NiAl Matrix Research Articles

Preparation and characterization of reactively sintered Ni 3Al–hBN–Ag composite coating on Ni-based superalloy

Ni 3Al–hBN–Ag intermetallic matrix composite coating was prepared on Ni-based superalloy by reactive sintering. The crystalline phase and microstructure of the coating were examined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that the hBN particles pretreated by electroless nickel plating and hot-dip aluminizing and Ag particles with a size of several micrometers were homogeneously dispersed in the Ni 3Al matrix, and the composite coating had strong interfacial bonding with a fine and dense microstructure. During the reactive sintering, an exothermic reaction between Ni and Al associated with a transient liquid phase occurred, leading to in situ synthesis of the densified Ni 3Al matrix in the coating with a high tensile strength of more than 70 MPa. The high-temperature tribological properties evaluated on a ball-on-disc test rig showed that the coating possessed self-lubricating properties from room temperature to 800 °C due to a synergetic lubricating action of Ag and hBN.

Effects of pre-strain on the compressive stress–strain response of Mo-alloy single-crystal micropillars

A NiAl–Mo eutectic was directionally solidified to produce composites with well-aligned single-crystal Mo-alloy fibers embedded in a NiAl matrix. They were pre-strained by compressing along the fiber axis and then the matrix was etched away to expose free-standing micropillars having different sizes (360–1400 nm) and different amounts of pre-strain (0–11%). Compression testing of the pillars revealed a variety of behaviors. At one extreme were the as-grown pillars (0% pre-strain) which behaved like dislocation-free materials, with yield stresses approaching the theoretical strength, independent of pillar size. At the other extreme were pillars pre-strained 11% which behaved like the bulk, with reproducible stress–strain curves, relatively low yield strengths, stable work-hardening and no size dependence. At intermediate pre-strains (4–8%), the stress–strain curves were stochastic and exhibited considerable scatter in strength. This scatter decreased with increasing pre-strain and pillar size, suggesting a transition from discrete to collective dislocation behavior.

Investigations on NiAl composites fabricated by matrix coated single crystalline Al2O3-fibers with and without hBN interlayer

The intermetallic compound NiAl has excellent potential for high temperature structural applications but suffers from low temperature brittleness and insufficient high temperature strength. One way to remove these deficiencies is the reinforcement by high strength ceramic fibers. Such intermetallic matrix composites can be conveniently fabricated by the hot pressing of matrix coated fibers. Al2O3 single crystal fibers show excellent chemical stability with the NiAl matrix, but the residual thermal compressive stresses during cool down dramatically degrades the fiber strength and thus, renders the composite useless for structural applications. We report on an experimental and computational study to mitigate this problem and to fabricate Al2O3/NiAl composites with sufficient high temperature strength. Analytical TEM, mechanical testing and push-out tests were employed to characterize chemistry, microstructure and mechanical properties of the composites. It will be shown that a processing window exists that allows producing intermetallic matrix composites with promising mechanical properties.

Electrodeposition of gold on tungsten nanowires present in NiAl–W eutectics

The formation of gold nanoelectrode arrays was investigated by electrodeposition of the metal on the tungsten nanowires present in directionally solidified NiAl–W eutectics. The tungsten fibres present in the eutectic were conditioned prior to the electrodeposition, and the NiAl matrix was simultaneously passivated by formation of Al 2O 3 oxide. The simultaneous oxidation of W to W(VI) species for the formation of nanopores, and growth of passive Al 2O 3 oxides on the NiAl matrix was studied for different polarisation potentials. Anodisation of the samples at potentials <0.7 V resulted in the formation of thin oxide films, and therefore the NiAl matrix remained partially active. In order for the matrix to be fully passive, and the fibres to dissolve and form nanopores, the initial polarisation must be run at 0.7 V vs. SHE. The electrodeposition of gold on the conditioned samples was afterwards optimised for the filling of the pores. When the deposition was run by application of reverse current pulses the obtained gold structures presented a hemispherical shape with a radius that increased with the duration of the cathodic pulse. The electrodeposition of gold from more diluted Au(I) solutions under potentiostatic conditions yielded the formation of thin Au deposits in the nanopores. The nucleation and growth mechanisms involved in the process were determined from the potentiostatic current transients in all the examined cases. It was found that instantaneous nucleation took place for the more concentrated solutions, whereas mixed processes were observed for diluted amounts of Au(I).

