B2 NiAl Research Articles

Effect of aging on coarsening- and creep resistance of a Ti-modified Fe–Ni–Al–Cr–Mo ferritic steel with L21/B2 composite precipitates

The FBB8-ferritic superalloy (Fe- 10Ni- 6.5Al- 10Cr- 3.4Mo- 0.25Zr- 0.005B, wt.%) modified with 2 wt.% Ti additions, was aged between 700 and 860 °C, leading to coarsening of its coherent L21-Ni2TiAl/B2–NiAl composite precipitates. The temporal evolution of these submicron precipitates upon aging, studied via scanning electron microscopy (SEM), is consistent with the diffusion-limited Lifshitz-Slyozov-Wagner (LSW) and Philippe-Voorhees (PV) coarsening models with an activation energy Q = 253 ± 72 kJ/mol and interfacial energies for the precipitates of 83, 78 and 75 (±26) mJ/m2 for 700, 780 and 860 °C, respectively. Creep tests performed at 700 °C reveal threshold stresses ranging from σth = 94 MPa (after over-aging at 860 °C, with significant coarsening) to σth = 187 MPa (after peak-aging at 700 °C). Creep resistance is reduced with increasing aging temperature and/or creep temperatures (from 700 to 860 °C) and the following mechanisms are identified: (i) composite L21/B2 precipitate coarsening, (ii) partial loss of coherency of precipitates by misfit dislocations at the L21/B2 precipitate/matrix interface, (iii) absence of nano-sized secondary precipitates. Nevertheless, the alloy exhibits a high creep resistance when aged and creep-tested at 780 °C, with a threshold stress σth = 67 MPa, which is similar to that exhibited at 700 °C, σth = 69 MPa, by the same alloy without 2% Ti addition (FBB8), which contains weaker, single-phase B2–NiAl precipitates.

Phase Stability and Thermo-Physical Properties of Nickel-Aluminum Binary Chemically Disordered Systems via First-Principles Study

The effect of Al content and crystal structures on ground state, phase stability, elasticity and thermodynamics of Ni1−xAlx (x = 0.25, 0.50 and 0.75) binary chemically disordered systems are investigated using first-principles method in combination with quasi-harmonic Debye-Gruneisen model. The special quasirandom structures are applied to model disordered body-centered cubic (bcc) and face-centered cubic (fcc) phases. The Gibbs free energy of mixing of equiatomic Ni0.5Al0.5 is the lowest. The nonmagnetic fcc structure’ Ni1−xAlx are predicted to be more favorable phases. Disordered Ni1−xAlx are less stable than ordered L21 Ni3Al and B2 NiAl, and L21 phase is the most likely to form a nuclear growth. The somewhat different impact of Al content on elastic properties has been extracted that the resistance to volume change, shear deformation and elastic deformation of Ni1−xAlx decrease with increasing Al content. For bcc and fcc phases, Ni0.75Al0.25 and Ni0.25Al0.75 are predicted to be ductile behavior, while Ni0.5Al0.5 exhibit brittleness. The structural, vibrational and electronic contributions are taken into account to study the thermodynamic properties at finite temperature. The lattice constants a and volumetric thermal expansion coefficient α of Ni1−xAlx systems increase with the increase of Al content. Nevertheless, it is decreasing for heat capacity Cv and C. The vibrational entropy Svib of bcc Ni0.25Al0.75 is the largest in considered temperature. The α, C and Svib of disordered Ni1−xAlx are larger than that of ordered Ni3Al and NiAl. Vibrational and electronic entropy are the dominating at finite temperature stabilization mechanism.

Site preference and brittle–ductile transition mechanism of B2-NiAl with ternary elements additions form first-principles calculations

The mechanism between ductility and brittleness of materials is particularly important in the design of devices and materials, and which is one of the least understood properties. In this paper, site preference and brittle vs ductile behaviors of ternary elements X (X=Fe, Co, Mo, W, Pt, Sc and Y) on B2 NiAl are investigated by using of the first-principles calculations. It is shown that alloyed elements can effectively improve the ductility and even there is a transition form intrinsic brittleness to ductility from elastic constants. Competition mechanism of microcrack and dislocation shows that brittleness to ductility transition is due to the suppressed fracture but activated dislocation emission. Finally, the results show that the improved ductility is own to the weakened covalent interactions between Ni-d and Al-p states from DOS structure.

