β-NiAl Phase Research Articles

Defect structure in Al-rich composition region in the β-NiAl intermetallic compound phase

Abstract The defect structure in the B2 type β-NiAl phase was investigated in the Al-rich composition region by means of powder X-ray diffractometry. In the analysis, earlier published density data were referred to for estimation of the total vacancy concentration in the alloy. As a result, the main constitutional defect was found to be the vacancy on the Ni-site (Ni-vacancy), supporting the defect structure model proposed previously by Bradley and Taylor. However, in a region of high Al concentration, some Al atoms substituted on the Ni-site, i.e. anti-structure defects of Al atoms (Al-ASD), as well as some vacancies on the Al-sites (Al-vacancies), were recognized to exist. Their amounts did not change with increasing temperature up to 1150 °C. These defect types of the Al-ASD and Al-vacancy had always been excluded in usual treatments based on the triple defect model. The reliability of the defect structure determined was examined by comparison with the triple defect model. Further, thermodynamic calculations based on the Bragg-Williams approximation were performed in order to reproduce the observations.

Structure and thermodynamic properties of nanocrystalline face centred cubic metals prepared by mechanical alloying

In this work the microstructures of gas atomised Ni75Al25 and Ni31.5Al68.5 at.% powder particles below 100 μm in size have been studied. The gas atomised Ni75Al25 powder particles are mainly spherical. The solidification of this alloy is very fast, and its microstructure consists of a dendrite and lamellar structure of partially ordered γ-(Ni), γ′-Ni3Al L12 phase, and β-NiAl phase. The order increases with the powder particle size. The gas atomised Ni31.5Al68.5 powder particles are also spherical in shape. The microstructure consists of Ni2Al3 dendrites with interdendritic peritectic NiAl3 and eutectic NiAl3 + α-Al. The amount of the Ni2Al3 increases as the cooling rate increases. NiAl phase is absent in the gas atomised Ni31.5Al68.5 powder.

Fracture Mechanisms in NiAlCr Eutectic Composites

ABSTRACTSelected eutectic compositions in Ni-Al-Cr ternary systems were processed by directional solidification (DS) with various growth rates. Fracture toughness tests were performed at room temperature and 400 °C; fracture surfaces of broken specimens were examined using SEM to investigate fracture behavior of each alloy. The alignment of eutectic phases was found to play an important role in composite toughening for the intermetallic matrix. Binary eutectic composites consisting of bcc α-Cr and B2 β-NiAl phases with a directional, well-aligned structure showed improved fracture properties over NiAl single crystals. Ternary eutectics, which contain an fcc γ-Ni phase, offered an excellent fracture resistance at room temperature.

Preliminary studies on NiAl/Nb2Be17 reaction and effectiveness of BeO as an interfacial reaction barrier

Alloys based on the ordered B2 NiAl phase are being considered as potential high-temperature structural materials. One drawback for this material is its lack of high-temperature strength,[1] which can be overcome by reinforcing the alloy with high-strength fibers. Like any other composite system, a suitable reinforcement material must have a matching coefficient of thermal expansion (CTE) with the matrix in addition to high-temperature strength and be chemically compatible with the matrix. Although there are many high-melting ceramic materials which are thermodynamicalry stable in the NiAl matrix, [2] the high CTE of NiAl,[3] 16 X 10−6 K−1 at 1200 K, makes it difficult to find a suitable ceramic reinforcement material with a matching CTE. Thus, there is a need to develop high CTE fibers for the NiAl matrix. One group of materials with matching CTE[3] to NiAl are the Be-rich intermetallic compounds called beryllides (CTEs in the range of 16 to 18 × 10−6 K−1 at 1200 K) of formula M2Be17, M2Be13, or M2Be12 (where M = Ta, Nb, Ti, Zr, Hf, or Y). Matching CTEs with the NiAl matrix along with their good high-temperature strength[4] and low densities[4] make Be-rich intermetallic compounds attractive as candidate reinforcement materials for the NiAl matrix.

Reaction of beta-phase Ni–Al alloys with CrB2

Reaction of Ni–Al alloys within the β-NiAl phase with CrB2 was studied at 1473 K as a function of Al concentration in the alloy. Reaction of 49–50 at. % Al alloys with CrB2 occurred by interdiffusion of Ni into CrB2 and Cr into the alloy without forming a new product phase. On the other hand, a new product phase, rich in Ni and B, formed by the reaction of alloys having Al concentrations 48 at. % or lower with CrB2. The reaction product was observed both at the CrB2/alloy interface and along the alloy grain boundaries.

