An Investigation on the Tribological Behaviors of Al 6061–B4C Metal Matrix Composites

Protection of Metals and Physical Chemistry of Surfaces - Ceramic reinforcements are used in metal matrix composite materials to enhance the mechanical properties. Ceramic particles also improve...     

Microstructural and Mechanical Properties of TiCX–Ni3(Al,Ti)/Ni Functionally Graded Composites Fabricated from Ti3AlC2 and Ni Powders

Metals and Materials International - In this paper, a novel type of functionally graded material (FGM) was successfully fabricated from pure Ni and Ti3AlC2 powder mixtures by hot-press sintering...     

Oxidation Behavior of ZrB<Subscript>2</Subscript>–SiC Composites at 1650 °C

The oxidation behavior of ZrB₂–SiC composites in air was studied at 1650 °C. Diffusion-controlled oxidation kinetics were found for the composites studied. A parabolic rate constant of 1.2 × 10^?8 g² cm^?4 s^?1 was measured for ZrB₂–10 % SiC composite. A transition in the oxidation kinetics was observed for ZrB₂–30 % SiC composite with the initial parabolic rate constant being 1.3 × 10^?8 g² cm^?4 s^?1. After exposure for 60 min, the parabolic rate constant was found to be 0.3 × 10^?8 g² cm^?4 s^?1. A single ZrO₂-rich oxide layer was found in the oxide scale structure of ZrB₂–10 % SiC composite. On the other hand, three-layer oxide structures, namely SiO₂-rich top layer, followed by ZrO₂-rich oxide scale and SiC-depleted layer, were found for ZrB₂–30 % SiC composite. The outer layer in the oxide scale structure of ZrB₂–SiC composite was tapered with enhanced oxidation at the corners of the sample. Vortex formation during the viscous flow of B₂O₃–SiO₂–ZrO₂ liquid near the corners on the surface was proposed as the root cause for enhanced oxidation at the corners of the sample.     

Effect of Ni4Ti3 precipitation on martensitic transformation in Ti–Ni

Precipitation of Ni₄Ti₃ plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni₄Ti₃ precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni₄Ti₃ precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni₄Ti₃ precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.     

Pre-oxidation Effect on Oxidation Behavior of F91 in Carbon Dioxide at 550 °C

Upon exposure to CO₂ at 550 °C, F91 tends to form rapidly growing scales consisting of an outer Fe oxide and an inner Fe–Cr spinel oxide. A comparative study has been carried out between the pre-oxidized and non-pre-oxidized F91, to determine the influence of pre-oxidation upon the oxidation behavior of F91 in CO₂. Formation of a rapidly growing scale and carburization could be inhibited by a pre-oxidation treatment in air prior to oxidation in CO₂. Although during exposure to CO₂, a fast growing scale still would form locally, pre-oxidation changed its structure. Effects of pre-oxidation time on the oxidation resistance in CO₂ are investigated.     

Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H₂O was investigated. The oxidation kinetics of Ni and the alloys in air + H₂O were very similar, but the mass gains of Ni and each alloy were smaller in air + H₂O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al₂O₃ precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H₂O. The NiO scale formed in air + H₂O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H₂O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H₂O.     

Effect of Thermal Cycling Treatment on Microyield Behavior of Al_2O_3p/Al Composite

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Effect of Cr Content on the Compression Properties and Abrasive Wear Behavior of the High-Volume Fraction(TiC–TiB_2)/Cu Composites

The(TiC–TiB2)/Cu composites with 50 vol% Ti C–Ti B2 ceramic particles were successfully fabricated by the combustion synthesis and hot press consolidation in a Cu–Ti–B4C–Cr system. The effects of the Cr content on the microstructures, hardness, compression properties, and abrasive wear behaviors of the composites were investigated. The final products consist of only Cu, Ti C, and Ti B2 phases, and the ceramic particles are distributed uniformly in these composites. The size of the ceramic particles decreases with Cr addition. As the Cr content increases, the yield strength,ultimate compression strength, microhardness, and abrasive wear resistance of the composites increase, and the fracture strain decreases.     

