Ll2 Structure Research Articles

Thermodynamic properties of Cu3Au alloy

Based on the general expression of the Helmholtz free energy of alloys with Ll2 structure obtained in previous works, we calculate the equations of the long-range order (LRO) parameter, and order-disorder transition (ODT) temperature in the Cu3Au alloy, and obtained thermodynamic quantities of Cu3Au alloy as simple functions of temperature and LRO-parameter, at pressures P = 0. Numerical results of the linear thermal expansion coefficient, isochoric and isobaric heat capacities, isothermal and adiabatic bulk modulus, are in good agreement with experiments.

Alloying Element Ta Effect on Microstructure of Co-Al-W Superalloy by Vacuum Arc Melting

Co-based high temperature alloys have been widely used in aeronautics and astronautics industry, because of its high strength at high temperature, excellent resistance of hot corrosion and oxidation. Unlike the traditional Co-based superalloys, strengthened by solution and carbide strengthening, the novel Co-Al-W superalloys are strengthened by a ternary compound with the Ll2 structure γ-Co3(Al,W). And the novel Co-Al-W superalloys showing high-temperature strength greater than those of conventional nickel-base superalloys, will become the candidates for next-generation high-temperature materials. We research alloying element Ta effect on microstructure of Co-Al-W superalloys by vacuum arc melting. Compare with the microstructure before and after adding alloying element Ta of Co-Al-W superalloy, we find that most of Ta element distributed in the γ-Co substrate phase, stabilizing and reinforcement the γ phase.

Theoretical investigations on phase stability, elastic constants and electronic structures of D022- and L12-Al3Ti under high pressure

Phase stability, elastic and thermodynamic properties, and electronic structure of titanium trialuminide (Al3Ti) with Ll2 and D022 structures under pressure up to 40GPa have been investigated using first-principles calculations. The equilibrium structure and formation energy show that L12-Al3Ti is stable when the pressure is higher enough, approximately above than 20–30GPa. The elastic constants, anisotropy index and Debye temperature of both phases increase with the pressure going up, and L12-Al3Ti has better ductility, smaller anisotropy and lower Debye temperature than D022-A13Ti. The pressure-induced Ti-3d delocalization can strengthen its orbital hybridization with Al(s,p), which leads to stronger atomic bonding, and subsequently makes the L12-Al3Ti more stable under high pressure.

Mechanisms of Al Self- and Al-Substituting Solute Diffusion in Ni3Al

ABSTRACTDiffusion of the Al-substituting elements Ga, Ge, Ti and Nb has been investigated in Ni3Al single crystals by the secondary ion mass spectrometry (SIMS). A model for the minority component diffusion by anti-structure defects has been suggested for the Ll2 structure. It involves both nearest neighbor jumps of the solute atoms on the Ni sublattice as anti-structure atoms and the formation of so-called anti-structure bridges which correspond to jumps between different sublattices. The model under consideration was shown to agree with the experimental data on the Al-substituting solute diffusion (X = Ga, Ge, Ti, Nb) in Ni3Al and it allows to explain also the observed ratio of the diffusivities DNi/DX, which may be higher and/or lower than unity in dependence on the solute and temperature.

Compacting and sintering of aluminium-lead powder mixtures : D.Nath et al. (Benaras Hindu University, Benaras, India.) PM Science Technol., vol 6, no 4, 1996, 25–31

The early decomposition stages of Al-7.8at.%Li, Al-10.4at.%Li and Al-11.8at.%Li alloys were studied by high resolution electron microscopy. In all the above alloys, it was confirmed that in the as-quenched stage small ordered domains of Ll2 structure were present surrounded by the disordered matrix. From this microstructure, it was concluded that the alloy was already decomposed as in the as-quenched stage. No clear evidence was found for congruent ordering which was proposed to occur prior to spinodal decomposition. Examination of the sublattices of the individual ordered domains also failed to give convincing evidence for congruent ordering. The present observations suggest that the kinetics of spinodal decomposition of this system is too fast to be detected by TEM. In order to slow down the decomposition kinetics, solution treated samples were quenched to just above the solvus line for δ′ and then to room temperature. By this heat treatment, a solution treated sample with muck weaker order spots could be obtained. However, this microstructure could not be interpreted as congruently ordered either based on the weak Ll2 spots. As there is a limitation to the quenching speed, it is suggested that ordering and spinodal decomposition progress concomitantly during the observable range of time.

