Al Sublattice Research Articles

Solubility and ordering of Ti, Ta, Mo and W on the Al sublattice in L12-Co3Al

Co–Al–W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L12-Co3(Al1−xWx) precipitates, which may form even though binary Co3Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L12-Co3Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40–80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta.

Effects of Mn partitioning on nanoscale precipitation and mechanical properties of ferritic steels strengthened by NiAl nanoparticles

The critical role of Mn partitioning in the formation of ordered NiAl nanoparticles in ferritic steels has been examined through a combination of atom probe tomography (APT) and thermodynamic and first-principles calculations. Our APT study reveals that Mn partitions to the NiAl nanoparticles, and dramatically increases the particle number density by more than an order of magnitude, leading to a threefold enhancement in strengthening. Atomistic structural analyses reveal that Mn is energetically favored to partition to the NiAl nanoparticles by preferentially occupying the Al sublattice, which not only increases the driving force, but also reduces the strain energy for nucleation, thereby significantly decreasing the critical energy for formation of the NiAl nanoparticles in ferritic steels. In addition, the effects of Mn on the precipitation strengthening mechanisms were quantitatively evaluated in terms of chemical strengthening, coherency strengthening, modulus strengthening and order strengthening.

Effects of Ni vacancy, Ni antisite, Cr and Pt on the third-order elastic constants and mechanical properties of NiAl

Effects of Ni vacancy, Ni antisite in Al sublattice, Cr in Al sublattice, Pt in Ni sublattice on the second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of the B2 NiAl have been investigated using the first-principles methods. Lattice constant and the SOECs of NiAl are in good agreement with the previous results. The brittle/ductile transition map based on Pugh ratio G/B and Cauchy pressure Pc shows that Ni antisite, Cr, Pt and pressure can improve the ductility of NiAl, respectively. Ni vacancy and lower pressure can enhance the Vickers hardness Hv of NiAl. The density of states (DOS) and the charge density difference are also used to analysis the effects of vacancy, Ni antisite, Cr and Pt on the mechanical properties of NiAl, and the results are in consistent with the transition map.

The First-principles Study on the Occupation Behavior and the Ductility Mechanism of Zr in Ni–Ni3Al System with Lattice Misfit

The influence of lattice misfit on the occupation behavior and the ductility effect of Zr in Ni–Ni3Al alloys were explored. It is found in energy analysis that the preferable site of Zr between Ni sublattice and Al sublattice will change under different lattice misfit, however, the Zr prefers to segregate Ni phase rather than Ni3Al phase in all lattice misfit range, which makes it impossible for Zr to go into Ni3Al phase to occupy Al sublattice in Ni–Ni3Al system. Bond order (BO) analysis shows that the localized ductility effect of Zr differs in different region, and the comparison between Zr-free and Zr-doped BO analysis successfully explain the mechanism of the embrittlement of Ni–Ni3Al alloys and the ductility effect of Zr.

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni70Al25Mo5 (composition 1) and Ni75Al20Mo5 (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ∼7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L12) at T ∼ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni3 Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.

Energy localization on the Al sublattice of Pt3Al with L12 order

A three-dimensional molecular-dynamics model of Pt3Al with L12 order was developed and found to support the excitation of discrete breathers (DBs) and energy localization on the Al sublattice. For an initial lattice temperature of 0 K, large-amplitude DBs polarized along [100] are found to be very weakly damped, retaining most of their initial energy for more than 1000 cycles, while DBs polarized along [111] damped out over ∼15 cycles. Because the DBs and their dissipation channels are confined to the Al sublattice, long-lived nonequilibrium states with large energy differences between the Al and Pt sublattices occur. Since collisions during irradiation more efficiently generate lattice vibrations in light atoms than heavy atoms, such nonequilibrium states may occur and alter the relaxation processes occurring during radiation damage.

