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SiCf/SiC composites are replacing superalloy components in hot sections of gas turbines. The high-velocity combustion gases in gas turbine hot sections contain water vapor. The mechanism of oxidative water vapor attack on SiC is vastly different from that of oxidation in static dry air. Therefore, it is of paramount importance to study the oxidation of these composites in high-velocity combustion environments. This study focuses on the high-temperature oxidation behavior of SiCf/SiC composites with a BN interfacial coating in both static air and combustion environments. The composite was prepared by the chemical vapor infiltration process (CVI). To elucidate the impact of water vapor in a combustion environment, an oxyacetylene flame apparatus was used. Oxidation of the SiCf/SiC composite was investigated at 1200, 1300 and 1400 °C, for up to 100 h in static dry air, and for up to 24 h in a combustion environment. The microstructure evolution and recession of the composite under combustion environment were investigated by scanning electron microscopy. The oxidation kinetics of the SiCf/SiC composite showed a parabolic nature in static air; however, in the combustion environment the composite showed accelerated mass loss owing to the concurrent effects of oxidation and recession. [ABSTRACT FROM AUTHOR]

Two types of high-entropy alloys (HEAs) AlCrCoFeNiTi and FeCrCoNiW0.3 + 5 at.% C are fabricated using atmospheric plasma spray (APS) technique. Laser surface processing (LSP) is performed on the developed alloys using Nd:YAG pulsed laser. Post processing the surface roughness of the alloys are reduced by ~ 29%. The impact of laser surface processing reveals the presence of a single BCC phase and FCC phase with the evolution of more W-rich and Cr-rich carbides in AlCrCoFeNiTi and FeCoCrNiW0.3 + 5 at.% C coatings, respectively. The microstructural study exhibits the formation of lamellar microstructure with minimum pores and interlaminar cracks. Post laser processing the microhardness of both the APS coated alloys are increased by 5%, nanoindentation results reveal an increase in the average elastic modulus (Er) by 12%, and average nanohardness by 18%. The FeCoCrNiW0.3 + 5 at.% C coatings achieved maximum wear resistance of 39.71% among the two alloys, indicating the improvement achieved through laser processing. Also the observed improvements in surface morphology of both the alloys are reported. [ABSTRACT FROM AUTHOR]

Aluminum-copper alloy system is extensively used in structural and aerospace applications for its high strength-to-weight ratio, good mechanical and tribological properties. Improving the properties of these alloys would likely widen their application area. In the present work, an attempt has been made to simultaneously enhance the wear resistance and mechanical properties of an Al-Cu alloy, AA2014 by imparting different levels of cryorolling strains and postroll aging treatment. The wear behavior of the material is studied under dry sliding condition by pin-on-disk experiments and mechanical properties are assessed by tensile test. Formation of high fraction of dislocation density and significant refinement of microstructure during cryorolling and nucleation of fine coherent Guinier-Preston (GP) zones of Al2Cu precipitates during postcryoroll aging has led to about 100% increment in the wear resistance of the material. Tensile test results proved that the synergetic effect of cryorolling and aging treatment led to 53% increment in strength (557 MPa) without compromising the material's ductility (22.5%). A detailed investigation on the various mechanisms responsible for the enhanced wear resistance and improved mechanical performance is presented based on the microstructural evidence. [ABSTRACT FROM AUTHOR]