Passivity of a Nanostructured Directionally Solidified NiAl–Re Alloy as Substrate for Electrodeposition of Gold

Directionally solidified NiAl–Re eutectic alloys can be employed as substrates for the formation of gold nano- and microelectrodes by electrodeposition of this metal in the nanofibers present in the eutectic structure. An initial polarization of the eutectic is required in order to passivate the NiAl matrix and thus confine the electrodeposition process to the fibrous phase. The effect of the applied polarization potential as well as the duration of the treatment were investigated and related to the nucleation process during electrodeposition. It was found that polarization at low anodic potentials resulted in a poor passivation of the NiAl matrix, thus yielding gold deposits randomly distributed over the whole surface of the sample. Polarization at high anodic potentials is essential to ensure that the matrix is fully passive, with the fibrous phase remaining as the only electrochemically active area. The impedance measurements carried out on the samples after polarization at for longer time intervals showed a decrease in resistance after a sufficiently long passivation, attributed to the formation of a porous oxide film and partial corrosion of the sample through the pores created during the process. The polarization time must therefore be confined to to avoid undesirable corrosion. Under these conditions, electrodeposition of gold takes place along the pores left by the dissolution of the fibrous phase and originates arrays of aligned gold microspheres.

Mechanical Properties of Directionally Arrayed Dendrites in the Ni&lt;sub&gt;3&lt;/sub&gt;Al Matrix Alloy

Ni3Al shows the unique feature of increasing strength with increasing temperature. However, it is too brittle to use as a structural material due to grain boundary weakness. Ductility could be enhanced by controlling grains using directional solidification. In order to increase the ductility or strength of Ni3Al alloys, a ductile γ (Ni-rich) phase of dendrite fibers or a strong β (NiAl) phase of dendrite fibers were arrayed in the γ´ (Ni3Al) matrix by directional solidification. The dendrite spacing could be controlled by varying the solidification rates, and the volume fraction of the γ or β phase could be changed by using alloy compositions, from 23 to 27 at. % Al-Ni alloy. With increasing solidification rates, the dendrite spacing decreased, which caused the tensile strength to be enhanced and the elongation to decrease, evidently due to the phase boundary augmentation. With increasing Al content, the γ dendritic microstructure changed to β dendrites in the γ´ matrix, which resulted in a decrease in elongation as a result of an increase in the volume fraction of the brittle β dendrites in the γ´ matrix.

Physicochemical laws of the interaction of nickel aluminides with alloying elements: II. Interaction of nickel aluminides with alloying elements and/or interstitial phases

The Ni-Al-X (X is an interstitial element or phase) phase diagrams are analyzed to reveal systems that can be used as the basis for designing promising alloys and natural composites based on nickel aluminides reinforced by interstitial phases (natural composites I). The most thermally stable materials are shown to be heterophase alloys and composite materials (CMs) located in the eutectic-type (including degenerate eutectic) pseudobinary sections of ternary or multicomponent phase diagrams. They exhibit insignificant (or zero) dissolution of interstitial phases at operating temperatures and the absence of an intense interaction between CM components (natural composites II). Natural composites I based on the NiAl-or Ni3Al-interstitial phase alloys produced upon cooling from a melt can be reinforced by the refractory thermally stable rigid interstitial phases, namely, borides and carbides, that are present in pseudobinary sections in equilibrium with these nickel aluminides, since the elements forming these phases dissolve completely in matrix melts and the mutual solubility of these phases in the solid state is low. Such borides are TiB2 and HfB2 in equilibrium with β-NiAl, and such carbides are, e.g., TiC and HfC in equilibrium with β-NiAl and La2C3, NbC, and TaC in equilibrium with γ′-Ni3Al. Natural composites II should be produced using solid-phase methods (NiAl with AlN, Y2O3, Al2O3) or a combination of methods, where a refractory interstitial phase of the Al2O3 or Y2O3 type is solid and the intermetallic NiAl or Ni3Al matrix is liquid. NiAl-TiB2 (HfB2), NiAl-Al2O3 (Y2O3), and Ni3Al-La2C3 (NbC, TaC) composites are considered as examples.

