Ni Al Cr Research Articles

The effect of microstructure on hardness measurements in the aluminium-rich corner of the Al–Ni–Cr system

Arc-melted Al-rich samples from the Al–Cr–Ni system were analysed using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. All of the expected binary phases were observed and identified. The binary phase with the largest extension into the ternary was Al 3Ni 2 at about 12 at.%. Three separate ternary phases were identified from the microstructures, and were found to agree reasonably with the compositions in the literature, although the phase fields had different shapes. Vicker’s microhardness measurements have indicated that the phase hardnesses in decreasing order are most likely: Al 3Ni 2, φ, (αCr), ρ, Al 4Cr, Al 7Cr, Al 11Cr 2, Al 9Cr 4, Al 3Ni and (Al). The φ, Al 4Cr, Al 11Cr 2 and Al 3Ni phases were brittle in massive form.

Thermal cycling creep behavior of NiAl–Cr alloy

Isothermal and thermal cycling creep behavior of a NiAl–Cr alloy was studied by compression tests. At low stresses, the thermal cycling creep rates were faster than the isothermal creep rates. The corresponding stress exponent under the thermal cycling condition was close to one. At high stresses, the stress exponent and the creep rate under the thermal cycling condition approached the isothermal values. The result indicates the occurrence of internal stress superplasticity at low stresses. The origin of internal stress was attributed to the difference in thermal expansion behavior of the constituent phases. Further, the thermal cycling creep rate was found to increase with increase in temperature amplitude and heating/cooling rate. The above trends were compared with the predictions of an internal stress superplasticity model. Good formability of the alloy under the thermal cycling condition was demonstrated through carrying out the thermal cycling compressive creep test to a large strain level.

Internal Stress Superplasticity in a In-Situ Intermetallic Matrix Composite

Processing of NiAl-Cr based eutectic in-situ composite via internal stress superplasticity has been explored. The alloy is heat treated to produce spherodized Cr particles in NiAl matrix. Results of thermal cycling creep experiments indicate that internal stress of considerable magnitude are generated between the NiAl matrix and Cr particles, which aids the external applied stress in deforming the alloy in viscous manner The origin of internal stress is attributed to the difference in thermal expansion behavior of the constituent phases. At low stresses, the thermal cycling creep rates are higher than the isothermal creep rates. However, the observed thermal cycling creep rates are an order of magnitude lower than values predicted by an internal stress superplasticity model based on micromechanics. The possible reasons for this discrepancy are discussed.

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′( L1 2)-particles (Ni 3Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.

Simulation of microsegregation and microstructural evolution in directionally solidified superalloys

A two-dimensional model for solidification and secondary phase precipitation in directionally solidified superalloys is presented, which makes use of a shape function approximation for the isothermal cross-section of the primary dendrites. The phase boundary is represented by a diffuse interface on a finite difference grid, as in phasefield methods. Thus, two-dimensional multicomponent diffusion can be calculated for all phases. Via a Fortran interface, the model is coupled to thermodynamic databases assessed using the ‘calculation of phase diagrams’ (Calphad) approach. In this way, for each time step actual data are available for partitioning, dendritic growth, and nucleation of secondary phases. The model is applied to Ni–Al–Cr as a ternary model system and to the commercial superalloy IN706.

Theoretical and experimental studies of the dislocation structure at the NiAl–Cr(Mo) interfaces

The dislocation structure at the NiAl/Cr(Mo) interfaces has been studied by high resolution electron microscopy (HREM). The (1̄10) and (111) cube-on-cube interfaces were examined. Both of the interfaces have been found to be partially coherent. Theoretical calculation of the expected dislocation structure at the interfaces was made based on the CSL/DSC lattice model. The experimental observations and theoretical prediction were shown to be in generally good agreement, with some differences with respect to the detailed structure.

Microstructure and Phase Stability Studies on Heusler Phase Ni2AlHf and G-phase Ni16Hf6Si7in Directionally Solidified NiAl–Cr(Mo) Eutectic Alloyed with Hf

Small additions of Hf to directionally solidified NiAl–Cr(Mo) eutectic resulted in precipitation of a high density of Heusler phase Ni2AlHf along with fine G-phase Ni16Hf6Si7. The Heusler phase was mainly located on the grain boundary region. The fine G-phase formed in the presence of Si, which was a contamination resulting from contact with ceramic shell molds during directional solidification of the alloy. These fine G-phases were cuboidal in shape and coherent with the NiAl matrix. After hot isostatic pressing and aging treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitated preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl matrix, and small amounts of them precipitated within the Cr(Mo) phase. The structures of the NiAl/Ni2AlHf and NiAl/Ni16Hf6Si7interfaces were investigated by high-resolution electron microscopy. The habit plane of the fine G-phase was {001}NiAl. This result was in good agreement with calculation based on the linear elastic theory. The misfit dislocation network on the NiAl/Ni2AlHf (110) interface was calculated from the O-lattice model and compared with the observation, which showed good agreement.

