Oxidation behavior of single-crystal Al2O3-fiber-reinforced Ni3Al-based composites

A series of single-crystal Al₂O₃-fiber-reinforced Ni3Al-based intermetallic matrix composites were fabricated by pressure casting. The matrices employed were binary Ni₃Al, Ni₃Al-0.5 at. pct Cr, and Ni₃Al-0.34 at. pct Zr. The development of microstructure upon oxidation in air at either 1100 °C or 1200 °C was investigated by optical, scanning, and transmission electron microscopy. In air-oxidized binary Ni₃Al, some of the fibers were fully or partially covered with a layer of oxide. A weak fiber/matrix bond in this system, which led to fiber debonding during composite processing, is believed to be responsible for the ingress of O into the composite and oxidation of the matrix in the debonded regions at the fiber/matrix interface. Addition of Cr to Ni₃Al resulted in an almost threefold increase in fiber/matrix bond strength. No oxidation of the interface was observed. A thick layer of oxide was formed around all the fibers when the composite was thermally cycled prior to isothermal annealing. Addition of Zr to Ni₃Al resulted in the formation of a layer of ZrO₂ on the surface of the fibers during composite processing. The ZrO₂ layer provided a fast path for the diffusion of O, which led to the formation of a rootlike oxide structure around the fibers. The rootlike structure consisted of a network of Al₂O₃-covered ZrO₂.     

Diffusion in the nickel-rich part of the Ni−Al system at 1000° to 1300°C; Ni3Al layer growth,diffusion coefficients,and interface concentrations

The formation of the Ni₃Al layer in NiAl (55 at. pct Ni)-pure Ni diffusion couples at temperatures above 1000°C has been found to be controlled almost completely by volume diffusion. At 1000°C and below, the relatively small grain size of the Ni₃Al compound in the layers caused such a large contribution from grain boundary diffusion, that the layer growth rates at 1000°C exceeded those at 1100°C and even those at 1200°C. In Ni₃Al (75at. pct Ni)-pure Ni diffusion couples the Ni₃Al compound rapidly converted into the solid solution of aluminum in nickel. Volume-diffusion coefficients calculated by the Boltzmann-Matano method yielded heats of activation of 55, 64, and 65 kcal·mol^?1 for NiAl, Ni₃Al and the solid solution of aluminum in nickel, respectively. In addition, eleven different types of diffusion couples were prepared from various Ni?Al alloys and annealed at 1000°C. Marker interface displacements and observations of porosity in these couples yielded a more detailed picture of the Kirkendall-effect than earlier work had done. The ratio of the intrinsic diffusion coefficients at the marker interface,D _NI/D _Al, is greater than one in the nickel-rich NiAl phase. For the Ni₃Al phase no statement can be made on the basis of this work. When the marker interface is located in the nickel solid solution,D _Ni/D _Al is smaller than one. The phase boundary concentrations in these couples did not show the expected deviation from the equilibrium concentrations in two-phase alloys; this finding is discussed with regard to the free-energycomposition diagram.     

Solidification of Highly Undercooled Hypereutectic Ni-Ni3B Alloy Melt

The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔT _p ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni₃B dendrite, rod eutectic, and precipitates was obtained. If ΔT _p < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni₃B and L → Ni/Ni₃B); if ΔT _p ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni₂₃B₆, Ni₂₃B₆ → Ni/Ni₃B, and L_r → Ni/Ni₃B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as L_r → Ni/Ni₃B; it also showed that this velocity decreases with increasing ΔT _r, which can be explained by reduction of the residual liquid fraction and decrease of Ni₂₃B₆ decomposition rate.     

A Study of the Effect of Nanosized Particles on Transient Liquid Phase Diffusion Bonding Al6061 Metal–Matrix Composite (MMC) Using Ni/Al2O3 Nanocomposite Interlayer

Transient liquid phase (TLP) diffusion bonding of Al-6061 containing 15 vol pct alumina particles was carried out at 873 K (600 °C) using electrodeposited nanocomposite coatings as the interlayer. Joint formation was attributed to the solid-state diffusion of Ni into the Al-6061 alloy followed by eutectic formation and isothermal solidification of the joint region. An examination of the joint region using an electron probe microanalyzer (EPMA), transmission electron microscopy (TEM), wavelength-dispersive spectroscopy (WDS), and X-ray diffraction (XRD) showed the formation of intermetallic phases such as Al₃Ni, Al₉FeNi, and Ni₃Si within the joint zone. The result indicated that the incorporation of 50 nm Al₂O₃ dispersions into the interlayer can be used to improve the joint significantly.     