First-principles study of point defects in Ni3Al

The energetics and structural properties of native, substitutional and interstitial defects in NiAl have been investigated by first-principles methods. In particular, we have determined the formation energies of composition conserving defects and established that the so-called penta defect, which consists of four vacancies on Ni sublattice and Ni antisite on the Al sublattice, is the main source of vacancies in NiAl. We show that this is due to the strong Ni-site preference of vacancies in NiAl. We have also calculated the site substitution behaviour of Cu, Pd, Pt, Si, Ti, Cr, V, Nb, Ta and Mo and their effect on the concentration expansion coefficient. We show the latter information can used for an indirect estimate of the site substitution behaviour of the alloying elements. The solution energy of carbon and its effect on the lattice constant of NiAl have been obtained in the dilute limit in the first-principles calculations. We have also determined the chemical and strain-induced carbon–carbon interactions in the interstitial positions of NiAl. These interactions have been subsequently used in the statistical thermodynamic simulations of carbon ordering in NiAl.

Ab initio study the effects of Si and Mg dopants on point defects and Y diffusion in YAG

The effects of Si and Mg dopants on point defects and Y diffusion in YAG are studied by the ab inito DFT calculations. Introduction of Si in YAG could largely decrease the formation energies of cation vacancies. It will improve the concentration of cation vacancies and enhance the lattice diffusion. Mg dopant has the effect of reducing O vacancy formation energies. In the codoping condition, the Si and Mg ions shows agglomeration in YAG. It indicates that using both MgO and SiO2 as the sintering aids would form MgO–SiO2 liquid phase, which could enhance the densification rate. The calculated electronic structures show that the Si or Mg dopant does not introduce defect state in the band gap of YAG. The Si shows a strong covalent property and Mg shows a strong ionic property in YAG. The charged states almost have no effect on local structure, electronic structure and effective charge of the dopants. We also studied the effect of Si and Mg dopants on Y diffusion via vacancy mechanism on the Y, Al(tet) and Al(oct) sublattices. The results show that Si and Mg dopants only slightly decrease the barrier energies of Y diffusion, indicating Si and Mg dopants enhance Y diffusion mainly through promoting the formation of cation vacancy or liquid phase.

О локализации энергии нелинейных и линейных колебаний атомов в модельной кристаллической решетке состава А3В

A three- dimensional molecular-dynamics model of Pt3Al with L12 order was developed and found to support the excitation of discrete breathers (DBs) and energy localization on the Al sublat-tice. For an initial lattice temperature of 0 K, large-amplitude DBs polarized along [100] are found to be very weakly damped, retaining most of their initial energy for more than 1000 cycles, while DBs polarized along [111] damped out over ~15 cycles. Because the DBs and their dissipation channels are confined to the Al sublattice, long-lived states with large energy differences between the Al and Pt sublattices occur.<br />

Phase transformations in Ni + Al and Ni + Al + Cr mixtures during mechanochemical synthesis and subsequent heating

Mechanochemical processing of elemental mixtures with the compositions Ni75Al25, Ni70Al25Cr5, and Ni75Al20Cr5 (5 at % Cr in the mixtures instead of the equivalent amount of Ni or Al) leads to the formation of nanocrystalline nickel-based solid solutions (crystallite size in the range ≃ 7–12 nm). Comparison of experimentally determined lattice parameters of the solid solutions with Vegard’s law values and with the lattice parameters evaluated using the Bozzolo-Ferrante rule, which takes into account the bulk moduli of constituent elements, suggests that the atoms in the solid solutions are bonded more strongly. Heating the synthesized ternary solid solutions in a calorimeter to 1000°C leads to the formation of an ordered γ′-phase (L12). Analysis of the relative intensity ratio of superlattice and fundamental reflections indicates that the Cr atoms always reside in the Al sublattice, independent of the composition of the starting mixture. When 5 at % Cr is incorporated instead of Ni, the chromium atoms force out aluminum from the Al sublattice, and the Ni deficiency in the Ni sublattice is compensated by the Al atoms. The ordered phases remain nanocrystalline (crystallite size in the range ≃ 40–70 nm).

Site occupation evolution of alloying elements in L12 phase during phase transformation in Ni75Al7.5V17.5

Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.

