Fcc Aluminum Research Articles

Cluster compounds formation in metal alloys crystals

The formation and physical origin of the pre-cluster complexes and pre-phase (cluster) compounds have been studied on cast magnesium alloys (in the liquid and solid states) as well as other primary solutions enriched with excess (non-intrinsic, structural) vacancies. The results produced by DSC, TEM observation, Auger-electron, UV-and X-ray photoelectron spectroscopy techniques were combined with positron annihilation, electroresistance, and internal friction measurements to evaluate different stages of the cluster compounds formation and to examine the strength of their chemical bonding in terms of a detailed analysis of electron-sensitive properties. The thermally activated deviations from classical (Arrhenius) behaviour in hydrogen-charged hcp and fcc metal crystals are found to be caused by the nucleation of the solute/excess vacancy complexes. Additional support for this short-range self-supported mechanism comes from data obtained by mechanical spectroscopy and positron annihilation. These pre-cluster separations are assumed to act as nuclei for the synthesis of the cluster compounds such as Mg16Ba2 and Al18Ba9 with own electronic (hybridized) structure in hcp magnesium- and fcc aluminium alloy crystals, respectively. A unified concept of the liquid/solid conversion for condensed cluster-forming alloys is considered. This approach is particularly useful for the development of chemically combined cluster compounds with own electronic configurations as functional materials for high-current electronics.

Texture Stability and Transition in an Accumulative Roll-Bonding-Processed Aluminum Single Crystal

The crystal plasticity finite element (CPFE) model was used to simulate the textural evolution in a FCC aluminum single crystal of the {1 1 2}〈1 1 1〉 (Cu) orientation processed by accumulative roll-bonding. The simulation was conducted up to nine cycles, and the predictions have been validated by the corresponding experimental observations. The dynamic balance between the destroyed and formed texture components (0 0 1)[1 1 0] and \( \left( { 4\; 4\; 1 1} \right)[ 1 1\; 1 1\;\bar{8} ] \), as representatives of shear-type texture and rolling-type texture, respectively, made their area fractions stable. Different deformation behaviors in the destroyed, formed, and preserved texture components (0 0 1)[1 1 0] and \( \left( {4\;4\;11} \right)[11\;11\;\bar{8}] \) were found and investigated.

Numerical investigation of evolution of earing, anisotropic yield and plastic potentials in cold rolled FCC aluminium alloy based on the crystallographic texture measurements

The mechanical anisotropy of FCC aluminium alloy is influenced by the polycrystalline orientations transformed by cold rolling. The relationship between mechanical anisotropy and evolution of crystallographic textures was established using the crystal plasticity finite element method (CPFEM) based on measured polycrystalline orientation density functions (ODFs) and used to determine the parameters of a phenomenological convex anisotropic yield function which are solved with an implicit finite element (FE) stress integration code. A multi-scale modelling system is generated by combining CPFEM and the phenomenological yield function enabling accurate simulations of forming operations without the need for conventional physical mechanical testing of the material.

An efficient Monte Carlo algorithm for determining the minimum energy structures of metallic grain boundaries

Sampling minimum energy grain boundary (GB) structures in the five-dimensional crystallographic phase space can provide much-needed insight into how GB crystallography affects various interfacial properties. However, the complexity and number of parameters involved often limits the extent of this exploration to a small set of interfaces. In this article, we present a fast Monte Carlo scheme for generating zero-Kelvin, low energy GB structures in the five-dimensional crystallographic phase space. The Monte Carlo trial moves include removal and insertion of atoms in the GB region, which create a diverse set of GB configurations and result in a rapid convergence to the low energy structure. We have validated the robustness of this approach by simulating over 1184 tilt, twist, and mixed character GBs in both fcc (Aluminum and Nickel) and bcc (α-Iron) metallic systems.

