Processing, microstructure, and mechanical properties of cast In-Situ Al(Mg, Ti)-Al2O3(TiO2) composite

In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by dispersion of externally added titanium dioxide (TiO₂) particles in molten aluminum at different processing temperatures. Alumina particles (Al₂O₃) form in situ through chemical reaction of TiO₂ particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten aluminum and results in the formation of intermetallic phase Ti(Al_1−x ,Fe _x )₃ during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic phase Ti(Al_1−x ,Fe _x )₃, (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile stress more than the yield stress. In the cast in-situ composite containing 3.31 ± 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly higher rate than that predicted by the shear-lag model.     

Local electrochemical studies after heat treatment of stainless steel: Role of induced metallurgical and surface modifications on pitting triggering

The morphology, the chemical composition, and the pitting corrosion resistance of a resulfurized stainless steel heated at 1000 °C for 2 hours were investigated at the microscale using ex-situ (field-emission-scanning electron microscope/electron dispersion spectrometer (FE-SEM/EDS) and secondary ion mass spectroscopy (SIMS)) and in-situ (electrochemical microcell technique and in-situ atomic force microscope (AFM) techniques. Although microcracks, which may have a deleterious effect, exist, the formation of a compound (Cr,Mn)₂(O,S)₃ instead of MnS is responsible for the better pitting corrosion resistance of sites containing an inclusion. Local electrochemical measurements indicate that no pitting was detected on these sites below 800 mV/SCE (saturated calomel electrode), whereas stable pitting was observed at around 350 mV/SCE before heating. Micropores were detected on the highly oxidized grains in which the ionic activity was found to be more marked than on the remaining surface (determined using in-situ AFM). Local electrochemical measurements revealed that the presence of such defects reduces significantly the corrosion resistance of the metallic alloy in NaCl-based media.     

Role of large-scale slip in mode II fracture of bimaterial interface produced by diffusion bonding

Bimaterial interfaces present in diffusion-bonded (and in-situ) composites are often not flat interfaces. The unevenness of the interface can result not only from interface reaction products but also from long-range waviness associated with the surfaces of the component phases bonded together. Experimental studies aimed at determining interface mechanical properties generally ignore the departure in the local stress due to waviness and assume a theoretically flat interface. Furthermore, the commonly used testing methods involving superimposed tension often renders the interface so extremely brittle that if microplastic effects were present it becomes impossible to perceive them. This article examines the role of waviness of the interface and microplastic effects on crack initiation. To do this, a test was selected that provides significant stability against crack growth by superimposing compressive stresses. Mode II interface fracture was studied for NiAl/Mo model laminates using a recently developed asymmetrically loaded shear (ALS) interface shear test. The ALS test may be viewed as opposite of the laminate bend test. In the bend test, shear at the interface is created via tension on one surface of the bend, while in the ALS test, shear is created by compression on one side of the interface relative to the other. Normal to the interface, near the crack tip, an initially compressive state is replaced by slight tension due to Poisson’s expansion of the unbonded part of the compressed beam.     

A Developed Method for Fabricating <Emphasis Type="Italic">In-Situ</Emphasis> TiC<Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>/Mg Composites by Using Quick Preheating Treatment and Ultrasonic Vibration

Quick preheating treatment of the Al-Ti-C pellets and high-intensity ultrasonic vibration are introduced in the fabrication of in-situ TiC _p /Mg composites. Al-Ti-C pellets are preheated for about 130 seconds in the furnace at 1023 K (750 °C), in which magnesium is melted as well. In this process, plenty of heat can be accumulated due to the reactive diffusion between liquid aluminum and solid titanium in Al-Ti-C, and a small amount of Al₃Ti phase is formed as well. After adding the preheated Al-Ti-C into the molten magnesium, thermal explosion takes place in a few seconds. In the meantime, high-intensity ultrasonic vibration is applied into the melt to disperse in-situ formed TiC particles into the matrix and degas the melt as well. Microstructural characterization indicates that in-situ formed TiC particles are spherical in morphology and smaller than 2 μm in size. Furthermore, a homogeneous microstructure with low porosity of the magnesium composite is obtained due to the effect of ultrasonic vibration. A novel approach using the quick preheating treatment technique and high-intensity ultrasonic vibration to synthesize in-situ TiC _p /Mg composites is proposed in our research.     