Deformability of powder materials for making composites by rolling : I.N.Zigalo et al. Poroshkovaya Metal. no 3/4, 1996, 19–24. (In Russian.)

The early decomposition stages of Al-7.8at.%Li, Al-10.4at.%Li and Al-11.8at.%Li alloys were studied by high resolution electron microscopy. In all the above alloys, it was confirmed that in the as-quenched stage small ordered domains of Ll2 structure were present surrounded by the disordered matrix. From this microstructure, it was concluded that the alloy was already decomposed as in the as-quenched stage. No clear evidence was found for congruent ordering which was proposed to occur prior to spinodal decomposition. Examination of the sublattices of the individual ordered domains also failed to give convincing evidence for congruent ordering. The present observations suggest that the kinetics of spinodal decomposition of this system is too fast to be detected by TEM. In order to slow down the decomposition kinetics, solution treated samples were quenched to just above the solvus line for δ′ and then to room temperature. By this heat treatment, a solution treated sample with muck weaker order spots could be obtained. However, this microstructure could not be interpreted as congruently ordered either based on the weak Ll2 spots. As there is a limitation to the quenching speed, it is suggested that ordering and spinodal decomposition progress concomitantly during the observable range of time.

Phase stability and microstructure of Al-Ti-Fe near Al3Ti

The phase stability and microstructure of Al-Ti-Fe near Al3Ti at 800 °C are studied by using high resolution electron microscopy and backscattered electron imaging. One of the most significant changes resulting from the addition of small amounts of iron in this system is to convert tetragonal Al3Ti or Al2Ti into cubic Ll2 structures. In addition, the added iron can change the stacking structure of Al2Ti and the domain size of the long-period one-dimensional antiphase domain structures. A partial ternary phase diagram in this region is also determined.

The Early Stages of Solid-State Reactions in Ti/Al Multilayer Films

ABSTRACTWe have investigated the solid-state reaction of Ti/Al multilayer films by x-ray diffraction (XRD) and differential scanning calorimetry (DSC), with focus on the early stages of the reaction provided by samples with pair thicknesses in the range 5 - 40 nm. This reaction, which results in formation of TiAl3 with metastable Ll2 structure, can be modeled by a nucleation and growth process on the basis of the Johnson-Mehl-Avrami theory, with a reaction-order parameter n ≈1. These observations indicate the significance of nucleation barriers even at early stages of solid-state reactions, and suggests that the phenomena of phase selection and formation of metastable phases can result from the presence of nucleation barriers.

Cohesive and electronic properties of ordered Li-Al intermetallic compounds

Self-consistent electronic structure calculations have been performed on ordered lithium-aluminium compounds using the tight-binding linear muffin-tin orbital (TBLMTO) method. The FCC-based ground-state superstructures (namely Ll2 and Ll0 structures) show some systematic trends in their cohesive and electronic properties, which are in reasonably good agreement with the available experimental data. We have also compared the density of states, band structures and total ground-state energies of equiatomic AlLi compounds, between the FCC-based Ll0 structure and the BCC-based B32 structure. While the former shows a two-dimensional metallic behaviour, the latter shows a resemblance to a tetrahedral-bonded covalent solid, and is more stable. After detailed comparison with some recent LAPW calculations, we conclude that the TBLMTO method can be used as an efficient and reasonably accurate first-principles tool for studying the phase stability and chemical bonding in ordered intermetallic compounds.

Nanoanalysis of precipitation in a modified Inconel 718 alloy

The nickel-base superalloy Inconel 718 (IN718) is strengthened primarily by precipitation of two types of intermetallic phases in the austenite (γ) matrix: metastable γ' (cubic Ll2 structure) and γ" (tetragonal D022 structure) precipitates. Minor changes of the chemistry of the IN718 alloy can significantly affect its mechanical properties at high temperatures. It has been found that the microstructural stability of the precipitates and the high temperature performance of IN718 depend critically on the (Ti+Al)/Nb and Al/Ti ratios. The small sizes of these intermetallic phases and the fact that γ' precipitates are frequently associated with γ" precipitates in modified IN718 make it difficult to determine their composition. In this study, nanometer resolution analytical electron microscopy techniques have been used to characterize the structure, the morphology and the composition of the precipitated phases.The fabrication and the heat treatment of the modified IN718 alloy and its high temperature mechanical properties have been reported elsewhere.

