The effect of temperature, matrix alloying and substrate coatings on wettability and shear strength of Al/Al2O3 couples

A fresh approach has been advanced to examine in the Al/Al₂O₃ system the effects of temperature, alloying of Al with Ti or Sn, and Ti and Sn coatings on the substrate, on contact angles measured using a sessile-drop test, and on interface strength measured using a modified push-off test that allows shearing of solidified droplets with less than 90 deg contact angle. In the modified test, the solidified sessile-drop samples are bisected perpendicular to the drop/Al₂O₃ interface at the midplane of the contact circle to obtain samples that permit bond strength measurement by stress application to the flat surface of the bisected couple. The test results show that interface strength is strongly influenced by the wetting properties; low contact angles correspond to high interface strength, which also exhibits a strong temperature dependence. An increase in the wettability test temperature led to an increase in the interface strength in the low-temperature range where contact angles were large and wettability was poor. The room-temperature shear tests conducted on thermally cycled sessile-drop test specimens revealed the effect of chemically formed interfacial oxides; a weakening of the thermally cycled Al/Al₂O₃ interface was caused under the following conditions: (1) slow contact heating and short contact times in the wettability test, and (2) fast contact heating and longer contact times. The addition of 6 wt pct Ti or 7 wt pct Sn to Al only marginally influenced the contact angle and interfacial shear strength. However, Al₂O₃ substrates having thin (<1 μm) Ti coatings yielded relatively low contact angles and high bond strength, which appears to be related to the dissolution of the coating in Al and formation of a favorable interface structure.     

Experimental Investigation and optimization of Process Parameters for Shear Strength of Compound Cast Bimetallic Joints

Joining of A356 alloy and magnesium was carried out by vacuum assisted sand mold compound casting process. Microstructure at the joint interface was studied by using optical microscope, scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffractometer. Characterization indicated that a relatively uniform joint interface was obtained. The joint interface was composed of three distinct layers containing Mg₂Al₃ on aluminum side, Mg₁₇Al₁₂?+?δ eutectic structure on magnesium side and Mg₁₇Al₁₂ as middle layer. As a result of interaction between silicon, present in A356 with magnesium, Mg₂Si compound was formed. Push out test was conducted on electronics universal testing machine to measure the shear strength across the joint interface. The important process parameters (grit size of sand paper, insert temperature, pouring temperature and vacuum pressure) were optimized to maximize the shear strength. Optimization was carried out by using response surface methodology, desirability analysis and genetic algorithm (GA) techniques. It was observed that the shear strength increased by 14.21, 8.60 and 4.80% with genetic algorithm, desirability analysis and regression model respectively. GA reported the optimal value of shear strength.     

Dispersion strengthened aluminum made by mechanical alloying

Dispersion strengthened aluminum with excellent combinations of electrical conductivity and tensile strength at room and elevated temperatures has been produced by mechanical alloying. The strength levels, obtained with only about 2.75 to 5.4 vol pct dispersoid (Al₂O₃ plus carbon), equal or surpass those of conventionally produced SAP containing 11.5 vol pct Al₂O₃. The electrical conductivity is considerably higher than that of SAP with comparable strength. It is concluded that these superior properties are due to a finer, more equiaxed dispersoid and a better dispersoid distribution than found in conventionally produced SAP.     

Electrocapillarity in the aluminum reduction cell

The effect of the current density on the interfacial tension between aluminum and cryolite containing melts was measured based on the sessile drop method and an X-ray radiographic technique. The experiments were carried out under constant current densities in graphite crucibles with BN lining. When the aluminum drop was the cathode, the interfacial tension was almost independent of the current density. During electrolysis, the interfacial tension increased with decreasing NaF/AlF₃ ratio in a similar manner to that observed when no electrolysis was performed. The interfacial tension between aluminum and an electrolyte containing between 5 to 10 wt pct A1F₃, 5 wt pct CaF₂, and 5 wt pct A1₂O₃ is 690 ± 60 mN/m for cathodic current densities between 0.1 and 0.6 A/cm₂. Interruption of electrolysis caused an instantaneous decrease in the interfacial tension followed by a slow increase with time. This sudden drop together with a decrease in interfacial tension with reversal of cell polarity indicate that the metallic side of the interface has an excess positive charge. The interface was enriched with NaF during electrolysis as indicated by the slow recovery of the interfacial tension after current interruption. T. UTIGARD, formerly Graduate Student, is now Research Engineer, Alusuisse, CH-3965 Chippis, Switzerland     