Elevated temperature microstructural stability of Al+-ion implanted nickel

Composition profiles were measured following 180 keV Al⁺-ion implantation of nickel at temperatures up to 600°C, and following 600°C annealing of nickel implanted at 25°C. The phases present in the implanted layers were identified and are compared to those found on the NiAl phase diagram. The implanted compositions are nearly temperature independent at fluences below 1.5 × 10¹⁸ ions/cm². The Al concentration reaches 70 at.% Al by 3 x 10¹⁸ ions/cm² fluence delivered at 25°C, and it extends to 3000Å depth. Beyond 1.5 × 10¹⁸ ions/cm² fluence delivered at 500°C, an Al composition of 25% extends to 6000Å depth, well beyond the ~ 100Å increase in depth due to radiation-enhanced diffusion. Similar results obtain for 25°C implanted specimens subsequently annealed at 600°C. TEM examination shows the defect-enriched region beyond 3000Å recrystallizes at elevated temperatures and enhances diffusion via the high diffusivity paths provided by the grain boundaries. Phases found in elevated temperature specimens often differ from those in 25°C implanted specimens. Comparing the former with the latter: Ni₃Al replaces the extended portion of the Al in Ni f.c.c. solution; Ni₃Al and NiAl replace an h.c.p. phase peculiar to lower temperatures; and phases richer than 50% Al are replaced by Ni₃Al due to compositional instabilities. Phase prediction based on local chemical composition and the phase diagram is shown to be valid, but can proceed only when the compositional instabilities are taken into account.     

Phase equilibria of the ternary Al-Cu-Ni system and interfacial reactions of related systems at 800 °C

A series of Al-Cu-Ni alloys of various compositions were made and annealed at 800 °C. The equilibrium phases were studied by metallography, X-ray diffraction (XRD) analysis, and electron probe microanalysis. The isothermal section of the ternary Al-Cu-Ni system at 800 °C was then determined based on these experimental results and the available phase relationship knowledge of the three constituent binary systems. No ternary compound was found. All three phases, AlNi₃, AlNi, and Al₃Ni₂, have very high ternary solubility, especially the AlNi phase, which almost reaches the binary Al-Cu side. However, no continuous solid solution was formed between the AlNi phase and any of the binary Al-Cu phases. Interfacial reactions of Al/Ni, Al/Cu, Al-Cu/Ni, and Al-Ni/Cu at 800 °C were investigated by using reaction couple techniques. The results showed that Al₃Ni and Al₃Ni₂ phases were formed in the Al/Ni couples; β-AlCu₄, γ ₁-Al₄Cu₉, and ɛ ₂-Al₂Cu₃ phases were formed in the Al/Cu couples. As for the results in the Al-2 at. pct Ni/Cu, Al-5 at. pct Ni/Cu, and Al-2 at. pct Cu/Ni, Al-4.5 at. pct Cu/Ni, and Al-6 at. pct Cu/Ni were similar to those in the binary Al/Cu and Al/Ni couples, respectively. A different reaction path was found in the Al-7.5 at. pct Cu/Ni couples, and an AlNi solid solution layer was formed instead of the Al₃Ni and Al₃Ni₂ phases.     

Near-net-shape forming of Al-Al3Ni functionally graded material over eutectic melting temperature

A possibility to make near-net-shape functionally graded material (FGM) products has been examined. The FGM billets having a graded volume fraction of Al₃Ni in thickness direction were machined from an Al-Al₃Ni FGM thick-walled tube manufactured by a vacuum centrifugal method. Billets, which were set in the container for the backward extruding, were heated to 650 °C to 680 °C, at which temperature the FGM becomes a mixture of molten aluminum eutectic and solid intermetallics. Then, billets were extruded successfully to FGM cups by a semisolid forming, except at 650 °C. Residual bulky Al₃Ni particles are found at higher temperature. Thus, an optimum operation temperature is found to be around 660 °C, because bulky Al₃Ni particles transform to fine spheroidal or fibrous shape after the forming. The volume fraction of intermetallics at the bottom region of the cup was condensed more than 60 vol pct in a proper billet setting.     

