Intermetallic NiAl Research Articles

Effect of pore structures on corrosion resistance of porous Ni3Al intermetallics

Porous Ni3Al intermetallics were fabricated by elemental powder reactive synthesis method, using carbamide powders as space holders. Corrosion behavior of porous Ni3Al intermetallics was investigated in a 6 mol/L KOH solution using electrochemical methods and immersion test. Effect of porous structures on the corrosion behavior of the porous Ni3Al intermetallics was studied. The results indicate that the porous Ni3Al intermetallics with higher porosities suffer more serious corrosion than the ones with lower porosities because the complicated interconnected porous structures and the large true surface areas exist in the samples with a higher porosity. But the corrosion rates of the porous Ni3Al intermetallics are not proportional to the true surface areas. The reason is that the pore size, pore size distribution and pore shape of the porous Ni3Al intermetallics change with the increasing porosity. All the porous Ni3Al intermetallics with different porosities exhibit excellent corrosion resistance in a strong alkali solution.

Site preference of Hf dopant in Ni3Al alloys: A perturbed angular correlation study

Perturbed angular correlation measurements of the hyperfine interactions of the 181Ta probe in the polycrystalline intermetallic Ni3Al doped with 0.5 and 5at.% Hf were performed in the temperature range 78–1230K. The observed local hyperfine fields at the 181Hf/181Ta probe showed that Hf atoms are substituting on Al sites in the L12 structure of Ni3Al. The two minor electric quadrupole interactions that were found for each Ni3Al alloy are discussed taking into account the L12 cubic structure and its two tetragonal distortions: D022 and L60. The presence of two tetragonal transformations of the parent cubic L12 lattice in 0.5at.% Hf and 5at.% Hf doped Ni3Al was revealed by X-ray diffraction. The ab initio calculations, performed with the projector augmented wave method correctly reproduce the experimental results and enable discussion on the structural and electronic properties of the polycrystalline alloys. The experimental and theoretical investigations showed that hafnium additions prefer aluminum sites in Ni3Al.

The Enhanced Tribological Properties of NiAl Intermetallics: Combined Lubrication of Multilayer Graphene and WS2

In recent years, reducing friction and wear-related adverse impacts on efficiency and durability in moving mechanical systems has gained increased attention. Herein, the search continues for novel materials and lubricants that can potentially reduce friction and wear. As one of the emerging self-lubricating materials, the tribological potential of graphene has been researched deeply. This article was dedicated to explore the combined lubrication of multilayer graphene (MLG) and WS2. The as-prepared sample of NiAl–1.5 wt% MLG–5 wt% WS2 (NB) exhibited excellent tribological properties. During the sliding process, a continuous lubricating film was formed to provide the low-strength junctions at the interface, reducing the friction coefficient and wear rate. Moreover, the MLG played the role of reinforcement particles and improved loading carrying ability.

Differential speed rolling of Ni3Al based intermetallic alloy—Analysis of the deformation process

Results of the first principles study on applying of a differential speed rolling (DSR) to a processing of the Ni3Al-based intermetallic alloy have been shown in the present paper. Obtained samples were free of cracks and shape defects, what indicates that selected parameters of the DSR method are suitable for the processing of Ni3Al intermetallics. It was also found that, the differentiation of rolling speed results with a prominent reduction and stabilization of the rolling force value during a rolling pass, and causes an increase of the imposed strain and a linear rise of the strain rate. Additionally, the DSR processed samples were characterized by a higher mechanical strength than the material deformed with an equal speed of both rolls.

Bulk Nanostructured Ni<sub>3</sub>Al Intermetallic and Ni<sub>3</sub>Al-Base Alloy

We show here that Ni3Al compound which is widely used as the base metal for advanced multipurpose hot-resistant alloys may be efficiently bulk nanostructured for improving its physical and strength characteristics. Developing the nanostructured component in the bulk of the intermetallic compound is achieved by plastic deformation of an SHS product during its synthesis and crystallization under conditions of thermal explosion of nickel/aluminum powder mixture of stoichiometric composition. It was shown that the nanosize component is formed on the basis of intermetallic Ni3Al synthesized by SHS under hot forging conditions from nickel/aluminum powder mixed with an inert binder component. Developing the nanosize structural components improves strength of the intermetallic alloy.