Characterization and tribological behavior of cast In-Situ Al (Mg,Mo)-Al2O3 (MoO3) composite

Aluminum alloy—based cast in-situ composite has been synthesized by dispersion of externally added molybdenum trioxide particles (MoO₃) in molten aluminum at the processing temperature of 850 °C. During processing, the displacement reaction between molten aluminum and MoO₃ particles results in formation of alumina particles in situ and also releases molybdenum into molten aluminum. A part of this molybdenum forms solid solution with aluminum and the remaining part reacts with aluminum to form intermetallic phase Mo(Al_1−x Fe _x )₁₂ of different morphologies. Magnesium (Mg) is added to the melt in order to help wetting of alumina particles generated in situ, by oxidation of molten aluminum by molybdenum trioxide, and helps to retain these particles inside the melt. The mechanical properties of the cast in-situ composite, as indicated by ultimate tensile stress, yield stress, percentage elongation, and hardness, are relatively higher than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The wear and friction of the resulting cast in-situ Al(Mg,Mo)-Al₂O₃(MoO₃) composites have been investigated using a pin-on-disc wear testing machine under dry sliding conditions at different normal loads of 9.8N, 14.7N, 19.6N, 24.5N, 29.4N, 34.3N, and 39.2 N and a constant sliding speed of 1.05 m/s. The results of the current investigation indicate that the cumulative volume loss and wear rate of cast in-situ composites are significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy, under similar load and sliding conditions. Beyond about 30 to 35 N loads, there appears to be a higher rate of increase in the wear rate in the cast in-situ composite as well as in cast commercial aluminum and cast Al-Mo alloy. For a given normal load, the coefficient of friction of cast in-situ composite is significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The coefficient of friction of cast in-situ composite increases gradually with increasing normal load while those observed in cast commercial aluminum or in cast Al-Mo alloy remain more or less the same. Beyond a critical normal load of about 30 to 35 N, the coefficient of friction decreases with increasing normal load in all the three materials.     

In vitro bonding of adhesive resin to dental alloys and dentine

The purpose of this in vitro investigation was to compare the shear bond strength of sandblasted, tin-plated and metal primed Type II gold alloy bonded to Ni-Cr alloy, as well as sandblasted Type II gold bonded to dentine, using two different adhesive resin cements. All bonding surfaces were treated with C&B-Metabond (Parkell, CB) or Imperva Dual (Shofu, ID), according to the manufacturers' instructions. In all, 20 sandblasted, 20 tin-plated and 20 metal-primed gold cylinders were bonded to Ni-Cr, and 20 sandblasted gold cylinders bonded to dentine. Bonds were stressed to failure using a shear load in an Instron testing machine. Data were calculated, statistically analysed (ANOVA and Student-t-test), and the fracture sites examined in a SEM. The CB and ID systems demonstrated significantly higher bond strengths (p < 0.01) when the gold was tin-plated, but CB demonstrated significantly lower bond strengths (p < 0.01) when the gold was pre-treated with metal primer. CB always demonstrated significantly higher metal (p < 0.01) and dentine (p < 0.05) bond strengths than ID.     

Application of electromagnetic separation of phases in alloy melt to produce <Emphasis Type="Italic">In-situ</Emphasis> surface and functionally gradient composites

The principle of electromagnetic separation of phases (primary phase) in alloy melt is that the electromagnetic force scarcely acts on the primary phases due to its low electric conductivity as compared to the melt. As a result, a repulsive force acts on the primary iron-rich phases to push them to move in the direction opposite to that of the electromagnetic force. The in-situ surface composite and the functionally gradient composite reinforced by primary Si are produced when the hypereutectic Al-Si alloy solidifies under electromagnetic force induced by static magnetic field and DC current. Similarly, the Al-Si-1.20 pct Fe-1.60 pct Mn alloy in-situ surface composite reinforced by primary iron-rich phase is produced. Based on this, a new method for production of in-situ multigradient composite with several layers, by electromagnetic separation of phases and directional solidification technique, is proposed.     

