TiC/Ni3Al Composites Manufactured by Self-Propagating High-Temperature Synthesis and Hot Isostatic Pressing

TiC–20 wt% Ni₃Al and TiC–40 wt% Ni₃Al composite materials were produced by self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP). In the SHS method the reacted powders were compacted by uniaxial pressing immediately after the reaction. The microstructure of the materials produced by SHS consisted of spherical carbides embedded in the Ni₃Al matrix, whereas the microstructure of the materials produced by HIPing was more irregular. A maximum hardness of 2010 HV₁ was measured for the material produced by HIP and a maximum fracture toughness of 10.5 MPa m^1/2 was measured for materials produced by SHS. High-temperature resistance was investigated by exposing the materials to 800°C in air for 110 h. The results obtained showed that the TiC + Ni₃Al composite materials can be recommended for use in environments consisting of oxidizing atmosphere at temperatures around 800°C where high wear resistance is required.     

Alloying element additions to Ni3Al: Site preferences and effects on elastic properties from first-principles calculations

First-principles calculations were performed to study the effects of alloying elements (Mo, Re, Ta, W, Ti, Co, Nb, Ru, Cr, Y) on the elastic properties of Ni₃Al. The site preferences of the alloying elements in Ni3Al at different temperature and concentrations were predicted. The influence of alloying elements on the lattice parameters of Ni₃Al were calculated and compared with the values fitted from experimental data. The effects of alloying elements on the elastic constants of Ni₃Al were present. The directional shear and Young’s moduli for single-crystal Ni₃Al alloys with alloying elements were estimated. The bulk, shear, and Young’s moduli of polycrystalline alloys were obtained. It is found that all the alloying elements occupy Al sites except Ru and Co, which may occupy both sites depending on concentration and temperature. All the elements increase shear and Young’s moduli of single-crystal Ni₃Al in all orientations except Cr, Co and Y. All the elements increase both bulk and shear moduli of polycrystalline Ni₃Al except Co and Y. The solute atoms with higher bulk modulus tend higher bulk modulus of Ni₃Al alloys, and the bulk modulus is related to the mole volume either.     

Study of Ni3 Al–Cr7 C3 coating fabricated by self-propagating high temperature synthesis casting route

Abstract

A coating on carbon steel was fabricated by a self-propagating high temperature synthesis (SHS) casting route. The process is described in detail. The phases in the coating were examined using X-ray diffraction (XRD). The microstructures of the coating and the substrate were analysed using electron probe microanalysis (EPMA) and optical microscopy, respectively. The wear behaviour and microhardness of the coating were determined. The experimental results show that the coating was composed of Ni₃ Al and Cr₇ C₃ phases and had a dense microstructure. Metallurgical bonding had formed between the coating and the substrate. The grain size of the substrate near the interface was coarsened. The wear resistance of the coating is better than that of a bearing steel or a Ni₃ Al coating. The microhardness of the coating is higher than that of a Ni₃ Al coating or the substrate.     

Fabrication of stable Ni–Al<Subscript>4</Subscript>Ni<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> superhydrophobic surface on aluminum substrate for self-cleaning,anti-corrosive and catalytic performance

A stable Ni–Al₄Ni₃–Al₂O₃ superhydrophobic surface (SHS) was fabricated on aluminum (Al) substrate via etching, subsequent replacement deposition and then annealing. The water contact angle of the SHS could reach to 155°, and the water sliding angle was less than 2°. The morphology and chemical composition of the samples were characterized using scanning electron microscopy, X-ray diffraction pattern, energy-dispersive spectroscopy and X-ray photoelectron spectroscopy. Anti-corrosion behaviors of the samples were investigated via Tafel extrapolation and electrochemical impedance, and the SHS showed a better corrosion resistance than that of pure Al. In addition, when the water drops impinged on the SHS, the drops could fully bounce, exhibiting exquisite self-cleaning property. The catalytic property of the samples was evaluated by the reduction of 4-nitrophenol (4-NP) at room temperature, in which the SHS served as the hydrogen generator as well as a catalyst. The degradation rate of 4-NP at the existence of the superhydrophobic samples in NaOH aqueous solution was 97%. And the reaction rate constant of 4-NP at the existence of the superhydrophobic samples was 9.51 × 10^?2 s^?1, which was about twice as large as that at the existence of pure Al. This work offers an effective strategy to fabricate SHSs and expands industrial applications for Al and its alloys.     