Investigation of Compression Deformation Behavior and Microstructure Evolution of NiAl-Cr (Mo)-Hf Alloy at Elevated Temperature

NiAl-based alloy is a promising material applied in the fields of aeronautic and astronautic instruments. In the paper the compression deformation behavior and microstructure evolution of NiAl-Cr(Mo)-Hf alloy at elevated temperature were studied. The results demonstrate that the alloy behaves good formability in the temperature ranging from 1320°C to 1360°C, in which the maximum initial strain rate is about 8.3×10-4s-1 and the maximum deformation resistance is lower than 40MPa. During compression at temperature between 1250°C and 1300°C the flow stress increased sharply with the increasing of the deformation degree. When compression deformation at 1320°C~1360°C, the flow stress decreased obviously and the flow stress decreased slightly after reached the maximum value. By analyzing the microstructure evolution during compression it can be concluded that as-casting microstructure was improved in deformation. The grains were refined and the brittle phases of lamellar Cr(Mo) existing at NiAl matrix were broken. The porosities in as-casting material were eliminated during compression and the density of the material increased. The fracture toughness of the alloy increased from 6.4MPa·m1/2 to 9.8MPa·m1/2 after compression.

Compressive Mechanical Properties of NiAl/Cr(Nb) Alloy Prepared by Rapid Solidification Processing

It is well known that inadequate strength at elevated temperatures and low fracture toughness at ambient temperatures are the major deficiencies preventing the B2 aluminide NiAl as an advanced structural material used at high temperatures. [1–3] In order to promote its practical use, a promising NiAl based alloy system, i.e., the IP alloy reinforced by Laves phase has been developed. [4–10] The common characteristics of these alloys are that all of them exhibit higher melting points (about 1460°C), lower densities (6.20 ∼ 6.35 g/cm 3 ), higher thermal conductivity and excellent oxidation resistance compared to the Ni-based superalloys. Furthermore, these alloys can be processed by a series of different production routes, which may be used for property variation within a wide range. [8–10] However, the previous relevant studies proved that the alloys prepared by conventional ingot casting showed coarse-grained microstructure and then exhibited serious brittle fracture at ambient temperatures. On the other hand, the alloys processed by powder metallurgy or mechanical heat treatment with finer microstructure usually improved room temperature brittleness but seriously lowered yield stresses at high temperatures. [6,7] Therefore, for these alloys, to find an efficient way to optimize mechanical properties is necessary to promote their practical use. It is suggested that the mechanical properties of these alloys depend sensitively on the grain size of NiAl matrix and distribution of strengthening Laves phase which is controlled by processing parameters, such as temperature, time and cooling rate. [7] Rapid solidification to produce bulk metallic alloys is known for a long time to result in refined microstructure, extended solubility, reduced microsegregation, and formation of metastable phases. Injection casting, as a popular rapidly solidified technique to fabricate bulk amorphous alloys, can provide relatively high cooling rates of about 10 2 ∼ 10 3 Ks –1 that is significantly higher than 10 –1 ∼ 10 –2 Ks –1 for conventional casting. [11] Sun et al. [12] and Dai et al. [13] have investigated the effects of refined eutectic interlamellar spacing and grain size on the mechanical properties of the Ti-based alloys, and found that the optimized microstructure induced by Cu mold casting was responsible for the improved room-temperature mechanical properties. Recently, Huai et al reported that the mechanical properties of eutectic NiAl/Cr(Mo, Hf) alloy could be optimized by rapid solidification process, i.e. suction casting. [14] However, the suction casting process could not provide enough high cooling rate as injection casting. Accordingly, in the current study, an IP alloy with the composition of NiAl-Cr/Nb alloy was chosen and prepared by injection casting, and its microstructural morphology and mechanical properties were investigated. Due to the limited amount of sample material available, compression tests were performed only. The compression testing results demonstrate that the resultant alloy exhibits improved compressive ductility and yield strength at room temperature as compared with that of the same alloy prepared by conventional ingot casting. Meanwhile, the yielding strength of resultant alloy is slightly lowered at high temperatures. The test results are quite reproducible.

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni3Al matrix was investigated in alloys containing 22.0–22.8at.% Al aged at 650–800°C for times exceeding 1800h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1±16.6kJmol−1, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.

Simultaneous improvement of strength and ductility in NiAl–Cr(Mo)–Hf near eutectic alloy by small amount of Ti alloying addition

Effects of Ti alloying addition on the microstructure and room temperature compression deformation behavior of a NiAl–Cr(Mo)–Hf near eutectic alloy were investigated by SEM, TEM, EDX and compression test. The results showed that compared with base alloy, the compressive fracture strain and 0.2% yield strength of the Ti-containing alloy were enhanced simultaneously. Disordered (Ti,Hf) solid solution phase together with the blocky Heusler phase Ni 2Al(Ti,Hf) precipitation presented at eutectic cell boundaries is responsible for the enhancement in ductility due to the better deformation ability of (Ti,Hf) solid solution phase. The improvement in strength depends mainly on the solid solution strengthening in NiAl matrix by large amount of Ti due to its larger solid solubility relative to Hf in NiAl.

