Interfaces in nickel aluminide/alumina fibre composites

Metal matrix composites based on the intermetallic alloy Ni₃Al and fibres of Al₂O₃ were fabricated by hot-pressing nickel aluminide powders and alumina fibres. Two matrix alloys were used in this investigation: Ni₃Al microalloyed with boron and Ni₃Al alloyed with 8 at% chromium and smaller amounts of zirconium and boron. The materials were studied using optical and transmission electron microscopy with particular emphasis placed on the characteristics of the matrix-fibre interface. The base Ni₃Al/Al₃O₃ composite displayed no evidence of chemical reaction at the interface, an intimate bond between matrix and fibre was observed, and the material exhibited 10% ductility at room temperature. Composites with the more complex matrix alloy were brittle, a phenomenon attributed to the formation of zirconia particles at the interface.     

Evolution of manufacturing parameters in Al/Ni3Al composite powder formation using blending and mechanical milling processes

Aluminu–matrix composites produced by Ni₃Al intermetallic particles are increasingly used in aerospace and structural applications because of their outstanding properties. In manufacturing of metal–matrix composites using powder metallurgy blending and milling are important factors. They control the final distribution of reinforcement particles and porosity in green compacts which in turn, strongly affect the mechanical properties of the produced PM materials. This paper studies different conditions for producing composite powders with uniform dispersion of Ni₃Al particles in aluminum powders and improved physical and mechanical properties. The results indicated that an intermediate milling time for fabrication of composite powder, better than prolonged and shortened ones, causes better microstructure and properties. It was shown that addition of 5 vol.% Ni₃Al particles, produced by 15 h mechanical alloying to aluminum powders, and then 12 h blending operation provides an appropriate condition for producing Al–Ni₃Al composite powder.     

Structural evolution of the Ti70Ni15Al15 powders prepared by mechanical alloying

Amorphization and crystallization behaviors of Ti₇₀Ni₁₅Al₁₅ powders during mechanical alloying (MA) and subsequent heat treatments are studied. Amorphous phase that cannot be obtained in the rapidly quenched ribbon is formed in the powders after MA for 60 h. Upon continuous heating of the amorphous powders in DSC, two exothermic events are observed. The first exothermic event corresponds to the crystallization of the amorphous matrix into a supersaturated α-Ti phase of hexagonal close-packed structure. The growth kinetic of the α-Ti phase is sluggish, resulting in the formation of nanostructured α-Ti matrix. The second exothermic event corresponds to the solid state transformation of the meta-stable α-Ti into the equilibrium phases, Ti₂Ni and Ti₃Al. Using the amorphous powders, Ti-based bulk materials with novel microstructures can be developed for structural applications.     

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.     

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.     

Fabrication of Intermetallic Compounds by Spark Plasma Sintering

The characteristics of spark plasma sintering, a new method of powder processing, were investigated. Four systems of intermetallic compounds—Ti-Al, Ti-Al-Cr, Mo-Si, and Mo-Si-Nb—were fabricated, and the formation process of compounds, the formed phases, and the microstructure of samples were observed. During the sintering of all the compositions of mixed powders, most of the compounds were formed by combustion reaction which occurred at almost the same temperature as the conventional combustion reaction temperature. The fabricated samples were well densified, however, the relative densities of the Mo-Si samples were lower than the Ti-Al samples. Ultrasonic images show that no internal defects were found in any sample and the grain size became finer with the increase in the Cr content in the Ti-Al system and Nb content in the Mo-Si system. The formed phases of Ti:Al=1:1 composition samples were TiAl and Ti₃Al phases, and Ti-Al added Cr samples consisted of TiAl, Ti₃Al, Cr₂Al, and Cr₉Al₁₇ phases. The sample synthesized with Mo:Si=1:2 mixed powders had only MoSi₂ phases, and Mo-Si samples with added Nb consisted of four phases: MoSi₂ with a small amount of Mo₅Si₃ phases in the matrix and Nb₅Si₃ with unreacted Nb for dispersed phases.     

Effect of barothermal processing on the structure and properties of a WC-Ni<Subscript>3</Subscript>Al alloy

We have studied the effect of barothermal processing (BTP) on the properties of samples prepared by sintering mixtures of tungsten carbide and aluminum nickelide powders (92 vol % WC + 8 vol % Ni₃Al) in vacuum and hydrogen. X-ray diffraction data indicate that BTP influences the unit-cell parameters of the WC and improves the mechanical properties of the alloy. In particular, it increases its fracture toughness, bending strength, compressive strength, and fracture work. BTP at 1450°C and 150 MPa produces the most significant changes in the properties of the alloy.     

NiAl formation by annealing of infiltrated aluminium-nickel precursors

Samples fabricated by pressure-infiltration of nickel powders with molten aluminium were heat treated at 1200 °C under vacuum or argon isostatic pressure. Reaction and diffusion in the asinfiltrated samples, which contained nickel, aluminium and varying amounts of Ni₂Al₃ and Al₃Ni, resulted in the formation of NiAl as a principal phase with nickel and Ni₃Al as minor phases. All samples exhibited macroporosity due to the formation of an interconnected transient liquid phase during heat treatment. Vacuum-annealed samples also showed extensive Kirkendall porosity in the nickel phase, which was, however, pore-free in hot isostatically pressed samples due to compaction during reaction. Concentration profiles of aluminium in these nickel regions were measured and are in good agreement with predicted values.     