Structure and mechanical properties of NiAl and Ni 3Al-based alloys

The investigation of Ni–Al–Fe–Ti–B alloys was carried out to determine the influence of iron and small titanium and boron additions on the phase composition, microstructure and mechanical characteristic, particularly with respect to high-temperature deformation conditions. These alloys, containing Al 35.8 at% and Fe 3.6–8.6–17.6 at% were prepared from high-purity components and Al master alloy containing Ti 2B particles. The influence of alloying additions of chromium and iron on the mechanical properties of directionally solidified Ni–Al–Cr–Fe alloy was investigated. Additions of both Cr 8 at% and Fe 2 at% result in higher strength than exhibited by unalloyed Ni 3Al. However, the ductility is reduced by the formation of the β′ phase. The typical, lamellar structure of Ni–20Al–8Cr–2Fe alloy undergoes coagulation during a high-temperature deformation process. The sequence of structural changes of NiAl and Ni 3Al-based alloys has been correlated with mechanical characteristics of high-temperature deformation process, determined in uni-axial compression tests. Two ranges of work hardening have been identified on the stress–strain curves of these alloys. It has been found that the first range of the deformation of Ni–Al–Fe–Ti–B alloys corresponds to the intergranular slip system operating within individual grains, while the second one is connected with transgranular slip. In the directionally solidified Ni–20Al–8Cr–2Fe alloy similar work hardening curves were observed in relation to the microstructural evolution from the lamellae shape, through elliptical shape into circular shape.

Brittle-to-ductile transition of a multiphase intermetallic alloy based on NiAl

The microstructure and mechanical behaviors of a multiphase β+γ+γ′+α-Cr Ni–Al–Cr alloy have been investigated. This alloy exhibits only 1% tensile ductility despite the presence of the continuous ductile phases. It is noted that both fractograph and BDTT are typically dependent on strain rate, with a two orders of magnitude increase in strain rate resulting in approximately a 150 K increase in BDTT. An asymmetry between the compressive and tensile yield strength was also observed at low temperature. SEM observation of the side surface of the fractured specimen indicated that the phase boundary seemed to be weaker than the fracture strength of the constituent phases at temperature below the BDTT; the random orientation relationship between the β phase and the γ/γ′ phases may result in difficulty in strain transfer from soft phase to hard phase, which, in turn, contributes to crack initiation at phase boundary.

Fabrication and thermal stability of a nanocrystalline Ni–Al–Cr alloy: Comparison with pure Cu and Ni

A Ni–Al–Cr alloy with an initial grain size of ∼60 μm was subjected to torsion straining to a strain of ∼7 at room temperature, thereby reducing the grain size to ∼34 nm. Similar torsion straining with samples of pure Cu and pure Ni gave grain sizes of ∼170 and ∼130 nm, respectively. Inspection of the Ni–Al–Cr alloy after torsion straining revealed highly strained regions containing dislocations associated with lattice distortions but with an absence of any Ni3Al ordered phase. The ultrafine grains in the Ni–Al–Cr alloy were extremely stable at high temperatures, and it was possible to retain a grain size of less than 100 nm after annealing at temperatures up to ∼900 K. By contrast, there was rapid grain growth in the samples of pure Cu and Ni at annealing temperatures in the vicinity of ∼500 K. The stability of the grains in the Ni–Al–Cr alloy is attributed to the formation of a Ni3Al-based ordered phase after annealing at ∼650–700 K. The presence of this phase also leads to an apparent negative slope in the standard Hall–Petch relationship.

Thermodynamic properties of the Ni–Al–Cr system

Phase equilibrium and thermodynamic data of the Ni–Al–Cr ternary system were reviewed and analyzed by thermodynamic modeling. The liquid, fcc(Al, Ni) and bcc(Cr) phases were modeled as substitutional solution phases. Two-sublattice models were used for the ordered B2 and L1 2 phases. The other intermetallic compounds were treated as binary phases without ternary solubilities. No ternary phases were included. A self-consistent thermodynamic description was obtained by optimization. In comparison with a previous Ni–Al–Cr thermodynamic description, the present one gives a better account of experimental data and, at the same time, has fewer model parameters with more reasonable values.