Microstructures and Mechanical Properties of Directionally Solidified Intermetallic Composites

This paper reviews two kinds of well-aligned fibrous microstructures produced by directional solidification of NiAl-Mo and Fe-Fe2Ta eutectics. In both these composites, fiber size and spacing decrese inversely as the square root of the growth rate. Tensile tests as a function of temperature showed that the NiAl-Mo composite has a higher yield strength and lower ductile-to-brittle transition temperature than the NiAl matrix. For the Fe-Fe2Ta composite, yield strengths in excess of 700 MPa were obtained at temperatures to 600°C, with elongations to fracture of ~3% which remained roughly constant at temperatures to 950°C.

TiC reinforced composite layer formation on Al–Si alloy by laser processing

Al–Si alloy was laser surface treated with various compositions of TiC and Ni under different laser processing parameters to produce a TiC reinforced composite layer on it. The study shows that TiC composite layer is formed when the composition is 75TiC–25Ni (wt-%) and laser power 2·5 kW and scan speed 0·2 m min−1. The layer exhibited an average microhardness of ∼750 HV, which is free from pores as well as cracks and contained TiC and Al–Si phases distributed in the Ni–Al matrix. Mechanical strength of layer is studied by the hardness test.

Fiber damage and high temperature tensile properties of Al 2O 3 fiber reinforced NiAl-matrix composites with and without hBN-interlayer

The fiber damage caused by thermal residual stress in NiAl-matrix composites reinforced by single-crystal Al 2O 3 fibers (sapphire) was studied. In the NiAl/Al 2O 3 ( V f = 0.3) composites, more fibers were broken into shorter segments because of the larger thermal axial compressive stress exerted on the fibers which was caused by the different thermal expansion coefficients of Al 2O 3 fibers and NiAl matrix. An increasing fiber volume fraction or addition of an hBN-interlayer decreased the thermal axial compressive stress and thus reduced the fiber damage. The tensile strength of NiAl/Al 2O 3 ( V f = 0.3) composites at 1100 °C reaches only about 80 MPa owing to the fiber strength degradation caused by severe fiber damage. By contrast, NiAl/Al 2O 3 ( V f = 0.5) composites showed a tensile strength of about 200 MPa at 1000 °C and 140 MPa at 1100 °C. The NiAl/BN/Al 2O 3 ( V f = 0.3) composites had a tensile strength of only 30 MPa at 1100 °C because of the low bonding strength between fiber and matrix and thus, ineffective load transfer.

Significant Improvement of Mechanical Properties in NiAl-Cr(Mo)/Hf Alloy by Suction Casting and Subsequent Hot Isostatic Pressing

The NiAl-28Cr-5.5Mo-0.5Hf eutectic alloy was prepared by the suction casting (SC) technique and subsequent hot isostatic pressing (HIP) treatment, and tested for compressive strength and fracture behavior in the temperature range of 300 to 1373 K. The microstructure of suction-cast alloy is characterized by fine interlamellar spacing, large area fraction of eutectic cell, and fine Heusler (Ni2AlHf) phase distributed semicontinuously at the cell boundaries. After HIP treatment, Ni2AlHf phase at the cell boundaries is transformed into Hf solid solution phase and distributed homogeneously within the NiAl matrix. Compared with the conventionally cast alloy, the room-temperature compressive strain and elevated temperature strength of suction-cast alloy are enhanced markedly after HIP treatment. The reason is that the HIP treatment causes Hf solid solution phase to distribute homogeneously and then strengthens the NiAl matrix.