Effect of Cobalt Content on the Oxidation and Corrosion Behavior of Ni–Fe-Based Superalloy for Ultra-Supercritical Boiler Applications

The oxidation and corrosion behavior of three model alloys with different cobalt contents (6–20 wt%) were investigated in static air and a simulated coal ash/gas environment at 750 °C. The model alloys follow a parabolic law approximately during the oxidation in static air. High cobalt level improves the oxidation resistance, however, without noticeable improvement in coal ash/gas corrosion resistance. The sample with the highest cobalt content grows the thinnest oxide layer and the fewest internal oxidation products in the oxidation test. Cobalt in the model alloys promotes the establishment of a protective chromium oxide scale during the oxidation test, but did not show much difference in restraining the inward diffusion of sulfur by increasing its content. The oxidation and corrosion products formed on the sample surface consist mainly of a protective chromium oxide film. Internal aluminum oxide particles have been found especially along the grain boundaries of the base alloy.     

Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and -Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this -Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this -Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.     

The Effect of Microstructural Evolution on Hardening Behavior of 2205 Stainless Steel in Long-Term Aging at 500 °C

The effect of microstructural evolution on hardening behavior of 2205 stainless steel in long-term aging at 500 °C was studied by optical microscope, scanning electron microscope, and transmission electron microscope. The results showed that the hardness of ferrite phase in matrix steadily increased with the aging time at the first stage of 4 months, presented a peak of hardness at about 5 months, and showed a downward trend for the aging time from 6 to 8 months, while the hardness of the austenitic phase remained constant. Analysis showed that the iron-rich α phase and the Cr-rich α′ phase generated by spinodal decomposition, Cr₂N precipitations, and Fe₂Mn (R-phase) were the main reasons for the generation of peak in hardness of ferrite phase. Further studies showed that some dislocation structure (changing with the aging time) in δ-ferrite of matrix is related to the microstructural evolution.     

Tensile properties of Ti3SiC2 in the 25–1300°C temperature range

Although significant progress has been achieved in understanding the mechanical behavior of bulk, polycrystalline Ti₃SiC₂ in compression and flexure, as far as we are aware there are no reports in the literature dealing with its mechanical response under tension. In this paper, we report on the functional dependence of the tensile response of fine-grained (3–5 μm) Ti₃SiC₂ samples on strain rates in the 25–1300°C temperature range. The tensile response of Ti₃SiC₂ is a strong function of strain rate and temperature. Increases in testing temperatures, and decreases in testing strain rates lead to large (≈25%) tensile plastic deformations. Strain-rate jump/drop tests and stress-jump creep tests confirm the high values for the strain-rate sensitivity coefficients (0.42–0.56) obtained from the tensile tests. These values are equal to, or greater than, the strain-rate sensitivity of most superplastic ceramics. The large strains to failure result primarily from a high degree of damage, not from a microstructure that remains self-similar throughout deformation (as in superplasticity). Another important distinction between superplasticity in ceramics and the deformation of Ti₃SiC₂ is that in the former the grains are typically about an order of magnitude smaller than the ones tested here.     

Effect of Al2O3 addition on properties of non-sintered SiC–Si3N4 composite refractory materials

Preparation of SiC–Si₃N₄ composite refractory materials without sintering entails only low energy consumption and incurs little cost compared with traditional preparation methods. This paper investigated the effect of Al₂O₃ addition on bulk density, apparent porosity, linear shrinkage and oxidation resistance of as-fabricated non-sintered SiC–Si₃N₄ composite refractory materials. Meanwhile, the compressive and flexural strengths both before and after heat treatment were analyzed. The mechanisms of oxidation resistance and cryolite resistance of the SiC–Si₃N₄ composite refractory materials are discussed. Increasing amounts of Al₂O₃ reduced linear shrinkage but increased oxidation resistance and cryolite resistance. Moreover, compressive and flexural strengths initially increased and then decreased, with maximum values achieved at an Al₂O₃ addition of 8% w/w.     

Effect of Al2O3 content on the synthesized products by the laser ignited self-propagating high-temperature synthesis reaction of Al2O3–Ti–C system

The effect of Al₂O₃ content on ignition temperature and combustion temperature, the phase composition, the density of the products and the grain size of TiC was investigated by self-propagating high-temperature synthesis reaction of Al₂O₃–Ti–C system. The results show ignition temperature increases and combustion temperature decreases with the increasing of Al₂O₃ content; the density of the products varies with Al₂O₃ content, TiC and Al₂O₃ are the two stable phases after SHS, TiC particle size decreases with the increasing of Al₂O₃ content, furthermore, the fracture type of the sintered specimens is a nearly completely intergranular mode.     

