Sliding Wear Behavior of TiC-Reinforced Cu-4 wt.% Ni Matrix Composites

The present investigation explores the effect of TiC content on the sliding wear properties of Cu-4 wt.% Ni matrix composites. Cu-4 wt.% Ni ? x wt.% TiC (x = 0, 2, 4 and 8 wt.%) metal matrix composites were developed by powder metallurgy route. Their friction and wear was studied under dry sliding at different loads of 5, 7.5 and 10 N and constant sliding speed of 2 m/s using a pin-on-disk machine. The metallographic observations showed an almost uniform distribution of TiC particles in the matrix. Hardness of the composites increased with increasing TiC content (up to 4 wt.%). Friction and wear results of TiC-reinforced composites show better wear resistance than unreinforced matrix alloy. However, the optimum wear resistance was observed for 4 wt.% TiC-reinforced composites. Worn surfaces of specimens indicated the abrasion as the primary mechanism of wear in all the materials investigated in the study. The observed behavior has been explained on the basis of (1) the hardness which results in a decrease in real area of contact in composites containing TiC particles and (2) the formation of a transfer layer of wear debris on the surface of the composites which protects underlying substrate by inhibiting metal-metal contact.     

Structural and Tribological Properties of Nanostructured Supersonic Cold Sprayed Ni-20 wt.% Sn Coatings

80-μm-thick nanostructured coatings consisting of a Ni solid solution, Ni₃Sn, Ni₃Sn₂, and metastable NiSn intermetallic phases were deposited via supersonic cold spraying onto inconel 718 alloy substrates. These coatings have complex nanostructured metallurgical phases as revealed by transition electron microscopy, scanning electron microscopy, and x-ray diffraction techniques. Their mechanical properties were determined by nanoindentation measurements. Furthermore, the wear behavior of these nanostructured sprayed coatings was compared to the one of the industrial bulk or sprayed coated benchmark materials. It was found that the nanostructured coatings exhibit higher wear resistance than the industrial benchmarks, thanks to an appropriate balance of hard intermetallic phases and soft Ni matrix, as well as to their nanostructuring. Their frictional characteristics under reciprocating sliding are mainly determined by the formation of an oxide-based tribo-layer, which was analyzed by x-ray photoelectron spectroscopy. The role of intermetallic phases in these coatings on the friction and wear is also discussed.     

Fracture Characteristics of SiC(p) / Al Composites

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Experimental and Thermodynamic Exploration of the High-Temperature Microstructures of Co-30 wt.% Cr- (2.5-5.0 wt.%) C Alloys

Cobalt alloyed with high contents in Cr and C potentially may lead to hard alloys candidate for high temperature applications involving reduced wear degradations. The Co-30wt.%Cr-xC system was explored from x = 2.5 to 5 wt.% by both experiments and thermodynamic calculations. The stable microstructures at 1000, 1100 and 1200 °C and the temperatures of solidus and liquidus were of interest. When the carbon content is between 2.5 and 3.5 the alloys display (hypo-)eutectic microstructures composed of FCC Co-based matrix and M₇C₃ carbides. Between 4 and 5 wt.%C, the microstructures contain coarse pro-eutectic M₇C₃ carbides, leading to a total volume fraction of M₇C₃ close to 50% or higher. The C-richest alloys also contain graphite, instead cementite as predicted by calculations. A better agreement between experiments and calculations about microstructures and solidus temperature can be obtained by forbidding cementite in the calculation conditions. The obtained hardness level is about 650 Hv but decreases when the heat-treatment temperature increases.     

Hot Workability and Flow Characteristics of Aluminum-5 wt.% B4C Composite

Flow behavior of aluminum-5 wt.% boron carbide (Al-B₄C) composite was investigated by carrying out compression tests over a range of strain rates (10^?4-10⁰ s^?1) and temperatures (200-500 °C). The flow stress data obtained from these tests at true strain 0.5 were used to develop processing map. The stable and instable flow regimes in the map were characterized by the microstructural examination using Scanning Electron Microscopy and Electron Backscattered Diffraction. The optimum condition for processing of Al-5%B₄C composite was found to lie between 425 and 475 °C at the strain rate of around 10^?4 s^?1. A strain-compensated Sellars-McG Tegart constitutive equation was established to model high-temperature deformation behavior of the material.     

Hot Tearing of Sand Cast Mg-5 wt.% Y-4 wt.% RE （WE54） Alloy

Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented ＂CRC＂ equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE （WE54） alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction （fs） in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.     

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10^?3 s^?1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (ε). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when ε > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for ε < 0.4. For ε values between 0.4 and 0.7, observed occurrences of microstructural changes and severe neck formation lead to the supposition that there is a transition in the deformation mechanism. These changes are more evident when ε > 0.7 as another mechanism is thought to take over.     

