NiAl Matrix Research Articles

Development of New Wear-Resistant Surface Coating at Elevated Temperature

Because of good oxidation resistance at high temperature and excellent mechanical properties of Ni3 Al and high hot hardncss, and good oxidation resistance of chromium carbide, chromium carbide particle reinforced Ni3 Al matrix composite would possess excellent wear resistance at elevated temperature. Cr7 C3 -NiAl-Ni welding wire was produced by pressureless sintering process in vacuum. When the welding wire was welded on the surface of carbon steel, under the action of the physical heat of arc, NiAl reacted with nickel to form Ni3 Al and carbide particle reinforced Ni3 Al matrix composite was formed on the welding layers. Cr3 C2 was dissolved during welding and dispersed Cr7 C3 was formed, which strengthened the Ni3 Al matrix significantly. The Cr7 C3 -Ni3 Al interface was broadened, and a zone of interdiffusion and a new phase M23C6 were formed, indicating that a good bond has been formed. The hardness of Cr7 C3/Ni3 Al composite at room and elevated temperatures is much higher than that of stellite alloys. In addition, Cr7 C3 /Ni3 Al composite possesses better high temperature oxidation resistance than stellite 12 alloy. So Cr7 C3 / Ni3 Al composite can become an attractive potential candidate for elevated temperature wear-resistant surface material.

Discrete dislocation simulations of precipitation hardening in inverse superalloys

The low-temperature yield stress of a γ′ (Ni3Al) matrix–γ (Ni) precipitate ‘inverse’ superalloy, containing 40% Ni precipitates (γ), is calculated by discrete-dislocation simulations. Two different precipitate sizes and two anti-phase boundary energies are considered. The results of these simulations are compared with corresponding results from γ–γ′ superalloys (S. Rao, T.A. Parthasarathy, D. Dimiduk, et al., Phil. Mag. 84 3195 (2004)). In general, the results show that precipitation hardening in inverse superalloys is weaker than for regular superalloys. Similar to studies of superalloys, many of these results can be rationalized from the results of simulations on simple homogenized precipitate structures. The Hirsch, Kelly and Ardell precipitation-strengthening model (Metall. Trans. A 16 2131 (1985); Phil. Mag. 12 881 (1965); Trans. Jpn. Inst. Metals 9 1403 (1968).), developed for low-stacking-fault-energy spherical precipitates in a high-stacking-fault-energy matrix, adapted for inverse superalloys, shows qualitative agreement with the simulation results for spherical γ precipitates.

Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy

Precipitation behavior of Heusler phase (Ni2AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni2AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni2AlHf phase was slightly reduced. In addition, the spherical Ni2AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni2AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni2AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl Ni2AlHf phase transformation.

Evolution of microstructure for multilayered Al/Ni composites by accumulative roll bonding process

Accumulative roll bonding process (ARB) has been used to prepare a multilayered Al/Ni composite from pure aluminum sheets and pure nickel strips, and structural evolution in the composite was observed throughout the process. As increasing the number of ARB cycle, thin Ni layer was necked and fractured. The structural formation was controlled by the co-deformation between harder Ni layer and softer Al matrix. The slip bands activated in Al matrix approached on the Ni layers, resulting in the fragmentation of Ni layers in a shearing manner. The heterogeneous strain distribution was caused by the frictions between roller and surface, as well as between Ni layer and Al matrix, which had an important effect on the structural evolution.

The effect of Ti-addition on plastic deformation and fracture behavior of directionally solidified NiAl/Cr(Mo) eutectic alloys