WC颗粒在堆焊过程中溶解机理的研究

研究了WC／Ni3Al表面强化功能复合材料的制备及WC颗粒的溶解机理。在堆焊过程中，WC／Ni3Al复合材料焊条中的WC直接溶解进入基体中，然后析出W2C，而非WC分解成W2C然后溶解。当焊条中含5％(质量分数)WC时，部分Al被氧化，WC溶解，析出W2C，形成碳化物包裹氧化物的球形复合析出物，基体转化成Nb(AlTi)C，形成碳化物包裹氧化物／金属间化合物的复合材料。随着WC含量增加，Al氧化减少。当焊条中的WC含量达到30％时，焊层中的Al不被氧化，表面层中的大部分WC颗粒溶解，析出针状W2C，形成碳化物／金属间化合物复合材料，耐磨性可达45钢的3倍以上。     

Simultaneous synthesis and densification of NiAl and Ni3Al by pressure-assisted combustion

Simultaneous synthesis and densification of NiAl and Ni₃Al from elemental powders of Ni and Al was investigated. Combustion synthesis was carried out under the uniaxial compressive pressure and preheating of die. It was shown that both the compressive pressure and preheating of die strongly affect the density, the grain size and the hardness of NiAl and Ni₃Al product. A high relative density of NiAl and Ni₃Al products as much as 99.8% was achieved under compressive pressure of 275 MPa and preheating temperature of 600°C.     

Phase quantification of impulse atomized Al<Subscript>68.5</Subscript>Ni<Subscript>31.5</Subscript> alloy

Powders of Al_68.5Ni_31.5 were produced using the impulse atomization technique. The molten droplets were cooled in-flight by the stagnant helium or nitrogen in the atomizing chamber, and the resulting powders were sieved into different size ranges. Scanning electron microscopy, X-ray diffraction, and neutron diffraction were used in order to study the microstructure and to quantify the phase fractions in the samples. The computer software GSAS was used to calculate the weight fraction of the existing phases, namely Al₃Ni₂, Al₃Ni, and Al, by the profile refinement method. X-ray micro-tomography and optical microscopy were used to study the porosity formation inside the particles. It was found that for particles having sizes decreasing from 925 to 256 μm (increasing cooling rate), the weight fraction of Al₃Ni and eutectic Al decreases while that of Al₃Ni₂ increases. Furthermore, the droplets formed at higher cooling rates yielded a lower volume fraction of porosity.     

Aluminization of nickel-formation of intermetallic phases and Ni2Al3 coatings

The occurrence and growth mechanisms of the various intermetallic phases of the Al-Ni system formed during pack aluminization of unalloyed nickel have been investigated with respect to the aluminium activity in the pack. Several types of coatings were obtained: (1) a Ni₂Al₃ coating formed by inward aluminium diffusion in a high activity cement of pure aluminium; (2) a Ni-rich NiAl coating formed by outward nickel diffusion in a low activity pack constituted by an Al-Ni alloy; (3) a mixed type of coating exhibiting the phases Ni₂Al₃, Al-rich NiAl, Ni-rich NiAl and Ni₃Al in four superposed layers, formed in a pack containing an Al-Cr alloy; (4) a high temperature, high activity type of coating formed above 950° C with an outer layer exhibiting a hypereutectic structure of NiAl₃ grains in a eutectic matrix due to precipitation from the liquid state. The optimum cementation conditions, for the production of maximum thickness and quality Ni₂Al₃ coatings were determined. The influence of surface reactivity and pack activity on the coating quality parameters was investigated.     

Microstructure and mechanical properties of Al-Si-Ni-Ce alloys prepared by gas-atomization spark plasma sintering and hot-extrusion

Al-Si-Ni-Ce alloys with the composition of Al_78.5Si₁₉Ni₂Ce_0.5, Al₇₆Si₁₉Ni₄Ce₁ and Al₇₃Si₁₉Ni₇Ce₁ were atomized and then sintered by using spark plasma method. The microstructure of the as-atomized powders, sintered and hot-extruded samples was analyzed. The influences of granularity and sintering parameters including time and temperature on the density of sintered alloy were also discussed. It is shown that the atomized powders are composed of Si, Al₁₁Ce₃, Al₃Ni and alpha Al. Tiny Al₃Ni particles precipitate from supersaturated matrix near the powder boundaries during SPS. Hot-extrusion process leads to the layer structure and more homogeneous distribution of precipitates. These alloys exhibit high comprehensive mechanical properties with combination of high Vicker's micro-hardness, moderate tensile properties and elongation, which provide a novel kind of promising engineering materials.     