Thermal properties of fcc titanium and aluminum thin films

The molecular dynamics method with many-body potential of interatomic interaction constructed in the embedded atom model is used to study the thermal characteristics of aluminum and metastable fcc titanium nanofilms with (001) and (110) surface orientation. It is shown that in Al films the linear coefficients of thermal expansion are positive in all the directions, except for the area close to the melting temperature. For fcc titanium the linear coefficients of thermal expansion in the film plane are negative in a wide temperature range. With increasing temperature, in Ti films the local vibrational densities of states of the surface atomic layers, polarized along the x,y,z axes, shift to the low-energy part of the spectrum only in those directions where a decrease in the lattice parameters is observed. The changes in the interface areas of Al films occurring with increase in temperature, lead to the growth of anharmonicity and a “softening” of the local vibration spectra near the critical temperature. It is shown that the negative coefficient of linear expansion is an indicator of the simulated system being in metastable state.

Thermal vibration contribution to continuum stress in the elastic regime

Homogeneous deformation of an ordered crystalline solid at finite temperature can cause non-affine transformation of atomic trajectories. In such a case, continuum measures based on affine transformation of trajectories are insufficient to ensure energetic equivalence between the atomic and continuum scales. We use molecular dynamics simulation of fcc aluminum in NVT ensembles to demonstrate that the second moments of atomic positions about equilibrium show increasing deviation from affine behavior with strain and temperature. While the Cauchy-Born rule enforces affine deformation of the crystal in the static sense, second moments have been used to quantify non-affinity in thermal vibrations due to atomic trajectory transformation under macroscopic deformation. The evolution of second moments with applied strain becomes highly non-linear at high temperatures in the tensile regime as a result of the anharmonic potential energy surface of the material. The vibrational entropy is computed using the time-averaged second moments of position from the molecular dynamics simulation. Stress computed using the resulting free energy is significantly lower than the virial stress in the high tensile strain – high temperature regimes.

Control of residual stress and distortion in aluminium wire + arc additive manufacture with rolling

The aluminium alloy wire 2319 is commonly used for Wire + Arc Additive Manufacturing (WAAM). It is oversaturated with copper, like other alloys of the precipitation hardening 2### series, which are used for structural applications in aviation. Residual stress and distortion are one of the biggest challanges in metal additive manufacturing, however this topic is not widely investigated for aluminium alloys. Neutron diffraction measurements showed that the as-built component can contain constant tensile residual stresses along the height of the wall, which can reach the materials' yield strength. These stresses cause bending distortion after unclamping the part from the build platform. Two different rolling techniques were used to control residual stress and distortion. Vertical rolling was applied inter-pass on top of the wall to deform each layer after its deposition. This technique virtually elimiated the distortion, but produced a characteristic residual stress profile. Side rolling instead was applied on the side surface of the wall, after it has been completed. This technique was even more effective and even inverted the distortion. An interesting observation from the neutron diffraction measurements of the stress-free reference was the significantly larger FCC aluminium unit cell dimension in the inter-pass rolled walls as compared to the as-build condition. This is a result of less copper in solid solution with aluminium, indicating greater precipitation and thus, potentially contibuting to improve the strenght of the material.

Stability of Cu-Precipitates in Al-Cu Alloys

We present first principle calculations on formation and binding energies for Cu and Zn as solute atoms forming small clusters up to nine atoms in Al-Cu and Al-Zn alloys. We employ a density-functional approach implemented using projector-augmented waves and plane wave expansions. We find that some structures, in which Cu atoms are closely packed on {100}-planes, turn out to be extraordinary stable. We compare the results with existing numerical or experimental data when possible. We find that Cu atoms precipitating in the form of two-dimensional platelets on {100}-planes in the fcc aluminum are more stable than three-dimensional structures consisting of the same number of Cu-atoms. The preference turns out to be opposite for Zn in Al. Both observations are in agreement with experimental observations.

