Effect of carbon on formation of mixed solid solutions during mechanochemical synthesis of Ni-Al-Mo-C mixtures and ordering of solutions during heating

Solid solutions Ni(Al, Mo, C) are formed via milling the Ni_2.8Al₁Mo_0.2 and Ni₃Al_0.8Mo_0.2 and graphite-containing Ni_2.8Al₁Mo_0.2C_{(0.25, 0.5)} and Ni₃Al_0.8Mo_0.2C_{(0.25, 0.5)} mixtures. In this case, some amount of Mo remains beyond the solid solution. Graphite added to a starting mixture decreases the Mo solubility and favors the amorphization of solid solutions. The complete amorphization was found for the mixture with the 5 at % C and 5 at % Mo, which was added instead of Ni. The heating of mechanically synthesized (MS) powder alloys leads to the ordering of carbon-free and carbon-containing solid solutions with the formation of the L1₂ and E2₁ structure, respectively. In the course of the ordering of the Ni(Al, Mo, C) solid solutions, Mo and carbon precipitate in the form of the molybdenum carbide (Mo₂C) second phase. The hardness of the MS three-phase Ni-Al-Mo-C solid solutions subjected to hot isostatic pressing is determined by the mass fraction of the formed Mo₂C carbide. It is shown that the carbon content in the multicomponent antiperovskite can be estimated by analyzing the ratio of integral intensities of superlattice reflections I ₍₁₀₀₎/I ₍₁₁₀₎.     

Solid State Interaction in Ni-Mo-B System During Mechanical Alloying and Annealing

This work presents the results of a study of Ni_87?x Mo _x B₁₃ alloys (x?=?7, 10 and 14?at.%), which were obtained by mechanical alloying (MA) of elemental powder mixtures in a MAPF-2M high-energy planetary ball mill. The x-ray diffraction analysis and differential scanning calorimetry measurements were used. The single-phase fcc solid solutions of Mo and B in Ni were formed by MA of Ni-Mo-B mixtures of various compositions for 6-8?h. The coherent domain sizes of solid solutions calculated from the x-ray peak widths were 12-14?nm. The exothermic effects on the DSC curves, which corresponded to the phase transformations of supersaturated Ni(Mo,B) solid solutions, were observed during heating of the synthesized alloys. After heating to 700?°C, the alloys contained a fcc Ni(Mo) phase and a metastable hexagonal MoB₄ phase. Thermodynamically stable phase composition of Ni₈₀Mo₇B₁₃ and Ni₇₇Mo₁₀B₁₃ alloys, containing three phases: fcc Ni (Mo), Ni₂₁Mo₂B₆ with cubic lattice and Ni₃B with orthorhombic lattice, was reached after the isothermal annealing at 1000?°C. The ratio between the amounts of these phases in the alloys corresponds to their location in a three-phase area of the Ni-Mo-B equilibrium phase diagram.     

Mechanical alloying as method for introducing carbon in Ni3Al intermetallide

The method for the mechanical alloying of Ni-Al-C and Ni₃Al-C mixtures was used to obtain nonequilibrium solid Ni(Al,C) solutions in which the carbon content varies from 2.9 to 8.5 at %. The relationship between carbon dissolution and the probability of appearance of deformation-induced stacking faults (SFs) in the formation of mixed (substitutional and interstitial) solid Ni(Al,C) solutions has been found based on an analysis of the diffraction spectra. SFs are assumed to serve as pathways of carbon penetration in nickel-based solid solutions. The effective carbon radius was found to be about 0.0616 nm in the formation of an antiperovskite phase Ni₃AlC _x . The method of calculating the amount of interstitial carbon was proposed based on the experimental lattice parameters of fcc solid Ni(Al,C) solutions and ordered phases L1₂ Ni₃Al and E2₁ (Ni₃AlC _x ). The temperature stability of the nonequilibrium solid Ni(Al,C) solutions was established. It was shown that the decomposition of the solid solutions proceeded according to a spinodal mechanism at a temperature of 400°C with separation into two phases, i.e., an antiperovskite carbide (Ni₃AlC _x ) and Ni(Al,C). At higher temperatures (600?C800°C), carbon precipitates from these phases with the formation of an antiperovskite Ni₃AlC_0.16, solid Ni(Al) solution, and nanocrystalline graphite.     