Oxidation Processes for Alloys in the Ni – Zr System. Part 2. Phase Composition of Scale Formed on Ni7Zr2

We have used x-ray and metallographic layer-by-layer phase analysis to study the structure and composition of scale formed on the alloy Ni₇Zr₂ during its oxidation in air over a period of 1 h and 10 h in the temperature range 500-1200°C. In the scale we find NiO, the cubic and monoclinic modifications of ZrO₂, and also Ni and Ni₅Zr. The phase components are nonuniformly distributed over the thickness of the scale. The outer scale consists of the oxides NiO and ZrO₂, while the composition of the inner scale includes Ni and Ni₅Zr in addition to monoclinic ZrO₂. Cubic ZrO₂ is formed on the surface of the specimen in the initial stages of its oxidation at 500-700°C. For T ≥ 900°C, on the surface of the scale we find both modifications of ZrO₂, while the nickel phase is itself a solid solution Ni(Zr). We note that the mechanisms for the formation of low-temperature (T ≤ 800°C) and high-temperature (T ≥ 900°C) scales are different. It is hypothesized that these differences are determined mainly by the fact that at high temperatures, diffusion of zirconium ions toward the outer boundary of the scale is superimposed on diffusion of oxygen toward the scale – alloy boundary.     

Reaction Pathways of Nanocomposite Synthesized in-situ from Mechanical Activated Al–C–TiO<Subscript>2</Subscript> Powder Mixture

In-situ Al matrix composite was synthesized from Al–TiO₂–C powder mixtures using mechanical alloying and heat treatment, subsequently. The effect of ball milling on reaction processes of the resulting nanocomposite was investigated. The evaluation of powder mixture without mechanical activation showed that at 900°C aluminum reduced TiO₂, forming Al₃Ti and Al₂O₃. After 20 h mechanical activation of powder mixture, Al₃Ti and Al₂O₃ were fabricated. After that, by increasing milling time up to 30 h, no new phases formed. The DTA analysis of 30 h milled powder indicated two peaks after aluminum melting at 730 and 900°C. The XRD results confirmed that at 730°C, molten Al reacted with TiO₂ and C, forming Al₃Ti, Al₂O₃ and Al₄C₃. After that, at 900°C, Al₃Ti reacted with Al₄C₃, causing TiC formation. This results proposed that the TiC formation is associated by a series of reactions between intermediate products, Al₃Ti and Al₄C₃ and the resultant nanocomposite was successfully synthesized after 30 h milling and heated by DTA analysis up to 1200°C.     

Determination of the isothermal sections of the Al-Ni-Si ternary system at 750 °C and 850 °C

The phase equilibria of the Al-Ni-Si ternary system at 850 °C and 750 °C have been investigated using scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA). Isothermal sections at 850 °C and 750 °C were constructed based on experimental data from 53 alloys heat treated at 850 °C for 1200 hours and at 750 °C for 1440 hours, respectively. The phase equilibria among the following intermetallics and solid-solution phases are described: Ll₂-Ni₃(Al,Si), B2-NiAl, Ni₅Si₂, δ-Ni₂Si, ϑ-Ni₂Si(τ ₄), Ni₃Si₂, NiSi, NiSi₂, Ni₂Al₃, NiAl₃, Ni₂AlSi(τ ₂), Ni₃Al₆Si(τ ₃), Ni₁₆AlSi₉(τ ₅), the fcc solid solution, and the diamond (Si) phase. In addition, a phase, temporarily designated as Ni₅(Al,Si)₃(τ ₆), was observed for the first time at both 750 °C and 850 °C. This phase is probably the stabilization of Ni₅Al₃ by Si to higher temperatures than the binary Ni₅Al₃, which is only stable at <∼700 °C.     

Laser melting treatment by overlapping passes of preheated nickel electrodeposited coatings on Al−Si alloy

Microstructure improvements of a nickel electrodeposited Al−Si alloy were studied after high-power laser melting treatment through a single pass or partially overlapping successive adjacent passes. In some cases, laser melting treatment was preceded by a 5-hour heating of the specimens at 500°C in argon atmosphere furnace. Microstructure observations and microhardness measurements were carried out on the specimens before and after laser melting treatment with and without preheating. Best results concerning microhardness, microstructural homogeneity, and porosity elimination, as well as adhesion of the nickel coating on the Al−Si alloy, were achieved when the specimens were first subjected to heating at 500°C in an argon atmosphere furnace for 5 hours and then submitted to a laser melting treatment through successive adjacent laser passes with an overlapping rate of 70 pct. Microstructure studies were carried out employing X-ray diffraction analysis (XRD) and energy-dispersive spectrometry (EDS). AlNi and Al₃Ni phases were detected in the diffusion area which resulted from the 5-hour heating. AlNi, Al₃Ni, and Al₃Ni₂ phases were identified in the laser melted zones (LMZs). Each one of the above phase was found to be the main phase under different conditions.     