Finite-temperature stress calculations in atomic models using moments of position

Continuum modeling of finite temperature mechanical behavior of atomic systems requires refined description of atomic motions. In this paper, we identify additional kinematical quantities that are relevant for a more accurate continuum description as the system is subjected to step-wise loading. The presented formalism avoids the necessity for atomic trajectory mapping with deformation, provides the definitions of the kinematic variables and their conjugates in real space, and simplifies local work conjugacy. The total work done on an atom under deformation is decomposed into the work corresponding to changing its equilibrium position and work corresponding to changing its second moment about equilibrium position. Correspondingly, we define two kinematic variables: a deformation gradient tensor and a vibration tensor, and derive their stress conjugates, termed here as static and vibration stresses, respectively. The proposed approach is validated using MD simulation in NVT ensembles for fcc aluminum subjected to uniaxial extension. The observed evolution of second moments in the MD simulation with macroscopic deformation is not directly related to the transformation of atomic trajectories through the deformation gradient using generator functions. However, it is noteworthy that deformation leads to a change in the second moment of the trajectories. Correspondingly, the vibration part of the Piola stress becomes particularly significant at high temperature and high tensile strain as the crystal approaches the softening limit. In contrast to the eigenvectors of the deformation gradient, the eigenvectors of the vibration tensor show strong spatial heterogeneity in the vicinity of softening. More importantly, the elliptic distribution of local atomic density transitions to a dumbbell shape, before significant non-affinity in equilibrium positions has occurred.

Phonon dispersion evolution in uniaxially strained aluminum crystal

The influence of loading upon the phonon dispersion of crystalline materials could be highly nonlinear with certain particular trends that depend upon the loading path. In this paper, we have calculated the influence of [100] uniaxial strain on the phonon dispersion and group velocities in fcc aluminum using second moments of position obtained from molecular dynamics (MD) simulation at 300 K. In contrast to nonlinear monotonic variation of both longitudinal and transverse phonon frequencies along the $$\Delta $$ , $$\Lambda $$ and $$\Sigma $$ lines of the first Brillouin zone under tension, transverse phonon branches along the $$\Lambda $$ line show inflection at specific wavevectors when the compressive strain exceeds 5%. Further, the longitudinal group velocities along the high-symmetry $$\Delta $$ line vary non-monotonically with strain, reaching a minimum at 5% compressive strain. Throughout the strain range studied, the equilibrium positions of atoms displace in an affine manner preserving certain static structural symmetry. We attribute the anomalies in the phonon dispersion to the non-affine evolution of second moments of atomic position, and the associated plateauing of force constants under the applied strain path.

Reproducibility of vibrational free energy by different methods

The paper reviews several modern methods for calculation of Helmholtz free energy, associated with atomic vibrations: quasiharmonic approximation, self-consistent lattice dynamics method, calculation based on velocity autocorrelation function, temperature-dependent effective potential and thermodynamic integration. The results of free energy calculation by these methods are compared with each other in a wide temperature range, and their applicability is discussed. As a tool we use classical molecular dynamics modeling on the example of bcc molybdenum and fcc aluminum. The relation is observed between how the vibrational spectrum changes with temperature due to the anharmonicity of the potential and how free energy is reproduced. All methods are consistent with each other within 10 meV/at. at relatively low temperatures. At high temperatures, the discrepancy reaches tens of meV/at., although the relative error is less than 5%.

The Effect of the Vertex Angles of Wedged Indenters on Deformation during Nanoindentation

In order to study the effect of the angle of wedged indenters during nanoindentation, indenters with half vertex angles of 60°, 70° and 80° are used for the simulations of nanoindentation on FCC aluminum (Al) bulk material by the multiscale quasicontinuum method (QC). The load-displacement responses, the strain energy-displacement responses, and hardness of Al bulk material are obtained. Besides, atomic configurations for each loading situation are presented. We analyze the drop points in the load-displacement responses, which correspond to the changes of microstructure in the bulk material. From the atom images, the generation of partial dislocations as well as the nucleation and the emission of perfect dislocations have been observed with wedged indenters of half vertex angles of 60° and 70°, but not 80°. The stacking faults move beneath the indenter along the direction [ 1 1 ¯ 0 ] . The microstructures of residual displacements are also discussed. In addition, hardness of the Al bulk material is different in simulations with wedged indenters of half vertex angles of 60° and 70°, and critical hardness in the simulation with the 70° indenter is bigger than that with the 60° indenter. The size effect of hardness in plastic wedged nanoindentation is observed. There are fewer abrupt drops in the strain energy-displacement response than in the load-displacement response, and the abrupt drops in strain energy-displacement response reflect the nucleation of perfect dislocations or extended dislocations rather than partial dislocations. The wedged indenter with half vertex angle of 70° is recommended for investigating dislocations during nanoindentation.