New Mo3Pb phase with a15 structure formed in solid solutions of film molybdenum-lead system

Ion-plasma sputtering and codeposition of Mo and Pb ultrafine particles have been used for the first time to prepare solid solutions that are alloys over the whole composition range of the binary system, which were obtained in the form of coatings; this confirms the thermal-fluctuation melting and coalescence of small particles. When coatings are formed by molybdenum and lead nanolayers less than 1 nm thick, the mutual dissolution of the components with the formation of solid solutions in each other takes place; in this case, beginning from a concentration of ～25 at % Pb in the alloy, lead atoms give their crystal symmetry for the formed lattice. A new phase was found that was prepared directly in the course formation; it was identified as the Mo₃Pb compound with the A15 body-centered cubic structure. X-ray diffraction data for the identification of the phases were determined. The upper temperature limit of the existence of the Mo₃Pb compound has been found and the unit cell whose volume is 0.1290 nm³ has been constructed.     

Generalized model for atomic site preference in crystal and its application in rare-earth alloys

A generalized atomic site preference (ASP) model for crystalline compounds is developed. The ASP state can be characterized quantitatively by the atomic distribution tensor or the ASP tensor. The freedoms and the value ranges of the ASP state parameters are analyzed and determined. Based on the present model, the ASP behaviors of atom Al in LaNi_5?xAl_x with CaCu₅ structure and atom M in SmCo_12?xM_x (M = Mn or Mo) with ThMn₁₂ structure are simulated by a thermodynamic mean field method and the inversed pair potentials. The calculated results show (1) in LaNi_5?xAl_x, the site preference degree of Ni on sublattice g decreases with the increasing of Al composition, and all Al atoms occupy sublattice g preferentially; (2) Mn and Mo prefer to locate on sublattice i in SmCo_12?xM_x alloys. The present results agree well with the experimental results and/or the reported theoretical calculations in literature.     

Effect of ternary alloying elements addition on atomic ordering characteristics of Fe–Al intermetallics

The energetic and structural characteristics of atomic ordering processes in Fe_0.5(Al_1−nX_n)_0.5 intermetallics have been qualitatively analyzed based on the statistico-thermodynamical theory of ordering by means of a quasi-chemical method combined with electronic theory in the pseudopotential approximation. The effects of ternary impurities on order–disorder phase transformation temperature and the characteristics of atomic short-range order in Fe–Al type intermetallics have been calculated. Impurity elements in Fe_0.5(Al_1−nX_n)_0.5 where X=Ni, Co, Mn, Cr, Ti, Si, Zr, Hf, Nb, Ta, Re, Mo or W, are considered up to 1 at.% concentration. The results of the calculation indicate that the impurity elements, X, with regard to their lattice site occupancy characteristics (SRO) can be divided into two groups; X_I=Ni, Co, Mn or Cr element atoms substitute mainly for Al sublattice sites, whereas X_II=Ti, Si, Zr, Hf, Nb, Ta, Re, Mo or W element atoms substitute preferentially for Fe sublattice sites in Fe_0.5(Al_1−nX_n)_0.5 intermetallics. It has been found that the absolute values of partial ordering energies of the W^Al–X(R₁) and W^Fe–X(R₁) have a profound effect on the order–disorder transition temperature of Fe_0.5(Al_1−nX_n)_0.5 alloys that would either increase or remain unchanged depending on the type and content of the ternary substitutional alloying elements. The impurities X=Zr, Hf, Nb, Ta, Re, Mo or W which are preferentially distributed Fe sublattice sites are more effective in increasing order–disorder transition temperature in Fe–Al(B2) intermetallics. The results of the present calculation are in good qualitative agreement with experimental observation for most of the third component impurity elements X in Fe_0.5(Al_1−nX_n)_0.5 intermetallics.     

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(Al_1−xW_x) 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.     

Electron-irradiation-induced solid-state amorphization in supersaturated Ni–Zr solid solutions

Electron-irradiation-induced solid-state amorphization (SSA) in supersaturated Ni–Zr solid solution models was investigated by molecular dynamics (MD) simulations, focusing on the threshold composition for the occurrence of SSA. The threshold composition for the MeV electron irradiation-induced SSA can be estimated to be around Ni₈₂Zr₁₈. A Ni₈₂Zr₁₈ solid solution shows SSA, while a Ni₈₃Zr₁₇ solid solution can maintain its original crystalline structure against irradiation. Thermal recovery of the lattice defects introduced during electron irradiation can be seen in both Ni₈₃Zr₁₇ and Ni₈₂Zr₁₈ solid solutions. There is a significant difference between Ni₈₃Zr₁₇ and Ni₈₂Zr₁₈ in terms of changes in the number of 12-faced volonoi polyhedrons, namely, the number of constituent atoms whose coordination number is 12. A change in the combination of various volonoi polyhedrons (a change in coordination number of constituent atoms) during the introduction of lattice defects and thermal recovery precedes SSA in the Ni–Zr solid solution model.     

