Al Sublattice Research Articles

Investigating interfacial segregation of [formula omitted]/Al in Al–Cu alloys: A comprehensive study using density functional theory and machine learning

Solute segregation at the interface between the aluminum (Al) matrix and the Ω (Al2Cu) phase decreases the interfacial energy, impedes the coarsening of precipitates, and enhances the thermal stability of such precipitates. In this study, we employ density functional theory to systematically calculate solute segregation energies of 42 solute elements at the coherent and semi-coherent interfaces between the two phases, as well as mixing energies of these elements within the Al and Cu sublattices of the Ω phase. Using correlation analysis and machine learning methods, we establish the relationship between the solute segregation energy and 20 selected atomic descriptors. Metalloid and late transition metal elements are predicted as potential candidates for enhancing the thermal stability of Al–Cu alloys. We observe that the solute segregation energy at the interfacial site of the semi-coherent interface correlates with the atomic size of solute atoms and their solubilities within the Ω phase. The developed machine learning models exhibit the potential to predict solute segregation energies at various sites of the coherent and semi-coherent interfaces. Overall, our study provides valuable insights into the stabilizing potential of individual elements at the Ω/Al interface in Al–Cu alloys.

Towards controllable formation of antiphase boundaries (APBs) in the B2 matrices of the Al-Fe based complex multicomponent alloys

Anti-Phase Boundaries (APBs) formation in B2 (ordered Body Centered Cubic) structures was investigated, focusing on the Al-Fe-Cr/Ni alloys. The current research goal was to propose strategies for controlling APBs formation in multicomponent alloys, following their positive effect on properties of High Entropy Alloys, stated in previous researches. This study is based on our previous investigation of the APBs formation mechanisms in binary B2 Fe50+xAl50-x alloys. There, stable APBs have formed at and above specific Long-Range order parameter. Here, it was established that stabilization of APBs requires disorder at only Al sublattice. Thorough analysis of dislocations revealed that higher complexity accelerates the decrease of order, promoting the formation of APBs. An assessment of disorder in multicomponent systems is challenging and sometimes even impossible, therefore suggested here usage of evaluation of dislocations is an interesting and useful experimental tool to estimate the disorder in these complex systems.

Preliminary study of the effect of Sn addition on microstructure and creep resistance of a β-solidifying TiAl alloy

In the present work, two β-solidifying γ-TiAl based intermetallic alloys doped with Nb, Zr, Hf (designated TNZ γ-TiAl alloy) and additionally Sn (designated TNZ-Sn) were investigated. Sn is known to be widely used as an alloying element in titanium alloys but almost has not been used for γ-TiAl alloys. In contrast to Nb, Zr and Hf atoms occupying only or preferentially the Ti sublattice, Sn atoms occupy the Al sublattice because Al and Sn belong to the same group of post-transition metals. Two microstructural conditions per alloy, duplex and fully or near lamellar, were obtained in the alloys by upset forging and heat treatments. Microstructure examination by SEM and XRD analysis revealed two basic phases in both alloys, γ-TiAl and α2-Ti3Al. The feature of the Sn-containing alloy was the presence of a small amount (0.5–1.5 vol%) of the phase enriched with Sn, Zr and Hf and located along grain/colony boundaries. After upset forging and heat treatment this phase had the average chemical composition 60(Ti + Nb + Zr + Hf)-40(Al + Sn) (at.%) and was identified as the α2 phase enriched with Sn, Zr and Hf and depleted with Al. The creep tests were performed for the produced microstructural conditions of the alloys. It was revealed that doping with Sn led to a strong increase in the creep resistance at 800 and 850 °C. The obtained effect was attributed to solid solution hardening due to substitution of Al by Sn and probably to a more ionic interatomic bonding taking into consideration an appreciably higher electronegativity of tin as compared to that of other alloying elements.

Materials properties of defect-stabilized off-stoichiometric τ-phase Al2Ge2Mg

We have performed a quantum-mechanical study of off-stoichiometric Al2Ge2Mg τ-phase intermetallic compounds. Our calculations focused on evaluating the formation energy of static lattices, their elastic properties as well as free energy of formation including phonon contributions. While the stoichiometric Al2Ge2Mg τ phase was found thermodynamically, mechanically and dynamically stable, we have identified an off-stoichiometric composition Al7Ge8Mg5 which exhibits the free energy of formation lower than the stoichiometric compound. The Al7Ge8Mg5 was found both mechanically and dynamically stable as well. This off-stoichiometric compound has Mg atoms located also at the Al sublattice, i.e. these defects exhibit a stabilizing effect. Phonon contributions into the free energy of formation have turned out to be crucial for a proper description of the thermodynamic stability. In contrast, several other off-stoichiometric states are significantly thermodynamically destabilized by anti-site atoms at the Ge sublattice. The formation energy and the homogenized polycrystal Young's modulus are found anti-correlated indicating a connection between the thermodynamic and mechanical stability. Regarding the theoretical values of homogenized polycrystal Young's moduli, our measurements resulted in values quite close to the predicted ones.