A curved pathway for oxygen interstitial diffusion in aluminum

The diffusion of interstitial oxygen (O) in fcc aluminum (Al) has been studied using first-principles and the diffusion coefficient has been calculated. Whereas interstitial atoms in fcc systems are typically found to hop directly between interstitial centres, the diffusion pathway for interstitial O in fcc Al was calculated to have a curved minimum energy pathway with an energy barrier of 0.95eV. The barrier was found to be off-centre of a neighboring octahedral site. Also unlike the majority of fcc metals, O prefers to sit in the tetrahedral interstitial site as opposed to the octahedral site. The calculated O diffusion coefficient is on the same order of magnitude of the diffusion coefficient of O in other fcc metals. The preferred interstitial site, diffusion pathway and vacancy binding energy were found to be related to the bond length of O with neighboring Al atoms.

Lattice mismatch modeling of aluminum alloys

We present a theoretical framework to accurately predict the lattice mismatch between the fcc matrix and precipitates in the multi-component aluminum alloys as a function of temperature and composition. We use a computational thermodynamic approach to model the lattice parameters of the multi-component fcc solid solution and θ′-Al2Cu precipitate phase. Better agreement between the predicted lattice parameters of fcc aluminum in five commercial alloys (206, 319, 356, A356, and A356+0.5Cu) and experimental data from the synchrotron X-ray diffraction (SXD) has been obtained when simulating supersaturated rather than equilibrium solid solutions. We use the thermal expansion coefficient of thermodynamically stable θ-Al2Cu to describe temperature-dependent lattice parameters of meta-stable θ′ and to show good agreement with the SXD data. Both coherent and semi-coherent interface mismatches between the fcc aluminum matrix and θ′ in Al-Cu alloys are presented as a function of temperature. Our calculation results show that the concentration of solute atoms, particularly Cu, in the matrix greatly affects the lattice mismatch.

Borane–Aluminum Surface Interactions: Enhanced Fracturing and Generation of Boron–Aluminum Core–Shell Nanoparticles

We present an experimental and theoretical study of borane–aluminum surface interactions that lead to rapid production of aluminum nanoparticles when Al balls are milled in the presence of diborane or pentaborane. Mass spectrometry was used to probe reactions of the boranes with aluminum fracture surfaces produced by milling collisions, which also generate local, transient high temperatures. Density functional theory was used to examine the interactions between a model aluminum surface and diborane and pentaborane, providing insight into the energetics of the first steps in the process that ultimately enables nanoparticle production. Further insight into the surface chemistry was obtained by analyzing the nanoparticles with X-ray photoelectron spectroscopy, scanning transmission electron microscopy with both electron-energy-loss and energy-dispersive X-ray spectroscopies, and dynamic light scattering. Particles were found to have fcc aluminum cores, capped by a ∼2-nm-thick shell, rich in both boron and hy...

The effect of temperature on the elastic precursor decay in shock loaded FCC aluminium and BCC iron

This article offers a comprehensive experimental and theoretical study of the causes of thermal hardening in FCC Al and BCC Fe at high strain rates, with the aim to shed light on important mechanisms governing deformation and failures in materials subjected to shocks and impacts at very high strain rates. Experimental evidence regarding the temperature dependence of the dynamic yield point of FCC Al and BCC Fe shock loaded at 107 s−1 is provided. The dynamic yield point of Al increases with temperature in the range 125K–795K; for the same loading and temperate range, the dynamic yield point of BCC Fe remains largely insensitive. A Multiscale Discrete Dislocation Plasticity (MDDP) model of both Fe and Al is developed, leading to good agreement with experiments. The importance of the Peierls barrier in Fe is highlighted, showing it is largely responsible for the temperature insensitivity in BCC metals. The relevance of the mobility of edge components in determining the plastic response of both FCC Al and BCC Fe at different temperatures is discussed, which leads to developing a mechanistic explanation of the underlying mechanisms leading to the experimental behaviour using Dynamic Discrete Dislocation Plasticity (D3P). It is shown that the main contributing factor to temperature evolution of the dynamic yield point is not the mobility of dislocations, but the temperature variation of the shear modulus, the decrease of which is correlated to the experimental behaviour observed for both FCC Al and BCC Fe.