Effect of heat treatments on <Emphasis Type="Italic">In-Situ</Emphasis> Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl<Subscript>3</Subscript> composites produced from squeeze casting of TiO<Subscript>2</Subscript>/A356 composites

In-situ Al₂O₃/TiAl₃ intermetallic matrix composites were fabricated via squeeze casting of TiO₂/A356 composites heated in the temperature range from 700 °C to 780 °C for 2 hours. The phase transformation in TiO₂/A356 composites employing various heat-treatment temperatures (700 °C to 780 °C) was studied by means of differential thermal analysis (DTA), microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). From DTA, two exothermic peaks from 600 °C to 750 °C were found in the TiO₂/A356 composites. The XRD showed that Al₂O₃ and TiAl₃ were the primary products after heat treatment of the TiO₂/A356 composite. The fabrication of in-situ Al₂O₃/TiAl₃ composites required 33 vol pct TiO₂ in Al and heat treatment in the range from 750 °C to 780 °C. The hardness (HV) of the in-situ Al₂O₃/TiAl₃ composites (1000 HV) was superior to that of nonreacted TiO₂/A356 composites (200 HV). However, the bending strength decreased from 685 MPa for TiO₂/A356 composites to 250 MPa for Al₂O₃/TiAl₃ composites. It decreased rapidly because pores occurred during the formation of Al₂O₃ and TiAl₃. The activation energy of the formation of Al₂O₃ and TiAl₃ from TiO₂ and A356 was determined to be about 286 kJ/mole.     

Phase Selection in Undercooled Ni-3.3 Wt Pct B Alloy Melt

The change of eutectic solidification mode in undercooled Ni-3.3 wt pct B melt was studied by fluxing and cyclic superheating. The eutectic structure is mainly controlled by the undercooling for eutectic solidification, ΔT ₂, instead of ΔT ₁, the undercooling for primary solidification. At a small ?T ₂ [e.g., 56 K (56 °C)], the stable eutectic reaction (L → Ni₃B + Ni) occurs and the eutectic morphology consists of lamellar and anomalous eutectic; whereas at a larger ?T ₂ [≥140 K (140 °C)], the metastable eutectic reaction (L → Ni₂₃B₆ + Ni) occurs and the eutectic morphology consists of matrix, network boundary, and two kinds of dot phases. Further analysis declares that the regularly distributed dot phases with larger size come from the metastable eutectic transformation and are identified as α-Ni structure, whereas the irregularly distributed ones with smaller size are a product of the metastable decomposition and tend to have a similar structure to α-Ni as it grows. Calculation of the classical nucleation theory shows that the competitive nucleation between Ni₂₃B₆ and Ni₃B leads to a critical undercooling, ΔT ₂ ^* [125 K < ΔT ₂ ^* < 157 K (125 °C < ?T ₂ ^* < 157 °C)], for the metastable/stable eutectic formation.     

Effect of Bonding Temperature on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

An investigation was carried out on the solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material prepared in vacuum in the temperature range from 973 K to 1073 K (700 °C to 800 °C) in steps of 298 K (25 °C) using uniaxial compressive pressure of 3 MPa and 60 minutes as bonding time. Scanning electron microscopy images, in backscattered electron mode, had revealed existence of layerwise Ti-Ni-based intermetallics such as either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) diffusion zone was free from intermetallic phases for all joints processed. Chemical composition of the reaction layers was determined in atomic percentage by energy dispersive spectroscopy and confirmed by X-ray diffraction study. Room-temperature properties of the bonded joints were characterized using microhardness evaluation and tensile testing. The maximum hardness value of ~800 HV was observed at TiA/Ni interface for the bond processed at 1073 K (800 °C). The hardness value at Ni/SS interface for all the bonds was found to be ~330 HV. Maximum tensile strength of ~206 MPa along with ~2.9 pct ductility was obtained for the joint processed at 1023 K (750 °C). It was observed from the activation study that the diffusion rate at TiA/Ni interface is lesser than that at the Ni/SS interface. From microhardness profile, fractured surfaces and fracture path, it was demonstrated that failure of the joints was initiated and propagated apparently at the TiA/Ni interface near Ni₃Ti intermetallic phase.     