In‐situ composite coating on powder metallurgy master alloy fabricated by vacuum hot‐pressing sintering technology

A material consisting of an in‐situ titanium carbide reinforced nickel‐aluminide (Ni₃Al) coating and a powder metallurgy master alloy was fabricated by vacuum hot‐pressing sintering technology. A metallurgical bonded, pores‐free interface between composite coating and powder metallurgy master alloy was formed at the sintering temperature of 1050 °C, pressure of 10^‐4 Pa and pressing pressure of 40 MPa. The phase, microstructure and wear behavior of composite coating were investigated. The results showed that polygonal titanium carbide particulates with various sizes were homogeneously distributed in nickel‐aluminide matrix. The sintering temperature, pressing pressure and heat from as‐reactions‐formed coating green compact facilitated the pore infiltration with transiently generated liquid phases and ensured the high‐intensity metallurgical bonding between composite coating and powder metallurgy master alloy. Due to the abnormal elevated‐temperature properties of nickel‐aluminide matrix, titanium carbide particulates reinforcement and the mechanically mixed layer protection, TiC/Ni₃Al‐coated parts demonstrated superior wear resistance and lower friction coefficient while compared with Ni₃Al‐coated parts and H13 steel.     

In situ fabrication of α-Al2O3 and Ni2Al3 reinforced aluminum matrix composites in an Al–Ni2O3 system

α-Al₂O₃ ceramic particles and Ni₂Al₃ intermetallic compound reinforced aluminum matrix composites were successfully fabricated via exothermic dispersion (XD) reaction in an Al–Ni₂O₃ system. Thermodynamic analysis indicated that the reaction between Al and Ni₂O₃ could occur spontaneously due to its negative Gibbs free energy. The reaction characteristic was discussed by using X-ray diffraction (XRD) method and differential scanning calorimetry (DSC) analysis. The results showed that the reactions of the Al–Ni₂O₃ system consisted of two steps as following: (1) the Al firstly reacted with Ni₂O₃ to form the stable α-Al₂O₃ particles and active Ni atoms; (2) the active Ni atoms further reacted with Al to form Ni₂Al₃. The values of activation energy of the two step reactions were around 457.3 and 282.4 kJ/mol, respectively. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) revealed that the Ni₂Al₃ blocks were uniformly distributed throughout the matrix, while the α-Al₂O₃ particles were slightly segregated in the matrix. The strength of the composite is controlled by the strength of Ni₂Al₃ phase, and the tensile strength and the elongation rate of the composite with 30 vol.% reinforcement volume fraction are 210 MPa and 8%, respectively.     

Weldability of Al4C3–Al composites via diffusion welding technique

In this study, Al–Al₄C₃ composites, produced by powder metallurgy in situ techniques, were joined by diffusion welding technique at 250 MPa pressure with various welding temperatures and durations. Microstructures and shear strengths of the joined areas were determined. Al powders were mixed with 2% carbon black and milled in a high energy ball mill (mechanical alloying) for up to 20 h. In order to obtain cylindrical blanks with 10 mm in diameter and 15 mm in height, powders were compacted in a single action press at 1000 MPa. Samples were sintered in Ar atmosphere at 650 °C and metal matrix composite (MMC) containing 8% Al₄C₃ particles were produced. Products were then joined to each other by using diffusion welding techniques. Scanning electron microscopy examination was carried out on the welded interfaces and shear tests were conducted to the sample interfaces to find out the effect of welding temperatures and duration on the weldability properties. It was found that high welding temperatures resulted in increase of both joined strength and shear properties. However, increase in welding duration did not make any detectable changes. Results indicated that MMC could be joined by diffusion welding technique successfully with the 88% strength of base material.     

The structure of consolidated rapidly solidified powders of Ni3Al

Rapidly solidified powders of stoichiometric Ni₃Al and Ni₃Al containing boron and titanium have each been consolidated by two distinct routes, namely hot extrusion or hot compaction. The resulting microstructures have been examined by optical microscopy, scanning electron microscopy and transmission electron microscopy. The consolidated material did not retain the non-equilibrium structure of the rapidly solidified powders. The addition of titanium and boron produced material with a larger final grain size, a decreased incidence of twinning and, in the extruded material, led to cracking. Although all processing was performed in inert atmospheres, oxides were present in all consolidated products.     

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti₃Al base material. And the Ti₃Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti₃Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni₃(Nb, Ti) + (Nb, Ti)Cr₂ and TiNi₃ phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.     

Reinforcements affect mechanical properties and wear behaviors of WC clad layer by gas tungsten arc welding

This work deals with the surface analysis, mechanical properties and wear performances of the clad layer, which is made from tungsten carbide (WC) powders on SKD61 die steel by the gas tungsten arc welding method. According to the experimental results, due to the high hardness and elastic modulus reinforcements (Fe₃W₃C and M₇C₃) existing in the WC clad layer, the WC clad specimen has excellent wear performance at different sliding speeds.According to the wear analysis, wear behaviors of the WC clad layer are two-body abrasion and oxidation wear. In addition, oxidation wear dominates the wear behaviors of the SKD61 die steel specimen at different sliding speeds.     

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.