Al Sublattice Research Articles

Effects of W and Si microadditions on microstructure and the strength of dilute precipitation-strengthened Al–Zr–Er alloys

The effect of single or combined micro-additions of tungsten (0.03 at.%) and silicon (0.02–0.40 at.%) on the evolution of Al3(Zr,Er) (L12) nanoprecipitates in a cast Al-0.11Zr-0.005Er (at.%) alloy is investigated utilizing isochronal (200–600 °C) and isothermal (400, 425 and 450 °C) aging treatments. Atom-probe tomography measurements reveal that the slow-diffusing W, upon aging, partitions to the L12-nanoprecipitates (average composition of 0.11–0.37 at.% W; partitioning ratio of 5–12), without altering their crystal structure or spheroidal morphology, as confirmed by transmission electron microscopy. First-principles calculations indicate that W occupies the Al sublattice sites of Al3Zr(L12), leading to a small increase in the lattice parameter of the nanoprecipitates. In the low-Si (0.02 at.%) alloys, the precipitation and coarsening kinetics of nanoprecipitates are unaffected by W additions. A small enhancement in nanoprecipitate coarsening resistance is achieved, however, by W additions in the high-Si (0.4 at.%) alloys. The W-modified alloys exhibit significant improvements in compressive creep resistance at 300 °C; the threshold stress increases from 11 MPa in the W-free alloy to 15 and 16 MPa in the low-Si and high-Si alloys with W additions, respectively. This is explained by an increased lattice parameter mismatch between the L12-nanoprecipitates and the matrix, due to Er enrichment in the nanoprecipitates in the presence of W. Silicon additions, in alloys with and without W, lead to a higher peak microhardness (ΔHV ~90–110 MPa) and higher creep threshold stresses (Δσth ~2–4 MPa) by increasing the L12-nanoprecipitate number density, but at the expense of their long-term coarsening resistance.

Lattice distortion inducing local antiferromagnetic behaviors in FeAl alloys

In this work, we study the local magnetic moment as a function of order degree in solid-solution FeAl alloys. Using the combination of ab initio method and similar atomic environment model, we find that the decrease of magnetic moment, even antiferromagnetic behavior, of the Fe atoms derives from the distorted local atomic clusters centered at Fe atoms on the Fe-atom sublattice sites in B2 FeAl alloys. While the local magnetic moment of Fe atoms is up to 2.2 μB on the Al and Fe solid-solution sublattice sites. The ordering results in the decrease of Curie temperature and magnetic moment of solid-solution FeAl alloys.

Site occupancy of alloying elements in γ′ phase of nickel-base single crystal superalloys

L12 structured γ′ phase is the main strengthening phase of Ni-base superalloys, which contain multiple alloying elements. Locations of these elements in the lattice are crucial for understanding the mechanical/physical properties of γ′ phases. Here, we reveal the site occupancy of various elements in γ′ phases of two multi-component Ni-base single crystal superalloys by using advanced scanning transmission electron microscopy and energy disperse spectroscopy. Cr, Mo, Re, Ta, W and Ti atoms preferentially locate at Al sublattices, and Co and Ru atoms prefer Ni sublattices. Moreover, the addition of Ru does not affect the site occupancy of other alloying elements.

Large anomalous Hall angle in the Fe60Al40 alloy induced by substitutional atomic disorder

The electronic and transport properties of the ordered B2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase which undergoes a continuous transition into the disordered A2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase are studied from first principles. The disordering is characterized as a gradual interchange of atoms between Fe and Al sublattices under the condition that the total amount of Fe and Al atoms is kept unchanged. This is the simplest model of gradual disordering of the ordered phase due to the ion irradiation. The physical properties are strongly influenced by varying local environment of Fe atoms on both sublattices. This leads to the transition between a high moment at large disorder and a very low moment in the ordered phase. Similar behavior is found also for anomalous Hall conductivity and anomalous Hall angle. Unusual behavior of the longitudinal conductivity as a function of degree of disorder is due to the shift of the Fermi level of majority states from the $sp$-like part to the $d$ states with increasing ordering. The disordered phase has a large anomalous Hall angle as contrasted with a negligible anomalous Hall angle for well ordered samples, which is in agreement with recent experiment.