Tracer diffusion of 63Ni in Ni3(Al,Ge) ternary intermetallic compound

Radio-tracer diffusion measurements of ⁶³Ni have been performed in Ni₇₅Al_xGe_25−x ternary intermetallic compounds at various temperatures. The tracer diffusivity was found to depend exponentially on the Ge content of the alloy. These compounds are ordered with the L1₂ structure, where the Ni atom diffusion proceeds mainly via Ni sublattice site jumps. The change of the diffusivity can be attributed to the change in vacancy concentration on the Ni sublattice as well as to the composition dependence of the saddle-point energy of the diffusion jumps.     

Site-preferences and local spin-polarization of transition metal solute atoms in B2 type Ni–Al alloys

In the framework of the Local Spin Density Approximation we study the electronic structure, site preference energies and magnetism in B2 Ni₅₀(Al_37.5Ni_12.5) alloys doped with 3d-transition metals using a Coherent Potential Approximation for treating effects of substitutional disorder. We find that the lattice expands also for non-magnetic Ti and V substitutions and thus this effect cannot entirely be related to magnetism as it was conjectured earlier. Except Co all TM atoms are found to prefer the Al sublattice. For the magnetic substitutions (Cr, Fe, Mn) we also find that the lattice expands independently on the state of magnetic order – only local atomic spin-polarization is important. An effective magnetic coupling between substituted atoms has been calculated and it is shown that it does not scale with the magnitude of the local moments.     

Interdiffusion in pseudobinary sections of Ni3Al–Co ternary system

Interdiffusion in Ni₃Al intermetallic alloyed by 0–19 at.% Co was studied in three pseudo-binary sections: c_Al=22, c_Co=3.5, and c_Co=16 (c in at.%). The experiments were carried out at temperatures between 1173 and 1473 K. An attempt was made to analyze the interdiffusion coefficient into intrinsic diffusivities of individual components. Since no detectable shift of tungsten inert markers was observed, it was concluded that the intrinsic diffusivities of element pairs with varying concentration (Al/Ni and Co/Ni) in respective pseudo-binary sections are approximately equal one to another. The interdiffusion in pseudo-binary alloys Al/Ni (c_Co=const.) runs slightly faster than that in pseudo-binary alloys Ni/Co (c_Al=const.). The relations between the measured interdiffusion coefficients in the Ni_3−xAlCo_x ordered γ′ phase agree well with literature data for the interdiffusion coefficients in both the ordered γ′ and γ′+Co phases and also for the disordered Al–Ni and Co–Ni solid solutions. They are qualitatively explained by mutual interaction between constituents and by their interaction with vacancies.     

Thermodynamic Description of the Al-Mo and Al-Fe-Mo Systems

The Al-Mo and Al-Fe-Mo systems were critically assessed using the CALPHAD technique. The solution phases (liquid, fcc and bcc) were described by a substitutional solution model. The non-stoichiometric compound AlMo₃ was described by a two-sublattice model (Al,Mo)(Al,Mo)₃ in the Al-Mo binary system and (Al,Fe,Mo)(Al,Fe,Mo)₃ in the Al-Fe-Mo ternary system. Other compounds Al₆₃Mo₃₇, Al₈Mo₃, Al₃Mo, Al₄Mo, Al₁₇Mo₄, Al₂₂Mo₅, Al₁₂Mo and Al₅Mo in the Al-Mo system were treated as stoichiometric compounds in the binary system and as line compounds Al _m (Fe,Mo) _n in the Al-Fe-Mo ternary system. The compounds μ and Fe₂Mo in the Fe-Mo system were treated as (Al,Fe)₇Fe₂(Fe,Mo)₄ and (Fe,Mo)₂(Al,Mo) in the Al-Fe-Mo system, respectively. Compounds Al₅Fe₄, Al₂Fe, Al₅Fe₂ and Al₁₃Fe₄ in the Al-Fe system were treated as (Al,Fe,Mo), Al₂(Fe,Mo), (Al,Fe)₅(Al,Fe,Mo)₂ and (Fe,Mo)_0.235Al_0.6275(Al,Va)_0.1375 in the Al-Fe-Mo system, respectively. Ternary compounds τ₁ and τ₂ were treated as Al₈(Al,Fe)Mo₃ and (Al,Fe,Mo)(Va)₃, respectively. A set of self-consistent thermodynamic parameters of the Al-Fe-Mo system was obtained.     