Microstructural and mechanical assessment of transient liquid phase bonded commercially pure titanium

Diffusion joining of commercially pure titanium was successfully prepared via transient liquid phase bonding in vacuum environment. The process was carried out using AMS 4772 silver-based filler alloy at 900–1000°C for various holding time under the vacuum of 6?×?10^?7?Torr. Optical and scanning electron microscopy equipped with an EDS analyzer was conducted for microstructural evaluations. Mechanical properties were also investigated by shear test, fractographic assessment and X-ray diffraction analyses. The tendency to achieve isothermally solidified joint increased by increasing bonding time. No sign of athermal solidification was detected of sample bonded at 1000°C for 90?min. Consequently, the bonding condition of a high quality joint was obtained. Elemental analyses revealed that filler alloy’s elements (Ag, Cu) distributed more uniformly in fully isothermal solidified bond, whereas the aggregation of these elements is considerable in athermally solidified bond. Shear test results represented that the highest shear strength attributed to the sample bonded in isothermal solidified condition (bonded at 1000°C for 90?min).     

Correlation Between Potentiodynamic Polarization Behaviors of Transient Liquid Phase Bonded 2205 Duplex Stainless Steel and the Bonding Constituents

In order to precisely evaluate the contribution of each bonding constituent to the pitting corrosion resistance of transient liquid phase (TLP) bonded 2205 duplex stainless steel (DSS), we have undertaken potentiodynamic polarization (PDP) and microstructural analytic measurements all across the TLP bonded area. The PDP results show that the pitting corrosion resistance of TLP bonded specimens is significantly affected by the presence of certain bonding constituents across the TLP bonded area. Electron microscopy analysis indicates that the formation of complex (Fe,Ni,Cr,Mo)₃P phosphide in the bonding zone (BZ) before the completion of isothermal solidification (IS) as well as the formation of P-rich sigma phase in the diffusion-affected zone (DAZ) following the completion of the IS provides the most preferential sites for the occurrence of pitting corrosion. The PDP results also confirm that the pitting potentials (E_pit) of the TLP bonded specimen before and after IS completion are, respectively, closer to the E_pit of the BZ and the E_pit of the DAZ rather than to those of other TLP BZs.

     

Processing and mechanical properties of AI2O3 fiber-reinforced NiAl composites

The mechanical properties of NiAl-matrix composites reinforced with 125-μm diameter single-crystal A1₂O₃ (sapphire) fibers have been examined over the temperature range of 300 to 1200 K. Composites were fabricated with either a strong or weak fiber-matrix interfacial bond strength. During fabrication, a fiber-matrix interaction occurred such that fibers extracted from the NiAl matrix were fragmented and significantly weaker than the as-received fibers. Tensile results of the weakly bonded composite demonstrated that the composite stiffness was greater than the monolithic at both 300 and 1200 K in spite of the weak bond. Room-temperature strengths of the composite were greater than that of the monolithic but below rule-of-mixture predictions (even when the degraded fiber strengths were accounted for). At 1200 K, the ultimate strength of the composite was inferior to that of the monolithic primarily because of the poor fiber properties. No tensile data was obtained on the strongly bonded material because of the occurrence of matrix cracking during fabrication. Primarily because of the fiber strength loss, sapphire-NiAl composite mechanical properties are inferior to conventional high-temperature materials such as superalloys and are currently unsuitable for structural applications.     

Delineating brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites

The fracture toughness of Nb-based in-situ composites typically decreases with increasing volume fractions of hard intermetallic phases, despite the presence of a ductile niobium solid-solution phase in the microstructure. For composites with a continuous intermetallic matrix, the fracture toughness can be more than double that of the monolithic intermetallics, but is still low in absolute terms, indicating that the solid-solution phase is not very effective in inducing ductile-phase toughening. The lack of enhancement of the fracture resistance appears to arise from an embrittlement effect instigated by the brittle phases in the microstructure, whose nondeformability results in a high plastic constraint acting on the ductile phase. In this article, an analytical model is developed for treating both brittle-phase embrittlement and ductile-phase toughening in terms of constituent properties and microstructural variables. The model is then used to (1) delineate brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites, and (2) design fracture-resistant in-situ composites based on Nb-Ti-Cr, Nb-Ti-Al, and Nb-Ti-Si systems.     