Optimizing precipitation hardening in a L12-strengthened medium-entropy alloy via tuning the anti-phase boundary energy

In present work, we investigate the elemental partition behaviors in L12 ordered intermetallics and its influence on mechanical properties in a Ni-Co-Cr-Al-Ta medium-entropy alloy (MEA) system through a combination of density functional theory (DFT) calculations and experimental investigation. Our finding reveal that Ta has a strong tendency to occupy Al sublattice, while Co prefers Ni sublattice, Cr, on the other hand, randomly occupy Al and Ni sublattices, resulting in the formation of the multicomponent (Ni, Co, Cr)3(Al, Ta, Cr) L12 intermetallics. Among the constituent elements, Ta is identified as an efficient element for enhancing hardening of the alloy. Furthermore, we demonstrate that improving Ta/Al ratio leads to a significant increase in anti-phase boundary (APB) energy, resulting in superior precipitation hardening effects and excellent mechanical properties. Our founding provide valuable insights for further the rationally design and rapid development of high performance L12-strengthened alloys.

Elemental Fluctuation in Gd3Al2Ga3O12:Ce Crystals Imposed by Li+ and Mg2+ Co-Doping: The Impact on Defects, Luminescence, and Scintillation Properties

This research revealed the response of Ga and Al sublattices to the incorporation of mismatching substituents in Gd3Al2Ga3O12:Ce single crystals. Incompatible in size and charge, Li+ and Mg2+ substituents violated configurational entropy. This led to lattice distortion and triggered structural rearrangements. The radial fluctuation of the Ga and Al elements was proven by multi-elemental energy-dispersive X-ray spectroscopy mapping and elemental composition analysis. Further evidence was observed by the shift of the exciton creation energy toward higher energy in the vacuum ultraviolet excitation spectra recorded with synchrotron radiation. In the Li+ and Mg2+ co-doped samples, the crystal core was depleted with Ga atoms and enriched with Al elements. The crystal rim showed the opposite behavior. The change in thermoluminescence peak positions revealed a different mechanism for the formation of localized traps. As a result, Li+ co-doping slightly improved the light yield value, but at the same time decelerated the scintillation decay time. On the contrary, Mg2+ co-doping markedly diminished scintillation parameters.

Elastic Properties of B2-NiAl with W addition: A first-principles study

The effect of tungsten alloying on the elastic properties of B2-NiAl at low-temperature and high-temperature distribution of W atoms on sublattices. By means of the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, the constants C11,C12,C44, Young's modulus E, shear modulus G, Cauchy pressure values, G/B ratios are calculated. Using phenomenological criteria of the correlation between ductility and elastic properties of solution phases, it has been shown that the addition of tungsten could yield improved ductility for B2-NiAl in both types of alloys. It is established that with the high-temperature distribution of W atoms on the Al sublattice, there is a loss of mechanical stability and a decrease in mechanical properties with increasing W concentration. In alloys with a low-temperature distribution of tungsten atoms between Al and Ni sites a unique combination of properties occurs: in addition to the ductility enhancement, a simultaneous increase of the elastic constants C44 and C11 and C11, shear modulus G, Young's modulus E with increasing W content is observed. The differences in the behavior of elastic constants in alloys with different types of tungsten distribution on NiAl sublattices are analyzed by calculating the density of electronic states. Keywords: NiAl, alloying elements, tungsten, elastic properties, ductility, first-principles calculations.

Упругие свойства B2-NiAl с добавлением W: исследование из первых принципов

The effect of tungsten alloying on the elastic properties of B2 NiAl at low-temperature and high-temperature distribution of W atoms on sublattices. By means of the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, the constants C11, C12, C44, Young's modulus E, shear modulus G, Cauchy pressure values, G/B ratios are calculated. Using phenomenological criteria of the correlation between ductility and elastic properties of solution phases, it has been shown that the addition of tungsten could yield improved ductility for B2 NiAl in both types of alloys. It is established that with the high-temperature distribution of W atoms on the Al sublattice, there is a loss of mechanical stability and a decrease in mechanical properties with increasing W concentration. In alloys with a low–temperature distribution of tungsten atoms between Al and Ni sites a unique combination of properties occurs: in addition to the ductility enhancement, a simultaneous increase of the elastic constants C44 and C11, shear modulus G, Young modulus E with increasing W content is observed. The differences in the behavior of elastic constants in alloys with different types of tungsten distribution on NiAl sublattices are analyzed by calculating the density of electronic states.