Yttrium-promoted selective oxidation of aluminium in the oxidation at 1100°C of an eutectic Ni–Al–Cr 3C 2 alloy

The effects of an yttrium addition of 0.2 wt% on the oxidation behaviour of an eutectic Ni–Al–Cr 3C 2 alloy of average composition Ni–12.3 Cr–6.9 Al–1.8 C (wt%) have been investigated at 1100°C in 1 atm oxygen. Experiments have been carried out under isothermal and thermal cycling conditions on the as-cast alloy, in which Cr 3C 2 was present as a finely dispersed eutectic phase, and on directionally-solidified specimens having an aligned structure. In all cases, Al 2O 3 formed almost entirely as an external scale. Although Cr 2O 3 formed in some regions of the surface in the early stages, compact subsurface Al 2O 3 layers generally developed beneath such locations, protecting the alloy from severe internal attack. This behaviour is quite different from that observed for similar yttrium-free alloys, where aluminium was oxidized internally beneath continuous external Cr 2O 3 scales. In the present case, while it was almost certainly formed during the transient stage, the growth rate of Cr 2O 3 was probably reduced due to the presence of the reactive element and the extent of chromium depletion in the alloy was thereby decreased. This yielded an increase in the concentration of chromium in the alloy at its interface with the transient Cr 2O 3 scale compared to the yttrium-free alloy, with a corresponding decrease in the oxygen activity at the same location. As a consequence, the inward flux of oxygen in the alloy was reduced, allowing a transition from internal to external oxidation of aluminium to become possible on longer periods of oxidation. The presence of carbides at the alloy/scale interface was highly detrimental to the adhesion of the oxide scale to the metal substrate, and the scales tended to crack and/or lift on cooling. However, no massive spalling occurred, even during thermal cycling for periods up to 124 h, while the Al 2O 3 scales tended to re-form rapidly after mechanical damage.

Precipitation of α-Cr in B2-ordered NiAl

Abstract A transmission electron microscope investigation has been performed on the precipitation of α-Cr in B2-ordered β-NiAl with different stoichiometry. By aging at temperatures around 1073 K after solution annealing at 1563 K, fine spherical particles of α-Cr appear in the NiAl matrix between the α-Cr which rapidly precipitated during quenching. The hardness of NiAl increases appreciably by the fine precipitation of α-Cr and overage softening occurs after prolonged aging. The perfect lattice coherency is kept at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, the loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles. The amount of age hardening of stoichiometric NiAl in which equal amounts of Ni and Al are replaced by Cr is smaller than that obtained in off-stoichiometric NiAl. Atom location by channeling enhanced microanalysis technique has been used to determine the site occupancy of Cr in NiAl.

A thermodynamic description of the Ni–Al–Cr–Re system

Abstract Thermodynamic properties of the Ni–Re binary system were analyzed using thermodynamic models for the Gibbs energy of individual phases of the system. The model parameters were optimized from limited experimental phase equilibrium data and estimated thermochemical data. Thermodynamic descriptions of the Ni–Al–Re and Al–Cr–Re were also discussed. A thermodynamic description of the Ni–Al–Cr–Re quaternary system was obtained by combining consistent thermodynamic descriptions of its lower-order systems. The calculations using these thermodynamic descriptions show good agreement with available experimental data of both the Ni–Al–Re and Ni–Al–Cr–Re systems.

