Al2O3 reinforced Al/Ni intermetallic matrix composite by reactive sintering

An aluminium-nickel reinforced Al₂O₃ particulate composite was fabricated by a powder metallurgy route, where 35wt% aluminium and 30wt% nickel powders were mixed with 35wt% Al₂O₃ particles and compacted at 548 MPa. Sintering was carried out at 850 °C, where the synthesis reaction was sustained by the transient liquid phase resulting from the exothermic reaction associated with the formation of intermetallic compounds, i.e. reactive sintering. The resultant microstructure was studied using X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). It was found that the initial distribution of individual constituent powders affect the outcome of the reactive sintering and that the inward diffusion of aluminium in nickel was responsible for nickel aluminide formation.     

Microjoining of Ni3Al based intermetallic thin foils

Abstract

A preliminary analysis of the joining ability of thin Ni₃Al (Zr, B) based foils (below 150 μm thickness) into 'honeycomb' structures by various techniques is described in this work. Resistance welding, CO₂ laser welding and microplasma arc welding, as well as explosive welding, were used in the investigations. The examinations conducted in this study of the joint microstructures obtained by each technique do not reveal the existence of a heat affected zone. Columnar grains with a longer axis, as determined by the direction of heat flow, were observed along the fusion lines. Changes in microhardness were also examined along with a microanalysis of the chemical composition of the joint cross-section. Preliminary tests of the joining of thin Ni₃Al foil into 'honeycomb' experimental structures were carried out.     

Laser-welding behaviour of cast Ni3Al intermetallic alloy

Ductile nickel aluminide, Ni₃Al+B, is an intermetallic alloy with high strength and ductility making it a promising structural material for both elevated temperature and cryogenic temperature applications. In order to be able to use this alloy over a spectrum of temperature-critical applications, it must be capable of being joined or welded. The weldability of a cast nickel aluminide alloy containing boron was studied using laser welding. Welding was carried out at laser beam traverse speeds ranging from 42.33–254 mms^–1 in the bead-on-plate and butt-joint configurations. Two types of surface preparation, namely chemical cleaning and mechanical polishing, were used prior to laser welding. The quality of the laser welds was evaluated through mechanical tests (hardness and tensile), X-ray diffraction and microscopical observations. High-magnification examination of the welds revealed fine columnar structures in the weld zone. The hardness of the weld zone was substantially higher than that of the base metal. Microscopic examination also revealed the welds to contain shrinkage cracks. For a constant set of laser parameters, the chemically etched surfaces provided deeper penetration than the mechanically polished surface. The performance of the laser-welded joint is rationalized.     

Microstructural investigations of Ni and Ni2Al3 coatings exposed in biomass power plants

Abstract

The present work investigates the corrosion resistance of Ni and Ni₂Al₃ coated austenitic stainless steel (TP347H) tubes, which were exposed in a biomass-fired boiler with an outlet steam temperature of 540 °C for 6757 h. The Ni₂Al₃ coating was produced by electroplating Ni followed by low temperature pack cementation. After exposure, microstructural investigations were performed by light optical and electron microscopy (SEM-EDS). Electroplated Ni coatings were not protective in straw firing power plants and exhibited similar corrosion morphology as uncoated tubes. For Ni₂Al₃ coatings, the nickel aluminide layer was no longer adherent to the tube and was only found within the deposit. However, Ni₂Al₃ coatings had provided some protection compared to uncoated and Ni coated tubes. The formation of nickel chloride binds aggressive chlorine and slows down the active oxidation mechanism. In local areas, sulphidation corrosion attack of Ni was detected.     

Up-hill diffusion of hafnium in Ni3Al intermetallic alloys

An up-hill diffusion of hafnium was observed during the interdiffusion of Ni-Ni₃Al(Hf, B) couples. The Hf atoms diffused backward from the original interface to the Ni₃Al end, instead of moving forward to the Ni end under the concentration gradient. The observation confirms that the Hf atoms partition preferentially to the Ni₃Al phase by substituting for Al atoms in their sublattice sites.     