Abstract: Extensive creep testing was carried out on 1Cr1Mo¼V low alloy forging and casing steels in as received (normalized and tempered) and aged condition. Both the steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 813 and 873K (540°C and 600°C). The primary stage, though present, was rather negligible in higher temperature ranges. Casting steel showed wedge type cavities which grew as cracks along the grain boundaries. In the case of forged steel, the voids were elliptical and flat which developed during the tertiary stage of creep deformation. Tertiary creep deformation and creep ductility of the two steels investigated have been analyzed based on the type of voids developed during the tertiary stage. Based on detailed microstructural investigations using scanning and transmission electron microscope, prevailing damage mechanisms such as structural transformation, particle coarsening, grain boundary thickening have been identi ied and applied for remaining life assessment of the two steels. Hardness and other mechanical properties were observed to decrease slightly on aging; both for rotor forging and casing casting steel. The softening occurred due to dissolution of M3C and Mo2C carbides and coagulation of others resulting in reduced creep strength. Gradual fall of creep strength at intermediate aging times was due to recovery in ferrite, gradual depletion of solid solution carbides from the ferrite matrix, metastability and transitional character of precipitated carbides. Creep crack growth studies have also been carried out on both the steels. Life assessment calculations have also been carried out using creep crack growth methodology. [Copyright &y& Elsevier]

Abstract: In this work nickel based hardfacing alloy (Colmonoy 5) was deposited on 316L (N) stainless steel substrate to study the effects of aging treatment on coating microstructure, wear and corrosion properties. Coatings, deposited through plasma transferred arc (PTA) welding process, were aged at 923K for 5000h. Microstructural characterization studies carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the coarsening of dendrites and precipitation of Cr23C6 particles in the aged coating. The wear behavior of the as deposited and aged coatings was compared in room temperature (RT) and high temperature (823K) under dry sliding wear condition (pin-on-disc configuration). At RT, aged coating experienced more wear loss when compared to as-deposited. At high temperature, the wear loss was almost same with similar operating wear mechanisms (tribo-oxidation) for both as-deposited and aged coating. From pitting corrosion studies, it was found that aged coatings are more prone to pitting when compared to the as-deposited coatings. [Copyright &y& Elsevier]

high-strength and wear-resistant alloyed gray iron with ausferritic microstructure on solidification directly from molten condition could be made in a Ni and Mo alloyed gray cast iron. The as-cast ausferritic cast iron was compared with two conventionally austempered gray iron with and without Ni and Mo additions. The various phase constitution and volume fractions were analyzed using optical, SEM and XRD analyses. The various aspects of the alloy chemistry and processing conditions have been correlated with the microstructure and mechanical properties obtained. The analysis showed that the Ni-Mo alloyed austempered gray iron and the directly as-cast austempered gray iron had similar phase constitutions. The strength of the direct as-cast alloy with ausferritic microstructure was higher than the others due to its higher austenite content and carbide distribution. The wear rate of the conventionally austempered Ni and Mo containing alloy and direct as-cast ausferritic alloys is 20% of the austempered gray iron without Ni and Mo with friction coefficient less than 0.4. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, influence of microstructure on high temperature dry sliding wear behavior of nickel base hardfaced coating is discussed in detail. Using plasma transferred arc (PTA) welding process, coating of nearly 4 to 5mm thickness was deposited on 316 L (N) stainless steel substrate without any defects. The as-deposited coating contains large quantity of precipitates (range from 100nm to 20μm in size) in the γ-nickel matrix which increased the hardness significantly. Sliding wear tests were carried out on the coating for different sliding distances at room temperature (RT) and also higher temperature (573K, and 823K). Results showed that the wear resistance of coating improved significantly with increase in test temperature, and the high wear resistance was observed at 823K. The coating exhibited different operating wear mechanisms depending on sliding distances and also test temperature. The wear mechanisms were identified based on friction and wear values along with worn surface analysis. The operating wear mechanisms at 823K are adhesion at shorter sliding distance and tribo-oxidation at longer sliding distance. [Copyright &y& Elsevier]