Influence of metastable Al9Ni2 phase on the sequence of phase transformations initiated by heating of Al/Ni multilayer foils produced by EBPVD method

Al/Ni multilayer foils (MF) undergo a cascade of phase transformations at heating, initiated by diffusion interaction of Al and Ni layers. It is found that phase transformations sequence at initial stages depends on the method of producing MF: at sputtering or ion-beam deposition of elements, metastable Al₉Ni₂ phase forms at phase transformations initial stages, and in the case of MF produced by electron-beam physical vapour deposition (EBPVD) method or cold rolling of laminates, this is Al₃Ni phase. Such a difference in phase transformations sequence is associated with the influence of the method of MF production on the possibility of intermixed zone (IZ) formation on layer interfaces. In the study it was suggested that such anomalously high diffusion mobility of atoms can be achieved in the presence of excess vacancies in MF structure. With this purpose, MF structure was produced by high-rate (up to 30 nm/s) layer-by-layer deposition of elements by EBPVD method. Phase transformations and MF were studied by the method of X-ray diffraction (XRD) and differential-thermal analysis (DTA). It is shown that irrespective of MF composition and modulation period, at their heating phase transformations start with formation of metastable Al₉Ni₂ phase. At further MF heating, a stable Al₃Ni phase forms alongside Al₉Ni₂ phase. Later on, Al₉Ni₂ and Al₃Ni phases turn into stable intermetallics characteristic of MF chemical composition.     

Microstructural changes of aluminized Alloy 617 during high-temperature aging

In this study, aluminized Alloy 617 was prepared by Al-pack cementation of high temperature high Al activity process. The microstructure evolution and microstructural changes of aluminide coating were investigated after Al-pack cementation and high-temperature aging. The aluminide coating was composed of Ni-aluminide layers, such as δ-Ni₂Al₃, β-NiAl, Cr₂Al, Al_3 + xMo_1 − x, and inter-diffusion zone by pack cementation. After high-temperature aging, the aluminide coating was transformed from the δ-Ni₂Al₃ to the β-NiAl because of outward Ni diffusion from substrate. The Cr₂Al and the Al_3 + xMo_1 − x were dissolved during aging. On the other hand, the α-(Cr, Mo) particles were precipitated during aging due to the low solubility of alloying elements in the β-NiAl. The β-NiAl newly formed by the outward Ni diffusion during aging and resulted in the formation of the inter-diffusion zone. The inter-diffusion zone consisted of β-NiAl, Ni₃(Al, Ti), Cr-rich M₂₃C₆ carbide, and sigma phases.     

Structure analysis of the constitutional phases in liquid phase sintered W–Mo–Ni–Fe heavy alloys

The crystal structures of constitutional phases in two different tungsten heavy alloys processed via different conditions were examined. The compositions of these two alloys were W–11.9Mo–17.0Ni–7.7Fe and W–29.6Mo–17.0Ni–7.7Fe (at.%), which were liquid phase sintered at 1500 °C for 5 or 240 min, and followed by either furnace-cooling or water-quenching. Increase in isothermal hold caused increased concentration of Mo but decreased concentration of W in the matrix phase, which did not affect the lattice parameter of the matrix phase to a significant extent. Quenching the specimen in water caused increase in the concentrations of both W and Mo in the matrix phase, and, consequently, increases in the lattice parameter of the matrix phase. A tungsten heavy alloy with a high alloying concentration of Mo was prone to induce the precipitation of an intermetallic phase during cooling, which was enhanced by increasing the isothermal hold at the liquid phase sintering temperature and decreasing the cooling rate. The structure of this intermetallic phase is analogous to that of MoNi, and can be designated as (W_xMo_1−x)(Fe_yNi_1−y). The composition of this intermetallic phase varied with the composition of the alloy and its cooling rate subsequent to sintering. For a furnace-cooling condition, the atomic ratio of W to Mo (x/1−x) in this intermetallic phase was about 0.47 times the atomic ratio of W to Mo of the original alloy composition. Such a proportional constant decreased to about 0.30 when the specimen was water-quenched.     