Composition dependence of the precipitation behavior in NiAl–Cr(Mo)–(Ti,Hf) near eutectic alloys

Influence of Ti and Hf combined additions on the Ni 2AlX (X = Ti,Hf) Heusler phase precipitation in as-cast NiAl–Cr(Mo) eutectics was investigated by XRD, SEM and TEM observation. The results show that with the increasing of Ti content, the Ni 2Al(Ti,Hf) precipitation with blocky morphology at NiAl/Cr(Mo) interface and cellular boundary is nearly suppressed; however, this precipitation with cuboidal or granular morphology within the NiAl matrix promoted. The mechanisms of this transition of precipitation behavior are attributed to the decrease of interfacial energies due to the coherency between NiAl and Heusler phase Ni 2Al(Ti,Hf) with the increasing of Ti addition. The size of Ni 2Al(Ti,Hf) coherent precipitates inside the NiAl matrix decreased progressively with the increasing of Ti content in NiAl phase, showing a Gibbs–Thomson relation-like trend. In addition, another (Ti,Hf) solid solution phase along with the Ni 2Al(Ti,Hf) precipitates was observed for lower Ti addition alloys, which is the precipitation from NiAl matrix prior to Ni 2Al(Ti,Hf) precipitation. This may be ascribed to the fast formation kinetics of (Ti,Hf) solid solution phase relative to the Ni 2Al(Ti,Hf) phase for lower level Ti-containing alloys.

Microstructure and abrasive wear performance of chromium carbide reinforced Ni 3Al matrix composite coating

A Ni–Al–Cr 3C 2 welding wire was produced by metal–powder–core technique. When the welding wires were welded on the surface of carbon steel, under the effect of the physical heat of arc, Ni reacted with Al to form Ni 3Al and carbide particles reinforced Ni 3Al matrix composite was formed. Cr 3C 2 was decomposed during welding and dispersed Cr 7C 3 with stripe shape formed, which strengthened the matrix significantly. The Cr 7C 3–Ni 3Al interface has broadened into a zone of interdiffusion and a new phase M 23C 6, which indicates that a good bond has been formed. The pin-abrasion wear test showed that the abrasion resistance of Cr 7C 3/Ni 3Al composite is six times higher than that of Stellite12 alloy at room temperature. The good wear resistance of Cr 7C 3/Ni 3Al composite coating can be attributed to large volume fraction of carbides, high hardness, and good phases interface bond.

Laser clad WC reinforced Ni-based intermetallic-matrix composites to improve cavitation erosion resistance

Nickel aluminide intermetallic coatings (IC) and nickel aluminide intermetallic matrix composite (IMC) coatings with a WC particulate reinforcement were successfully clad onto AISI 420 Martensitic stainless steel using a 4 kW Nd:yttrium-aluminum-garnet laser with rotary-disk powder feeding of Ni/Al powder and a Ni/Al+WC powder mixture, respectively. Under optimized processing with pre- and postheat treatment conditions, the nickel aluminide clad layers were free of cracks and pores. The microstructure of the IMC was mainly composed of reprecipitated and refined WC particles dispersed uniformly throughout the Ni–Al matrix with a few partially dissolved and undissolved WC particles. The average hardness of the IC was increased from 330 HV0.2 to 420 HV0.2 by the WC reinforcement. The normalized cavitation erosion resistance (Ren) of the IC and IMC were about 5.2 and 284 times that of the stainless steel substrate, respectively. The superior cavitation erosion resistance of the laser clad layers were explained in terms of microstructure, microhardness and work hardenability.

TiC/Ni 3Al coating on steel via combustion synthesis during casting

TiC-reinforced Ni 3Al intermetallic matrix composite coating on steel was fabricated successfully using the combustion synthesis reaction of 3Ni–Al–Ti–C system during casting. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectrum (EDS) are used to analyze phases, microstructure and composition of the coating and the interface. The results show that the coating is composed of Ni 3Al and TiC. In the coating, TiC particulates with dimension of 1–2 μm are relatively uniformly distributed in Ni 3Al matrix. The coating is compactly formed on steel with a metallurgical bonding interface. The wear resistance of the coating is 4.3 and 23.4 times higher than that of steel substrate at the room temperature and 400 °C respectively.

Fabrication and mechanical properties of microalloyed and ceramic particulate reinforced NiAl-based alloys

NiAl-based alloys and their composites reinforced with in situ formed TiC and externally added ceramic particles are fabricated by hot-pressing. Their microstructures and mechanical properties are evaluated. Comparatively, the Ti- and/or C-alloyed NiAl and the ceramic particulate reinforced composites possess a significant improvement in both flexural strength fracture toughness at room temperature. Nevertheless, the NiAl–TiC-nano-Al 2O 3 composite has low strength, mainly due to the existence of residual porosity, inhomogeneous distribution and severe agglomeration of nano-scaled Al 2O 3 particle within the NiAl matrix.