Phase-partitioning and site-substitution patterns of molybdenum in a model Ni-Al-Mo superalloy: An atom-probe tomographic and first-principles study

Atom-probe tomography (APT) and first-principles calculations are employed to investigate the partitioning of Mo in the γ(f.c.c.)-and γ′(L12)-phases in a model Ni-6.5Al-9.9Mo at. % superalloy. Mo is experimentally observed to partition preferentially to the γ(f.c.c.)-matrix, which is consistent with the smaller value of the γ(f.c.c.)-matrix substitutional formation-energy, with a driving force of 0.707 eV for partitioning as determined by first-principles calculations. APT measurements of the γ′(L12)-precipitate-phase composition and Al-, Mo-centered partial radial distribution functions indicate that Mo occupies the Al sublattice sites of the Ni3Al(L12) phase. The preferential site-substitution of Mo at Al sublattice sites is confirmed by first-principles calculations.

Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys

The effects of adding 0.02 or 0.06at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al3(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4h at 275 and 300°C, compared with the 0.02at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al3(Sc,Zr) (L12) precipitates in the high-Si alloy subjected to double aging (8h/300°C for Sc precipitation and 32days/400°C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al3(Sc,Zr) (L12) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al3Sc; and (iii) Si partitions preferentially to Al3Sc (L12) rather than Al3Zr (L12).

Local structure of NiAl compounds investigated by extended X-ray absorption fine-structure spectroscopy

The local structures of pure NiAl and Ti-, Co-doped NiAl compounds have been obtained utilizing extended X-ray absorption fine-structure (EXAFS) spectroscopy. The results provide experimental evidence that Ni antisite defects exist in the Ni-rich NiAl compounds. The site preference of Ti and Co has been confirmed. Ti occupies the Al sublattice, while Co occupies the Ni sublattice. The structure parameters obtained by EXAFS were consistent with the X-ray diffraction results. Owing to the precipitation of α-Cr, the local structure of NiAl-Cr has not been obtained, making the site preference of Cr unclear.

Study of effect of Mn addition on the mechanical properties of Ti2AlC/TiAl composites through first principles study and experimental investigation

The effect of Mn on the mechanical properties of the Ti 2 AlC/TiAl composites is investigated by experiments and first principles calculations. The Mn atoms preferentially occupy the Al sublattice sites and can cause the lattice contraction of TiAl along its c -axes. As a result, the lattice tetragonality of TiAl, which is related with its intrinsic brittleness, could be reduced. The calculation results are consistent with the experimental observations. The addition of Mn remarkably improves the ductility of the composites. Moreover, the strength and work-hardening rate are also enhanced due to the solid solution of Mn and the grain refinement of the TiAl matrix. The strength and the work-hardening rate increase with the increase in the Mn content, while the ductility increases firstly and then decreases. The composite with the addition of 2 at.% Mn possesses the best compression properties, of which the average compression strength is 256 MPa higher than that of Mn-free sample, and the average compression fracture strain increases from 16.4% to 19.9%. ► Effect of Mn on the mechanical properties of the Ti 2 AlC/TiAl composites is studied. ► Mn addition improves the compression properties of the Ti 2 AlC/TiAl composites. ► The work-hardening behavior of the Ti 2 AlC/TiAl composites was discussed in detail.

Disorder trapping during crystallization of theB2-ordered NiAl compound

Using molecular dynamics simulations, disorder trapping associated with solidification is studied for the (100), (110), and (111) growth directions in the B2 NiAl ordered alloy compound. At the high interface velocities studied we observe pronounced disorder and defect trapping, i.e., the formation of antisite defects and vacancies in the crystal at higher than equilibrium concentrations upon rapid solidification. The vacancies are located primarily on the Ni sublattice and the majority of antisite defects are Ni atoms on the Al sublattice, while the concentration of Al on the Ni sublattice is negligibly small. The defect concentration is found to increase in an approximately linear relationship with increasing the interface velocity. Further there is no significant anisotropy in the defect concentrations for different interface orientations. Our results suggest that the currently available models of disorder trapping should be extended to include both antisite defects and vacancies.