Precipitation behavior of Cu/Ni(Mn,Al) co-precipitates in the medium-Mn dual-phase steel

• High Mn content promotes the partition of Mn to NiAl by occupying Al sublattice. • Cu/Ni(Mn, Al) co-precipitates are formed just in the martensite of the DP steel. • The growth of precipitates doesn’t follow volumetric diffusion-controlled process. The precipitation behavior of Cu/Ni(Mn,Al) co-precipitates in the medium-Mn dual-phase (DP) steels was investigated through a combination of atom probe tomography (APT) and small-angle neutron scattering (SANS). The results reveal that high Mn content of the DP steel promotes the partition of Mn to NiAl nanoprecipitates by occupying the Al sublattice, thereby forming a Cu/Ni(Mn,Al) co-precipitates in the martensite. The growth of the co-precipitates doesn’t follow the volumetric diffusion-controlled process, which is controlled by the coupled diffusion of multiple chemical elements induced by the synergistic phase transformation. The coarsening coefficient of the co-precipitates in the DP steel are ten times more than it in the previous martensitic steel, leading to the change of precipitation strengthening effect. The mechanism for the co-precipitates’ coarsening is also addressed and discussed.

Wurtzite Al[formula omitted]Ga[formula omitted]P[formula omitted]N[formula omitted] barrier layer growth for high electron mobility transistors

Wurtzite AlPyN1−y and Al1−xGaxPyN1−y was grown on GaN using Metal-Organic Vapour Phase Epitaxy using tertiary-butylphosphine (tBP). To incorporate more than 5% P, the temperature needs to be higher than 1100 °C and the tBP/(tBP+NH3) ratio must exceed 0.2 at 20 kPa and 0.3 at 5 kPa reactor pressure. This limits the V/III ratio to less than 20, which can result in high unintentional Ga incorporation due to the decomposition of the initial GaN surface. Thus, pure AlPyN1−y is preferably grown with nitrogen carrier gas and fast growth rates. Since P can also incorporate on the Al sub-lattice, tensile stain through coherent growth on GaN is mandatory. Too high P/N ratios or too thick layers lead to relaxation and subsequent non-stoichiometric AlxPx+yN1−y growth. A coherent 5 nm thin layer of nominal AlP0.07N0.93 showed a two-dimensional electron gas with sheet carrier densities above 1013cm−2 at 30K.

Quasiparticle self-consistent GW band structures and phase transitions of LiAlO2 in tetrahedrally and octahedrally coordinated structures

A first-principles computational study is presented of various phases of LiAlO$_2$.The $\beta$ and $\gamma$ tetrahedral phases are found to be very close in energy with the $\gamma$ phase having the lowest energy. The octahedral $\alpha$ phase is a high-pressure phase and the transition pressure from the $\gamma$ and $\beta$ phases to $\alpha$ is determined to be about 1 GPa. The electronic band structures are determined using the quasiparticle self-consistent (QS) $GW$ method. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy $\gamma$ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at $\Gamma$ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase and their anisotropies are discussed. Si doping on the Al site is investigated as a possible $n$-type dopant in $\gamma$-LiAlO$_2$ using a 128 atom supercell corresponding to 3.125 \% Si on the Al sublattice in the generalized gradient approximation and a smaller 16 atom cell with 25 \% Si in the QS$GW$ approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, of order a few 0.1 eV in the hydrogenic effective mass approximation.

Diffusion of Tracer Atoms in Al4Ba Phases Studied Using Perturbed Angular Correlation Spectroscopy

The Al4Ba crystal structure is the most common structure among binary intermetallic compounds. It is well suited for accommodating large atoms of group II elements and is often the intermediate phase closest to the terminal phase. It is, therefore, of interest to characterize diffusion properties of compounds with this tetragonal crystal structure. In the present study, 111In perturbed angular correlation spectroscopy was used to study solute site occupation and atom movement in In4Ba, Al4Ba, Al4Eu, Al4Sr, and Ga4Sr. The indium tracer and its daughter cadmium were found to occupy only the two Al-type sublattices in these compounds through detection of nuclear quadrupole interactions with axially symmetric EFGs. Measurements with increasing temperature revealed merging of signals due to dynamical averaging of these interactions as Cd atoms jumped at increasing rates between alternating sublattices. The jump rates were estimated to be between 8 kHz and 2 MHz at about 350 °C for Al4Eu and at about 450 °C for In4Ba and Al4Ba. Fits of spectra using Blume’s stochastic model allowed determination of activation enthalpies for average Cd jump rates between alternating Al sublattices in Al4Sr and Ga4Sr to be 1.16(3) eV and 1.47(3) eV, respectively. This result was used to estimate transverse diffusivities of Cd.

Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti–48Al–4Nb–2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ → α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γ equiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti–48Al–4Nb–2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γ grain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and Σ5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.

Stability and ordering of bcc and hcp TiAl+Mo phases: An ab initio study

Atomic ordering in bcc and hcp TiAl+Mo alloys near equiatomic TiAl composition is investigated by different ab initio tools. We show that small addition of Mo, about 5 at. %, is enough to make bcc alloys with more than 50 at. % of Ti stable with respect to the hcp alloys. Moreover, such alloying also leads to stabilizing the B2 ordered structure with respect to its B22 modification, which is the bcc-based ground state structure of binary TiAl. The site preference of Mo in the B2 and B19 ordered alloys is investigated by different methods: in the dilute limit, using the transfer energy formalism; in concentrated alloys, from the total energies of disordered and partially ordered alloys in the mean-field coherent potential approximation; and from Monte Carlo simulations. These methods produce consistent results for the B2 phase predicting a strong preference of Mo to Al sublattice. The site preference of Mo in the B19 phases varies from a weak preference for Al sites in the single impurity calculations to a quite strong preference for Ti sites in the mean-field approximation and finally to a strong Al preference in Monte Carlo simulations. Mo alloying dramatically increases the order–disorder transition temperatures in bcc and hcp Al-deficient Ti0.5Al0.5−xMox alloys.

Experimental and theoretical characterization of Al3Sc precipitates in Al–Mg–Si–Cu–Sc–Zr alloys

Abstract The structure of Al3Sc precipitates in Al –Mg– Si–Cu – Sc –Zr alloys annealed at 450 °C was investigated using transmission electron microscopy and energy-dispersive X-ray spectroscopy. The Al3Sc particles contain mainly Sc and small amounts of Zr, Si, and Cu. The addition of Zr limits the size of the Al3Sc precipitates to about 10– 30 nm, and these precipitates are coherent with the matrix. Density functional energy calculations showed that exceptionally small energies are required to dissolve Zr and Cu in Sc and Al sublattices, respectively. On the contrary, Cu, Mg, and Si are difficult to be dissolved in the Sc sublattice.

Interactions of oxygen with intrinsic defects in L1[formula omitted][formula omitted]-TiAl in presence of substitutional solutes: Influence on diffusion kinetics

This work reexamines the insertion of O atoms in the L10γ-TiAl system using first-principles calculations and thermodynamic modeling in the independent point defect approximation. It includes a study of intrinsic point defects, the insertion of many alloying elements (more than twenty were considered), as well as a study of their interaction with oxygen. The formation of complex defects composed of either vacancies, anti-sites or solute elements is then studied. Results at the atomic scale show a high segregation of oxygen in titanium-rich environments: oxygen easily segregates onto Ti anti-sites (TiAl) and alloying elements are located in the vicinity of Al sub-lattices. DFT point-defect energetics shows that there is a clear correlation between the nature and site preference of an alloying element, and the oxygen segregation energy in the vicinity of this solute. The thermodynamic model shows that at equilibrium, oxygen does not occupy isolated interstitial sites but prefers to be located in the vicinity of Ti anti-sites or alloying elements. The effect of this strong segregation on oxygen diffusivity is discussed hereinafter. Results show a strong slowdown in oxygen diffusivity due to intrinsic defects. For Ti/Al >0.5 ratios, the traps for O diffusion are mainly constituted by Ti anti-sites, and the addition of solutes does not contribute much to the trapping of diffusing O atoms. For Ti/Al < 0.5 ratios however, the contribution of solutes to trapping phenomena can be very important, and a decrease by 1–2 orders of magnitude of effective O diffusion coefficients can be observed for temperatures around 800–1100 K.