Acetylene hydrogenation on Ni–Al–Cr oxide catalysts: the role of added Zn

Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

A coupled thermodynamic/kinetic model for diffusional processes in superalloys

A coupled thermodynamic/kinetic model for diffusional processes in superalloys is described. Use is made of the generalised force-flux equations, and therefore an accurate knowledge of thermodynamical properties of the system is a prerequisite for the calculations. Calculations can be carried out in either the lattice-fixed (Kirkendall) or the mass-invariant (Matano) frame. In order to model transformations, e.g. involving the γ and γ′ phases, the concept of an interface sub-system is introduced. The numerical accuracy and stability associated with the treatment of the moving boundary is shown to be comparable to more traditional analyses which incorporate the Murray–Landis transformation. The results from the simulations are compared with experimental information, which includes (i) concentration profiles, (ii) movement of inert markers and (iii) observations of porosity formation. Particular attention is paid to the simulation of Ni–Al–Cr interdiffusion in the f.c.c. phase, for which reliable thermodynamic and kinetic information is available. Similar comparisons have been made for precipitation reactions involving the γ′ phase, although the available information is very sparse. Observations made on a number of Ni–Ni 3(Al,Ti) couples can be simulated with a reasonable degree of accuracy. Particular advantages of the method are (i) the interface between phases is treated in a novel way, which avoids numerical difficulties arising from the estimation of concentration gradients at phase boundaries, (ii) it can be readily extended to multicomponent systems, and (iii) the treatment of the Kirkendall drift of vacancies. It is clear that there is a great need for experimentation aimed at deducing kinetic data, e.g. diffusional mobilities, particularly for the γ′ phase.

Microstructural development in cast alloys based on the -NiAl–β′-Ni2AlTi– ′-Ni3Al– -Cr system

An investigation of microstructural development in three arc-cast Ni–Al–Cr–Ti multiphase alloys derived from the B2 type β-NiAl phase is presented. Detailed microstructural characterization of Ni–25 at % Al–20 at % Cr–15 at % Ti, Ni–26 at % Al–21 at % Cr–11 at % Ti and Ni–25 at % Al–24 at % Cr–15 at % Ti materials by transmission electron microscopy (TEM), is described. Microstructural development is examined in both the as-cast condition and after 140 h homogenization treatments at both 850 and 1100 °C. The formation of a eutectic between an L21-type β′ phase (Heusler phase, with a nominal composition of Ni2AlTi) and elemental α-Cr is examined. The precipitation of α within β(′) and vice versa, in both the inter- and intradendritic regions, is considered. The formation of L12-type γ′ (nominally Ni3Al) precipitates within the β and β′-phases is discussed, as is the transformation of β to two-phase β/β′ during ageing. © 1998 Chapman & Hall

Coulomb blockade and superconductor-insulator transition in ultrasmall single tunnel junctions

We studied ultrasmall Al single tunnel junctions with various tunnel resistances and lead resistances. The leads were made of linear arrays of Al junctions or NiCr thin films. The Coulomb blockade in normal single junctions was observed clearly when both the tunnel resistance and the lead resistance were higher than the resistance quantumR K=h/e 2=25.8kΩ, otherwise it was suppressed. The superconductor-insulator (SI) transition in single Josephson junctions was also observed. We claim that this transition is a manifestation of the dissipative phase transition predicted by the theory.

Adhesion In NiAl-Cr From First Principles

AbstractWe have calculated the work of adhesion (i.e. energy for rigid fracture) and peak interfacial stress for NiAl, Cr, and NiAl–Cr using self-consistent density functional calculations to obtain the complete energy vs. separation curve for each system. Our calculations indicate that the work of adhesion is largest for Cr and smallest for NiAI while those for interfaces of NiAI with Cr are intermediate. We have also estimated that segregation processes could alter the work of adhesion for the AlNi/Cr interface by up to 20% since Al tends to segregate to the free NiAl surface while Ni tends to segregate to the AlNi/Cr interface.

Processing and mechanical properties of in-situ composites from the NiAlCr and the NiAl(Cr,Mo) eutectic systems

Abstract In-situ composites based on the NiAl Cr eutectic system have been successfully produced by containerless processing and evaluated. Molybdenum additions of 0.6 to 6 at.% were used to change the eutectic microstructure. The NiAl Cr alloys had a fibrous microstructure, while the NiAl (Cr,Mo) alloys containing 1 at.% or more of molybdenum exhibited a lamellar structure. The room temperature fracture toughness of the different eutectic alloys was evaluated. The toughness values of the directionally solidified eutectics were similar regardless of composition or eutectic morphology, but all the directionally solidified alloys exhibited superior toughness compared to binary NiAl or conventionally cast eutectics. However, the principal mechanism responsible for the improved toughness of the directionally solidified alloys was dependent on the second phase morphology. The effect of eutectic morphology on the 1300 K creep strength was also investigated by testing a typical fiber reinforced and a lamellar reinforced eutectic. A molybdenum-doped alloy with the lamellar eutectic morphology exhibited the best creep resistance. Due to the promising creep behavior of this NiAl-28Cr-6Mo alloy at 1300 K, additional creep testing was performed at 1200 and 1400 K. This NiAl (Cr,Mo) eutectic displays promising high temperature strength while still maintaining a reasonable room temperature fracture toughness when compared to other NiAl-based eutectics.