Site Occupation of Nb in γ-TiAl: Beyond the Point Defect Gas Approximation

Microalloying is an effective approach to improve the mechanical properties of γ-TiAl intermetallic compound. Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect. Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected. In the present work, we include the point defect interaction energy in the thermodynamic model, which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation. The model is applied to the γ-TiAl–Nb alloy. We show that, at low temperature, the site occupancy of Nb atoms depends on the composition of the alloy: Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy. The fraction of Nb atoms occupying Al sublattice in the Ti-rich alloy decreases drastically, whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature. At high temperature, Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys. The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.

Experimental Chemistry and Structural Stability of AlNb₃ Enabled by Antisite Defects Formation.

First-principles evolutionary algorithms are employed to shed light on the phase stability of Al–Nb intermetallics. While the tetragonal Al3Nb and AlNb2 structures are correctly identified as stable, the experimentally reported Laves phase of AlNb3 yields soft phonon modes implying its dynamical instability at 0 K. The soft phonon modes do not disappear even upon elevating the temperature in the simulation up to 1500 K. X-Ray diffraction patterns recorded for our powder-metallurgically produced arc cathodes, however, clearly show that the AlNb3 phase exists. We propose that AlNb3 is dynamically stabilised by ordered antisite defects at the Al sublattice, leading also to a shift of the Nb content from 75 to ∼81 at.%. Unlike the defect-free AlNb3, the antisite-stabilised variant hence falls into the compositional range consistent with our CALPHAD-based phase diagram as well as with the previous reports.

Atomic size effect of alloying elements on the formation, evolution and strengthening of γ′-Ni3Al precipitates in Ni-based superalloys

Mechanical properties of Ni-based superalloys strongly depend on phase and site preferences of alloying elements which influence bonding strength within γ′-Ni3Al precipitates and microstructural characteristics of these unique class of materials. In the current work, therefore, besides disclosure of the phase partitioning behaviours of alloying X elements (i.e. X = Co, Cr, Nb, Ta or Ti) among γ′ and γ phases, their site occupancy tendencies in γ′ precipitates (determined via first-principles ab initio calculations at 0 K) and effects on the microstructural evolution of Ni80Al15X5 alloy systems (exposed to aging at 800 °C for 4, 16, 64 and 256 h, respectively) have been examined. Bonding features of Ni-Al, Ni-X and Al-X atomic pairs within Ni3Al-X intermetallics have been simulated by utilizing charge density difference (CDD) method, which reveals site preferences of alloying X elements as well. Present theoretical and experimental investigations have shown that mechanical strength of Ni-based superalloys is predominantly affected by bonding properties within γ′ precipitates. As atomic radii of alloying X elements become closer to that of Al atom, energy change parameter, ENi→AlX, values decrease and more Al sublattice sites are preferentially occupied in γ′ precipitates. Correspondingly, bonding strength of Ni-X atomic pairs along <110> directions of Ni3Al-X phases and micro-hardness properties of both as-cast and pre-aged Ni80Al15X5 alloy systems enhance in the order of X = Co < Cr < Ti < Nb < Ta additions. Nevertheless, with increasing aging time, mechanical strength of alloys weakens in parallel with increasing size of γ′ precipitates simultaneously evolved from near-spherical to irregular forms.

Dispersoid composition in zirconium containing Al-Zn-Mg-Cu (AA7010) aluminium alloy

Zirconium (Zr) is used in modern aluminum alloys to form dispersoids that control grain structure. The interaction of these dispersoids with the alloying elements used to strengthen aluminum remains poorly understood. We have used high resolution imaging and composition analysis via electron microscopy to study the Zr-rich dispersoids in AA7010, a commercial Al-Zn-Mg-Cu alloy, addressing this knowledge gap.We show that the dispersoids are not of the ideal Al3Zr stoichiometry, and contain Zn up to approximately 15 at%. Copper also concentrates in the dispersoids up to approximately 6 at%. Atomistic simulation was used to predict the partitioning, demonstrating favourable substitution of Zn and Cu onto the Al sublattice in the dispersoid phase, consistent with the measurements. We have also observed larger, facetted dispersoids, which are not of a phase observed in the binary Al–Zr system. Instead, we have found a previously unreported dispersoid structure (tI10 Ni4Mo structure type), which we propose is stabilized by the presence of Zn.We have calculated the total loss in Zn and Cu due to partitioning into the dispersoids. We show this is too small to directly have a detrimental effect on age hardening, but may have a secondary effect in promoting heterogeneous nucleation for undesirable quench induced precipitation.