Elemental partitions and deformation mechanisms of L12-type multicomponent intermetallics

Multicomponent high-entropy alloys (HEAs) with compositionally disordered elemental arrangement have attracted extensive attention because of their excellent mechanical properties. However, these alloys usually exhibit low strength at elevated temperatures. Recent works suggest that HEAs strengthened by L12 ordered intermetallics of the Ni3Al type possess a superior synergy of strength and ductility. The performance of such ordered-intermetallics-strengthened HEAs depends critically on the composition and deformation mechanism of the strengthening component, which will inevitably suffer partial disorder in HEAs. In this work, we study the elemental partitions and deformation mechanisms in typical multicomponent intermetallics (MCIs) based on density functional theory (DFT) calculations. Informed by experiments, systems considered in this work include L12-type (Ni,Co,Fe)3(Al,Ti,Fe) and its six derived subsystems. We first determine the site preference of different elements in MCIs using Monte-Carlo simulations with the energies obtained from DFT calculations (DFT-MC). The results confirm that Ti prefers Al sites. In contrast, most Co and Fe atoms tend to occupy Ni sublattice. With the established elemental occupations, we build quasirandom structures with partial disorders in either or both sublattices of the L12 structures. DFT results show that incorporation of Ti in the Al sublattice dominates the increase of stable planar fault energies (γSPF) in the considered MCIs. In contrast, Co in the Ni sublattice decreases the energy cost for planar fault formation. We further uncover quantitative relations between γSPF and geometrical structures as well as electronic structures in MCIs. The analytical results show that the incorporation of Ti decreases the adaptability of (Ni,Co,Fe)3(Al,Ti,Fe) both geometrically and electronically. Specifically, we propose that γSPF can be described by the ability of the alloy to accommodate planar faults, which is measured by the interlayer spacing changes between {111} planes [Δd(111)] and charge density redistribution (Δρ) at critical points (cps). We address the fundamental understandings of the deformation behavior of MCIs by pinpointing the contributions of different species to their local geometrical and electronic structures, which pave the way for rationally designing ordered-precipitate-strengthened HEAs.

Imogolite: Curvature‐Induced Hospitality for Trivalent Dopants

Imogolite—a congregation of aluminosilicate nanotubes—has shown a great promise for fabrication of advanced materials. Imogolite nanotubes hold the record narrow diameters distribution and demonstrate one of the highest curvatures among noncarbon nanotubes. Current experimental and theoretical prospects for their modification are related mostly to surface functionalization or to progressive Si substitution on Ge, whereas a doping of Al sublattice is steadily neglected. The empirical rules of Hume–Rothery disallow such doping due to a large size mismatch between Al3+ and the majority of metal cations. Using the quantum‐chemical calculations, herein, the difference in coordination environment is unveiled for 12 sorts of trivalent cations at octahedral Al sites in both a highly curved imogolite layer and the flat gibbsite layer. The curvature of natural imogolite acts provoking for violation of the Hume–Rothery rules, sterically promoting the doping of Al sublattice by cations with radii 1.0–1.2 Å typical for rare‐earth elements. Theoretically, the synthesis of a metal‐doped imogolite is restricted only by the overlap of the phase stability fields for precursor reactants or by kinetic factors. This study might be inciting for the synthesis of chemically modified imogolite and related Ge‐imogolites.

Interactions between cadmium and Al3Li precipitates: A new mechanism of accelerating dissolution and transformation of phases in Al–Li–Cu alloy

Abstract The interactions between Cd and δ′-Al3Li precipitates and their impacts were reported for the first time. Transmission electron microscopy and X-ray diffraction results suggested that a trace (0.06 at. %) addition of Cd significantly altered the microstructural evolution and the hardening behavior of high-lithium aluminum alloys. The Cd addition not only led to a lower peak-hardness and shorter peak-aged time, but also caused a sharp decrease of hardness and the dissolution of δ′ precipitates in over-aged state. Atom probe tomography results and first-principles calculations showed that Cd atoms preferentially partitioned to δ′ precipitates, occupying the Al sublattice. Such configuration was shown to be energetically unfavorable, thus would accelerate the dissolution of δ′ precipitates, leading to the decrease of hardness. The dissolution of δ′ subsequently provided ample Li atoms in the matrix, promoting the formation of T2-Al6CuLi3 phase. Eventually, spherical δ′ precipitates completely dissolved, replaced by T2 phases of larger sizes and irregular shapes.

NiAl thin film growth on Ni(001) substrate using molecular dynamics simulations

We have studied thin film growth of NiAl on Nickel (001) substrate using molecular dynamics simulations (MD) based on the Embedded Atom Method (EAM) potential. An incidence energy of 0.06 eV at 800 K, 900 K and 1000 K was considered. After the deposition process, we have obtained a B2-NiAl structure film with different percentages; 32.6% for the temperature 1000 K, 30% for 900 K and 25% for 800 K. Our investigation has prompt us to analyze the crystalline structure. During the evolution of deposited film, we observe the formation of grains with different orientation, as well as the appearance of vacancies in Ni and Al sublattices and antisites.