Comparative Analysis of the Microstructural Features of 28 wt.% Cr Cast Iron Fabricated by Pulsed Plasma Deposition and Conventional Casting

The effect of pulsed plasma deposition (by an electrothermal axial plasma accelerator) followed by post-heat treatment on the structure and microhardness of a 28 wt.% Cr white cast iron is analyzed and discussed with respect to the microstructure of the conventionally cast monolithic counterpart. The cast iron (as deposited on a 14 wt.% Cr cast iron substrate) had a microhardness of 630-750 HV_0.05; it had layered light contrast/dark contrast structure where dark contrast layers contain fine carbide network. Pulsed plasma deposition followed by heat treatment resulted in a substantial refinement of the microstructure: eutectic M₇C₃ coarse acicular plates in the conventional cast iron were replaced by fine M₇C₃, M₃C₂, M₃C particles (Cr depleted in favor of Fe), while the initial carbide particle of 2-3 μm was reduced to 0.6 μm. Secondary dendrite arm spacing decreased from 15 to 1.3 μm, accordingly. The carbide volume fraction in the post-heat-treated coating remarkably increased with respect to the conventional counterpart resulting in a substantial increase in the coating hardness (1300-1750 HV_0.05). The heat-treated coating displayed higher resistance to three-body abrasion than the as-deposited coating and similar resistance with that of the conventionally cast iron.     

Effect of Ti3SiC2 Content on Tribological Behavior of Ni3Al Matrix Self-Lubricating Composites from 25 to 800 °C

The self-lubricating composites of Ni₃Al-Ti₃SiC₂-TiC-C (NMC) with varying Ti₃SiC₂ contents were fabricated by spark plasma sintering technique. Dry sliding pin-on-disc friction and wear tests of NMC against Si₃N₄ ceramic ball were undertaken at 25, 200, 400, 600, and 800 °C in air, respectively. The results showed that NMC with 15 wt.% Ti₃SiC₂ lubricant owned the excellent tribological properties over a wide temperature range from 25 to 800 °C, whose friction coefficients and wear rates were about 0.17-0.58 and 0.31-4.2 × 10^?5 mm³/N/m, respectively. A possible explanation for these results was that the subsurface microstructure self-refinement and the special stratification morphology of the tribo-layer were beneficial to the reduction of friction coefficient. Meanwhile, the protective action of the tribo-layer for the frictional surface could also decrease the wear rate.     

Electrochemical Corrosion Behavior of Magnetron-Sputtered Al-Mo Gradient-Coated Steel in 3.5 wt.% NaCl Solution

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion properties of the coating were studied in 3.5 wt.% NaCl solution using electrochemical techniques, whereas the hydrogen-induced cracking (HIC) resistance was examined by constant-load tests using notched tensile specimens. These results were compared with conventional electroplated cadmium-coated steel. The results show that the gradient Al-Mo coating exhibits better corrosion and HIC resistance when compared to electroplated cadmium. This was due to the excellent corrosion resistance of the bottom aluminum-rich layer, while the top Mo-rich layer provided good lubrication properties.     

Competitive Effect of Water Vapor and Oxygen on the Oxidation of Fe–5 wt.% Al Alloy at 1073 K

The effect of oxygen on the oxidation of Fe–5wt.% Al alloy was investigated at 1073 K in N2–12.2 vol.% H₂O, O₂–12.2 vol.% H₂O, and N₂–O₂–12.2 vol.% H₂O with various amounts of oxygen. The results showed S-shaped oxidation curves that consisted of three stages: slow-incubation, rapid transition, and relatively slow oxidation. The amount of oxidation increased with increasing oxygen contents up to 0.9 vol.% O₂ and then rapidly decreased. On the oxygen-rich side, a slow incubation oxidation stage was observed and its duration increased with increasing oxygen content. The extent of oxidation decreased gradually with decreasing oxygen content from the critical value and the incubation period disappeared. In the transient period, Fe₂O₃ was formed on the lean oxygen-content side and elongated voids were formed in the outer Fe₃O₄ and FeO layer. It was suggested that the differences in the morphology of Fe₂O₃ formed on the surface affected by the dissociation and gas-transport process due to differences in oxygen partial pressure at the gas–scale interface.     

Microstructure,Tensile and Fatigue Properties of Al–5 wt.%Mg Alloy Manufactured by Twin Roll Strip Casting

This study investigated the microstructure, tensile and fatigue properties of Al–5 wt.%Mg alloy manufactured by twin roll strip casting. Strips cast as a fabricated (F) specimen and a specimen heat treated (O) at 400 °C/5 h were produced and compared. In the F specimen, microstructural observation discovered clustered precipitates in the center area, while in the O specimen precipitates were relatively more evenly distributed. Al, Al₆(Mn, Fe), Mg₂Al₃ and Mg₂Si phases were observed. However, most of the Mg₂Al₃ phase in the heat-treated O specimen was dissolved. A room temperature tensile test measured yield strength of 177.7 MPa, ultimate tensile strength of 286.1 MPa and elongation of 11.1% in the F specimen and 167.7 MPa (YS), 301.5 MPa (UTS) and 24.6% (EL) in the O specimen. A high cycle fatigue test measured a fatigue limit of 145 MPa in the F specimen and 165 MPa in the O specimen, and the O specimen achieved greater fatigue properties in all fatigue stress conditions. The tensile and fatigue fracture surfaces of the above-mentioned specimens were observed, and this study attempted to investigate the tensile and fatigue deformation behavior of strip cast Al–5 wt.%Mg based on the findings.     