Wear mechanisms of WC–10Ni3Al carbide tool in dry turning of Ti6Al4V

The current investigation studied the possible beneficial effect of Ni3Al binder on WC-based cemented carbides in dry machining of a titanium alloy. The WC-10wt.% Ni3Al carbide tool was prepared through spark plasma sintering (SPS) technique. The cutting performance and wear mechanism of WC–10Ni3Al in continuous turning of Ti6Al4V were also discussed by analyzing the morphologies and compositions of worn surfaces of the carbide. The commercial available WC–8Co has also been tested under the same conditions for the purpose of comparison. The results showed that the WC–10Ni3Al has better cutting performance than WC–8Co, especially at the higher cutting speed of vc=100m/min. The excellent crater and flank wear resistance of WC–10Ni3Al was attributed to its synergistic mechanism of chemical inertness and high hardness induced by the intermetallic alloy Ni3Al binder at elevated temperature.

Tribological characteristics of an A356 aluminium alloy laser surface alloyed with nickel and Ni–Ti–C

Abstract Laser surface alloying of an A356 aluminium alloy was performed in order to improve the tribological behaviour of its surface using nickel and Ni–Ti–C powders to provide a reinformcement of intermetallics and ceramic phases. Alloying with nickel resulted in the formation of Al–Ni intermetallics whilst alloying with Ni–Ti–C led to the formation of Al–Ni and Al–Si-Ti intermetallics together with TiC particles produced by an in-situ reaction within the melt. The surfaces achieved hardness values of up to 3.5 times that of the untreated material, and these values were related to the overall content of alloying elements in the alloyed surface. The sliding wear resistance of surfaces alloyed with nickel and Ni–Ti–C was highly improved when using powder flow rates of more than 0.04 g/s, attaining mass loss due to wear at least 15 times less than that of the untreated material. A reduction of plastic deformation and adhesive wear with increasing alloying addition was the main reason for this improvement.

NiAl intermetallics dispersion-strengthened with silica, alumina, and mullite: Synthesis and characterization

Ni-Al intermetallics dispersion-strengthened with silica, alumina, and mullite were prepared by SHS under load and characterized by XRD and SEM/EDS. The observed formation of a transition zone between the embedded mullite particles and NiAl matrix in combustion-synthesized NiAl-mullite composites can be expected to improve the service parameters of thus synthesized material.

Tensile behavior of directionally solidified Ni3Al intermetallics with different Al contents and solidification rates

Despite the excellent high temperature mechanical properties of the Ni3Al intermetallic compound, its application is still limited due to its inherently weak grain boundary. Recent research advances have demonstrated that the tensile ductility can be enhanced by controlling the grain morphology using a directional solidification. In this study, a series of directional solidification experiments were carried out to increase both the tensile ductility and the strength of Ni3Al alloys by arraying either the ductile phase of γ-Ni-rich dendrite fibers or the hard phase of β-NiAl dendrite fibers in the γ′-Ni3Al matrix. The dendrite arm spacing could be controlled by the solidification rate, and the volume fraction of the γ or β phase could be altered by the Al content, ranging from 23 at.% to 27 at.%. With an increasing Al content, the γ dendritic microstructure was transformed into the β dendrite in the γ′ matrix, thereby reducing the tensile ductility by increasing the volume fraction of brittle β dendrites in the γ′ matrix. With an increasing solidification rate, the dendrite arm spacing decreased and the tensile properties of Ni3Al varied in a complex manner. The microstructural evolution affecting the tensile behavior of directionally solidified Ni3Al alloy specimens with different solidification rates and Al contents is discussed.