Dispersoid Composition in Zirconium Containing Al-Zn-Mg-Cu (Aa7010) Aluminium Alloy

Zirconium (Zr) is used in modern aluminum alloys to form dispersoids that control grain structure. The interaction of these dispersoids with the alloying elements used to strengthen aluminum remains poorly understood. We have used high resolution imaging and composition analysis via electron microscopy to study the Zr-rich dispersoids in AA7010, a commercial Al-Zn-Mg-Cu alloy, addressing this knowledge gap. We show that the dispersoids are not of the ideal Al3Zr stoichiometry, and contain Zn up to approximately 15 at%. Copper also concentrates in the dispersoids up to approximately 6 at%. Atomistic simulation was used to predict the partitioning, demonstrating favourable substitution of Zn and Cu onto the Al sublattice in the dispersoid phase, consistent with the measurements. We have also observed larger, facetted dispersoids, which are not of a phase observed in the binary Al-Zr system. Instead, we have found a previously unreported dispersoid structure (tI10 Ni4Mo structure type), which we propose is stabilized by the presence of Zn. We have calculated the total loss in Zn and Cu due to partitioning into the dispersoids. We show this is too small to directly have a detrimental effect on age hardening, but may have a secondary effect in promoting heterogeneous nucleation for undesirable quench induced precipitation.

Atom probe tomography study of Fe-Ni-Al-Cr-Ti ferritic steels with hierarchically-structured precipitates

The ferritic Fe-Ni-Al-Cr-Mo steel (FBB8) has good creep properties up to 700 °C due to B2-NiAl nanoscale precipitates and its creep resistance can be further improved by additions of 2 or 4 wt.% Ti, as a result of sub-precipitates within the main precipitates. Here, the hierarchical structure of the precipitates is studied in the light of phase separation via transmission electron microscopy (TEM) and atom probe tomography (APT). For FBB8-2Ti (with 2% Ti added) exhibiting B2-NiAl precipitates with L21-Ni2AlTi sub-precipitates, APT analysis shows strong partitioning of Ni, Al and Ti from the ferritic matrix into the B2/L21 precipitates and, within the precipitates, partitioning of Ti and Fe within the L21 sub-precipitates. Based on the published pseudo-binary phase-diagram between (Ni,Fe)Al and (Ni,Fe)Ti, this hierarchical precipitate microstructure is discussed based on the known miscibility gap between the B2 and L21 phases, due to partitioning of Ti into the L21 phase and ordering of Al and Ti on the Al sub-lattice of the B2 structure. For FBB8-4Ti (with 4% Ti added), by contrast, the L21 precipitates exhibit bcc sub-precipitates rich in Fe and Cr, with a composition close to that of the matrix; the absence of the B2 structure is consistent with an increase of Ti and Fe concentrations, to 18.2 and 19.3 at.% respectively, as measured via APT, in the L21 precipitates.

Tuning planar fault energies of Ni3Al with substitutional alloying: First-principles description for guiding rational alloy design

We determine effects of substitutional alloying, at Al or Ni sublattice, with 3d, 4d and 5d series of transition elements on the energies of anti-phase boundary (APB), superlattice intrinsic stacking fault (SISF) and unstable stacking fault (USF) of Ni3Al using first-principles density functional theoretical calculations. We show that USF and APB energies are maximal for the systems wherein elements having half-filled d-orbitals are substituted at the Al site. Analysis of the data on alloys studied reveals a correlation between SISF and APB energies, which along with d-orbital occupancy can be useful as descriptors in rational design of improved Ni-Al alloys.

Enhanced magnetocaloric properties and critical behavior of (Fe0.72Cr0.28)3Al alloys for near room temperature cooling

Magnetic cooling is an environmentally friendly, energy efficient, thermal management technology relying on high performance magnetocaloric materials (MCM). Current research has focused on low cost, corrosion resistant, rare earth (RE) free MCMs. We report the structural and magnetocaloric properties of novel, low cost, RE free, iron based (Fe0.72Cr0.28)3Al alloys. The arc melted buttons and melt spun ribbons possessed the L21 crystal structure and B2 crystal structure, respectively. A notable enhancement of 33% in isothermal entropy change (−ΔSm) and 25% increase in relative cooling power (RCP) for the ribbons compared to the buttons can be attributed to higher structural disorder in the Fe–Cr and Fe–Al sub-lattices of the B2 structure. The critical behavior was investigated using modified Arrott plots, the Kouvel–Fisher plot and the critical isotherm technique; the critical exponents were found to correspond to the short-range order 3D Heisenberg model. The field and temperature dependent magnetization curves of (Fe0.72Cr0.28)3Al alloys revealed their soft magnetic nature with negligible hysteresis. Thus, these alloys possess promising performance attributes for near room temperature magnetic cooling applications.

