Grain Boundaries In Molybdenum Research Articles

First-principles study of oxygen segregation and its effect on the embrittlement of molybdenum symmetrical tilt grain boundaries

Elemental segregation can pronouncedly affect the cohesion of grain boundary (GB), which shows tremendous potential in altering the thermal stability and mechanical properties of metals. In this work, we systematically investigate the segregation behavior of oxygen (O) atoms at a series of GBs in molybdenum (Mo), using first principles calculations, to reveal the effect of O segregation on the embrittlement of Mo GBs. It is found that O atoms energetically prefer to segregate at sites in both the GB core and the vicinity of the GB plane. The strengthening energy is strongly associated with the GB structure, and it linearly decreases with the continuous deformation of the GB. O segregation can lead to an increase in the volume of polyhedron sites and a decrease in the charge density among adjacent Mo atoms across the GB plane. The weakened Mo-Mo bonds are believed to disrupt the GB cohesion, thereby increasing the intergranular cleavage tendency of Mo. Interestingly, O segregation does not always trigger GB embrittlement. The Σ5(310)[001] symmetric tilt grain boundary (STGB) with an O atom doped at the cap trigonal prism (CTP) site possesses a strength as strong as the clean GB. O segregation promotes the generation of Mo-O bonds across the GB plane, which counteracts the adverse effect of weakened Mo-Mo bonds. This work deepens the understanding of the possible effect of O segregation on GB cohesion of Mo from the atomic and electronic scales, providing guidance for enhancing the mechanical properties of Mo via GB structure optimization.

Segregation of interstitial light elements at grain boundaries in molybdenum

We investigated the segregation of H, O, N and C at the grain boundaries in molybdenum using ab initio calculations. These light elements have the short-distance interactions with GBs and show the preferential to segregate around GBs. The most energetically favorable trapping sites were found to locate right at the open space of boundaries. Such segregation is associated with the low electron density at the GBs, where the segregation energy is the lowest. Our studies would help understand the possible strengthening or weakening caused by impurities in molybdenum.

Influence of crystal orientation and Berkovich tip rotation on the mechanical characterization of grain boundaries in molybdenum

In this study, nanoindentation experiments with a continuously measured stiffness are performed in the vicinity of grain boundaries in technically pure molybdenum. A significant change in hardness as a function of indentation depth is put in context with the corresponding deformation patterns in the indented crystals. The difference in hardness increase of 18% compared to 5% of two different grain boundaries with similar misorientation angle could be explained in this way. Five degrees of freedom have to be determined to fully describe a grain boundary. However, the influence of the rotation angle of a pyramidal indenter geometry around the loading axis as yet another loading parameter is assessed by backscattered electron micrographs and orientation deviation maps. Indentation experiments with different rotation angle at the same grain boundary confirm that a hardness increase close to the grain boundary is only apparent if a significant amount of plastic deformation is introduced towards the interface.

Cohesive strength of zirconia/molybdenum interfaces and grain boundaries in molybdenum: A comparative study

We present calculations within density functional theory on the thermodynamic stability and mechanical properties of t-ZrO2(001)/Mo(001) interfaces. The interfacial strength is evaluated by applying energy-based (work of separation) and stress-based (theoretical strength) criteria for different cleavage planes. The lowest energy for crack propagation is obtained for a cut creating a stoichiometric ZrO2(001) surface. Our results reveal that molybdenum grain boundaries contaminated with oxygen are less stable against brittle fracture than pure Mo GBs. Addition of Zr, however, can strengthen Mo grain boundaries that contain oxygen by forming an ultrathin zirconia film between Mo grains. The stress required to cleave an ultrathin zirconia film is equal to that required for a pure Mo GB.