Laser-induced microstructural modifications in a vacuum plasma sprayed NiCoCrAlYTa coating

Laser remelting of a vacuum plasma sprayed MCrAlY-type coating was carried out using a solid state YAG laser. The resulting laser-induced microstructural changes in the coating are presented. A concentric two-zone structure with a columnar and an equiaxial two-phase microstructure respectively was observed. The composition of the β-NiAl phase in that structure was found to be different from that in the coating after post-coating diffusion treatment. The diffusion of alloying elements from substrate through the coating, when the re-fusion affects the substrate, is shown.

An investigation of the Ni5Al3 phase

The crystallography of the phases obtained by heat treating a Ni-34.7 at. % Al alloy with 0.9 at. % Hf and 0.04 at. % B was studied in detail using x-ray diffraction techniques. Particular emphasis was placed on characterizing the Ni5Al3 phase, which is obtained by quenching the B2 NiAl phase and aging the resultant martensite. The structure of the Ni5Al3 phase is confirmed to be orthorhombic with space group D192h (Cmmm) and lattice parameters of a0 = 0.7475 nm, b0 = 0.6727 nm, and c0 = 0.3732 nm. The transformation from B2 NiAl to L10 martensite is shown to involve a Bain distortion of the c/a ratio from 0.707 to 0.853, and the further ordering transformation to Ni5Al3 involves a further increase of this c/a ratio to 0.900.

L12-type Ni-Al-Cr alloys processed by rapid solidification

A study of the effect of Cr additions (5, 10, and 12.5 at. pct) on the microstructure of melt-spun Ni3Al-base alloys has been carried out using analytical electron microscopy. The analyses showed that the formation of the β-NiAl phase could not be totally avoided at all three levels of Cr additions studied. However, its volume fraction, morphology, and distribution were affected by the Cr concentration as well as by changes in the rapid solidification conditions. The ordering structure of the matrix γ contains anisotropic antiphase boundaries or cube-plane-oriented thin layers of disordered γ, depending on the local chemistry. A series of microchemistry measurements along the radius of a grain was used to trace the route of Al and Cr segregation, and an unexpected reverse segregation with a partition coefficient greater than 1 was observed for Cr. The bend ductility and tensile properties of the melt-spun ribbons were also studied. It was shown that the Cr additions substantially increased the ribbon strength and modified the mode of failure toward transgranular fracture. The improved tensile properties were partially attributed to the fine matrix γdomains which had mixed interfaces of antiphase boundaries and y thin layers.

The influence of reducing conditions on the sodium sulfate-induced corrosion of pure and alloyed βNiAl

This study concerns the corrosion of the β-NiAl phase (pure or with additions of Co and Cr) by Na 2SO 4 in reducing atmospheres ( P o 2 = 10 −16 atm), at 850°C. With pure NiAl, an alumina scale impregnated with Na 2S is formed at the surface, whereas in the alloy, aluminum depletion leads to the formation of a very porous scale of the γ′Ni 3Al phase. With chromium additions, sodium chromite is observed on the alumina scale and sodium thiochromite at the γ′/oxide interface. A high corrosion rate is observed, which is probably due to the formation of a liquid sulfide filling pores in the γ′ layer.

Microstructure and phase solubility extension in rapidly solidified NiAl-Cr quasibinary eutectic

The microstructure of melt-spun ribbon of the NiAl-Cr quasibinary eutectic composition has been characterized by optical and transmission electron microscopies. The eutectic composition is Ni-33 at.%Al-34 at.%Cr and is of interest because of the similarity of crystal structures (CsCl for β-NiAl and b.c.c. for α-Cr) and lattice parameters of the two phases in the eutectic. The rapidly quenched microstructure consists of 0.5μm diameter columnar grains with a composition near that of the eutectic. These grains exhibit a fine spinodal decomposition structure. Based on the presence of coarse antiphase domains within the columnar grains it is concluded that they originally formed from the melt as supersaturated β-NiAl. Between the grains a fine rod-type eutectic structure of the β-NiAl and α-Cr phases is observed with eutectic spacings as fine as 12 nm. A solidification model for the appearance of a supersaturated β-NiAl phase near the eutectic composition, rather than a supersaturated α-Cr phase, is presented based on the T 0 curves for this alloy system.