Effect of Mo addition on microstructure,ordering, and room-temperature mechanical properties of Fe-50Al

The effects of Mo addition on microstructures, phase relationships, order–disorder phase-transition temperatures and room-temperature mechanical properties of Fe₅₀Al_50-nMo_n alloys (n=1, 3, 5, 7, and 9, mole fraction, %) were investigated after solidification and heat treatment. Structural characterization of the samples was performed via X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry. Room-temperature mechanical properties were investigated by conducting compression and microhardness tests. Mo₃Al particles precipitated in all alloys because of the limited solid solubility of Mo in the Fe-Al-based phases. The as-cast Fe₅₀Al_50-nMo_n alloys exhibited brittle behavior with high yield strength and limited fracture strain at room temperature. Compared with the as-cast alloys, all the heat-treated alloys except for the Fe₅₀Al₄₁Mo₉ alloy exhibited enhanced mechanical properties at room temperature. The heat-treated Fe₅₀Al₄₃Mo₇ alloy exhibited the highest fracture strain and compressive strength of 25.4% and 2.3 GPa, respectively.     

The phase equilibria of the Al–Ce–Ni system at 500 °C

The phase equilibria at 500 °C in the Al–Ce–Ni system in the composition region of 0–33.3 at.% Ce are investigated using XRD and SEM/EDX techniques applied to equilibrated alloys. The previously reported ternary phases and the variation of the lattice parameters versus the composition for different solid solution phases are investigated. It is confirmed that τ₂(Al₂CeNi) exists at 500 °C, while τ₃(Al₅Ce₂Ni₅) does not exist at 500 °C. A new compound τ₉ with composition of about Al₃₅Ce_16.5Ni_48.5 is found. The solubility of Ni in Al₁₁Ce₃ and αAl₃Ce is generally about 1 at.%, while the solubility of Ni in Al₂Ce is measured to be 2.7 at.%. The solubility of Ce in Al₃Ni, Al₃Ni₂, AlNi and AlNi₃ is all less than 1 at.%. The solubility of Al in CeNi₅, Ce₂Ni₇ and CeNi₃ is measured to be 30.4, 4.8 and 9.2 at.%, respectively, while there is no detectable solubility for Al in CeNi₂. A revised isothermal section at 500 °C in the Al–Ce–Ni system has been presented.     

Stability,phase transformation and deformation behavior of Ti-base metallic glass and composites

Melt-spun ribbons and copper-mold cast cylinders of (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)_100−xMo_x bulk glass-forming alloys are prepared. Both Ti₅₀Cu₂₃Ni₂₀Sn₇ and (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ melt-spun glassy ribbons exhibit large supercooled liquid regions, high reduced glass transition temperatures, and good thermal stabilities. During continuous heating of the melt-spun ribbons, both alloys present a two-stage crystallization behavior. Mo slightly lowers the glass-forming ability but significantly decreases the temperature of the second stage crystallization. For both alloys, the stable phases after heating are Ti₂Ni, TiCu, Ti₃Sn and β-(Cu,Sn). As-cast Ti₅₀Cu₂₃Ni₂₀Sn₇ cylinders contain dendritic hcp-Ti solid solution precipitates, as well as interdendritic glassy and Sn-rich crystalline phases. The ultimate compression stress reaches 2114 MPa with 5.5% plastic strain for 2-mm diameter cylinders. Yielding occurs at 1300 MPa, and Young’s modulus is 85.3 GPa. Mo improves and stabilizes the precipitation of a β-Ti solid solution but prevents glass formation in as-cast (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ bulk alloys. The bulk samples contain dendritic β-Ti solid solution precipitates, Ti₂Ni particles and Sn-rich phases. The ultimate compression stress is 2246 MPa with about 1% plastic strain for a 3-mm diameter cylinder. σ_0.2 is about 1920 MPa and Young’s modulus is 104 GPa. The high strength is attributed to both Mo solution strengthening and Ti₂Ni particle strengthening. The limited ductility is induced by the precipitation of brittle Ti₂Ni particles.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.     

Microstructure and composition of the grain/binder interface in WC–Ni3Al composites

The microstructure and composition of WC/Ni₃Al interface were studied. An orientation relationship of [100]_WC//[110]_Ni3Al and (001)_WC//(001)_Ni3Al with a good coherence, besides many random orientation relationships between WC and Ni₃Al, has been repetitively found by selected area electron diffraction and high resolution TEM observations. The XRD pattern of WC–Ni₃Al composites indicated that the major binder phase was Ni₃Al and showed possibility of coherence between WC and Ni₃Al as common interplanar spacings existed. Electron probe microanalysis results revealed that the atomic ratio of Al:Ni is close to 1:3 in binder phase and WC/Ni₃Al interface in the WC–Ni₃Al composites has a sharper compositional gradient and a smaller width of transition region than the WC/Co interface in WC–Co composites.