To improve the high-temperature strength of NiAl/Cr(Mo) eutectic alloys, the effect of Ti-addition on microstructure and mechanical properties was examined. Three directionally solidified (DS) alloys with the composition of Ni–(33 − x)Al–31Cr–3Mo– xTi ( x = 0, 3 and 5 at.%, respectively), denoted 0Ti-, 3Ti- and 5Ti-alloys hereafter, were prepared. Temperature dependence of the yield stress and the room temperature fracture toughness of these DS alloys was examined. The aligned lamellae with B2-NiAl and A2-Cr(Mo) were formed in 0Ti-alloy, but the formation of lamellar structure was hindered by the Ti-addition. Cellular microstructures containing short plate shapes of Cr(Mo) phases were obtained in 3Ti- and 5Ti-alloys. In 5Ti-alloy, the precipitation of the L2 1-Ni 2AlTi was confirmed in NiAl matrix phase after the DS treatment. The Ti-addition induced a significant increase in high-temperature strength accompanied by a large deterioration of room temperature fracture toughness. The fracture toughness of 5Ti-alloy showed the low value of about 4 MPa m 1/2 because of the disturbance of microstructure.

Relationship between Microstructure and Tensile Strength in the Directionally Solidified (23-27) at. % Al-Ni Alloys

Directional or single crystal technique was applied to enhance the ductility, and two phases of γ (Ni) phase or β (NiAl) phase in γ‘(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50µm/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the γ dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the γ‘ single phase in the eutectic composition of 24.5 at. % Al, and the β dendrite fibers in the γ‘ matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including γ dendrites with γ‘ matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the γ dendritic microstructure changed to the β dendrite in the γ‘ matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle β dendrites in the ductile γ’ matrix.

Fabrication of Lotus-Type Porous Ni3Al with and without Boron

Ni3Al with and without boron was melted and unidirectionally solidified in pressurized hydrogen atmosphere by using a continuous zone melting method. For Ni3Al without boron, cylindrical pores elongated in the direction parallel to the solidification direction, are formed in Ni3Al matrix. On the other hand, no pore is formed in Ni3Al with 0.23 mol% B solidified under the same condition. This result suggests that the solubility gap of hydrogen between liquid and solid Ni3Al with boron is extremaly small.

Elevated Temperature Mechanical Properties of a Directionally Solidified NiAl/Cr(Mo)-Hf Alloy

The microstructure of directionally solidified (DS) NiAl-28Cr-5Mo-1Hf eutectic alloy was investigated by SEM and TEM. The eutectic alloy is composed of NiAl matrix , Cr (Mo) phase and a small amount of Heusler phase. Tensile testing of this alloy was performed from room temperature to 1373K. The brittle-to-ductile transition temperature (BDTT) is dependent on strain rate, with two orders of increase on strain rate resulting in a100K increase in transition temperature. The observation of fracture surface showed that at temperature above BDTT, the fractograph changes from NiAl cleavage and debonding along NiAl/Cr(Mo) interface boundary to ductile in nature.

Diamond cutting tools with a Ni3Al matrix processed by reaction pseudo-hipping

Nickel aluminide, Ni3Al, has high hot strength, which could help overcome the high heat and the interrupted vibrations that diamond cutting tools encounter during operation. Reaction pseudo-hipping, on the other hand, require only a short dwell time at high temperatures, which are detrimental to the diamond grits. Thus, it is promising to combine the unique nickel aluminide with the unique reaction pseudo-hipping process and replace the commonly used cobalt matrix. This study reports for the first time the process and application of reaction-pseudo-hipped Ni3Al matrix for diamond tools. In this work, mixtures of elemental nickel, aluminum, boron powder, and diamond particles are reaction-pseudo-hipped. Densities greater than 99 pct and mechanical properties comparable to those of the cobalt are attained. With high-grade diamond grits, the tools thus prepared show, under dry cutting conditions, a grinding ratio 118 pct higher than that with the cobalt matrix.

Fabrication of rhenium nanowires by selective etching of eutectic alloys

Rhenium nanowires have been fabricated via directional solidification and selective etching of a eutectic alloy. A NiAl–1.5 at.%Re eutectic alloy was directionally solidified using a constant growth rate and temperature gradient, in the Bridgman-type directional solidification furnace. The selective dissolution of the NiAl matrix was achieved with a mixture of HCl:H 2O 2, and produced an anisotropic etching of the eutectic, with the favoured etching directions aligned in parallel. The corroded surface was dominated by long rhenium fibres (diameter ∼400 nm), although shorter, and sometimes more rectangular wires were also observed in some sections. Digestion of the NiAl–Re eutectic in sulphuric acid, on the other hand, produced mainly long rhenium fibres of consistent shape and length. Both etching procedures might subsequently be applied for the preparation of nanodisc electrode arrays by embedding the obtained Re nanowires into a polymer and grinding until the wires are exposed. The reduction on the electrode area inherent in the use of such nanoelectrodes would allow a considerable increase in the signal-to-noise ratio, thus favouring the system for its application in analytical sensors. The use of rhenium in the electrode formation might also favour its application in high-temperature measurements.