Ab-initio search for cohesion-enhancing solute elements at grain boundaries in molybdenum and tungsten

The influence of all transition element dopants on grain boundary (GB) cohesion in both W and Mo is investigated by means of high throughput density functional theory (DFT) calculations. We present site specific segregation energies to GBs and free surfaces (FS) of W and Mo and evaluate the strength of embrittlement of the individual solutes. The segregation properties for a consistent set of solutes allow an unprecedented comparison to previous DFT data and phenomenological models for segregation. Our DFT data show that the trends are very similar in W and Mo for the different solutes, while the comparison with available phenomenological models highlights the limits of band-filling and elastic models for GB segregation and strengthening.

Influence of Mo/MoSe2 microstructure on the damp heat stability of the Cu(In,Ga)Se2 back contact molybdenum

The degradation behavior of Mo/MoSe2 layers have been investigated using damp heat exposure. The two studied molybdenum based films with different densities and microstructures were obtained by lifting off Cu(In,Ga)Se2 layers from a bilayer molybdenum stack on soda lime glass. Hereby, a glass/Mo/MoSe2 was obtained, which resembles the back contact as present in Cu(In,Ga)Se2 solar cells. The samples were degraded for 150h under standard damp heat conditions and analyzed before, during and after degradation. It was observed that the degradation resulted in the formation of needles and molybdenum oxide layers near the glass/Mo and the Mo/Cu(In,Ga)Se2 interfaces. X-ray Photoelectron Spectroscopy measurements have shown that the sodium was also present at the surface of the degraded material and it is proposed that the degraded material consists mostly of MoO3 with intercalated Na+. This element has likely migrated from the soda lime glass. This intercalation process could have led to the formation of NaxMoO3 ‘molybdenum bronze’ following this redox reaction:xNa++MoO3+xe−↔NaxMoO3It is proposed that the formed oxide layer contains NaxMoO3 with different Na+ contents and different grades of conductivity. This intercalation process can also explain the high mobility of Na+ via the grain boundaries in molybdenum. It was also observed that the molybdenum film with a top layer deposited at a high pressure is more susceptible for damp heat degradation.

Atomistic modelling of zirconium and silicon segregation at twist and tilt grain boundaries in molybdenum

We investigate the influence of Zr and Si segregation on the cohesive strength of grain boundaries (GBs) in molybdenum using density functional theory calculations. A tilt \(\Sigma \)5(310)[001] and twist \(\Sigma \)5[001] GB in bicrystal geometry are chosen as structural models. We determine the site preference of Zr and Si for segregation in these GBs and define the segregation energy. We quantify the effect of solutes on the stability of the GBs against brittle fracture by means of the Griffith criterion (work of separation). Additionally, the intrinsic bond strength of the GB containing a solute is quantified by means of the theoretical strength. The results show that Zr and Si tend to segregate at the GBs if the low-energy insertion sites are available. However, the work of separation is decreased by the presence of Zr and Si and even in the presence of oxygen, there is no increase of the Griffith energy. Contributions of strain and chemical energy are analysed in order to explain our findings.

Energies of formation and structures of point defects at tilt grain boundaries in molybdenum

The structure and formation energy of vacancies and interstitials at symmetrical tilt grain boundaries in molybdenum have been investigated by means of classical molecular dynamics. The dependence of the formation energy of these boundaries on the grain misorientation angle has been calculated. The structures of defects, energies of their formation, and diffusion mechanisms have been determined for the most stable grain boundaries.

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.