Interaction of refractory compound nanoparticles with a surfactant in a nickel melt: II. Surface tension and density

The surface tension and density of Ni-S melts with Al₂O₃ or TiN nanoparticles are studied by the sessile drop method using a digital photographic camera and computer processing of images with special-purpose computer programs. The dependences of the surface tension and density of (Ni-S) + (Al₂O₃, TiN) melts on the temperature and the type of introduced refractory compound nanoparticles are determined, and the inversion of the temperature dependence of the surface tension of the Ni-S-Al₂O₃ system is detected. Metallographic analysis of polished sections demonstrates the presence of aluminum, nickel, and sulfur in nonmetallic inclusions at grain boundaries in the first series of experiments and the presence of titanium, nickel, and sulfur in globular nonmetallic inclusions in the second series of experiments.     

Wettability of silicon carbide ceramic by Al2O3/Dy2O3 and Al2O3/Yb2O3 systems

Wettability is an important phenomenon in the liquid phase sintering of silicon carbide (SiC) ceramics. This work involved a study of the wetting of SiC ceramics by two oxide systems, Al₂O₃ /Dy₂O₃ and Al₂O₃ /Yb₂O₃, which have so far not been studied for application in the sintering of SiC ceramics. Five mixtures of each system were prepared, with different compositions close to their respective eutectic ones. Samples of the mixtures were pressed into cylindrical specimens, which were placed on a SiC plate and subjected to temperatures above their melting points using a graphite resistance furnace. The behavior of the melted mixtures on the SiC plate was observed by means of an imaging system using a CCD camera and the sessile drop method was employed to determine the contact angle, the parameter that measures the degree of wettability. The results of variation in the contact angle as a function of temperature were plotted in graphic form which showed that the curves displayed a fast decline and good spreading. All the samples of the two systems presented final contact angles of 40° to 10° indicating their good wetting on SiC in the argon atmosphere. The melted/solidified area and interface between SiC and melted/solidified phase were evaluated by scanning electron microscopy (SEM) and their crystalline phases were identified by X-ray diffraction (DRX). The DRX analysis showed that Al₂O₃ and RE₂O₃ reacted and formed the Dy₃Al₅O₁₂ (DyAg) and Yb₃Al₅O₁₂ (YbAg) phases. The results indicated that the two systems had a promising potential as additives for the sintering of SiC ceramics.     

Relation between steel/slag interfacial tension and the sulphur absorption rate in dependence of the Si and Mo concentration

The relation between the interfacial tension at the steel/slag interface and the sulphur transport through the phase boundary has been investigated. The experiments have been carried out at 1550°C. The steel/slag system consisted of Armco iron in combination with a synthetic slag containing 40% CaO, 40% SiO₂ and 20% Al₂O₃. The controlled change of the interfacial tension was performed by additional alloying of the surface active element silicon and surface inactive molybdenum. The different influences of silicon and molybdenum on interfacial tension are directly reflected by the sulphur transport rate. Higher interfacial tensions generally cause a lower sulphur transport rate. A linear relationship was only obtained in presence of silicon.     

