimodal microstructure was twice as shallow as that measured in the sample with a lamellar microstructure. This effect is attributed to the higher hardness of the sample with a bimodal microstructure and the larger amount of elastic recovery of scratch grooves in this sample. On the basis of the results of molecular dynamics simulation, a mechanism was proposed, which associates the recovery of the scratch grooves with the inhomogeneous vanadium distribution in the β-areas. The calculations showed that at a vanadium content typical for Ti-6Al-4V alloy, both the body-centered cubic (BCC) and hexagonal close-packed (HCP) structures can be more energetically favorable depending on the atomic volume. Therefore, compressive or tensile stresses induced by the indenter could facilitate β→α and α→β phase transformations, respectively, in the vanadium-depleted domains of the β-areas, which contribute to the recovery of the Ti-6Al-4V alloy subjected to scratching.

Theoretical development and experimental validation on the measurement of temperature by extended X-ray absorption fine structure

A systematic investigation on the theoretical framework of the ultra-fast measurement of temperature by extended X-ray absorption fine structure (EXAFS) applied in laser-driven-compression experiments has been carried out and a new temperature measurement scheme based on the EXAFS cumulant expansion analysis and anharmonic correlated Debye model has been advanced. By considering the anharmonic effect of thermal vibration and avoiding the employment of the empirical model as well as parameters which have large inherent uncertainties in the temperature determination, this new scheme is theoretically more accurate than traditional ones. Then the performance of the new measurement scheme and traditional methods were validated on a synchrotron radiation platform by temperature-dependent EXAFS (TDEXAFS) experiments on Au, Fe, V and Ti; the results showed that the new scheme could provide the most accurate measured temperatures with much lower uncertainties. This accurate scheme gives a firmer physical ground to the EXAFS temperature measurement technique and can expect to be applied in laser-driven compression experiments and promote the development of matter state research at extreme conditions.

Formation free energies of point defects and thermal expansion of bcc U and Mo

[Formula: see text]-U is a high temperature body-centred cubic (bcc) phase of uranium which is mechanically unstable at T  =  0 K. The point defect properties in pure bcc uranium are not sufficiently well studied. In this work we use classical molecular dynamics simulations with the thermodynamic integration approach to calculate the formation free energies of vacancies and interstitials in bcc uranium and, for comparison, in bcc molybdenum. Contrary to the majority of other metals where the formation free energy is (much) higher for interstitials than for vacancies, our results show that in [Formula: see text]-uranium interstitials are the dominating type of defects in thermal equilibrium. We discuss the possible implications of this finding in the context of the thermal expansion data for [Formula: see text]-U that provide a certain supporting evidence.

Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials

Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.

Hydrogen contamination in metals during sample preparation for high-resolution microscopy remains a challenge, especially when hydrogen itself is being investigated. Here, the authors show that using cryogenic milling significantly reduces hydrogen pick-up during sample preparation of titanium and titanium alloys.