Thermal-expansion behavior of a directionally solidified NiAl–Mo composite investigated by neutron diffraction and dilatometry

The thermal expansion of directionally solidified NiAl–Mo eutectic alloys consisting of nanoscale Mo fibers embedded in a NiAl matrix was analyzed by neutron diffraction and dilatometry. From room temperature to 800°C, perpendicular to the fiber direction, the NiAl and Mo phases expand independently with average coefficients of thermal expansion (CTEs) of 16.0×10−6°C−1 and 5.8×10−6°C−1, respectively. Parallel to the fiber direction, they coexpand up to 650°C with an average CTE of 12.8×10−6°C−1, but above this temperature the Mo fibers expand more than the NiAl matrix. This anomalous behavior is the result of the load transfer to the Mo fibers when the NiAl matrix softens. The average CTE of the composite parallel to the fiber direction was determined by dilatometry to be 13.0×10−6°C−1, which is approximately 11% lower than the value predicted by a simple rule of mixtures using the CTEs of the constituent phases.

Electrochemical production of nanopore arrays in a nickel aluminium alloy

Directional solidification of a eutectic is a novel route for the production of nanostructures. This method was applied to the quasibinary NiAl–Re system. Re as the minor phase forms fibers, which are parallel aligned in the NiAl matrix. At a temperature of 1690 ± 10 °C using a thermal gradient of 40 °C cm −1 and a growth rate of 30 mm h −1, the fibers formed had a diameter of about 400 nm. An electrochemical method is presented here that simultaneously passivates the NiAl matrix and selectively electrodissolves the Re. In this manner, it was possible to form an array of nanopores each with the same diameter of 400 nm. The mechanisms behind this procedure, as well as the potential of this method for the production of nanoelectrode arrays or nanofilters are discussed.

Coarsening of ? (Ni?Al solid solution) precipitates in a ?? (NiAl) matrix; a striking contrast in behavior from normal ?/?? alloys

Abstract Coarsening of γ precipitates in a γ′ (Ni3Al) matrix depends strongly on volume fraction, in dramatic contrast with coarsening of γ′ precipitates. Coalescence of γ precipitates is easy, unlike the resistance to coalescence of γ′ precipitates. We regard this as indirect proof that anti-phase relationships among γ′ precipitates inhibit coalescence.

Trans-interface diffusion-controlled coarsening

Accurate theoretical predictions of the volume-fraction dependence during diffusion-controlled coarsening of a polydisperse assembly of particles have proved difficult. Here, a new model of coarsening is presented, involving diffusive transport through the coherent interface between ordered and disordered phases, which atomistic calculations show has a ragged structure. The interface is a diffusion bottleneck when the ordered phase is dispersed. It is predicted that the square of the average radius grows linearly with time, that the depletion of solute decreases as the inverse square-root of time, and that there is no effect of volume fraction on kinetics and the scaled particle-size distributions. These differ dramatically from predictions of modern theories of diffusion-controlled coarsening. Data on coarsening in Ni-Al alloys is examined. We show that no other theory is consistent with the experimentally observed absence of an effect of volume fraction on coarsening of ordered gamma' (Ni3Al) precipitates in a disordered Ni-Al (gamma) matrix, and the strong volume-fraction dependence of coarsening of gamma precipitates in an ordered gamma' matrix.