Tensile behaviour of polycrystalline Ni3Al intermetallic: influence of environment

Alloys based on the nickel aluminide intermetallic compound are particularly attractive for a spectrum of applications because of their high strength and resistance to oxidation. The inferior ductility of polycrystalline nickel aluminide intermetallic alloy was improved by controlled microalloying with boron. This important development has engendered considerable scientific interest in the use of this novel engineering material for temperature-critical and environment-sensitive applications. In this study, an attempt has been made to rationalize the influence of environment on the tensile behaviour of polycrystalline Ni₃Al alloy containing boron and zirconium. Slow strain tensile tests were performed on cold-deformed and annealed samples in environments of laboratory air, distilled water, and aqueous 3.5% sodium chloride solution. A comparison of the tensile properties and fracture behaviour of the polycrystalline intermetallic alloy in the different environments is made to highlight the role of environment in tensile behaviour.     

Interaction of hydrogen with the intermetallic compounds Sc2Al and Sc2Ni

The reactions of the intermetallic compounds Sc₂Al and Sc₂Ni with hydrogen have been investigated. The results demonstrate that Sc₂Ni reacts with hydrogen even at room temperature, to form amorphous Sc₂NiH₅. The reaction is irreversible at room temperature. Vacuum extraction of hydrogen between room temperature and 900°C leads to partial decomposition of Sc₂NiH₅ and the formation of crystalline ScH₂ and ScNi₂. Sc₂Al is nonreactive with hydrogen at room temperature. Heating at a hydrogen pressure of 5 MPa leads to Sc₂Al hydrogenolysis starting at 225°C and the formation of ScH₂ and metallic Al.     

Reactive sintering of boron-doped Ni76Al24 intermetallic

A preliminary investigation into the formation of boron-doped nickel-rich Ni₃Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni₃Al) from elemental powders by reaction synthesis was carried out. The application of reaction synthesis was seen as a low-energy alternative to the production of Ni₃Al/boride composite suitable for wear applications. X-ray diffraction, Neutron diffraction, SEM/EDS,WDS, Image analysis, Archimedes principle and Rockwell hardness measurements; were used to study the effect of boron addition on the final microstructure, average grain size, bulk density and hardness of as-prepared Ni₇₆Al₂₄. Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni₃Al, however, at a boron content of 0.5 wt% an apparent transition from a single phase microstructure to a two-phase intermetallic/boride composite microstructure was observed, which dominated when the boron content increased, up to 2 wt%. The two-phase microstructure was identified as Ni₃Al (particles) within an Ni₄₁Al₅B₁₂ boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni₃Al via two mechanisms. Below the solubility limit, the increase in hardness was due to solution hardening. Above 0.5 wt%B, solution hardening in addition to the formation of the harder boride phase, were found to amount to up to 50% increase in the hardness compared with boron free Ni₃Al. The extrusion of semi-molten beads at the surface of the compact at high B-content may be a limiting factor, in the formation of Ni₃Al/boride composites via this route.     

金属间化合物Ni3Al薄膜的制备及性能研究

采用直流磁控共溅射法,在衬底温度为450℃的SiO2基体上制备了厚度为500nm的Ni3Al薄膜,X射线衍射(XRD)和透射电子显微镜(TEM)等测试表明,薄膜为(111)取向的L12型晶体结构金属间化合物。采用纳米压痕方法测试了薄膜的力学性能,其硬度为8.00GPa,弹性模量为200GPa。为克服亚微米级薄膜氧化增重难以测量的困难,采用四探针测试金属薄膜电阻的方法,间接给出了薄膜的腐蚀性能和高温氧化程度。结果表明Ni3Al金属间化合物薄膜的氧化速率为2.28×10-13g2/(cm4.s),薄膜具有良好的高温抗氧化性能。     

The growth of single crystals of the intermediate phases NiAl and Ni3Al

Single crystals of stoichiometric NiAl, nickel-rich Ni₃Al, and aluminium-rich Ni₃(Al, Ti) have been grown by a modified Bridgman technique; a strain-anneal method has also been used to produce small, single crystals of non-stoichiometric NiAl.     