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo₃Si–Mo₅SiB₂ at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO₃ and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO₂ phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.     

Superb Strengthening Effect of Net-Like Distributed Amorphous Al2O3 on Creep Resistance of (B4C + Al2O3)/Al Neutron-Absorbing Materials

B₄C reinforced Al composites are widely used as neutron absorbing materials (NAMs) due to excellent neutron absorbing efficiency, however, such NAMs exhibit poor high-temperature properties. To meet the requirement for structure-function integration, NAMs with enhanced high-temperature mechanical properties are desired. In this work, a novel (B₄C + Al₂O₃)/Al NAM with netlike distribution of Al₂O₃ was fabricated by powder metallurgy method and subjected to high-temperature tensile creep test. It was shown that the creep resistance was enhanced by several orders of magnitude via the addition of only 2.1 vol.% netlike-distributed Al₂O₃. (B₄C + Al₂O₃)/Al exhibited high apparent stress exponents ranging from 16 to 25 and high apparent activation energy of 364 kJ/mol. The creep behaviour could be rationalized using the substructure-invariant model and its rupture behaviour could be described by the Dobes-Milicka equation.     

Effect of solution annealing on weldability of aged alloy 800/25Cr–35Ni steel dissimilar welds

Abstract

Dissimilar welding of the aged alloy 800 and the as cast 25Cr–35Ni (wt-%) heat resistant steel was investigated. Microstructures, mechanical properties and weldability of the dissimilar welds were characterised using optical microscopy, scanning electron microscopy and transition electron microscopy equipped by energy dispersive X-ray spectroscopy, and Varestraint test. Since such dissimilar welding was susceptible to crack formation in the heat affected zone of the aged part, the effects of a preweld solution annealing, heat input, interpass temperature and type of filler metal on the weldability of two alloys were investigated. It was found that during the solution treatment, the precipitates produced in the service stage were decomposed and that TiC was formed. In addition, tensile strength and hardness were reduced, but ductility and toughness increased. It was concluded that the most important step to improve weldability and to reduce cracking susceptibility was solution annealing. A suitable annealing treatment was then proposed. The best weldability was found under conditions of solution annealing, low heat input, low interpass temperature and using Inconel 82 or 617 for filler metals.     

Effect of Zn Additions on the Oxidation of Cu–2 at.% Al and Cu–4 at.% Al Alloys in 1 atm O2 at 800 °C

Four ternary Cu–Zn–Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 °C in 1 atm O₂, and their behavior has been compared with that of the corresponding binary Cu–Zn and Cu–Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu–2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu–5Zn and Cu–10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.     

Effect of Phase Composition on the Resistance of Hydrogen-Charged Ti – 6% Al Alloy to Compressive Strain

Effect of hydrogen (0.004 – 1.0%) on the ductility and strain resistance of titanium alloy Ti – 6% Al in a temperature range of 400 – 1050°C at a deformation rate of 5 × 10^{– 3}, 5 × 10^{– 2}, and 5 × 10^{– 1} sec^{– 1} is studied. The relation between the quantitative and qualitative phase compositions forming in the alloy as a result of its alloying with hydrogen and the main parameters of the deformation process are determined. Hardening and softening of the alloy in the single-phase - and -ranges and the double-phase ( + )-range are investigated. The temperature range of maximum ductility of hydrogen-charged alloy Ti – 6% Al is determined.     

Effect of Mg addition on mechanical and thermoelectrical properties of Al–Al2O3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al–Mg/5%Al₂O₃ nanocomposite with different Mg contents (0, 5%, 10% and 20%) produced by mechanical alloying were studied. Scanning electron microscopy analysis (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to characterize the produced powder. The results show that addition of Mg forms a predominant phase (Al–Mg solid solution). By increasing the mass fraction of Mg, the crystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutional sites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electrical conductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermal conductivity increases with the increase of Mg content.