Microstructure evolution and oxidation behaviour of Tif/TiAl3 composites

In this paper, Ti_f/TiAl₃ composites were fabricated by infiltration–in situ reaction method and its oxidation behaviours were investigated by cyclic oxidation testing at 700 °C, 800 °C and 900 °C. The microstructure evolution and oxidation of Ti_f/TiAl₃ composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray diffraction (EDX). The reaction between Ti₃Al particles and Al was more violent than that of Ti fibres and Al. Ti₃Al/Al reaction consumed a large amount of Al and inhibited the reaction of Ti fibres indirectly. Reactant of Ti fibres was TiAl₃ at 700 °C, and four reaction layers surrounding Ti fibre (Ti₃Al, TiAl, TiAl₂ and TiAl₃ from inner to outside) were observed above 800 °C. The thickness of the total reaction layers increased little with temperature and time, while the thickness of inner reaction layers increased remarkably. A model corresponding to the microstructure evolution process was drawn schematically. Oxidation resistance of Ti_f/TiAl₃ composites decreased with increasing of temperature, and changed from cubic law at 700 °C to parabolic law at 900 °C. The oxidation weight gain of Ti_f/TiAl₃ composite was dominated by the exposed Ti fibres. Due to outward diffusion of Ti and Al element, the oxide of Ti fibre at 900 °C changed to mushroom-shape. Fortunately, when TiAl₃ was oxidized, a thin and continuous Al₂O₃ layer was formed, protecting matrix from further oxidation.     

Pack formation and long term oxidation kinetics of TiAl3 coating on γ-TiAl

Abstract

The aim of this study is to produce microcracking free TiAl₃ coatings on γ-TiAl alloy by the pack cementation process and to determine the long-term oxidation kinetics and thermal stability of the coating at high temperatures. It was shown that microcracking free coatings could be prepared in the AlCl₃ activated packs containing 4 wt-%Al as the depositing source. The conditions required for the formation of a microcracking free coating are discussed in relation to the pack chemistry at high temperatures. The TiAl₃ coatings formed were oxidised in air for more than 6200 h, during which weight gains were measured at regular intervals. The major oxide in the scale was Al₂O₃. It was observed that a TiAl₂ phase zone formed in the subsurface of the scale as a result of preferential oxidation of Al in the TiAl₃ coating. It was found that a linear relationship existed between the weight gain and logarithm of time of oxidation at 800°C: Δm _t = k _lln(αt+ 1). The thermal stability of the coating was assessed by measuring the growth kinetics of the TiAl₂ interlayer at the boundary between the TiAl₃ coating and substrate, which was determined to be d^1·4 = 0·1t (d and t in μm and h respectively) at 800°C using the experimental data measured over a diffusion annealing period of more than 6200 h.     

Plastic flow instabilities of L12 Ni3Al alloys at intermediate temperatures

The serrated plastic flow of L1₂ Ni₃Al alloys at intermediate temperatures was investigated using tensile tests. The effects of temperature, strain rate and composition were examined. The serrated plastic flow accompanied by the lowest (negative) strain-rate sensitivity was observed most strongly at 673 K and at a strain rate of 3.2 × 10^–4 s^–1. The serrated plastic flow became more significant as the alloy departed from a stoichiometric composition. The static strain aging at 673 K resulted in a reduced flow strength. The activation energy of the serrated plastic flow was estimated to be about 66 kJ/mol, which suggests that it is smaller than that for lattice diffusion of solutes in L1₂ lattices. The serrated plastic flow behavior of the Ni₃Al alloys was compared with that of the L1₂ Co₃Ti and Ni₃(Si,Ti) alloys, and is qualitatively explained on the basis of the dynamics of solutes in the core of dissociated screw dislocations.     

Effects of B+ and Cr+ ion implantation on the oxidation of Ni3Al

Ni₃Al samples were implanted with different doses of 150keV B⁺ and Cr⁺ ions to modify the surface region and the high-temperature oxidation behaviour was tested. The surface layer structure was investigated by Auger electron spectroscopy, and transmission electron microscopy, secondary ion mass spectroscopy and optical microscopy before and after testing. The experimental results show that boron atoms exist in the form of interstitial atoms. No evidence was found that any new phase existed in boron implanted Ni₃Al. Implanted Ni₃Al alloy has better oxidation resistance than the unimplanted ones at 900°C. For B⁺-implanted Ni₃Al, the oxide layer is basically composed of fine-grained NiO inner layer and an a-Al₂O₃ outer layer. Boron is oxidized into B₂0₃ of comparatively larger grain size. B₂0₃ particles are enriched at grain boundaries and defects. This curtails the short-circuit transportation of oxygen and improves the oxidation resistance of Ni₃Al. Implantation with Cr⁺ and B⁺ combines the good effects of both elements and produces a remarkable improvement on the oxidation resistance. The effects of implanted elements and the possible reaction mechanisms are discussed.     