Directional melting of alumina via polarized microwave heating

Dynamical instabilities and melting of crystals upon heating are fundamental problems in physics and materials science. Using molecular dynamics simulations, we found that drastically different melting temperatures and behaviors can be achieved in α-alumina using microwave heating, where the electric field is aligned with different crystallographic orientations. Namely, alumina melts much earlier at lower temperatures when the electric field is parallel to the c-axis. The atomistic mechanism was identified as selective liberation of the Al sublattice due to the shear instability along the c-axis. This directional melting concept may be used for triggering distinct dynamical instabilities and melting of dielectric crystals using polarized microwave fields.

Investigation of the elemental partitioning behaviour and site preference in ternary model nickel-based superalloys by atom probe tomography and first-principles calculations

The topologically close-packed phase formation is heavily influenced by the concentrations of Co, Ru, and Cr in Ni-based superaloys. For this purpose, we investigate the partitioning behaviour and site preference of these additions in the model Ni–Al–X single crystal superalloys by atom probe tomography and first-principles calculations. Compared with the commercial multicomponent superalloys, Co and Ru still strongly partition to the phase, while Cr weakly partitions to the phase in the ternary model alloys. Based on the phase composition analyses, Ru and Cr atoms preferentially substitute for the Al sublattice sites, whereas Co atoms substitute at both Al and Ni sublattice sites in the ordered - phase. First-principles calculations on the substitutional formation energies at 0 K were performed to understand the site preference, and the partitioning coefficient of additions was given in a quantitative model. The partitioning behaviour and site preference of Co, Ru, and Cr by theoretical calculations are consistent with the experimental results.

Preferential site occupancy of alloying elements in TiAl-based phases

First principles calculations are used to study the preferential occupation of ternary alloying additions into the binary Ti-Al phases, namely, γ-TiAl, α2-Ti3Al, βo-TiAl, and B19-TiAl. While the early transition metals (TMs, group IVB, VB, and VIB elements) prefer to substitute for Ti atoms in the γ-, α2-, and B19-phases, they preferentially occupy Al sites in the βo-TiAl. Si is, in this context, an anomaly, as it prefers to sit on the Al sublattice for all four phases. B and C are shown to prefer octahedral Ti-rich interstitial positions instead of substitutional incorporation. The site preference energy is linked with the alloying-induced changes of energy of formation, hence alloying-related (de)stabilisation of the phases. We further show that the phase-stabilisation effect of early TMs on βo-phase has a different origin depending on their valency. Finally, an extensive comparison of our predictions with available theoretical and experimental data (which is, however, limited mostly to the γ-phase) shows a consistent picture.

Dynamics of Discrete Breathers in a Pt3Al Crystal

The discrete breathers in a Pt3Al crystal, which exhibit soft (DB1) and hard (DB2) nonlinearity, are shown to possess a number of principal differences. Unlike an immobile and stable DB1, a DB2 breather is mainly localized on four Al atoms and is stretched along one of the close-packed rows of crystals. On the other hand, DB2 can displace hundreds of nanometers along one of the directions of close packing. Having localized a considerable amount of energy, both DB1 and DB2 breathers slowly emit it during their lifetime. A collision of DB1 and DB2 results in part of their energy being released into the Al sublattice, the larger part lost by DB2 that is destroyed faster than DB1. The DB2 breather can effectively transport the energy throughout the crystal, and a collision of DBs results in its considerable localization in the crystal. A capability of transferring the energy can thus give rise to structural transformations far from the focus of excitation of such localized objects.

Site preference and lattice relaxation around 4d and 5d refractory elements in Ni3Al.

X-ray absorption spectroscopy is employed to investigate site preference and lattice relaxation around Mo, Ru, Hf, W and Re dopants in Ni3Al. The site occupation preference and the measured distances between the refractory elements as dopants and the nearest host atoms are compared with the results of ab initio calculations within the density functional theory. Combined experimental and theoretical results indicate that Mo, Hf, W and Re atoms reside on the Al sublattice in Ni3Al, while Ru atoms occupy the Ni sublattice. A more pronounced lattice relaxation was detected in the case of Hf and Ru doping, with a strong outward relaxation of the nearest Ni and Al atoms.