Mechanical properties of NiAl-Y2O3-based powdered alloys produced by directional recrystallization

The mechanical properties of NiAl-Y2O3-based powdered composite alloys (0.5–7.5 vol %), including those with an NiAl intermetallic matrix alloyed with 0.5 wt % Fe and 0.1 wt % La have been studied. Structures with various aspect ratios (AR, the ratio of the grain length to the grain diameter) are formed using deformation and subsequent annealing. A combination of the optimum amount of strengthening phase (2.5 vol % Y2O3) and a quasi-single-crystalline structure with a sharp axial texture with the (100) main orientation and AR ≈ 20–40 provides the maximum short-term strength and life at temperatures up to 1400–1500°C. An NiAl-Y2O3 alloy (2.5 vol %) has the best strength properties among all known nickel superalloys at temperatures higher than 1200°C and can operate under moderate loads at temperatures higher than the working temperatures of nickel superalloys (by 100–400°C) and their melting points. Additional alloying with 10 wt % Co and 2 wt % Nb makes it possible to increase the ultimate tensile strength of an intermetallic NiAl matrix at 1100°C by a factor of 1.3–1.4.

Effect of temperature on solid-state formation of bulk nanograined intermetallic Al3Ni during high-pressure torsion

A bulk form of nanograined intermetallic Al3Ni was produced by severe plastic deformation using high-pressure torsion (HPT). Powder mixtures of 75 mol% Al and 25 mol% Ni were processed by HPT at a selected temperature in the range of room temperature (RT) to 573 K under a pressure of 6 GPa. X-ray diffraction analysis revealed that the Al3Ni intermetallic formed after processing for 50 revolutions at RT but, as the processing temperature increased, less revolutions (i.e. lower imposed strain) were required for the formation of Al3Ni. Observations by transmission electron microscopy showed that the microstructure consists of ultrafine grains having a size of 300–2000 nm after 3 and 10 revolutions. Once the Al3Ni formed after a higher number of revolutions, equiaxed nanograins with a size of ~30 nm prevailed with a significant increase in hardness. The increase in hardness was more significant when processed at higher temperatures because of increasing the fraction of Al3Ni. It was shown that the solid-state formation of Al3Ni occurred due to enhanced diffusion (i.e. decreased activation energy for diffusion) through the presence of high density of lattice defects.

Production and characterization of Al-xNi in situ composites using hot pressing

In this study, a new metal matrix composite of aluminium was designed with the addition of nickel alloy particles. To produce in situ intermetallic formation, aluminium-nickel powder mixtures with different ratios ranging from 5 to 40 wt% Ni were consolidated at 550?C for 15 minutes under 40 MPa pressure. The interlayer phase formed during sintering was determined using X-ray diffraction and energy dispersive X-ray spectroscopy. The effect of nickel and Al-Ni intermetallics on the mechanical properties of the material was studied. The results demonstrated that the addition of nickel enhanced the hardness and wear behaviour of aluminium by forming a strong bonding interface between the aluminium and nickel particles.

Deformation behavior of perfect and imperfect nanomaterials made of intermetallic compounds with bcc lattice

The paper reviews the literature devoted to the stress-strain behavior of nanomaterials made of NiAl and FeAl intermetallics having B2 superstructure with body-centered cubic (bcc) lattice. Such nanomaterials demonstrate elastic deformation up to the point of their failure. In a concave region of a strain dependence of the potential energies P(e), the materials demonstrate thermodynamic instability and the non-homogeneous elastic deformation which is accompanied by the appearance of domains of two strain levels. Stretching behavior of NiAl and FeAl nanomaterials depends on lattice defects and sample geometry. An introduction of lattice defects which result in an appearance of residual stresses in structures can lead to the nanomaterial strengthening.

