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.     

Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 aluminium matrix composite

An Al-Mg-Si alloy matrix composite reinforced with 10 vol.% of alumina whiskers (Al₂O₃w) has been processed by powder metallurgy and investigated. The Al₂O₃w were produced as single crystal c-axis alpha-alumina fibres at pre-pilot scale via vapour-liquid-solid (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this is the first report of the utilization of whiskers of Al₂O₃ as reinforcing elements for Al alloys. Tensile tests have been performed on the composite at room and high temperatures. Results show that the AA6061 alloy reinforced with the as-produced Al₂O₃ whiskers has remarkably high mechanical properties at room temperature. This is attributed to the high quality of the Al₂O₃ single crystals and to the strong bonding attained between them and the 6061 alloy matrix.     

Effects of short mullite fibres on aging response of Al–4.5Cu based metal matrix composite

Abstract

Short mullite fibre reinforced Al–4.5Cu composite and its monolithic alloy have been produced by squeeze casting. The age hardening behaviour at various aging temperatures, aging precipitation characteristics and micromorphologies of the composite and the base alloy have been investigated by means of hardness measurement (HB), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), respectively. It is shown that the aged hardness of the reinforced composite is always higher than that of the unreinforced base alloy during the whole aging procedure and at the various temperatures, indicating that short mullite fibres can reinforce Al–4.5Cu binary alloy. The aging response of 3Al₂O₃.2SiO_2f/Al–4.5Cu composite is accelerated considerably by short mullite fibres, compared with Al–4.5Cu alloy, i.e. the precipitation of both θ″ and θ′ phases is apparently accelerated in the composite, based on DSC analysis and TEM examination. But Guinier–Preston zone formation is heavily suppressed in the reinforced composite owing to the high density of dislocations in the near vicinity of the fibre/matrix interface.     

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.     

Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel

The diffusion bonding of a Ni₃Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al₃Ni has been detected at the Ni₃Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni₃Al alloy has been observed.     

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.     

Aluminium based composites strengthened with metallic amorphous phase or ceramic (Al2O3) particles

Two methods were used to obtain amorphous aluminium alloy powder: gas atomization and melt spinning. The sprayed powder contained only a small amount of the amorphous phase and therefore bulk composites were prepared by hot pressing of aluminium powder with the 10% addition of ball milled melt spun ribbons of the Al₈₄Ni₆V₅Zr₅ alloy (numbers indicate at.%). The properties were compared with those of a composite containing a 10% addition of Al₂O₃ ceramic particles. Additionally, a composite based on 2618A Al alloy was prepared with the addition of the Al₈₄Ni₆V₅Zr₅ powder from the ribbons used as the strengthening phase. X-ray studies confirmed the presence of the amorphous phase with a small amount of aluminium solid solution in the melt spun ribbons. Differential Scanning Calorimetry (DSC) studies showed the start of the crystallization process of the amorphous ribbons at 437 °C. The composite samples were obtained in the process of uniaxial hot pressing in a vacuum at 380 °C, below the crystallization temperature of the amorphous phase. A uniform distribution of both metallic and ceramic strengthening phases was observed in the composites. The hardness of all the prepared composites was comparable and amounted to approximately 50 HV for those with the Al matrix and 120 HV for the ones with the 2618A alloy matrix. The composites showed a higher yield stress than the hot pressed aluminium or 2618A alloy. Scanning Electron Microscopy (SEM) studies after compression tests revealed that the propagation of cracks in the composites strengthened with the amorphous phase shows a different character than these with ceramic particles. In the composite strengthened with the Al₂O₃ particles cracks have the tendency to propagate at the interfaces of Al/ceramic particles more often than at the amorphous/Al interfaces.     

A high-resolution-electron-microscopy study of γ/γ′-α directionally solidified eutectic alloy

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into γ/γ′-α alloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of α fibres still display the Bain orientation relationship with the γ′/γ matrix. In a few cases, however, the so-called Nishiyama-Wasserman (NW) orientation relationship is found in specimens aged at 850 °C for 2000 h. Different microdomain structures of the γ phase, corresponding to different ageing temperatures, were revealed. Orthorhombic Ni₃Mo phase, with a size of tens of nanometres, was found to precipitate inside α fibres after ageing at both 850 and 650 °C. Occasionally, an γ′-Ni₃Al phase with lamellar twin structure was also found to coexist with Ni₃Mo precipitate inside the α fibres. The orientation relationships between the precipitates and the α fibres were determined. Energy dispersive X-ray analysis (EDX) showed that the precipitation of Ni₃Mo and Ni₃Al is due to solid solution of Ni and Al in the α fibres.     

Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites

Simultaneous measurements of mechanical damping, or internal friction (Q ^–1 ), and dynamic Young's modulus (E) were made near 80 kHz and at strain amplitudes () in the range 10^–8 to 10^–4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al₂O₃) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al₂O₃ and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT –80 M Pa° C^–1 ) was the same for both materials and the reduction in modulus of the Al/Al₂O₃ was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10^–5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al₂O₃ results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al₂O₃ specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10^–6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface.     

Effect of metal coating of reinforcements on the microstructure and mechanical properties of Al-Al2O3 nanocomposites

The effects of various methods of reinforcement modification on the microstructure and mechanical properties of Al–Al₂O₃ nanocomposites were investigated. Alumina nanoparticles were modified by electroless deposition of Cu, Ni and Co. Subsequently, aluminium matrix nanocomposites reinforced with uncoated and coated nanoparticles were produced by the stir casting method. The results of microstructural analysis showed improved wettability of coated nanoparticles in the molten aluminium alloy. Furthermore, coated nanoparticles exhibited a more desirable interface with the matrix and were homogenously distributed within it. The mechanical properties of the nanocomposites were improving significantly when coated nanoparticles were used as reinforcements. Among the reinforcement modification methods, Ni-coating was recognised as being more effective for improving the mechanical properties of Al–Al₂O₃ nanocomposites.     

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.     

Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.     

Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

Formation of nickel aluminide/nickel hybrid coatings on alloy steels by two step process of nickel plating and low temperature pack aluminisation

Abstract

The present study investigates the conditions required for forming a hybrid coating consisting of an outer nickel aluminide layer and an inner nickel layer on alloy steels. A commercial alloy steel of 9Cr–1Mo was used as a substrate. Electroless and electronickel plating processes were used to form an initial nickel layer on the steel. The AlCl₃ activated packs containing pure Al as a depositing source were then used to aluminise the nickel deposit at temperatures ≤650°C. The effect of phosphorus or boron content in the initial nickel layer deposited with the electroless nickel plating solutions using hypophosphite or boron–hydrogen compound as reducing agent was investigated in relation to the spallation tendency of the coating either immediately after the aluminising process or during the thermal annealing post-aluminising process. Under the aluminising conditions used, the outer nickel aluminide layer formed was Ni₂Al₃. For the electroplated nickel deposit, the growth kinetics of the outer Ni₂Al₃ layer during the pack aluminising process was found to obey the parabolic rate law with a parabolic rate constant being 12·67 μm at 650°C for 2 wt-%AlCl₃ activated pack containing 4 wt-% pure Al as a deposition source.     

Kohlenstoff-faserverstärktes Al2O3 und Mullit

Carbon Fibre Reinforced Al₂O₃ and Mullite The hydrolysis process from metalalkoxides has been investigated to prepare carbon fibre reinforced Al₂O₃ and mullite composites. Carbon fibres (SIGRAFIL HF with 10,000 monofilaments) were im pregnated with metalalkoxides (Al-tri-sec-butylate to form Al(OH)₃, mixtures of tetramethoxy-silan and Al-tri-sec-butylate to form mullite), hydrated in air and hot-pressed in a graphite mold at about 1700 °C, 10 min. Beside this also the preparation of composites with chopped fibres is described. The mechanical properties were measured in a three point bending test. Composite material with more than 10 vol% fibres show plastic deformation with matrix cracking during testing. For Al₂O₃ an additional SiC-interlayer (0.5–1 μm SiC by a CVD-process from CH₃SiCl₃ and H₂ at 1280 °C) is necessary to obtain a bonding between fibre and matrix. Only in this case with fibre content higher 10% it is possible to increase the fexural strength of Al₂O₃. The strength values lie on a line with 60% of the fibre strength. The rule of mixture cannot be applied for Al₂O₃ as matrix. Low fibre contents decrease the flexural strength. It appeares that in this case the Al₂O₃ matrix is damaged before testing. But with mullite as matrix the measured values of flexural strength lie on a line which is 65% of the rule of mixture. This is caused by a relative good bonding between fibre and matrix and is independend of an additional SiC-interlayer. The Young-modulus of Al₂O₃ is drastically decreased with 10% carbon fibre content (for mullite matrix: 28 vol%). It was possible to calculate the beginning of the formation of cracks in the Al₂O₃ matrix caused by the mismatch of the components. This value does agree with the measured decrease of Young-modulus. The toughness of both materials measured by the critical intensity factor K_ICV (for a composite material) with notched speciments (0.1 mm notch width) is increased with fibre content. (These K_ICV-values are a function of notch length and width in a range of the probable error and therefore the uncorrected values are considered). For Al₂O₃ matrix the K_ICV-values increase only if SiC-scoated fibres are used. For mullite as a matrix K_ICV increases with fibre content for both of the SiC coated and uncoated fibre composites. With higher fibre volume than 50% the values decrease because the interspace between the fibres is not filled with matrix. The increased K_ICV for these composites compared to that of the matrix alone can be explaned by energy absorption at the fibre-matrix interface. In the case of mullite matrix this mechanical behaviour is caused by a good bonding between the components. Therefore the fracture energy of these mullite composites is drastically increased with fibre content. This means that a strong mullite material with good toughness is formed. These properties are a function of the preparation method. The hydrolysis process formes a very fine grain starting material, which formes a relative dense mullite by reaction sintering between the carbon fibre monofilaments. During this reaction also a good bonding between the fibres and the matrix is formed.     