Effects of increased alloying element content on NiAl-type precipitate formation, loading rate sensitivity, and ductility of Cu- and NiAl-precipitation-strengthened ferritic steels

Two experimental bcc-Cu- and B2–NiAl-precipitation-strengthened ferritic steels with 6.3 at. % and 12.4 at. % Cu + Mn + Ni + Al, 950 MPa and 1600 MPa yield strength respectively, were studied. Atom probe tomography showed that the volume fraction and number density of NiAl-type precipitates in the heavier alloyed steel (designated as CF-9) is ∼60–70 times greater than those in the lighter alloyed steel (designated as CF-2). This is attributed to the smaller lattice misfit between these NiAl-type precipitates and the ferritic matrix in CF-9 due to more incorporation of Mn atoms on the Al sub-lattice in the B2 NiAl unit cell. Loading rate sensitivity of hardness was measured for CF-2, CF-9 and SAE-1090, which does not have bcc-Cu precipitates. Results show that even though CF-2 and CF-9 have double and triple the strength of SAE-1090 respectively, their hardness shows weaker dependence on loading rate. This is attributed to the presence of bcc-Cu precipitates in CF-2 and CF-9 providing athermal activation of nearby screw dislocation motion. Auger electron spectroscopy studies of CF-9 samples reveal Cu segregation on grain boundaries. The observed Cu segregation is believed to be partly responsible for the lower elongation-to-failure of CF-9 compared with CF-2.

Magnetism of ordered and disorderedNi2MnAlfull Heusler compounds

Based on ab initio calculations and Monte Carlo simulations, we present a systematic study of the magnetic ground state and finite temperature magnetism of ordered and disordered ${\mathrm{Ni}}_{2}\mathrm{MnAl}$ full Heusler compounds. By increasing the degree of the long-range chemical disorder between the Mn and Al sublattices, the magnetic order progressively changes from the ferromagnetic state in the ordered $L{2}_{1}$ phase toward a fully compensated antiferromagnetic state in the disordered $B2$ phase and we also conclude that the Ni atoms exhibit induced moments. We determine the Mn-Mn interactions by using the magnetic force theorem and find dominating, but rather weak ferromagnetic couplings in the ordered $L{2}_{1}$ phase. We used a recently proposed renormalization technique to include the weak Ni moments into the spin model, which indeed remarkably increased the nearest-neighbor Mn-Mn interaction. In accordance with the total energy calculations, in the disordered compounds, strong antiferromagnetic site-antisite Mn-Mn interactions appear. Determining the spin-spin correlation functions from Monte Carlo simulations, we conclude that above the transition temperature, short-range antiferromagnetic correlations prevail between the Mn atoms. In view of the potential application of disordered ${\mathrm{Ni}}_{2}\mathrm{MnAl}$ as a room temperature antiferromagnet, we calculate the magnetic anisotropy energies of tetragonally distorted samples in the $B2$ phase and find that they are smaller by two orders in magnitude than in the frustrated antiferromagnet ${\mathrm{IrMn}}_{3}$.

Simulation of the interaction between discrete breathers of various types in a Pt3Al crystal nanofiber

It is known that, in a molecular dynamics model of Pt3Al crystal, a discrete breather (DB) with soft type nonlinearity (DB1) can be excited, which is characterized by a high degree of localization on a light atom (Al), stationarity, as well as a frequency that lies in the gap of the phonon spectrum and decreases with increasing amplitude of the DB. In this paper, it is demonstrated that a DB with hard type nonlinearity (DB2) can be excited in a Pt3Al nanofiber; this DB is localized on several light atoms, can move along the crystal, and has a frequency that lies above the phonon spectrum and increases with the DB amplitude. It is noteworthy that the presence of free surfaces of a nanofiber does not prevent the existence of DB1 and DB2 in it. Collisions of two DBs counterpropagating with equal velocities, as well as a collision of DB2 with a standing DB1, are considered. Two colliding DBs with hard type nonlinearity are repelled almost elastically, losing only insignificant part of their energy during the interaction. DB2 is also reflected from a standing DB1; in this case, the energy of the breathers is partially scattered into the Al sublattice. The results obtained indicate that DBs can transfer energy along a crystal over large distances. During the collision of two or more DBs, the energy localized in space can be as high as a few electron-volts; this allows one to raise the question of the participation of DBs in structural transformations of the crystal.