Atom probe study of grain boundary segregation in technically pure molybdenum

Molybdenum, a metal with excellent physical, chemical and high-temperature properties, is an interesting material for applications in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is limited because of the poor oxidation resistance at high temperatures and a brittle-to-ductile transition around room temperature, which is influenced by the grain size and the content of interstitial impurities at the grain boundaries. Due to the progress of the powder metallurgical production during the last decades, the amount of impurities in the current quality of molybdenum has become so small that surface sensitive techniques are not applicable anymore. Therefore, the atom probe, which allows the detection of small amounts of impurities as well as their location, seems to be a more suitable technique. However, a site-specific specimen preparation procedure for grain boundaries in refractory metals with a dual focused ion beam/scanning electron microscope is still required.The present investigation describes the development and successful application of such a site-specific preparation technique for grain boundaries in molybdenum, which is significantly improved by a combination with transmission electron microscopy. This complimentary technique helps to improve the visibility of grain boundaries during the last preparation steps and to evidence the presence of grain and subgrain boundaries without segregants in atom probe specimens. Furthermore, in industrially processed and recrystallized molybdenum sheets grain boundary segregation of oxygen, nitrogen and potassium is successfully detected close to segregated regions which are believed to be former sinter pores.

Ab initio calculation of traction separation laws for a grain boundary in molybdenum with segregated C impurites

We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the Σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction–separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy or WoS of the grain boundary, while the strength of the Σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction–separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy–displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities.

Segregation, Precipitation and Wetting of Tilt Grain Boundaries in Molybdenum

The intrinsic structure of different tilt grain boundaries in bcc molybdenum is determined by electron microscopy and compared to the ones obtained after an annealing treatment of the same boundaries in presence of different impurities like carbon and nickel. Specially grown bicrystals with tilt axes parallel to [001] and [011] are used. The boundaries correspond to the major coincidence relationships ∑ = 5, ∑ = 3 and ∑ = 11. Their experimental atomic structure is compared to calculated ones. After the treatments in presence of carbon or nickel the new structure is determined by electron microscopy from the structural and chemical aspect. After a treatment in presence of carbon the ∑ = 5[001]{310} boundary contains either a segregation or a very thin precipitate layer of a new MoCx quadratic phase. In presence of nickel, the physical phenomenon is possibly a wetting of the boundary. The different [011] tilt boundaries have a different behavior according to their respective energy.

High-resolution and conventional electron microscopy study of a Σ = 3, [101]{121} twin grain boundary in molybdenum

Abstract Complete structural analysis of a Mo bicrystal containing a Σ = 3, [101](121)A grain boundary (GB) was achieved by means of transmission electron microscopy from the microscopic to the nanoscopic level. Secondary GB dislocations compensating a small deviation from the exact Σ = 3 concidence were observed and their character was determined. Rigid-body displacements (RBDs) were determined directly from high-resolution electron microscopy images using samples with surface normal parallel and perpendicular to the tilt axis. Measured values of RBDs are compared with recent theoretical calculations.

Atomic structure of the Σ5 (310)/[001] symmetric tilt grain boundary in molybdenum

Atomistic simulations offer an important route towards understanding and modeling materials behavior. Incorporating the essential physics into the models of interatomic interactions is increasingly difficult as materials with more complex electronic structures than f.c.c. transition metals are addressed. For b.c.c. metals, interatomic potentials have been developed that incorporate angularly dependent interactions to accommodate the physics of partially filled d-bands. A good test of these new models is to predict the structure of crystal defects and compare them with experimentally observed defect structures. To that end, the Σ5 (310)/[001] symmetric tilt grain boundary in Mo has been fabricated and characterized by HREM. The experimentally observed structure is found to agree with predictions based on atomistic simulations using angular-force interatomic potentials developed from model generalized pseudopotential theory (MGPT), but disagrees with predictions based on radial-force potentials, such as those obtained from the Finnis–Sinclair method or the embedded atom method (EAM).

Structure and Stability of Grain Boundaries in Molybdenum with Segregated Carbon Impurities

AbstractThe segregation of interstitial impurities to symmetrical tilt grain boundaries (STGB) in bodycentered cubic transition metals is studied by means of ab-initio electronic-structure calculations based on the local density functional theory (LDFT). Segregation energies as well as changes in atomic and electronic structures at the ΣE5 (310) [001] STGB in Mo caused by segregated interstitial C atoms are investigated. The results are compared to LDFT data obtained previously for the pure Σ5 (310) [001] STGB in Mo. Energetic stabilities and structural parameters calculated ab initio for several crystalline Molybdenum Carbide phases with cubic, tetragonal or hexagonal symmetries and different compositions, MoCx, are reported and compared to recent high-resolution transmission electron microscopy (HRTEM) observations of MoCx, intergranular films and precipitates formed by C segregation to a Σ5 (310) [001] STGB in a Mo bicrystal.