Reaction sintering and joining nickel aluminide to AISI 316 stainless steel by self-propagating high temperature synthesis

Abstract

Self-propagating high temperature synthesis (SHS) is a process whereby reactants are ignited to spontaneously transform to products in an exothermic reaction. The aim of this study is to propose a method to join nickel aluminide with AISI 316 stainless steel by SHS and to study the combustion synthesis of nickel aluminide. From the heat of combustion synthesis junctions were formed between annular AISI 316 stainless steel and a powder metallurgy compact of Ni and Al blends. The Al mole ratio for testing the joining grade in the initial powder mixture varied from 25 at.-% to 40 at.-%. In order to check the sufficiency of the SHS reaction, the test temperature was compared with the thermodynamic calculation values. The metallographic analysis indicated that NiAl and Ni₃Al were formed in the joint layer.     

Cyclic oxidation behavior and recrystallization of cold-rolled Ni3Al foils

The floating zone (FZ) method makes it possible to fabricate Ni₃Al thin foils by cold rolling up to 96% without any intermediate annealing steps or boron addition. For the practical application of Ni₃Al foils, oxidation behavior, in addition to mechanical properties, at high temperature should be investigated. The cyclic oxidation properties at 1000 °C of cold-rolled Ni₃Al foil were investigated, and compared with those of bulk Ni₃Al alloy produced by conventional casting. The Ni₃Al foil exhibited more stable oxidation behavior than the bulk Ni₃Al alloy; in addition, the more stable oxidation behavior of Ni₃Al foil relative to bulk Ni₃Al alloy was attributable to an abrupt increase of grain boundaries due to recrystallization.     

High temperature interfacial evolution of Ti3Al/Al reaction couples

Abstract

High temperature interfacial evolution by reaction between liquid Al and solid Ti₃Al has been studied by observation of reaction couples isothermally reacted at 700, 800 and 900°C for different time intervals between 1 and 60 min respectively. Only TiAl₃ phase was detected in Al/Ti₃Al reaction couples up to 900°C. The reaction zone consisted of aluminide particles dispersed in an Al rich matrix. However, microstructures in the inner, central and marginal regions of reaction zone were different and were continuously changing across the width of reaction zone. A model corresponding to evolution progress was drawn schematically. A water quenched experiment confirmed the restriction effect of dispersed TiAl₃ particles to Al matrix and suggested that TiAl_3p/Al composite could be fabricated by this in situ reaction method.     

Pack codeposition of Al and Cr to form diffusion coatings resistant to high temperature oxidation and corrosion for γ-TiAl

Abstract

Thermochemical analyses were carried out for a series of pack powder mixtures formulated for codepositing Al with Cr to form diffusion coatings on γ-TiAl resistant to high temperature oxidation by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for codepositing Al with Cr to form diffusion coatings with an adherent and coherent coating structure. The results of the thermochemical calculations performed indicated that codeposition of Al and Cr is possible with CrCl₃.6H₂O and AlCl₃ activated pack powders containing elemental Al and Cr as depositing sources. However, experimental results obtained at 1100°C revealed that CrCl₃.6H₂O is not suitable for use as an activator for codepositing Al with Cr on γ-TiAl. It caused a significant degree of degradation indicated by weight losses instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer Cr doped TiAl₃ layer containing Al₆₇Cr₈Ti₂₅ phase and an inner layer containing TiAl₃ and TiAl₂ phases were successfully codeposited at 1100°C using pack powder mixtures activated by AlCl₃. It is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and chromium. Conditions that affect the pack codeposition process, and hence need to be carefully controlled, are discussed in relation to the mechanism of the formation of diffusion coatings with an integral structure free from microcracking on γ-TiAl.     

A study of vacancy-type defects in pure and boron-doped Ni3Al alloys

Positron lifetimes have been measured for two sets of pure and boron-doped Ni₃Al alloys. The alloys were large-grain polycrystals and had compositions of Ni_75+xAl_25−x (x = −1, 0, + 1) with 0, 100 and 500 wt ppm boron added. Lifetime parameters for samples of composition Ni_75+xAl_25−x (x= ± 1) with 0 and 500 wt ppm boron added were measured after initial thermal conditioning and after a subsequent cold-work anneal treatment. Positron trapping (≈20%) was observed in all unprocessed alloys. The vacancy concentration was calculated to be ≈ 5 × 10⁻⁶ and showed little, if any, systematic dependence on either alloy composition or boron concentration. Cold-worked fully annealed samples contained no detectable vacancies, i.e. the trapped state intensity was observed to be zero. The results are at variance with previously published data. During the annealing procedure (> 350°C) carbon was observed to diffuse out of the cold-worked samples. It is therefore possible that carbon stabilizes vacancies in Ni₃Al alloys. There is, however, no evidence to suggest that boron interacts with constitutional vacancies in Ni₃Al.     