Alloyed grey cast iron is an important automotive material used in clutch and brake applications. In order to understand the influence of microalloying additives on the graphitic morphology, mechanical and wear behaviour, a series of microalloyed cast iron melts were taken. The laboratory scale ingots made were characterised for microstructure, mechanical properties and wear behaviour. All the cast irons made were hypereutectic with a microstructure consisting of class A type graphite in a pearlitic matrix with eutectic grain boundary carbides. The microalloyed cast irons showed two- to threefold improved wear resistance over a base unalloyed material at comparable mechanical strengths. The wear loss was found to be linear with increasing sliding distance, and the wear resistance is attributed to the combined influence of dense pearlitic matrix with carbides and good distribution of graphite flakes. Cast iron microalloyed with Ni (0·62%) with Ce inoculation showed best overall properties compared to other alloys. [ABSTRACT FROM AUTHOR]

Abstract: 1Cr1Mo1/4V low alloy forging and casting steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 813 and 873K (500 and 600°C). The primary stage, though present, was rather negligible in higher-temperature ranges. Casting steel showed wedge-type cavities which grew as cracks along the grain boundaries. In the case of forging steel, the voids, which developed in the tertiary stage of creep deformation, were elliptical and flat. Tertiary creep deformation and creep ductility of the two steels have been analysed based on the type of voids developed in the tertiary stage. [Copyright &y& Elsevier]

• Hypereutectic Al-Si processed through spark plasma sintering (SPS) and cast route. • Interfacial eutectic melting in SPS produces microstructure different from cast. • Finer eutectic Si in SPSed microstructure substantially enhances the failure strain. • SPSed microstructure prevents anodic dissolution and improves corrosion resistance. Spark plasma sintered hypereutectic Al-Si alloy, free from characteristic eutectic microstructure was evaluated for its mechanical and corrosion behaviour in comparison with conventional cast microstructure having coarser primary Si and acicular eutectic Si. In SPSed microstructure, Al region free from acicular eutectic imparts substantial failure strain under compression loading. Absence of coarser primary Si and acicular eutectic Si in SPSed microstructure prevents the rate of anodic dissolution, specifically at the Al-Si interface and enhances corrosion resistance than the cast microstructure in 3.5 wt% NaCl corrosive medium. [ABSTRACT FROM AUTHOR]

In the present work, it was envisaged to develop thermally stable Al–Cu alloy by modifying its chemistry. AA2219 alloys with different levels of scandium (Sc), magnesium (Mg) and zirconium (Zr) were prepared by melting with inert gas (argon) tungsten arc welding. Sc was added in three levels (0·2, 0·4, 0·8%), Mg one level (0·45%) and Zr one level (0·2%). Optical, scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM) were carried out to study the microstructural details. Long time aging studies and hot tensile tests were used to study the high temperature strength of the alloys. High temperature tensile test was performed on the specimens at 250, 350 and 400°C. Room temperature tensile test was carried out for comparative purpose. Electron probe microscopy analysis (EPMA) was carried out to find out the copper segregation in the alloys. Microstructural studies showed that raising Sc content from 0·2 to 0·8% resulted in significant grain refinement. Grain refinement was much more effective with the addition of 0·2%Zr to AA2219 alloy containing 0·8%Sc and 0·45%Mg. Addition of Sc, Mg and Zr was found to increase in eutectic temperature and decrease in freezing range. Hot tensile results were found to be in agreement with results of hardness data. Among all the compositions, AA2219+0·8%Sc+0·45%Mg+0·2%Zr alloy was found to be the best in terms of its high temperature strength. [ABSTRACT FROM AUTHOR]

In the present investigation, a nickel based hardfacing alloy (AWS NiCr–B) was deposited on an austenitic stainless steel substrate 316LN using the plasma transferred arc welding process. The deposit was characterised by hardness measurements, microstructural examination and sliding wear assessment. Identification of precipitates was carried out using X-ray diffraction and SEM/EDAX. These studies revealed the presence of chromium rich carbides and borides in a γ-Ni matrix. Down to a distance of 1 mm from the interface, the hardness of the deposit was found to be 52 HRC. The sliding wear behaviour of the hardfacing alloy was investigated in air in the room temperature to 550°C range, with a pin on disk configuration using a cylindrical pin with tip radius of 3 mm under loads of 30, 40 and 50 N. Wear experiments were conducted up to a sliding distance of 180 m at a sliding speed of 0·1 m s-1. The elastic modulus and Poisson's ratio of the hardfaced deposits were evaluated by the ultrasonic method and these values were used for calculating initial Hertzian contact stress. The study showed that, while significant wear loss occurred at room temperature, there was practically no measurable weight loss at temperatures of 300 and 550°C. This could be attributed to the formation of an oxide layer at the surface during wear testing. [ABSTRACT FROM AUTHOR]