Atomic morphology of the γ/γˊ interface

Nickel-based superalloys are well known for their high temperature strength. These alloys consist of two phases, γ and γˊ, which in combination produce an alloy of higher strength than either phase alone. A significant factor in this strength increase is the interface between the two phases. The γ phase is a fcc solid solution of nickel, while the γˊ phase has a Ll2 structure based on Ni3Al. The difference in lattice parameter between these two phases is very small, producing large regions of coherent interface between the two phases. This interface between γ and γˊ was examined by high resolution electron microscopyThe nickel-based superalloy (René N4) was generously supplied by Dr. R. Field of GE Aircraft Engines. TEM samples were cut from this oriented single crystal, mechanically thinned, then electropolished in a solution of 10% perchloric acid in ethanol. A Philips 400T TEM was used for conventional imaging and a JEOL 4000EX was used for high resolution imaging.

Diffusion of cobalt, chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni3Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D(Co)= (4.2 ± 1.2) × 1O-3exp {-325 ± 4 (kJ/mol)/RT} ~D(Cr) = (1.1 ± 0.3) × 10-1 exp {-366 ± 3 (kJ/mol)/RT} and D(Ti)= (5.6 ± 3.1) × 101 exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll2 lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll2 structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.

Strength anomaly and dislocation structure at 4.2 k in ni3(si, ti) single crystals

Abstract The critical resolved shear stress (CRSS) at 4.2 K for the (111)[01] slip system activation in Ni3(Si, Ti) single crystals with the Ll2 structure was measured by compression testing and the corresponding dislocation structures were investigated by means of TEM strong-and weak-beam methods. The CRSS showed strong orientation anisotropy and correspondingly a positive temperature dependence at this temperature. [01] screw superdislocations were dominant and sessile in all the observed crystal orientations. The superpartial dislocations combined with the APB tended to dissociate more widely onto the (010) cross-slip plane as the crystal orientation changed from [001] through [011] to [11]. It is suggested that the Kear-Wilsdorf locking is the most likely mechanism by means of which the strength anomaly observed at this temperature can be explained.

CALCULATION OF ORDERED PARAMETERES IN LI2 STRUCTURE BY MONTE-CARLO METHOD

The calculation of order parameteres with variant temperature in Ll2 structure has been carried out in this work by means of Monte Carlo method statistical physics. The order interaction of the nearest neighbour J1 and three kinds of order interaction of next neares' neighbour J2= 0, - 0.3J1 as well as 0.3J1 were considered in this calculation. The results show that the calculation value of the critical temperature Tc (order-disorder transition temperature) of the first order phase transformation is in good agreement with that obtained by some other theories, such as quasi-chemical method; and Tc is sensitively affected by J2, that may be regarded as an interpretation about the action of interstitial atoms in Ll2 structure.

A SISF formation mechanism in Ni3Al

In the ordered Ll2 structure, the superlattice intrinsic stacking fault (SISF) has often been observed to be formed through two steps, the formation of antiphase boundary (APB) by dissociation of superlattice dislocation at first and then the conversion of the APB to the SISF(1,2). The present paper studied the SISF in NijAl by TEM and found that the SISF can also be formed by superlattice dislocation dipoles.The tensile deformation of the directionally solidified Ni3Al (of composition in wt% of 82.88Ni, 8.5A1, 0.8Zr, 7.8Cr, and 0.02B) specimens was performed at room temperature. Thin foils were made normal to the specimen axis (parallel to [001]) and studied at 200 kv in H-800.As shown in Fig.l, a SISF is formed at the end of a superlattice dislocation dipole, according to the SISF-type dissociation [10] =1/3 [11]+1/3[2l] on (111) plane. In Fig.2, a superlattic dislocation dipole is moving away from a SISF.

Phase Stability in the Al(6−x)CuxZr2 System for 0 < × < 1.

AbstractWe have determined the structures and thermal stability of AI(6-x) CuxZr2 (0≤ × ≤ 1) powders prepared by mechanically alloying elemental powder mixtures. In the as mechanically alloyed condition the structure is L12 for all ×. After heating to 750°C the alloy is single phase D023 for 0 < × < 0.2, is a two-phase mixture of D023 and L12 for 0.2 < × < 0.6, and remains in the Ll2 phase for 0.6 < × < 1. Al5CuZr2 is stable in the Ll2 structure up to 1300°C. The lattice parameter of the annealed L12 phase is Independent of ×.