Nanocrystalline aluminium particles inside Mg-4Li-4Al-2RE magnesium alloy after severe plastic deformation

Detailed microstructure characterization of ultra-fine grained LAE442 magnesium alloy processed by ECAP was performed by transmission electron microscopy and positron annihilation spectroscopy. Formation of FCC aluminium particles in the magnesium matrix and intensive segregation of aluminium at dislocations was observed. In situ annealing in the transmission electron microscope together with the analysis of the positron annihilation spectrum showed the limited stability of Al particles and Al segregation at temperatures below the processing temperature. It was suggested that the formation of observed Al nanoparticles is caused by the synergic effect of high external stresses during ECAP and the presence of Li atoms dissolved in the Mg matrix.

Initial dislocation density effect on strain hardening in FCC aluminium alloy under laser shock peening

The effect of initial dislocation density on subsequent dislocation evolution and strain hardening in FCC aluminium alloy under laser shock peening (LSP) was investigated by using three-dimension discrete dislocation dynamics (DD) simulation. Initial dislocations were randomly generated and distributed on slip planes for three different dislocation densities of 4.21 × 1012, 8.12 × 1012 and 1.26 × 1013 m−2. Besides, variable densities of prismatic loops were introduced into the DD cells as nanoprecipitates to study the dislocation pinning effect. The flow stresses as a function of strain rate obtained by DD simulation are compared with relevant experimental data. The results show a significant dislocation density accumulation in the form of dislocation band-like structures under LSP. The overall yield strength in FCC aluminium alloy decreases with increasing initial dislocation density and forest dislocation strengthening becomes negligible under laser induced ultra-high strain rate deformation. In addition, yield strength is enhanced by increasing the nanoprecipitate density due to dislocation pinning effect.

Mobility law of dislocations with several character angles and temperatures in FCC aluminum

We study the mobility law of dislocations in aluminum as an important building block for the development of a multiscale method that couples an atomistic model with discrete dislocation dynamics in 3d (e.g., CADD3d). Straight dislocations of arbitrary character angles are modeled with classical molecular dynamics at several temperatures. The obtained mobility results are analyzed and validated by comparisons to theoretical models. A critical velocity parameter identified by the analytic models is correlated to the material dispersive nature. We revisit the interpretation of this constant by considering character angles that were not studied previously. Finally, the obtained mobility law is implemented and employed in the discrete dislocation dynamics simulation of a dislocation loop. Our results highlight the importance of including several angles when constructing the mobility law to produce consistent results.

Effect of the metal content and annealing on the structure, phase composition, and nanohardness of fullerite–aluminum films

The effect of thermal annealing on the structure, phase composition, and nanohardness of fullerite–aluminum films with different metal contents (from 1 up to 40 at % Al) prepared from the combined atomic–molecular f low on monocrystal silicon substrates has been studied by atomic force microscopy, X-ray diffraction, Raman spectroscopy, and nanoindentation. It is established that the heterophase composite formed by sputtering and containing the hexagonal phase of fullerite, fcc aluminum, and fullerite-like structures with nanoparticles incorporated into the interstices has increased nanohardness in comparison with pure fullerite and aluminum. Thermal annealing at 650 K leads to the enlargement of the structural elements more than 1.5–2 times compared with the freshly precipitated films, the share increase of aluminumphase fullerene, and a slight decrease in nanohardness. Also, the dependence of structural element proportion, the phase composition, nanohardness, the distribution of local rigidity, and the lateral forces on the surface of the films upon the metal content and thermal annealing was established.