Compatibility between alumina fibres and aluminium

An investigation is carried out on the interfacial wetting behaviour and reactions between aluminium and alumina fibres (85mass% Al₂O₃ and 15mass% SiO₂). Aluminium is coated onto alumina fibres by a vacuum evaporation technique and the surface of the fully coated fibres and the edge of the partially coated fibres are examined by scanning electron microscope after heat treatments at various temperatures. Within a temperature regime between 943 and 1273 K, occurrence of such interfacial reactions as 4Al(I) + Al₂O₃(s) 3Al₂O₃(g) and 4Al(I) + 3SiO₂(s) 2Al₂O₃(s) + 3Si(s) are detected. It is found that molten aluminium can cover the alumina fibre surface but it peels off near the edge of the coating film on a partially coated fibre, showing the very weak interface cohesion. This is ascribed to the lack of a stable compound formation at the interface. Results of tensile test show that the strength of the coated fibres is degraded after heat-treating at above the melting point of aluminium. The culprits for the tensile failure of alumina fibres are evaluated by the Weibull distribution theory.     

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.     

A phenomenological model for lifetime design of Ni2Al3/Ni hybrid coating formed on creep resistant ferritic steels

The phase layer transformation kinetics in the Ni₂Al₃/Ni hybrid coating formed on creep resistant steel P92 has been studied via a series of prolonged isothermal annealing experiments at 650 °C. All the intermediate phase layers of NiAl, Ni₅Al₃ and Ni₃Al formed in the coating by interdiffusion during isothermal annealing process. The phase layers of NiAl and Ni₃Al formed at the very beginning of isothermal annealing at the interface between Ni₂Al₃ and Ni, but the Ni₅Al₃ phase layer formed at the interface between the NiAl and Ni₃Al phase layer only at an annealing time at which the outer Ni₂Al₃ phase layer was completely consumed. The growth and consumption of the Ni₂Al₃ and NiAl phase layers and the growth of the Ni₃Al₅ phase layer were all parabolic, but the growth of the Ni₃Al phase layer obeyed the power rate law d = kt ^1/n. The growth kinetics of an intermediate phase layer was found to be faster than the kinetics of its subsequent consumption. The rate constants in both the growth and consumption kinetics need to be determined for each of the intermediate phase layers at a particular temperature. The lifetime of the coating with an outer Ni₂Al₃ phase layer of any specified initial thickness and a sufficiently thick inner Ni layer can then be estimated using the lifetime design model delineated in this study.     

Deformation behaviour and microstructure of a 20% Al2O3 reinforced 6061 Al composite

Deformation and microstructural behaviours of a 20% (volume percent) particle reinforced 6061 Al matrix composite have been studied by torsion from 25 to 540°C with strain rates of 0.1, 1 and 5 s⁻¹. The logarithmic stress versus reciprocal temperature relationship exhibits two slopes indicating different deformation mechanisms. The 20% Al₂O₃/6061 Al composite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal much smaller subgrain size and higher volume of non-cellular substructures, as well as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the 10% Al₂O₃/6061 Al composite and matrix alloy the same test condition. The torsion fracture surface was studied and compared to the three point bending failure specimens.     

Microstructure and bond strength of Ni–Cr steel/Al2O3 joints brazed with Ag–Cu–Zr alloys containing Sn or Al

Abstract

Sintered Al₂O₃ was joined to Ni–Cr steel by the active metal brazing route with Ag–Cu–Zr brazing alloys containing Sn or Al. A single ZrO₂ layer with a monoclinic structure was formed at the Al₂O₃ /brazement interface by the migration of Zr in the molten brazing alloy to the Al₂O₃ surface, followed by a redox reaction between the Al₂O₃ and Zr. The remainder of the brazement formed a Cu–Ag eutectic alloy. Precipitates CuZr₂ and Cu–Zr–Al were formed in the brazements of the Ni–Cr steel/ Al₂O₃ joints brazed with Ag–Cu–Zr alloys and Al containing Ag–Cu–Zr alloys, respectively. On the other hand, no precipitates were formed in the brazement of the Ni–Cr steel/Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys. The Ni–Cr steel/ Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys showed much higher fracture shear strengths than those brazed with Ag–Cu–Zr alloys or Al containing Ag–Cu–Zr alloys.