Computer Simulation and High Resolution Electron Microscopy Study of the Σ=5 (210) [001] Symmetric Tilt Grain Boundary in Molybdenum

We present the results of a parallel study of the atomic structure of the Σ = 5 (210) [001] symmetric tilt grain boundary in bcc Mo, by computer simulation and High Resolution Electron Microscopy (HREM). Excess energy values of different boundary configurations are obtained via a quasi-dynamic minimisation scheme while cohesion is described by a new n-body, central-force, phenomenological potential which satisfactorily reproduces static and dynamic properties of the bulk material. HREM observations and numerical modelling both show the symmetric configuration of the Σ = 5 (210) [001] symmetric tilt grain boundary (GB) to be of lowest energy.

Atomic-scale APFIM and TEM investigation of grain boundary microchemistry in Astroloy nickel base superalloys

Electron microscopy and atom-probes techniques, including 3D (three-dimensional) atom-probe, were applied to the investigation of grain-boundary (GB) microchemistry and interfacial segregation phenomena in nickel base superalloys, Astroloy. Diffraction patterns and EDX microanalyses exhibit the presence of intergranular M 23C 6 chromium enriched carbides as well as intragranular TiC precipitates. Complex phases containing Zr, C and S were also observed. Three-dimensional images as provided by the tomographic atom-probe revealed the presence of a strong segregation of both boron and molybdenum at grain boundaries. Considerable chromium enrichment at γ′-γ′ grain boundaries and slight carbon segregation to GBs, whatever their chemical nature, were also detected. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1200 and 800°C. Boron, chromium and molybdenum GB enrichments are thought to be mainly controlled by an equilibrium segregation process. No segregation of zirconium was detected.

Significance of non-central forces in atomistic studies of grain boundaries in bcc transition metals

Abstract The effects of non-central forces in atomistic studies of grain boundaries in molybdenum and tungsten, the transition metals with half-filled d-band, are investigated. For this purpose we have used two different types of potential which include different number of moments of the local density of electronic states when evaluating the total energy: the central-force Finnis-Sinclair potentials which include the scalar second moment and the potentials constructed by Carlsson which include the fourth and the matrix second moments. The energy terms associated with these two moments represent non-central interactions and assure that the bcc-fcc structural energy difference is reproduced with good accuracy. For the three boundaries studied, the non-central forces have been found to be very important in determining the lowest energy structures. In particular, the energy differences between multiple structures depend on specific orientations and geometries of the atomic clusters at and near the interface. ...

Structure determination of planar defects in crystals of germanium and molybdenum by HREM

The possibility of the structure determination of planar defects in crystals by high-resolution electron microscopy (HREM) is presented. The optimum conditions for a direct visual interpretation of the projected structure are discussed using two typical examples: a Σ = 5 tilt grain boundary in germanium and a Σ = 41 grain boundary in molybdenum. It is shown that two particular defect images taken at the Scherzer defocus and at reverse contrast enable an approximate input structure to be found which can be employed to start a trial-and-error method. A practical procedure improving this input structure is proposed on the basis of the calculated positioning errors. It is demonstrated that positioning errors are mainly due to the electron microscope transfer function. The effect of crystal tilt is shown to modify greatly the atomic column visibility at the interface plane. Finally three-dimensional analysis of an interface is proposed by observation of it along two (or more) low-index axes. This procedure is illustrated on the Ge Σ = 5 grain boundary observed along [001] and [310].

High resolution study of Σ=41 grain boundary in molybdenum

High resolution study of Σ=41 grain boundary in molybdenum