Investigation into the thermal behaviour of the B2–NiAl intermetallic alloy produced by compaction and sintering of the elemental Ni and Al powders

A cubic B2–NiAl alloy was synthesized by mixing of elemental Ni and Al powders followed by cold compacting and sintering of some samples at 750 °C and other samples at 1300 °C. The alloys sintered at 1300 °C exhibited a brittle B2–NiAl structure easily crushable into powder, while the 750 °C sintered alloys were not nearly as brittle. Electron Backscatter Diffraction (EBSD) results show that the B2–NiAl alloy sintered at 750 °C for a longer time (120 h) has larger grain size associated with the <111>-oriented grains encountering high mobility boundaries than the alloy sintered at 750 °C for a shorter time (48 h). Samples morphologies were analyzed using the Scanning Electron Microscope (SEM). Structural development of the alloy was studied via the x-ray diffraction (XRD) technique. The B2–NiAl intermetallic developed a thin scale of stable Al2O3 alloy due to oxidation in air at 750 °C for 120 h. As a result, further oxidation on the sample's surface was restricted, except the traces of Al2O3 formed via intergranular oxidation transformed into a metastable monoclinic oxynitride phase due to nitrogen (N) contamination.

Evolution of B2 and laves phases in a ferritic steel under Fe2+ ion irradiation at 475 °C

Microstructural evolution in a novel ferritic steel (Fe–12Cr–3W–3Ni–3Al–1Nb, in wt.%) computationally designed to contain B2 and Laves phases after 4 MeV Fe2+ ion irradiation up to 220 dpa at 475 °C was characterized using transmission electron microscopy in conjunction with x-ray energy dispersive spectroscopy. The ferritic matrix phase exhibited dislocation loops and tangled dislocations, but our focus was on stability of two types of intermetallic precipitates. The B2–NiAl precipitates ∼13 nm in size remained crystalline and appeared to have slightly lower Al concentration after irradiation. The Laves phase, (Fe,Cr)2(Nb,W), were present in two size ranges: coarse micron-scale precipitates which were amorphized with a slight composition change at irradiation doses above ∼ 30 dpa, while the finer precipitate particles ∼ 100 nm in size were partially disintegrated with a noticeable composition change at doses above ∼ 70 dpa. Meanwhile, many Nb/Cr-enriched particles ∼8 nm in size formed within a few hundreds of nanometers from the disintegrated particles. The understanding of the phase stability would help design advanced steels and engineer microstructures that are stable against high irradiation doses, while retaining good high temperature strength.

Grain size effects on Ni/Al nanolaminate combustion

Abstract

Threshold displacement energy in Ni, Al and B2 NiAl

For simulation of radiation effects and defects in the Ni-Al system, a new many body potential was developed by joining the equilibrium part of the Mishin’s EAM potential with the universal function of Ziegler, Biersack and Littmark at a suitable interatomic spacing and the corresponding pairwise energy from the DFT calculations. To assess the qualities of this potential, we performed molecular dynamics simulations to calculate the threshold displacement energy of pure metals Ni, Al and their NiAl B2 phase superalloy at ambient temperature. It was found that the threshold energy of displacement of nickel in nickel is higher than that in the B2 long-range ordered alloy NiAl in contrast to aluminium, which has lower threshold energy than B2 NiAl. The threshold displacement energy ranges between 14±2 eV and 189±2 eV depending on the crystallographic direction. The lowest threshold energy of two fcc metals (14±2 eV for Al and 28±2 eV for Ni) was found in the direction <101>, these values correspond to the mean value in the easiest directions of atomic displacements and they are in good agreement with many reports from the literature.

Phase composition of macroporous Ni-Al alloys obtained by combustion synthesis

This paper discusses the phase composition of macroporous Ni-Al alloys obtained by self-propagating high-temperature synthesis. The alloys have been synthesized in a nonstationary combustion mode. The combustion wave consists of super adiabatic foci, and macroporous structure is realized under the action of capillary hydrodynamic effects. The influence of reaction mixture composition of aluminium in the range of 13.5-31.5 wt.% is also discussed. The specific requirements to obtain the single phase B2 NiAl and LI2 Ni3Al as well as biphase gas permeable alloys with the average size of structure elements in the range of 1.2-3.15 mm are described in the paper.

Effects of Point Defects on Properties of B2 NiAl: A First-principles Study

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal were analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B0 and Cauchy pressure parameters C12-C44 values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.

Effect of microalloying constituents–A novel approach to ordering phenomenon

A conscientious study by X-Ray diffractometry has been done to determine long-range order parameter, the lattice constant, and crystallite size of B2 NiAl (Cu) as a function of Ni concentration. Addition of Zr and Ti as micro-alloying elements enhances the lattice parameter as well as ordering behaviour, particularly at higher Ni concentration in the alloys. Our present investigation shows that a broad concentration range of Ni content is suitable for the formation of B2 structure. Ordering behaviour in NiAl is explained by the formation of the antiphase boundary based on the data of transmission electron microscopy (TEM) study and enthalpy of formation of the corresponding compositions.