Tensile behaviour of Ni3Al+B intermetallic compound: influence of cold deformation and annealing

Nickel aluminide, intermetallic compound Ni₃Al, is a promising structural material on account of its high strength at elevated temperatures. The influence of cold deformation on the tensile behaviour of an Ni₃Al alloy containing zirconium and boron is presented. The undeformed material, in the as-cast condition, was subjected to varying levels of cold deformation ranging from 11.4%–61.4%, and tensile tests performed. The tensile properties and fracture behaviour of the cold-deformed material are compared with undeformed material to highlight the influence of cold deformation on strength, ductility and fracture behaviour. Tensile tests were performed on cold-deformed plus annealed samples and properties compared with the cold-deformed counterpart in order to elucidate the influence of annealing on tensile behaviour. The intrinsic effects of cold deformation and annealing on microstructure, tensile properties and fracture behaviour are highlighted.     

In situ synthesis of TiN/Ti3Al intermetallic matrix composite coatings on Ti6Al4V alloy

The aim of this paper was to develop an in situ method to synthesize the TiN reinforced Ti₃Al intermetallic matrix composite (IMC) coatings on Ti6Al4V alloy. The method was divided into two steps, namely depositing pure Al coating on Ti6Al4V substrate by using plasma spraying, and laser nitriding of Al coating in nitrogen atmosphere. The microstructure and mechanical properties of TiN/Ti₃Al IMC coatings synthesized at different laser scanning speeds (LSSs) in laser nitriding were investigated. Results showed that the crack- and pore-free IMC coatings can be made through the proposed method. However, the morphologies of TiN dendrites and mechanical properties of coatings were strongly dependent on LSS used in nitriding. With decreasing the LSS, the amount and density of TiN phase in the coating increased, leading to the increment of microhardness and elastic modulus and the decrement of fracture toughness of coating. When the LSS was extremely high (i.e., 600 mm/min), only a thin TiN/Ti₃Al layer with thickness around of 100 μm was formed near the coating surface.     

Nanostructured Ni3Al layers and Ni3Al/Ni multilayers obtained by magnetron sputtering

Intermetallic Ni₃Al thin layers and Ni₃Al/Ni multilayers were deposited on a Si wafer by means of magnetron sputtering. The structure and morphology of the layers have been characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The polycrystalline films are textured in the (111) direction and have grain sizes below 20 nm. Superlattice reflections due to chemical order have been observed in the electron microscope. It is shown by x-ray diffraction that the multilayers grow coherently on the amorphous substrate.     

Mathematical and experimental investigation of the self-propagating high-temperature synthesis (SHS) of TiAl3 and Ni3Al intermetallic compounds

One-dimensional mathematical modeling was used to describe the self-propagating high-temperature synthesis (SHS) process for preparing TiAl₃ and Ni₃Al intermetallics. The kinetic parameters (activation energies and pre-exponential factors) for the two compounds were obtained by matching experimental measurement and the numerical solution. The results thus obtained were compared with rate parameters obtained using different methods. The activation energy was 483 and 283 kJ mol^?1 for the formation of TiAl₃ and Ni₃Al, respectively. The temperature profiles calculated using the mathematical model were compared with experimental measurements for both aluminides which indicated reasonable agreement. Fine particle size and moderate preheating increase the SHS rates.     

Hydrogenation of the intermetallic compound Zr2Ni

We have determined conditions for the preparation of hydride phases with the composition Zr₂NiH_～5 by reacting the intermetallic compound Zr₂Ni with hydrogen or ammonia and identified the products of the reaction between the intermetallic compound and ammonia in the temperature range 150–500°C in the presence of NH₄Cl as an activator. The results demonstrate that the use of ammonia at 500°C leads to decomposition of the intermetallic compound and formation of zirconium hydride, zirconium nitride, and metallic nickel.