• Characteristic eutectic free microstructure obtained in hypereutectic Al-Si through SPS. • Current activated eutectic melting at Al-Si interface is responsible for densification. • Current-Voltage evolution during SPS is employed to elucidate densification, for the first time. • Rapid heat transfer during SPS resulted in suppression of characteristic eutectic in hypereutectic Al-Si. Spark plasma sintering aids in characteristic eutectic structure free microstructural evolution in hypereutectic Al-Si. The mechanism of suppression of characteristic eutectic was elucidated through current-voltage evolution during sintering, electron backscatter diffraction phase mapping and high-angle annular dark field imaging. The electric current activated eutectic melting at the Al-Si interface followed by rapid heat transfer lead to a microstructural evolution under far from equilibrium condition and free from conventional eutectic structure. [ABSTRACT FROM AUTHOR]

This study investigates the feasibility of using cold spray for repair of Inconel 718 (IN718) components. The effect of cold spray parameters on the particle velocity, splat morphology, deposition efficiency (DE), thickness, and porosity was evaluated. Thick coatings of approximately 4 mm were deposited onto ground and polished IN718 substrates using N2 gas heated to 1000 °C with gas pressure of 5 and 7 MPa. The coatings were subjected to standard double aging (DA) and a combination of solutionizing and double aging (STDA) heat treatment. The microstructure, hardness, porosity, tensile strength, and adhesive strength of as-sprayed and heat-treated coatings were evaluated. Additionally, the tensile properties of the coating–substrate combination (sandwich samples) were also evaluated. It was observed that higher gas pressure led to increased particle velocity, decreased porosity, enhanced hardness, and improved adhesion/tensile strength. The splat size increased with higher particle velocity, indicating greater particle deformation. The DE decreased with increase in number of deposited layers and increased with increase in gas pressure. The AS coating microstructure exhibited a deformed powder microstructure having fine dendritic/cellular structure. Following the DA treatment, the dendritic features were preserved, accompanied by the precipitation of γ′, γ′′, and δ phases. After STDA treatment, a homogeneous microstructure with the presence of γ′, γ′′, and Nb and Ti carbides without any δ phase was observed. The most favorable outcomes were achieved with the 7 MPa STDA sample, yielding a minimal porosity level of 0.4 ± 0.1% and a tensile strength of 1325 ± 10 MPa with a failure strain of 6.1 ± 0.6%. The tensile strength of 7 MPa sandwich sample was found better (1049 ± 8.5 MPa) compared to stand-alone coating (965 ± 24 MPa) after DA treatment. [ABSTRACT FROM AUTHOR]