Heat Treatment of Closed-Cell A356 + 4 wt.%Cu + 2 wt.%Ca Foam and Its Effect on the Foam Mechanical Behavior

In this investigation, aluminum-silicon alloy foam is developed by adding certain amounts of copper and calcium elements in A356 alloy. Addition of 4 wt.%Cu + 2 wt.%Ca to the melt changed bubbles morphology from ellipsoid to spherical by decreasing Reynolds number and increasing Bond number. Compression behavior and energy absorption of the foams are assessed before and after aging. Solid solution treatment and aging lead to the best mechanical properties with 170% enhancement in yield strength and 185% improvement in energy absorption capacity as compared to non-heat-treated foams. The metallographic observations showed that bubbles geometry and structure in the A356 + 4wt.% Cu + 2 wt.%Ca foam are more homogeneous than the A356 foam.     

High Temperature Corrosion of a Pt-30 wt.% Rh Alloy in a Phosphorizing Gas

High temperature corrosion of a Pt-30 wt.% Rh alloy in a phosphorizing gas was isothermally investigated at 1285 K using a gas switching technique. Diffusion of P into the alloy created an outer layer of Pt-rich liquid and blocky (Pt, Rh)₂P precipitates along with an inner layer of fcc and (Pt, Rh)₂P plates in a cellular microstructure. Concentration profiles measured by SEM-WDS and EPMA across the layers at room temperature showed that there were three fcc phases: first was a 12 at.% Rh phase in the outer layer; second was a 37 at.% Rh phase in the cellular microstructure; and third was the initial 43 at.% Rh alloy. Also, the EPMA data registered approximately 0.1 at.% P in fcc of these layers. Based on the surrounding binary phase diagrams and the experimental data obtained in this study, a partial Pt-Rh-P phase diagram was constructed. A diffusion path for the corrosion microstructure was drawn on the partial phase diagram to help develop a step by step model for how the microstructure evolved. Growth kinetics of the inner layer were used to calculate a P diffusivity of about 10^?12 m²/s in the Pt-Rh alloy at 1285 K, suggesting rapid diffusion by either an interstitial or interstitialcy mechanism.     

Toughness Enhancement in Roll-Bonded Al6061-15 vol.% SiC Laminates via Controlled Interfacial Delamination

Researchers have examined different approaches to improve damage tolerance of discontinuously reinforced aluminum (DRA). In this study, three-layer DRA laminates containing two exterior layers of Al6061-15 vol.% SiCp and an interlayer of Al1050 were fabricated by hot roll bonding. Interfacial adhesion between the layers was controlled by means of rolling stain. The results of shear test revealed that, the bonding strength of laminates was influenced by number of rolling passes. Considering this effect, the role of interfacial bonding on the toughness of laminates was studied under three-point bending in the crack divider orientation. The quasi-static toughness of the laminates was greater than that of the monolithic DRA. Plastic deformation of the ductile interlayer and interfacial delamination were found as the major sources of energy absorption in this fracture process. It was shown that interfacial adhesion in these laminate does not alter the initiation energy in quasi-static test. Propagation energy under same loading condition, however, illustrated significant sensitivity to the interfacial bonding. The results of the current study reveal that improving the interfacial adhesion by means of rolling strain eliminates the ease of plastic deformation of the ductile interlayer and thus reduces the contribution of this mechanism in quasi-static toughness of the laminate.     

Erosion-Corrosion Behavior of Electroless Ni-P Coating on Copper-Nickel Alloy in 3.5 wt.% Sodium Chloride Solution

The present investigation concerns electroless Ni-P coatings prepared onto the copper-nickel alloy (CuNi 90/10) to enhance the erosion-corrosion resistance. X-ray diffraction technique (XRD), scanning electron microscopy, and electrochemical methods were utilized to study the heat treatment effects on the erosion-corrosion resistance and the physical properties of the coatings. The results indicated that both the as-plated and coating with post-heat treatment (PHT) showed much higher self-passivation ability and much lower current densities than that of the copper-nickel alloy (CuNi 90/10). XRD results showed that both the as-plated and coating with PHT had amorphous structure and the effect of PHT at 190 °C on the structure of Ni-P coating was not significant. Compared with coating with PHT, the as-plated exhibited lower erosion-corrosion resistance due to some small pits and cracks. The coating with PHT demonstrated significant improvement of erosion-corrosion resistance in 3.5 wt.% NaCl solution, which was attributed to better protective film, lower porosity of coatings, and high micro-hardness compared with Ni-P coating without PHT.