Intermetallic-reinforced light-metal matrix in-situ composites

Transition-metal trialuminide intermetallics such as Al₃Zr and Al₃Ti, having low densities and high elastic moduli, are good candidates for the in-situ reinforcement of light-metal matrices based on Al and Mg alloys. In this work, in-situ composites based on Al and Al-Mg matrices reinforced with an Al₃Zr intermetallic were successfully processed by conventional ingot metallurgy. The microstructural studies showed that “needle” or “feathery”-like particles of Al₃Zr phase, whose volume fraction increased with increasing concentration of Zr, were formed in the Al matrix in the investigated range of Zr contents from 0.9 to 11.6 at. pct. Properties of Al-Zr alloys were investigated as a function of volume fraction of Al₃Zr. It is shown that the density, hardness, and yield strength of the in-situ Al/Al₃Zr composites can be quite adequately described by the composite rule-of-mixtures (ROM) behavior. Alloying of a binary Al-2.4 at. pct Zr alloy with Mg up to ∼25 at. pct reduces profoundly its density and, additionally, strengthens the matrix by a Mg solid-solution strengthening mechanism.     

Modification of Cast Al-Mg<Subscript>2</Subscript>Si Metal Matrix Composite by Li

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in-situ prepared Al-15 pct Mg₂Si composite were investigated. Adding 0.3 pct Li reduced the average size of Mg₂Si primary particles from ~30 to ~6 μm. The effect of cooling rate was investigated by the use of a mold with different section thicknesses from 3 to 9 mm. The results show a refinement of primary particle size as a result of both Li additions and cooling rate increases, and their effects are additive. Similarly, both effects increased ultimate tensile stress (UTS) and elongation values. The thin sections show somewhat unexpectedly low and scattered tensile results attributed to the casting defects observed in fracture surfaces. The Li-modified alloy displays serrated yielding behavior that is not fully explained here. The refinement by Li and enhanced cooling rate is explained in terms of an analogy with the effect of Sr and cooling rate in Al-Si alloys, and is ultimately attributed to the effect of the alkali and alkaline earth metals deactivating oxide double films (bifilms) suspended in Al melts as favored substrates for intermetallics.     

Adhesion of a compomer to dental structures

The objective of this study was to evaluate the bond strength of a compomer to dental enamel, dentin, and cementum. Flat surfaces of these tissues were obtained from recently extracted human teeth. The different substrates were either treated with PSA (a primer and adhesive) or acid etched (35% phosphoric acid gel) and treated with PSA. Cylindrical specimens of compomer were then bonded to the substrates. Shear bond strength was determined after a 24-hour immersion in 37 degrees C water. Significant differences were found between both treatments on enamel, while none were found on dentin or cementum. The use of acid etchant on enamel as a surface-conditioning step previous to priming with PSA allowed a better bond between Dyract compomer and that substrate; acid etching was not particularly needed on dentin and cementum.     

<Emphasis Type="Italic">In-Situ</Emphasis> Fracture Observation and Fracture Toughness Analysis of Ni-Mn-Ga-Fe Ferromagnetic Shape Memory Alloys

The fracture property improvement of Ni-Mn-Ga-Fe ferromagnetic shape memory alloys containing ductile γ particles was explained by direct observation of microfracture processes using an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber. The Ni-Mn-Ga-Fe alloys contained a considerable amount of γ particles in β grains after the homogenization treatment at 1073 K to 1373 K (800 °C to 1100 °C). With increasing homogenization temperature, γ particles were coarsened and distributed homogeneously along β grain boundaries as well as inside β grains. According to the in-situ microfracture observation, γ particles effectively acted as blocking sites of crack propagation and provided the stable crack growth, which could be confirmed by the R-curve analysis. The increase in fracture resistance with increasing crack length improved overall fracture properties of the Ni-Mn-Ga-Fe alloys. This improvement could be explained by mechanisms of blocking of crack propagation and crack blunting and bridging.     

Hydride embrittlement and irradiation effects on the hoop mechanical properties of pressurized water reactor (PWR) and boiling-water reactor (BWR) ZIRCALOY cladding tubes: Part III. Mechanical behavior of hydride in stress-relieved annealed and recrystallized ZIRCALOYs at 20 °C and 300 °C