Biomedical grade Ti–6Al–4V alloy, is the best candidate material for bioimplants due to its outstanding corrosion resistance. Additive manufacturing processes such as laser-based powder bed fusion (LPBF) are trending technologies that challenge conventional manufacturing methods in producing customized implants. However, the Ti–6Al–4V alloy fabricated by LPBF consists of a non-equilibrium phase (α′-martensite), that tends to degrade the corrosion resistance, and therefore a post-treatment is required to stabilize the phases. In this work, the annealing is done in the α+β range of the Ti–6Al–4V alloy. The corrosion performances of the as-built (AB) and annealed samples in 0.9 wt% NaCl solution are investigated through electrochemical measurements. The relationship between the microstructure and its corresponding corrosion behaviour is elucidated. The rate of corrosion increased by around 12 % following low temperature annealing, but decreased by more than 50 % following high temperature annealing above 850 ᵒC. Overall, the AB structure shows an inferior corrosion behaviour compared to the annealed ones. The electrochemical results show that post-annealing has a positive effect on corrosion performance that can be ascribed to various factors such as phase fraction, chemical composition, morphological features of microstructural and presence of defects. The findings show that a microstructure with increased β-phase fraction, a fine lamellar structure with low defect density, and a comparatively coarser grain size is beneficial in improving corrosion resistance. This can be achieved by annealing the Ti–6Al–4V alloy at 850 ᵒC or higher. [Display omitted] • The as-built Ti–6Al–4V microstructure showed inferior corrosion resistance, owing to the presence of α′-martensite phase. • The corrosion resistance was improved more than 50 % after annealing above 850 ᵒC. • A fine α- and β-lamellar microstructure with increased β-phase fraction is found to improve the corrosion resistance. • A reduced defect density is beneficial for an improved corrosion resistance. [ABSTRACT FROM AUTHOR]

Abstract: Alumina coatings were prepared using detonation gun from commercial powder (CP) and high energy ball milled powder (MP) with 83nm crystallite size. The sliding wear rate and wear coefficient of friction were lower in the nanocrystalline powder coating. The coating from the as-received commercial powder consisted of thick splats, and a paucity of nanoparticulate inter-splat regions whereas milled powder coating has thinner splat morphology with nanoparticulate inter-splat regions. The nanoparticulate material in the inter-splat regions and the finer microstructure of the MP coating is believed to be responsible for improved wear resistance. [Copyright &y& Elsevier]

Abstract: A novel surface modification process namely controlled ball impact peening was developed for synthesizing a nanostructured surface layer and to impart compressive residual stresses on metallic materials in order to enhance the overall surface properties. This article demonstrates the microstructural evolution, surface hardening and introduction of the residual stresses in the ball impact peened aluminium alloy surfaces, AA6063-T6. Hardened steel balls were impinged in controlled manner inducing high strain rates on the aluminium samples which are precisely moved using independent programmable logic controlled linear actuators in the controlled ball impact peening process. Mechanical properties of the nanocrystalline surface layer were investigated using dynamic ultra micro-hardness tester. The hardness of the nanocrystalline surface layer is (~1.3GPa) improved compared to the matrix (~0.58GPa) and the depth of the hardened layer is about ~350μm depending upon the peening conditions. The amount of compressive residual stress developed by the treatment is also studied using depth sensing indentation method. The surface compressive residual stresses induced in the ball impact peened samples is about 70–127% of yield strength of the target material depending upon the peening conditions. X-ray diffraction analysis and transmission electron microscope analysis revealed the formation of nanograin crystalline structure on the ball impact peened surface layer. The mean grain size of the peened sample determined by transmission electron microscope is about 8±2nm in the top surface layer. High strain rate and repeated directional loading imparted in the contact zone generates the various dislocation activities and microstructural features which were responsible for the formation of the randomly oriented nanostructured grains on the metallic materials. With increasing strain, the various microstructural features produced in the ball impact peened aluminium samples are deformation twins, multiple shear bands, high density dislocation and dislocation pile-up at the grain boundaries as investigated by transmission electron microscope. Grain refinement on the ball impact peened aluminium surfaces resulted in the formation of high density dislocation associated with the subdivision of original grains into subgrains. The peening coverage and number of overlapping impacts depend upon the sample travelling velocity, which in turn affects the hardness, compressive residual stresses induced and grain size formed for a given ball diameter and impact velocity. [Copyright &y& Elsevier]

Abstract: Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (∼210nm) with high surface hardness (∼0.88GPa), compressive residual stress (−102±7MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ∼400μm. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior. [Copyright &y& Elsevier]

Abstract: The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360°C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6×10−7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described. [Copyright &y& Elsevier]