Ti/Al/Ti/Ni/Au ohmic contacts on AlGaN/GaN high electron mobility transistors with improved surface morphology and low contact resistance

Optimizing surface morphology of ohmic contacts on GaN high electron mobility transistors continues to be a challenge in the GaN electronics industry. In this study, a variety of metal schemes were tested under various annealing conditions to obtain contacts with optimal qualities. A Ti/Al/Ti/Ni/Au (20/120/40/60/50 nm) metal scheme demonstrated the lowest contact resistance (Rc) and a smooth surface morphology, and the mechanisms were investigated by materials analysis. A Ti/Al/Ti/Ni/Au metal scheme with optimized Ti and Ni thicknesses can result in formation of a larger proportion of Al-Ni intermetallics and a continuous TiN interlayer, which results in smooth surface and low Rc.

Effect of Milling Time on the Formation of NiAl Nanostructure Intermetallic Produced by the Mechanical Alloying Process

In the present study, NiAl nanostructure intermetallic produced by mechanical alloying process was investigated. Pure Al and Ni powders with the ratio of 50 at.% were mixed and milled in a planetary ball mill for different milling times in the range of 8 to 128 hours. The milled powder particles were investigated by XRD technique and scanning electron microscope. Williamson-Hall equation was employed to calculate lattice strains and crystallite sizes. The results showed that NiAl intermetallic formed after some milling times. The observation of flame or explosion in the vial just after the uncovering of it, in some cases, showed intense interaction between Al and Ni atoms after homogenization during milling. After longer milling time no flame and explosion was observed while NiAl intermetallics had been formed. Therefore, NiAl intermetallics were formed under different conditions with different structures and properties. The crystal sizes of obtained NiAl particles were less than 10 nm. The hardness changes in particles and also their internal strains were affected by milling operation at early stages of milling but they were mainly affected by transformation process of (Ni,Al) solid solution into intermetallic NiAl at longer milling times.

Characterization of Al–Ni intermetallics around 30–60 at% Al for TLPB application

Interest on the Al–Ni equilibrium diagram along the latest years is associated with the attractive properties of its intermetallic phases, such as high thermal stability, high corrosion resistance and high strength to density ratio. The Transient Liquid Phase Bonding (TLPB) is a technological process which can be applied to manufacture new pieces and to perform reparations. Morphology, composition profiles, growth kinetic and hardness as a function of temperature and composition of the Intermetallic Layers (ILs) were analyzed, especially focused on solid–solid interactions during isothermal annealing in reactive diffusion couples Ni/Al (800–1170°C). The study yields to the following association of the Al–Ni Intermetallic Phases (IPs) to the ILs: L1 (Al3Ni), L2 (Al3Ni2), L3 (Ni-poor AlNi), L4 (Ni-rich AlNi) and L5 (AlNi3). The composition ranges of L3 and L4 are 36–46 and 53–58at% Al, respectively. Martensitic transformation was found in the half thickness of L4 (L4M and L4S) at 1170°C. Kinetics show diffusion controlled growth for L2 and L5 and interface reaction control for L4 at 800–1170°C, while L3 revealed a mixed kinetic behavior: parabolic at 800–1000°C and linear at 1170°C. The growth rate constants presented temperature dependence according to the Arrhenius model. Vickers microhardness values decrease with annealing temperature and Ni concentration for ILs, and put in evidence different mechanical properties of L3, L4M and L4S.

Enhancement of high-temperature oxidation resistance and mechanical properties of Ni3Al thin films by inserting ultrathin Cr layers

Ni3Al thin films and Ni3Al/Cr multilayer films with a chemically ordered intermetallic Ni3Al phase were deposited on NO6625 alloy substrates by magnetron sputtering at 400 °C. Compared with the single layers, the multilayer samples show superior high-temperature oxidation resistance and mechanical properties. Both of the samples can prevent the substrate from being drastic oxidized even at 1100 °C. Their oxidation kinetics was found to follow the logarithmic law at 900 °C and be converted to the parabolic law at 1000 °C and 1100 °C. The oxidation rate can be reduced by one order of magnitude after inserting the Cr ultrathin layer. Nanoindentation measurements revealed that the hardness was decreased from 8.4 GPa to 8.0 GPa while the elastic modulus was increased from 191.4 GPa to 219.2 GPa. Furthermore, nano-scratch test indicated that the multilayer exhibited improved scratch toughness than that of the Ni3Al thin film.