Uniaxial deformation of face-centered-cubic(Ni)-ordered B2(NiAl) bicrystals: atomistic mechanisms near a Kurdjumov–Sachs interface

Creating tailored interfaces between soft and hard materials is a promising route to simultaneously enhance ductility and strength of multicomponent materials. Here, we study deformation mechanisms in a model bicrystal, with a Kurdjumov–Sachs (KS) interface, between face-centered-cubic Ni and ordered-B2 NiAl slabs using molecular dynamics simulations. The bicrystals were uniaxially deformed by strain rates of $$10^7$$ and $$10^9\,\hbox {s}^{-1}$$ by holding temperatures constant at 300, 500, 700, and 900 K for each strain rate. Our simulations reveal atomistic processes that create sessile and glissile dislocations, and their reactions during high-strain rate deformation. At $$10^9\,\hbox {s}^{-1}$$ strain rates, dislocation processes enhance ductility and cause large-scale atomic rearrangements in the KS interfacial region. This subsequently causes nucleation, growth, and coalescence of nano-voids into cracks inside the harder B2-ordered phase bordering the interface. Our results suggest that interfaces between “soft”–“hard” materials likely withstand high-strain rates better.

Revisiting intrinsic brittleness and deformation behavior of B2 NiAl intermetallic compound: A first-principles study

For NiAl intermetallic compound with B2 structure, there is still no calculation combining models of single and multiple layers while using the same basic set. Furthermore, some recently proposed criteria for brittleness and toughness have not been used to analyze its deformation behavior. Thus, first-principles calculation was applied to comprehensively study the elastic properties, ideal strength, generalized stacking fault energy and surface energy of B2-NiAl intermetallic compound. The results suggest that calculations based on the current basic set give more accurate lattice parameters and elastic moduli. The Pugh criterion and Cauchy pressure cannot be used to interpret the intrinsic brittleness of NiAl. In comparison, the ductility parameter based on the strain energy under elastic instability and ZCT and Rice criteria based on generalized stacking fault energy and surface energy successfully identify the intrinsic brittleness of the NiAl intermetallic compound. The reason why [111] slip always occurs in the deformation along [100] direction was clarified by examining the critical value for brittle-ductile transition. The results of density of state indicate shear deformation has less impact on structural stability, and the change of charge density difference implies that <001> shear induces more intensive redistribution of charge density, which is well correlated to the brittleness and deformation behavior of NiAl intermetallic compound.

Diffusion in B2 NiAl and FeAl Intermetallics and Their Alloys

Both FeAl and NiAl with B2 crystal structure are envisaged for their usage in high temperature applications and hence, availability of diffusion data in these intermetallics is crucial in designing their alloys and processes as well as deciding their in-service performance. A comprehensive overview of diffusion data available in B2 FeAl and NiAl and their alloys is provided in this article. Nearest neighbor vacancy jumps in B2 intermetallic lead to a local disorder in the lattice and hence it is not necessarily the unit step of diffusion in these structures. Several mechanisms of diffusion proposed in the literature are discussed including nearest neighbor jumps, next nearest neighbor jumps, six-jump vacancy cycle, triple defect and antisite bridge. Relevance of these mechanisms in FeAl and NiAl is discussed. An overview is given on the self-and solute diffusion and interdiffusion data available in both binary FeAl and NiAl. Due to wide solubility range of both FeAl and NiAl as well as their alloying requirements for improved properties, it becomes pertinent to study the multicomponent diffusion in the alloys based on these B2 itnermetallics. Hence, in the latter part of the article, various methods used for determining multicomponent diffusion data are reviewed. A detail overview is also provided on the diffusion studies available in literature on ternary alloys based on FeAl and NiAl with an emphasis on highlighting the diffusional interactions observed in these systems.

Microstructures and electrothermal properties of AlxCrFeNi multi-component alloys

AlxCrFeNi multi-component alloys were fabricated by vacuum arc-melting to study the effect of homogenization on microstructures of as-cast and homogenized alloys using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Besides, electrical resistivity and thermal conductivity of as-cast and homogenized alloys were also investigated. The results showed that no obvious differences of crystal structures and phase compositions between these as-cast and homogenizing alloys were presented. However, transmission electron microscope indicated that the ordered B2 NiAl intermetallic nanoparticles, which were distributed on the disordered BCC [Fe, Cr] solid solution of as-cast Al1.2CrFeNi alloy, disappeared after homogenization. For AlxCrFeNi alloys, the electrical resistivity increased with Al addition and dramatic increase of thermal conductivity was presented with increasing the homogenization temperatures. However, homogenization could decrease both electrical resistivity and thermal conductivity of as-cast alloys. The main factors influencing the changes of electrothermal properties of AlxCrFeNi alloys were the electron scattering affected by crystal defects, phase boundaries and temperatures.