Thermal stability of electroless-nickel/solder interface: Part A. interfacial chemistry and microstructure

The thermal stability of the interface between the Sn-Pb eutectic alloy and an electroless Ni-P coating was examined by cross-sectional transmission electron microscopy (TEM). The interface was formed by reflowing the eutectic Sn-Pb solder alloy between electroless Ni-P deposits. The microchemistry and microstructure of the interface were analyzed in the as-reflowed, mild-aged, and overaged conditions, by energy-dispersive spectroscopy (EDS), selected-area electron diffraction (SAD) convergentbeam electron diffraction (CBED), and bright- and dark-field imaging. In the as-reflowed condition, the interfacial microstructure consisted of a thin Ni₃Sn₄ intermetallic layer and a thin P-rich layer. The P-rich layer was composed of two phases, face-centered cubic (fcc) Ni and Ni₃P, and the excess P was primarily due to the ejection of P from the electroless Ni-P when it reacted with Sn in the reflow process. Following the mild aging, a trilayer interfacial microstructure was found, including a coarsened Ni₃Sn₄ layer, a P-rich layer with increased P concentration, and a P-deficient layer. With overaging, a multilayer interfacial microstructure was developed, which consisted of two Ni-Sn intermetallic layers, Ni₃Sn₄ and Ni₃Sn₂, and three distinct P-rich layers, Ni₁₂P₅, Ni₁₂P₅ + Ni₃P, and Ni₃P + Ni.     

The influence of carbon on the interdiffusion of Mo and Ni

Experiments are carried out with Ni samples containing different amounts of carbon. The temperature range used is 800 to 1295°. In the beginning of the diffusion process a layer with a two-phase character is formed, for a high carbon content(i.e., 0.4 wt pct) at all temperatures and for a low carbon content (0.06 wt pct) below 1200°. The layer consists of the phases Mo₂C and an Ŋ-carbide Mo₅₄Ni₃₀C₁₈. Above 1000° this Ŋ-carbide is present also as a single-phase layer adjacent to this two-phase layer. After an incubation time a layer consisting of another Ŋ-carbide Mo₆M₆C is formed in the couples; this has a homogeneity range of only 0.5 at. pct Ni. There exists an intricate relationship between the two-phase layer and the layers of the pure Ŋ-carbides. After a very long annealing time at 1200°, σ-MoNi appears as a part of the Mo₆Ni₆C layer.     

Fabrication of Al2O3-reinforced Ni3Al composites by a novel in situ route

A novel in situ technique has been used to fabricate an Al₂O₃-reinforced Ni₃Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al₂O₃ fibers in a Ni₃Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al₂O₃ fiber by the Zr present in the matrix and the formation of a layer of ZrO₂ on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for 10 days. Air annealing led to the formation of networks of Al₂O₃ around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni₃Al particles in a matrix of disordered α-Ni(Al). The Al₂O₃ networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.     

Self-propagation Combustion Behavior with Varying Al/Ni Ratios in Compression-Bonded Ni-sputtered Al Foil Multilayers

Compression-bonded Ni-sputtered Al foil multilayers with various atomic ratios of Al/Ni are fabricated with various bilayer thicknesses. The microstructures that are formed after the self-propagating combustion using laser ignition result in equilibrium phases in the Al/Ni binary system. Homogeneous intermetallic compounds for Al₃Ni₂ and AlNi are obtained for the first time in the micrometer-scale multilayers through controlling the Ni layer thickness. The onset temperatures of the multilayers are below 800 K (527 °C) for all multilayer samples. The maximum temperatures correspond to the liquidus temperatures of the intermetallic compounds. The self-propagating direction is divided into a transverse propagating direction and a gross propagating direction. The measured gross propagation velocities vary widely without exhibiting a clear trend. However, the transverse propagation velocity is dependent on the measured maximum temperatures, while the effects of the bilayer thickness are not discernible. The measured transverse propagation velocities are similar to the reported propagation velocities for sputtered multilayers with similar bilayer thicknesses.