Size-Dependent Mechanical Properties in Ni–Al Intermetallic Nano-particles Studied Using Embedded Atom Method

The energy and elastic constants C11, C12, and C44 as a function of particle size for Ni–Al intermetallic nano-particles are investigated by using embedded-atom method. We find that for nano-particles of large size, the size-dependent change in C12 is slightly different from those in C11 and C44, which is attributed to the internal strains. We also find that for the nanoparticles of size larger than 2.5 nm, the calculated C11, C12, and C44 of 1.52, 0.88 and 0.77 eV/A3 are in agreement with the available experimental results.

Investigation of a Nickel-Aluminum Reactive Shaped Charge Liner

A nickel/aluminum (NiAl) reactive powder system has been investigated to determine its mechanical properties under quasi-static and high rate compression to understand its deformation behavior. A shock recovery system has been used to define shock reaction thresholds under a triaxial loading system. Two nickel/aluminum (NiAl) shaped charge liners have been fired into loose kiln dried sand to determine whether the jet material reacts during the formation process. A simple press tool was developed to press the liners from a powder mixture of nickel and aluminum powder and a simple conical design was used for the liner. The shaped charge jet particles were recovered successfully in the sand and subjected to a detailed microstructural analysis. This included X-ray diffraction (XRD) and optical and electron microscopy on selected particles. The analysis demonstrated that intermetallic NiAl was detected and all the aluminum was consumed in the particles examined. In addition, different phases of NiAl were detected as well as silicon oxide in the target material. There was also some evidence that the aluminum had melted along with evidence of a dendritic microstructure. This is the clearest evidence that the shaped charge jet material has reacted during the formation process. Simulations have been performed using the GRIM Eulerian hydrocode to compare with flash X-rays of the jet.

SHS Joining Via Combustion of Ti-Containing Systems

The joining of intermetallic parts to the metals (Ti, Al, Cu, Ni) or different intermetallics (e.g. NiAl, TiAl) is important for high-temperature applications. Traditional fusion welding is difficult to apply due to the large thermal expansion mismatch between the intermetallics to be joined and the filler material.<br />Brazing and diffusion bonding are also inefficient, energy-consuming, and must be carried out in vacuum or protective atmosphere. Meanwhile, Self-propagating High-temperature Synthesis (SHS) that is widely used for production of powders, compact materials and parts holds considerable promise as a method of joining various component parts. We report on a new SHS-based approach to joining TiAl and NiAl intermetallics with metal substrates Ti and Ni. The adiabatic combustion temperatures for the systems under study and stoichiometric ratios of green mixtures were determined by thermodynamic calculations. Experiments were performed in the modes of pressure-assisted thermal explosion and combustion wave propagation, with the samples placed between the punches of experimental set up. Investigated were the processes taking place in thermally treated Ti or Ni plates mechanically impregnated<br />(by Mechanically Activation or Vibration Milling) with a mixture of metallic particles. Vibratory treatment of an Al and Ti surfaces in a powder mixture of metals was found to result in destruction of the surface oxide film and formation of extensive physical contacts between the Ni or Ti matrix and the metallic particles<br />mechanically impregnated into the near surface layer of Ni or Ti.<br />Subsequent thermal treatment was then used to launch chemical reaction yielding intermetallides within the impregnated layer. Performing SHS reactions yielding melted intermetallides in the coatings (or pellets) deposited (or placed) onto the surface of thus treated Ti or Ni plates, one can obtain strong weld joining<br />between solidified SHS products and substrates. This approach can also be used for deposition of coatings with a desired composition onto substrates.