Partial Transient Liquid-Phase Bonding, Part I: A Novel Selection Procedure for Determining Ideal Interlayer Combinations, Validated Against Al2O3 PTLP Bonding Experience

Partial transient liquid-phase (PTLP) bonding is a bonding process that can bond hard-to-join materials, such as ceramics. The process uses a multi-layer interlayer composed of a thick refractory core and thin diffusant layers on each side. Upon heating, the diffusant material melts, and diffusion occurs until the liquid isothermally solidifies. Selecting interlayer materials is a key problem in producing strong, reliable PTLP bonds; materials are usually selected empirically or system by system. This article presents a novel selection procedure that provides a generalized, comprehensive, first-principles-based approach. Components of the selection procedure are linked directly to key characteristics of PTLP bonding. A filtering routine that provides structure for the selection procedure is summarized in this article and detailed in a companion article. Specific capabilities of the routine, such as non-symmetric bonds, add to its effectiveness in identifying additional PTLP bond candidates. By way of example, output from the selection procedure, in conjunction with sessile drop data, is used to analyze all Al₂O₃ PTLP bonds in the current literature. All analyzed bonds are included in various outputs from the selection procedure, validating its comprehensiveness. Also, Al₂O₃ PTLP bonds are analyzed as a whole, leading to the identification of important trends that result in increased bond strength. Finally, additional interlayer combinations for PTLP bonding of Al₂O₃ are presented based on output from the selection procedure and existing sessile drop data.     

Effect of exogenous refractory nanophases on the structural properties of nickel melts

The surface tension and density of nickel and Ni-S melts containing Al₂O₃ and TiN refractory-phase nanoparticles (RPNPs) are studied by the sessile drop method using a digital camera and computer processing of images. The dependences of the structural properties of Ni, Ni-(Al₂O₃,TiN), Ni-S, and Ni-S-(Al₂O₃,TiN) melts on temperature and the type and size of RPNPs are determined. It is found that the surface tension decreases with increasing temperature, the temperature dependence of the surface tension is inverted, the degree of loosening in the Ni-S-(Al₂O₃,TiN) melts decreases, and Ni-S-RPNP ensembles affect the melt structure.     

Ultra high vacuum diffusion bonded NbAl2O3 and CuAl2O3 joints—The role of welding temperature and sputter cleaning

Polycrystalline niobium and copper are welded in UHV (2 × 10⁻¹⁰mbar) to alumina (99.7 wt%) at various temperatures. Prior to joining the surfaces of the metal and ceramic to be welded are sputter-cleaned by argon ions at 3–5 keV with a maximum dose of 5 × 10¹⁹Ar⁺/cm². The amount of bonded area at the interface depend on welding temperature and welding time. After 1 h welding time the fraction of bonded area is 98% for NbAl₂O₃ joints at 0.65T_m (T_m = melting point of the metal in K). The amount of unbonded regions at the interface of CuAl₂O₃ joints decrease from 20% after 1 h welding time to 5% after 3 h welding time at 1010°C. Plastic flow and creep determine the pre closure mechanism at the metal-ceramic interface. Fracture energy G_c and the fracture resistance K_c of the UHV bonded metal-ceramic joints depend strongly on welding temperature and the amount of bonded area. For CuAl₂O₃ joints sputter-cleaning is a prerequisite for reliable measurements of the fracture energy. Without sputter-cleaning most of these joints did not withstand the cutting procedure during fabrication of four-point bend test beams. The fracture energy of NbAl₂O₃ joints manufactured without sputter-cleaning is low and can only be determined for joints welded above 1500°C. Impurities at the metal-ceramic interface are assumed to be responsible for the decrease in bond strength of joints manufactured without sputter-cleaning.     

Tensile behavior of AI2O3/FeAI + B and AI2O3/FeCrAIY composites

The feasibility of Al₂O₃/FeAl + B and Al₂O₃/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the single-crystal A12O3 fibers. The interfacial shear strengths were low for Al₂O₃/FeAl + B and moderate to high for Al₂O₃/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites. The strength of the A1₂O₃ fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al₂O₃/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in coefficient of thermal expansion (CTE). Development of the Al₂O₃/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing and elevated temperature service.     