First—principles study of stacking fault energies in Ni3Al intermetallic alloys

The first-principles method based on the projector augmented wave method within the generalized gradient approximation was employed to calculate the superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF) energies of the binary Ni3Al alloys with different Al contents and the ternary Ni3Al intermetallic alloys with addition of alloying elements, such as Pd, Pt, Ti, Mo, Ta, W and Re. The results show that the energies of SISF and CSF increase significantly with increase of Al contents in Ni3Al. Addition of Pd and Pt occupying the Ni sublattices does not change the SISF and CSF energies of Ni3Al markedly in comparison with the Ni–23.75Al alloy. While addition of alloying elements, such as Ti, Mo, Ta, W and Re, occupying the Al sublattices dramatically increases the SISF and CSF energies of Ni3Al. The results suggest that the energies of SISF and CSF are dependent both on the Al contents and on the site occupancy of the ternary alloying element in Ni3Al intermetallic alloys.

Simulation of V Atom Substitution Character in Ni-Al-V Alloy

Simulations are performed on atom substitution character in Ni-Al-V alloy based on microscopic phase-field model at 1046K. It was showed that the ordering of both Al and V atoms take place simultaneously during the precipitation process of Ni-Al-V alloy, V atoms substitute the Al sublattice in L12phases, and they prefer β-sites to α-sites. Both of Al and V occupy the β-sites and complex phases Ni3(Al1-xVx)are formed; V atoms substitute the Al sublattice, the D022phases are formed at the boundary of L12phases.

Nanoscopic Al 1−xCe x phases in the NaH + Al + 0.02CeCl 3 system

The NaH + Al + 0.02CeCl 3 system has been studied by high-resolution X-ray synchrotron diffraction and transmission electron microscopy (TEM), after planetary milling under hydrogen and hydrogen (H) cycling. Isothermal absorption kinetics were determined at 150 °C, and compared with the NaH + Al + 0.02TiCl 3 system, indicating that CeCl 3 and TiCl 3 are equally effective additives, with CeCl 3 preferred on the basis of hydrogen storage capacity. After milling, AlCe contains 100% of the Ce. After the first H absorption, we observe two Al 1−xCe x phases. The first, AlCe, contains ca. 60% of the originally added Ce atoms. The AlCe phase observed after milling and H cycling is chemically disordered, with complete exchange between the Al and Ce sublattices occurring, yielding zero intensity in ordering reflections such as (100). In the absorbed state after H cycling, the remaining 40% of Ce atoms are contained in a cubic Al 1−xCe x phase not previously observed in the Al-Ce phase diagram. Indexing yields a primitive cubic unit cell of dimension 7.7111 Å, in space group P23. Lineshape analysis indicates the AlCe and unknown cubic Al 1−xCe x phases are ca. 35 nm and 30 nm in dimension respectively. High resolution TEM imaging confirms that both Al 1−xCe x phases are embedded on the NaAlH 4 surface, and localised energy dispersive X-ray spectroscopy (EDS) indicates a ca. 2:1 Al:Ce ratio for the unknown cubic Al 1−xCe x phase.

Ab initio Calculations of Elastic Constants of Superalloys

Using the-state-of-the-art ab initio method we have studied elastic constants of alloys potentially interesting for high temperature applications. We have shown that Cr substitutes the Al sublattice in B2 NiAl at concentration up to 40 at. %, but at higher Cr content it prefers the Ni-sublattice. Alloying of NiAl with Cr yields reduced strength but improves the ductility of the alloys. Alloying of NiAl with W which substitutes the Al sublattice, leads to a strong decrease of the shear modulus, and near 50 at. % of W the alloy becomes mechanically unstable as elastic constant C' is negative. This is in agreement with our phonon calculations where we found soft modes along the [110] direction for B2 NiW. According to our calculations in (Ru,Ni)Al alloys the shear modulus is almost constant up to 40 at. % of Ni, at higher Ni concentrations it is drastically reduced. We have shown that the changes in elastic properties of (Ru–X)Al alloys are due to electronic topological transitions.