Initial study of strengthening of NiAl–Cr(Mo)–Hf alloys by strong magnetic field

It was found that after treatment under 10T magnetic field and temperature 900 °C the bending strength of NiAl–Cr(Mo)–Hf alloy is increased by about 75%. The fracture section observation shows that the specimens' ductility is enhanced by the magnetic field treatment. Electron probe microanalysis shows that after treatment of the magnetic field the Heusler phases at NiAl/Cr(Mo) grain boundaries are partially dissolved and the small Heusler phases particles at Cr(Mo) and matrix phases interfaces almost dissolved into NiAl matrix.

Tensile Creep Behavior of NiAl-9Mo Eutectic Alloy

The tensile creep behavior of NiAl-9Mo eutectic alloy has been investigated over a stress range of 50 to 100MPa at the temperatures ranging from 850 to 950°C. All of the creep curves exhibit the very long steady-state stage. The creep parameters and TEM observations indicates the kinetics of the steady-state creep deformation is governed by dislocation climb in the NiAl matrix phase. The crack origination and development at the colony boundary results in the onset of tertiary creep stage and final fracture of the alloy.

Microstructure, brittle–ductile transition temperature and elevated temperature compressive behavior of the directionally solidified NiAl–Cr(Mo)–Hf alloy

The directionally solidified NiAl–27Cr–5.6Mo–0.4Hf (at.%) alloy was hot isostatically pressed at 1573 K, 100 MPa for 2 h and then annealed at 1323 K for 40 h. Microstructure examination revealed that the present alloy was mainly composed of three phases, i.e. the NiAl matrix, the lamellar Cr(Mo) and Heusler phase (Ni 2 AlHf). The interfaces between the NiAl–Cr(Mo), the NiAl–Ni 2 AlHf were atomically flat. The mechanical properties including the brittle–ductile transition temperature and the high temperature compressive behavior were also tested. The BDTT of the alloy was about 1233 K at a strain rate of 1.04 × 10 −4 s −1 and dependent on strain rate. This strain-rate dependent BDT can be explained by the cooperative dislocation generation model. The investigation on the high temperature compressive behavior between 1200 and 1300 K showed that the deformation behaviors of the alloy could be adequately described by the standard power-law. The possible strengthening mechanism was discussed.

Microstructures and mechanical properties of a directionally solidified NiAl–Mo eutectic alloy

A NiAl–Mo ternary eutectic alloy, having the nominal composition Ni–45.5Al–9Mo (at.%), was directionally solidified in a high-temperature optical floating zone furnace. Well-aligned rod-like microstructures were obtained, consisting of NiAl matrix and 14% (by volume) continuous Mo fibers having a square cross-section. With increasing growth rate (from 20 to 80 mm/h), the spacing and size of the Mo fibers decreased, from ≈2 to 1 μm (fiber spacing) and 800 to 400 nm (fiber size). X-ray microprobe analyses revealed that the NiAl matrix contained essentially no Mo (<0.1 at.%) and had the composition Ni–45.2Al (at.%), while the composition of the Mo fibers was Mo–10.1Al–3.9Ni (at.%). From electron backscatter diffraction patterns, the following orientation relationships were obtained: 〈1 0 0〉 NiAl//〈1 0 0〉 Mo, and {0 1 1} NiAl//{0 1 1} Mo. The growth directions and Mo/NiAl interface boundaries were found to be parallel to 〈1 0 0〉 and {0 1 1}, respectively, in both Mo and NiAl. Nanoindentation was used to probe the mechanical properties of the individual phases in the eutectic microstructure and the modulus and hardness of the NiAl matrix were determined to be 180 and 2.9 GPa, respectively, and those of the Mo fibers 270 and 4.8 GPa. Tensile tests were used to investigate the temperature dependence of the strength and ductility of the composite. Its ductile-to-brittle transition temperature was found to be ∼675 °C, and its yield strength about 25–30% higher than that of 〈1 0 0〉 NiAl single crystals over the temperature range 600–1000 °C.