Molecular Dynamics Studies of B2 NiAl Alloy Melting Point

In this paper, we perform a computer simulation to investigate the melting point at zero pressure of B2 NiAl intermetallic alloy by using LAMMPS with EAM potential developed by Mishin et al. Simulation box contains 20×20×20 unit cells with 16000 atoms, periodic boundary conditions are applied in three directions. To verify the quality of Mishin potential we first conduct several simulations to calculate defects formation energy, cohesive energy, equilibrium lattice constant and elastic constants of this alloy at absolute zero. The main simulation is performed by using one-phase method in NPT ensemble. Simulation results are analyzed and visualized by Ovito using radical distribution function and common neighbor analysis method. We observe B2 NiAl bulk alloy that begins to melt at 1840 K and crystallizes at 1153 K with critical cooling rate higher than those of almost other alloys. The good agreement between simulation results and experiment suggests that we should continue using Mishin potential for further work in B2 NiAl case study with more sophisticate simulation.DOI 10.14258/izvasu(2017)4-01

Combustion synthesis in the Ni–Al–Nb ternary system: A Time-Resolved X-ray Diffraction study

Combustion synthesis of intermetallics, using the thermal explosion mode, in the Ni-Al-Nb ternary system is presented, with a 40:40:20 atomic ratio. The kinetic pathway is determined using Time-Resolved X-ray Diffraction, with a time-step resolution of 1s and demonstrated a first formation of the B2 NiAl structure followed by progressive dissolution of Nb to yield NiAlNb intermetallic Laves phase, representing 35w% of the final product. SEM observations show a double dendritic (coarse and fine) microstructure, resulting from eutectic crystallization. Due to a high cooling rate, Nb dissolution is not complete at the surface, and yields slightly more complex microstructure, including the Ni2AlNb Geissler phase, the (Ni,Al)2Nb Laves phase, and (Ni, Al)7Nb6.

Effect of alloying on elastic properties of ternary Ni-Al-Ti system: Experimental validation

Using combustion synthesis approach we fabricated B2 NiAl intermetallic compound as well as quasi-binary Ni(Al50Ti50) alloy, where half Al atoms are randomly substituted by transitional metal Ti. Young’s modulus for synthesized materials was measured and appeared to be 222 ± 10 GPa for NiAl and 175 ± 15 GPa for Ni(Al50Ti50) phases. Using first-principles simulations in the framework of the Density Functional Theory, we investigate the elastic properties of Ni(Al1−xTix) system, including single-crystal, as well as polycrystalline elastic moduli. Direct comparison of the experimental and theoretical values of the Young’s modulus demonstrates that the employed theoretical approach allows carefully predict elastic properties of NiAl-based intermetallics. In particular, we predict that alloying NiAl with Ti should increase the ductility of the intermetallic phase.

A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing

In order to promote the performance of B2 NiAl by texture control of orientation during in situ processing, phase transformation in laminated NiAl with bimodal grain size distribution manufactured by reaction annealing of Ni and Al foils has been studied. It turned out that there existed a Kurdjumov–Sachs orientation relationship (K–S OR) between parent Ni and product NiAl by crystallography analysis according to the electron backscatter diffraction (EBSD) results. The parent Ni did not transform to the product NiAl directly but via the formation of Ni3Al firstly according to the transmission electron microscope (TEM) observation of the interface. This led to a new K–S OR between Ni3Al and NiAl with a small atomic misfit, which made less residual stress generated through the formation of Ni3Al than directly from the parent Ni.

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.

Solid State Reaction Mechanism and Microstructure Evolution of Ni-Al Powders during High Energy Ball Milling Revisited by TEM.

Microstructure evolution during the formation of B2-NiAl by high energy ball milling of equiatomic elemental mixtures was studied by X-ray diffractometer, scanning electron microscopy, and transmission electron microscopy (TEM). The crystallite size, lattice defects and ordering of the B2-NiAl were monitored via TEM as function of milling time. The diffusion reaction, Ni+Al→NiAl3 or/and Ni2Al3, occurred during high energy ball milling, and to a certain extent offered the stored energy for the explosive exothermic reaction, Ni+Al→B2-NiAl. The fine microstructure of newly formed B2-NiAl after 5 h milling involved high density defects, e.g. antiphase boundary, long range ordering domains, vacancies, and dislocations.