Thermodynamics of oxygen solutions in the complex reduction of Fe-Co melts

Thermodynamic analysis of the complex reduction of metal melts is considered. The proposed analytical method identifies the influence of the weaker reducing agent in amplifying the effect of the stronger reagent. The curves of oxygen solubility pass through a minimum. Analysis of the extremal curves of oxygen concentration in the melt as a function of the content of reducing agents yields a formula for the content of the stronger reducing agent such that the oxygen concentration is minimal. Thermodynamic analysis of the combined influence of aluminum and silicon on the oxygen solubility in Fe-Co melts indicates that the reaction products may contain both mullite (3Al₂O₃ · 2SiO₂) and kyanite (Al₂O₃ · SiO₂). The presence of silicon in the melt intensifies the reducing action of aluminum: slightly when mullite is formed and significantly when kyanite is formed. When kyanite is formed, the curves of oxygen solubility pass through a minimum, whose position depends on the aluminum content in the melt but not on the silicon content. The aluminum content at the minimum declines slightly from iron to cobalt, as for Fe-Co-Al systems. Further addition of aluminum elevates the oxygen concentration. The formation of the compounds Al₂O₃, 3Al₂O₃ · 2SiO₂, Al₂O₃ · SiO₂, and SiO₂ is investigated as a function of the Al and Si content in the melt.     

Interfacial Molten Reactivity of Aluminum in Contact with (0001) Al2O3 and (111) Al2MgO4 Single Crystal Surfaces

The present work studies (0001) Al₂O₃ and (111) Al₂MgO₄ wetting with pure molten Al by the sessile drop technique from 1073 K to 1473 K (800 °C to 1200 °C) under Ar at PO₂ 10^?15 Pa. Al pure liquid wets a smooth and chemically homogeneous surface of an inert solid, the wetting driving force (t,T) can be readily studied when surface solid roughness increases in the system. Both crystals planes (0001) Al₂O₃ and (111) Al₂MgO₄ have crystallographic surfaces with identical O^?2 crystalline positions however considering Mg²⁺ content in Al₂MgO₄ structure may influence a reactive mode. Kinetic models results under similar experimental conditions show that Al wetting on (0001) Al₂O₃ is less reactive than (111) Al₂MgO₄, however at >1273 K (1000 °C) (0001) Al₂O₃ transformation occurs and a transition of wetting improves. The (111) Al₂MgO₄ and Al system promotes interface formations that slow its wetting process.     

Analysis of the complex deoxidation of carbon steel melts

The joint complex deoxidation of carbon steel melts is analyzed. A procedure is proposed to calculate the equilibrium oxygen concentration in a melt. Rail steel is used as an example to study the joint complex deoxidation of a melt by aluminum and silicon. Mullite (2Al₂O₃ · 3SiO₂) and kyanite (Al₂O₃ · SiO₂) are considered as the reaction products. Thermodynamic calculations demonstrate that the deoxidizing capacity of aluminum is increased in the presence of silicon in a melt. In this case, a substantial increase in the deoxidizing capacity in the concentration range 0.001–0.1 wt % Al is achieved when kyanite (Al₂O₃ · SiO₂) forms in the reaction products. The results of laboratory and industrial experiments on complex deoxidation are shown to agree well with the calculated data. These results demonstrate that the proposed calculation procedure can be recommended to determine the equilibrium oxygen concentration in a melt in the presence of several deoxidizing elements.     