The Effect of Scandium on the Phase Stability of Al3Nb AND Al3Zr

AbstractA series of Al-25 (Nb,Sc), at.%, and Al-25 (Zr,Sc), at.%, alloy buttons were arccast in argon and homogenized for 2 h in vacuum at temperatures ranging from 1473 to 1573K. X-ray powder-diffraction indicated that almost all the Nb in Al-25 Nb (DO22) had to be replaced by Sc in order to obtain the Ll2 structure. In the case of Al3Zr much less Sc was required - the single phase Ll2 composition is approximately Al16Zr8Sc.Recent calculations [1,2] show that once the tetragonality is properly included the phase stability of the trialuminide transition-metal binary alloys can be understood in terms of their electronic density of states. The dominating feature is a deep minimum in the density of states just below the major transition metal d-band peak. The exact position of the minimum changes with structure type (i.e., Ll2, DO22, or DO23, and with c/a). The alignment of the Fermi energy with the minimum appears to determine the equilibrium structure. The results of linearized-muffin-tin-orbital [3] (LMTO) electronic structure calculations are compared to the rigid band model and checked against the experimentally determined phase boundaries.

TEM images of overlapped precipitates

The Ni3Al precipitates with a cubic shape and densely aligned along the [100] direction are always observed in Ni-Al alloys and nickel base superalloys. It is quite frequently observe the overlap of two or more separate Ni3Al particles, which locate at different depths in a thin foil specimen during a TEM examination. However, no one has paied much attention to the variations of intensity and contrast on the overlapped area of two separated particles in TEM dark field image. Hence, the analyzed results of dark field microscopy may give an unfaithful determination of particle size as well as a miscount of the total particle number. This investigation has been concentrated on the image forming mechanism and the explanation of intensity change on the overlap area of two Ni3Al particles.Fig.1 indicates a unit cell with Ll2 structure in Ni3Al precipitate A, and an identical one in precipitate B.

Cleavage fracture in an Al3Ti-based alloy having the Ll2 structure

Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al–23Ti–6Fe–5V alloy, which is an Al3Ti-based alloy having the Ll2 structure. Alloys like this one are of scientific and technological interest because they fracture by brittle transgranular cleavage, despite the availability of more than five independent slip systems in the Ll2 structure and their relatively low hardness when compared to ductile Ll2 alloys like Ni3Al. Homogenized bars of the levitation zone melted and directionally solidified alloy were fractured at room temperature in three point bending. The fracture surfaces, which consisted almost entirely of brittle transgranular cleavage facets, were examined in a scanning electron microscope (SEM) equipped to take channeling patterns. An optical microscope with a short depth of focus was inserted in the SEM column just below the objective lens, and by focusing it on several points on the cleavage facets, it was possible to orient the facets normal to the optic axis of the SEM prior to taking the channeling patterns. Of the eight cleavage facets examined in this study, six were of the {110} type and two were of the {111} type. Although it cannot be said that these are the only two cleavage planes in this alloy, the availability of more than one plane with low cleavage strength contributes to the brittleness of this alloy. These results are examined in the light of theoretical treatments of cleavage fracture, and comparisons are made with earlier studies on other Ll2 materials.

The effect of composition and cooling rate on the structure of rapidly solidified (Fe, Ni)3Al–C alloys

There is controversy in the literature regarding the existence of the metastable γ′ phase with an ordered Ll2 structure in rapidly solidified Fe–Ni–Al–C alloys. In this study, the quench rate–metastable structure dependence was examined in the Fe–20Ni–8Al–2C (weight percent) alloy. The effect of silicon on the kinetics of phase formation was studied by adding two weight percent silicon to a base alloy of Fe–20Ni–8Al–2C. Samples were rapidly solidified in an arc hammer apparatus and examined by transmission electron microscopy. In the Fe–20Ni–8Al–2C alloy, the nonequilibrium γ′ and γ phases were found in foils 65 to 100 μm thick. At higher quench rates, i.e., thinner samples, the matrix was observed to be disordered fcc γ with K-carbide precipitates. Samples containing silicon were found to have a matrix composed of γ′ and γ structures when the foils were thicker than 40 μm. At higher quench rates, the matrix was disordered fcc γ with K-carbide precipitates. The nonequilibrium γ′ and γ structures are present in samples with or without silicon, but are observed at higher cooling rates with the addition of silicon. This sensitivity to cooling rate and composition in resulting metastable structures may explain the differences reported in the literature for these rapidly solidified materials.