Study on Ti fiber reinforced TiAl<Subscript>3</Subscript> composite by infiltration-in situ reaction

This study is concerned with investigation of forming Ti fiber reinforced TiAl₃ composite by infiltration-in situ reaction. The as-cast material was obtained by pressing molten pure Al into a preform which was composed of Ti particles and Ti fibers. Based on the differential scanning calorimetry (DSC) result, in situ reaction samples were obtained by heating as-cast materials to 660, 950, and 1300 °C, and held for 1 h, respectively. The microstructure evolution of in situ reaction samples was analyzed by scanning electron microscope and Energy dispersive X-ray (EDX). In addition, the phase composition of products was inspected by X-ray diffraction (XRD). Experiment results show that TiAl₃ was formed initially, which was the unique product between Ti and Al. While at high temperature, products of Ti fibers and Al were complex, and Ti _x Al_1−x (0.25 < x < 0.75) compounds were formed around Ti fibers. Finally, TiAl₃ decomposed, and oxidation occurred. The mechanism of in situ reaction between Ti and Al in this system was discussed.     

Microstructure and refinement mechanism of TiB2/TiAl3 in remelted Al-5Ti-1B system

In this work, we propose a new method (by remelting Al-5Ti-1B) to investigate the grain refinement mechansim. It is found that the morphology and size of TiAl₃ phase had little effect on the grain refinement of pure Al. Therefore, further experimental studies were carried out to understand the potency of TiB₂ particles. The high-resolution transmission electron microscopy (HRTEM) observation has confirmed the existence of an atomic layer on the surface of (0001) TiB₂, which is possibly a two-dimensional (2D) (1-12) TiAl₃. Crystallographic study indicates that it is a more suitable nucleation sites for α-Al than other particles. The TiB₂ particle with TiAl₃ 2D acts as the best effective nucleation sites for α-Al.     

Joining of TiAl intermetallic by self-propagating high-temperature synthesis

This study reports on a novel approach for joining TiAl by self-propagating high-temperature synthesis (SHS). The Ti, Al and C powders were applied in the joining process with the assisted electromagnetic field. The microstructure analysis revealed that a dark TiAl₃ reaction layer existed at the interface. The TiC particles and multi-layer granular structures of the Ti–Al system were found in the reaction products. It was noted that the porosity was inevitable in direct SHS joining. In order to improve the joining quality, Ag-based brazing foil was inserted between the powder compacts and the TiAl substrate. Molten brazing alloy during SHS reaction improved the wettability of the interlayer to TiAl substrate and filled well into the holes in the reaction products. With the application of Ag-based brazing alloy, the density increased and the joining quality improved.     

Formation of TiB2 particles during dissolution of TiAl3 in Al–TiB2 metal matrix composite using an in situ technique

A new technique has been developed in an aluminum based metal matrix composite in order to reveal the mechanism of formation of TiB₂ particles by mixing molten master alloys i.e., Al–8Ti and Al–4B in the Ti:B weight ratio of 5:2. A composite containing fine TiB₂ particles produced by this technique. In this approach, the progress of In situ formation of TiB₂ was carried out using hot stage microscope, energy dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction analysis. From the experimental observations obtained; it was proposed that the formation of TiB₂ particles occurred via diffusion of Boron atoms through TiAl₃ particles interface, thereby reacting to form fine TiB₂ particles. Studies indicate that since the primary TiB₂ particles on the surface of TiAl₃ are appreciably free and movable and because of boron diffusion across boundary layer towards TiAl₃, TiB₂ particles produced during growth with the primary ones formed agglomeration rings. A model was schematically developed to explain the formation of TiB₂.     

TEM Studies of High Temperature Corrosion Behaviour of TiAl Intermetallics with Surface Modifications

The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl₃ diffusion coating was studied in an environment of H₂/H₂S/H₂O at 850^oC. The kinetic results demonstrate that the TiAl₃ coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al₂O₃ scale on the surface of the TiAl₃‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800^oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.