Creep and fracture in model niobium-alumina laminates under shear loading

A simple experimental technique was developed to study the near-interface deformation and fracture behavior in ductile-phase-toughened brittle-matrix laminates subjected to elevated-temperature shear loading. In the study, specimens of Nb-foils bonded to Al₂O₃ blocks were subjected to shear loading parallel to the Nb/Al₂O₃ interfaces. The fracture path was controlled by the applied stress, the temperature and the thickness of the ductile Nb layers. At high shear stresses failure took place by brittle fracture within the Al₂O₃ phase with concurrent shear creep in the Nb, and multiple crack branching/arresting toward the interface. At lower stresses, shear-creep and ductile fracture within the Nb were the dominant damage modes. Shear deformation was found to localize along the mid-plane of the Nb, due to strengthening of the Nb adjacent to the interface via solid solution and precipitation resulting from interdiffusion. With thin 20 μm Nb-layers the fracture energy was low, similar to that found for pure Al₂O₃. Our findings suggest that the ductile-phase toughening of laminated brittle matrix composites depends critically on the thickness of the ductile phase. A concept of brittle-ductile transition to assist in the understanding of the toughness enhancement provided by ductile phase additions into a brittle matrix.     

Physicochemical aspects of technology of the Al2O3-Al layered cermet obtained using reaction sintering

Certain features of the technology of the Al₂O₃-Al layered cermet obtained by the reaction sintering (RS) in air of powdered billets that were fabricated by pressing the charge from the PAP-2 plateletshaped aluminum powder. The RS process was activated by the introduction of a dry residue of liquid glass (DRLG) into the charge (C = 3?C28%). According to the data of the X-ray phase analysis, the sintered material contains (vol %): Al (61?C82), ??-Al₂O₃ (8?C25), Na₂Si₂O₅ (2?C15), and Si (1?C14). Oxide phases and silicon are nanodimensional morphological objects (13?C100 ??m) in the layered aluminum host. The cermet density is 2.1?C2.45 g/cm³, tensile strength is 45?C90 MPa, and the ultimate bending strength is 320 MPa. The activation of RS by a small DRGL additive (3%) retains the layered cermet structure. It is leveled at a considerable increase in the RS time (up to 600 min) or C (up to 28%) because of recrystallization and involves plateletshaped aluminum particles in the chemical interaction.     

Effect of interfacial reaction on bending strength of Al18B4O33 whisker-reinforced aluminum composites

The present study deals with the mechanism of the interfacial reaction and the microstructure of the interface in aluminum borate whisker-reinforced pure aluminum matrix composites prepared by a squeeze casting process. By means of X-ray diffraction analysis (XRDA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM), it has been found that no interfacial reaction occurred under the as-cast condition, whereas aluminum reacted with the whiskers when the composite was reheated at a temperature higher than 726 °C. The reaction produced γ-Al₂O₃. Based on HRTEM observation, it is considered that the whiskers bond to the aluminum matrix directly after squeeze casting. The observation also shows that there is a specific orientation relationship between the reaction product and whisker, {002} Al₁₈B₄O₃₃ ‖ {220} γ-Al₂O₃, <200> Al₁₈B₄O₃₃ ‖ <111> γ-Al₂O₃, which leads to a coherent interface with a mismatch of less than 1 pct. The interfacial bonding state changed after heating under different conditions. Propagation of cracks in each interfacial state was observed by SEM, and the effects of the interfacial reaction on the bending strength were studied with the microstructure of the interface.     

Control of Non‐metallic Inclusions by Slag‐metal Reactions for High Strength Alloying Steels

A laboratory study was carried out to investigate non‐metallic inclusions in high strength alloying steel refined by high basicity slag. The results indicated that the inclusions were mainly of the CaO? MgO? Al₂O₃ system, Al₂O₃? MgO and MgO‐based inclusions. The steel/slag reaction time and Al₂O₃ content in slag had a great effect on inclusions characteristics. With the reaction time increasing from 30 to 180 minutes, inclusions experienced a transformation process: from mainly Al₂O₃? MgO system and MgO‐based inclusions to spherical CaO? MgO? Al₂O₃ system inclusions surrounded by a lower melting temperature surface layer of CaO? Al₂O₃. Formation and transformation mechanisms of the inclusions were given based on the results. It was also found that with Al₂O₃ content in slag reduced from 40% to 30%, [Mg] contents in steel melts were increased and MgO in slag reached saturation, which contributed to the formation of more MgO‐based inclusions and a more scattered inclusion composition distribution after 90 min reaction.