This series of articles on the hydride embrittlement effect in ZIRCALOYs consists of three parts. Part III deals with the mechanical properties of some hydrides in some ZIRCALOYs at two temperatures (20 °C and 300 °C). The damage and fracture mechanisms were investigated by SEM in-situ testing. Based on the scanning electron microscopy (SEM) in-situ observations and the mechanical modeling, the mechanical behavior of hydrides confined within a matrix was determined. The hydrides can undergo significant plastic deformation under certain conditions. They have higher strength than the surrounding ZIRCALOY matrix. The radiation effect on hydrided ZIRCALOYs and the combined effect were also discussed. Part I is concerned with the general influence of hydrides on the mechanical properties and damage and fracture mechanisms of ZIRCALOYs at two temperatures (20 °C and 300 °C) and for hydrogen content up to 4500 ppm. In Part II, the morphology and crystallographic structure of hydrides in two ZIRCALOYs are examined at different magnifications and by different techniques (SEM, transmission electron microscopy (TEM), and X-ray diffraction) as a function of their pre-heat treatment and their hydrogen content.     

Improving the fracture toughness of constituent phases and Nb-based <Emphasis Type="Italic">in-situ</Emphasis> composites by a computational alloy design approach

A computational alloy design approach has been used to identify a ductile matrix for Nb-based in-situ composites containing Ti, Hf, Cr, Si, and Ge additions. Candidate alloys in the form of cast buttons were fabricated by arc melting. Coupon specimens were prepared and heated treated to vary the microstructure. Backscattered electron (BSE) microscopy, quantitative metallography, energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were utilized to characterize the morphology, volume fraction, composition, and crystallography of individual phases in the microstructure. The fracture toughness of the composites was characterized by three-point bending and compact-tension techniques, while the fracture toughness of individual phases in the in-situ composites was determined by an indentation technique. The composition, crystallography, and volume fraction of individual phases were correlated with the fracture-toughness results to assess (1) the role of constituent properties in the overall fracture resistance of the composites and (2) the effectiveness of the computational design approach. The results indicated that the effects of alloy addition and plastic constraint on fracture toughness were reasonably predicted, but the conditions for relaxing plastic constraint to attain higher fracture toughness were not achieved.     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Experimental Investigation of the Influence of Moisture on the Bond Behavior of FRP to Concrete Interfaces

The effects of moisture on the initial and long-term bonding behavior of fiber reinforced polymer (FRP) sheets to concrete interfaces have been investigated by means of a two-year experimental exposure program. The research is focused on the effects of (1) moisture at the time of FRP installation, in this paper termed “construction moisture,” consisting of concrete substratum surface moisture and external air moisture; and (2) moisture, in this paper termed “service moisture,” which normally varies throughout the service life of concrete. Concrete beams with FRP bonded to their soffits were prepared. Before bonding, concrete substrates were preconditioned with different moisture contents and treated with different primers. The FRP bonded concrete beams were then cured under different humidity conditions before being subjected to combined wet/dry (WD) and thermal cycling regimes to accelerate the exposure effects. Adhesives with different elastic moduli were used to investigate the long-term durability of each adhesive when subjected to accelerated WD cycling. Pull-off tests and bending tests were conducted at the beginning of the cycling and then again after 8 months, 14 months, and 2 years of exposure so as to evaluate the tensile and shear performance of the FRP-to-concrete interfaces. It was found that the effect of the concrete substrate moisture content on short-term interfacial bond performance could be eliminated if an appropriate primer was used. All FRP-to-concrete bonded joints failed at the interface between the primer and concrete after exposure while those not exposed usually failed within the concrete substrate. After exposure to an environment of accelerated WD cycles, it was also found that the interfacial tensile bond strength degraded asymptotically with the exposure time while the flexural capacity of the FRP sheet bonded plain concrete beams even increased. The mechanism behind the above, which is an apparently contradictory phenomenon, is discussed.     

Effect of Sn Alloying on the Diffusion Bonding Behavior of Al-Mg-Si Alloys

Effect of Sn as an alloying element on the diffusion-bonding behavior of Al-Mg-Si alloy has been studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical testing of the diffusion-bonded joint. XRD results revealed the formation of Mg₂Sn and (Sn) phases during solidification following induction casting. DSC results showed local liquid (Sn) formation during the bonding process for Sn-containing alloys, where its amount was found to be increasing with the increasing Sn content. Results revealed that Sn addition leads to an increase in the bond shear strength of the diffusion-bonded joints and elimination of the irregularities formed on the bonded interface. Fractured surfaces showed that formation of (Sn) layer at the bonded interface causes the fracture to transform from the ductile to the mixed fracture mode.