Investigation of the scuffing characteristics of candidate materials for heavy duty diesel fuel injectors

The objective of this study is to characterize and understand the evolutionary processes that produce changes in the friction and surface damage in materials for possible use as heavy duty diesel fuel injector plungers. This work has involved the development of test methods to impart reciprocating motion to various metals, ceramics, and coated specimens in the presence of diesel fuel-like fluids. Commercial and candidate plunger materials, including 52100 steel, zirconia, cermets (TiC in Ni 3Al matrix), and TiN coatings, were evaluated on a crossed-cylinders-like scuffing test we call the ‘pin-on-twin’ geometry. Contacts were lubricated by on-highway #2 diesel and Jet A aviation fuels. Using friction-based criteria, the material ranking was in good agreement with field experience with actual injectors from the diesel engine industry. Zirconia and cermets exhibited promising scuffing resistance in both fuels. Scuffing generally became more severe in the Jet A fuel. Experimental results indicated that smoother surfaces that are required to sustain higher injection pressures could be more vulnerable to scuffing due to their thinner lubricant films. Material transfer was the major scuffing mechanism of zirconia, cermets, and TiN coatings against steel. Micro-scratches were also observed on the matrix material of cermets.

Microstructural characteristics of NiAl/TiC composites with high TiC content prepared by pressureless melt infiltration

The ordered intermetallic compound B2-NiAl is an attractive structural material for high-temperature application due to its high melting point (1638 ◦C), low density (5.90 g/cm3), high Young’s modulus (190 GPa), and excellent oxidation resistance at high temperature. Additionally, the thermal conductivity of NiAl is 4–8 times greater than that of Ni-based superalloys [1]. However, it is inherently brittle and has inadequate elevated temperature strength. Researchers have made efforts to incorporate a reinforcing phase in the NiAl matrix to overcome its shortcomings. Mechanical alloying, hot-pressing aided exothermic synthesis, hot-pressing and reaction synthesis, and so on for the incorporation of second phase such as TiC, Al2O3, TiB2, ZrO2, or AlN into NiAl, are the successful techniques so far [2–7]. Tuan [2] reported that the strength of NiAl/Al2O3was higher than the values predicted by the rule of mixtures. Xing [7] also reported that NiAl/(0–20 vol%)TiC showed higher compressive and tensile strength from room temperature to 1000 ◦C than polycrystalline NiAl. However, little information is available on NiAl/ceramic with low NiAl content, which can be expected to have higher wear resistance and better high temperature properties. In the present study, pressureless melting infiltration [8, 9], a simple and economic route to produce ceramic/metal composites, is used for preparing NiAl/TiC composite with about 75 vol%TiC. The microstructural characteristics, crystalline structure properties of the composites were investigated by optical microscopes (Ziess, Jenaphot 2000, Germany), transmission electron microscopy (TEM, Hitachi, H-9000NA, Japan), energy dispersive spectroscopy (EDS, Rontec) attached to the TEM and X-ray radiation (XRD) on Rigaku, Geigerflex/D. In the present study, the starting materials were TiC powder (H. C. Strack, Germany, D50 = 1–2 μm) and stoichiometric NiAl intermetallic smelted in our laboratory. TiC powder was de-agglomerated in 2-propanol medium with WC/Co balls, then dried and sieved through a 100 mesh sieve before forming. Disk-shaped TiC powder preform of 19 mm × 7 mm, and was uniaxially pressed in a steel die up to 32 MPa, followed by cold isostatic pressing at 200 MPa. The relative density of the as-pressed TiC was 60% of the theoretical density of TiC (4.93 g/cm3), which was determined by immersing in mercury. Infiltration experiments were performed in a graphite furnace in argon atmosphere. A piece of NiAl was placed on the top of the as-pressed TiC preform in an alumina crucible. Taking the sintering of TiC preform before NiAl melting and infiltrating into account, the amount of the NiAl piece for the infiltration was equal to 25 vol% of the TiC preform. Infiltration schedule, heating at 25 ◦C/min, dwelling at 1750 ◦C for 5 min or for 40 min, and cooling at 10 ◦C/min, was used. Experiments showed that NiAl infiltrated into the porous TiC preform completely in just 5 min at 1750 ◦C. The densification degree of the composite was estimated from the following equation,