Abstract: Alloyed gray cast irons were made with and without misch metal inoculation (0.1%). The mechanical and wear properties were compared with conventional gray cast iron used for a typical clutch application in heavy commercial vehicles. Alloyed gray iron without misch metal showed higher volume fraction of pearlite (89%) and lower flake graphite (11%). Misch metal inoculated gray irons showed higher volume of flake graphite (15%) with 85% pearlite as matrix. Alloyed gray irons produced tensile strength from 300 to 344MPa and hardness in the range of 221–247VHN. Misch metal inoculation has slightly increased the graphite volume (40–60%) with corresponding decrease in strength and hardness (9–13%) in alloyed gray irons. The specific wear rates of all alloyed gray irons are significantly lower (<34%) compared to unalloyed base at two different sliding speeds (1.6m/s and 2.5m/s). The friction coefficient is less than 0.4 for alloyed gray irons as against 0.5–0.7 for unalloyed base gray iron at both the sliding speeds. This is attributed to the presence of alloying additives within the matrix which resists adhesive and abrasive wear loss. Among the inoculated alloyed gray irons, the alloy with lower S content (0.08%) showed higher wear rate at higher sliding speed due to lower graphite flake density compared to higher S containing iron (0.12%). This indicates that the wear rate is influenced by the amount of graphite which is released into the interface during sliding to provide lubrication and reduce wear. Inoculation with rare earth misch metal has a positive influence over graphite morphology in gray iron. [Copyright &y& Elsevier]

Abstract: In the present study, flux-cored arc welds of modified 9Cr–1Mo (P91) steel were produced using four types of shielding gases with rutile based flux-cored wire. Artificially prolonged post-weld heat treatment and GTAW surface melting methods improved the toughness of welds having high inclusion content. Fusion zone toughness was correlated with inclusion size and its distribution, shielding gas composition, chemical composition and post-weld heat treatment process parameters. It was found that 95% argon+5% CO2 was the ideal shielding gas medium for FCA process to meet the toughness requirements with better process characteristics. [Copyright &y& Elsevier]

Abstract: Modified 9Cr-1Mo steel is widely used in the construction of power plants. Flux-shielded processes result in inadequate weld metal toughness due to the presence of inclusions. An acidic-coated electrode with primary constituent of rutile in flux coating tends to produce inferior toughness due to the presence of coarse microinclusions in the resultant weld. In the present study, welds produced using acidic-coated electrodes were given a surface melting using TIG process to refine the inclusions. There was significant reduction in number of coarse microinclusions and increase in number of fine microinclusions. Charpy V-notch test was conducted at room temperature to evaluate the toughness of weld. Surface melted welds have superior toughness compared to unmelted welds. Fractographic features correlate well with the observed impact energy values of welds. [Copyright &y& Elsevier]

The interdendritic Laves phase and the microsegregation have been investigated in Alloy 718 fusion zone cooled with liquid nitrogen during welding. Conventional GTA welding process was employed with modified waveform and two types of shielding gas and filler metal (solid solution and age hardenable). The weld cooling rate was enhanced using liquid nitrogen cooling during Gas Tungsten Arc welding process. The resultant fusion zone microstructures were characterized using the metallurgical tools. Dendrite remelting phenomenon was observed from the optical micrographs. It was found that the enhanced cooling rate with liquid nitrogen reduced the interdendritic phases which were confirmed in both the electron microscopic and the X-ray diffraction analysis. The elemental mapping in scanning electron microscope-energy dispersive spectral analysis also confirmed the reduced microsegregation. The dendrite arm spacing was reduced from the range of 15–54 μm (CCPHE–CCAR, conventional) to 3–17 μm (CCPHE–CCAR, liquid nitrogen cooled) for the employed process variables. The computed weld cooling rate was found to be enhanced from seven to fifteen times than the conventional welding process. [ABSTRACT FROM AUTHOR]