Investigations of Defective β-NiAl by Transmission Electron Microscopy

The β-NiAl phase, which is an ordered B2 structure, covers a wide range of compositions extending from ∼46 at.% Ni to ∼60 at.% Ni. For non-stoichiometric β-NiAl, studies (1,2) have shown that, for compositions below 50 at.% Ni, structural vacancies are introduced on the Ni sublattice (denoted as VNi), but for compositions above 50 at.% Ni,excess Ni atoms replace A1 atoms on the A1 sublattice (denoted as NiAl). In this work, defect structures in compositions of 45, 45.5, 46, 47, 50, 55, 58 and 60 at.% Ni were studied by electron diffraction techniques in a Siemens 102 TEM.Figs. 1 and 2 show selected area diffraction patterns at the exact [011] and [001] zone axes respectively.

A Diffraction Contrast Study of Displacement Faults in an Ordered Ni-Al-Ti Alloy

A study has been made of Ni-Al-Ti alloys with compositions along the tie-line between the β-NiAl and Ni2AlTi phases; Ni2AlTi has the Heusler-type structure. The micrographs in Figs. 1-4 are from an alloy with composition 63.8 wt% Ni, 20.5 wt% Al, and 15.6 wt% Ti. A fine precipitate forming a “tweed” or modulated structure was observed (see Figs. 2, 3, and 4), and in the diffraction pattern strong Heusler ordering reflections were evident. of particular interest are the unusual displacement faults, which unlike those observed by Ball in Al-rich NiAl generally do not lie in specific crystallographic planes, but are high convoluted as shown in Fig. 1.In Fig. 2 each fault in the array A to B terminates at a dislocation, and it was deduced that these dislocations had a ao[001] Burgers vector since they were invisible when g = [110], and [200].

Further studies on the nickel–aluminium system. I. β-NiAl and δ-Ni2Al3phase fields

New lattice parameter and density results have been obtained for alloys in the β-NiAl and 6-Ni2Al3 phase fields of the nickel–aluminium system. The lattice parameter of the β-NiAl phase (CsCl-type) falls linearly from 2.8870 A at 50 at.% Ni to 2.8618 A at 66. at.% Ni, with 2.00 atoms per unit cell. On the other hand, the lattice parameter on the Al-rich side of NiAl falls linearly from 2.8870 A to 2.8652 A, while the number of atoms per unit cell falls from 2.00 to 1.817 by the creation of vacancies in normally nickel sites. The trigonal 6-Ni2Al3 phase-structure, which is essentially an extension of cubic β-NiAl, but with every third plane of nickel atoms perpendicular to the trigonal axis missing, shows a minimum in the a and c spacings at stoichiometric Ni2Al3. Density measurements indicate that the vacancies formed by the missing planes are progressively filled as nickel is added to Ni2Al3, but that a substitutional solid solution is formed on the aluminium-rich side of stoichiometric Ni2Al3 with aluminium replacing nickel atom by atom. In the NiAl phase, the number of valence electrons increases from 2.28 per unit cell at 61.9 at.% Ni to 3.00 at stoichiometric NiAl, and remains constant at 3.00 as the vacancies form until Ni2Al3 is reached, at which stage the number of vacancies will have reached a maximum when there are only 1.67 atoms per pseudo-cubic cell. The number of electrons per pseudo-cubic unit cell then begins to rise and reaches the phase boundary value of 3.12 at 37.6 at.% Ni.

Tracking signals from “echo” satellite

Twenty disk-shaped aluminized (β-NiAl) CMSX-4 single crystal superalloy specimens were oxidized in air at 788, 871, 954, and 1010 °C for 1000, 2500, 5000, 7500, and 10,000 h. Phase constituents and the residual stress of thermally grown oxide (TGO), as well as microstructural degradation in the aluminized layer of CMSX-4, were examined by using photostimulated luminescence spectroscopy (PSLS), optical microscopy, scanning and transmission electron microscopy (SEM and TEM). The compressive residual stress in the TGO scale increased with increasing oxidation temperature and time until local spallation, after which a large standard deviation in the magnitude of residual stress was observed due to stress relief associated with TGO scale spallation. Whisker-shaped metastable Al2O3 phases were detected by PSLS and were observed by SEM and TEM. The presence of metastable Al2O3 phases on scale developed at 788 °C caused localized spallation of TGO within the grains of β-NiAl. At higher temperatures, the TGO scale primarily consisted of equilibrium α-Al2O3, and spallation occurred along the vaulted asperities associated with grain boundary ridges. The Al-rich β-NiAl phase was dissolved and transformed into Al-depleted phases with high-temperature oxidation. A finite difference model was developed to simulate the change in Al concentration, the phase constituents in the diffusion aluminide coatings, and the thickness of the TGO scale during oxidation. This model can serve as a method to predict the lifetime of these coatings.