The aim of this review is to update and to offer an overview of the investigations that have more recently been made on intermetallics, to highlight their importance for several applications in many areas and to examine topics for future research. The main subject is the application of intermetallics as thermal spray coatings. This will allow balancing the discussion of whether they are promising as a surface medium for materials that work at high temperatures. The general intermetallics classification includes iron, nickel, and titanium aluminides, as well as transition metal silicides. As nickel aluminides have been important in issues related to bonding coats, their research trajectory has been revised. Moreover, as the Fe-Al system has also been studied in terms of thermal spray processing, its good characteristics have also been assessed. The third intermetallic this article focuses on is NiTi, which appears not to be included in general classifications of intermetallic compounds, but it deserves to be mentioned mainly due to its shape memory effect and biocompatibility properties. © 2013 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.

An overview of intermetallics research and application: Status of thermal spray coatings

Selective laser melting was considered in this study with the goal to manufacture dense parts with mechanical properties comparable to those of bulk materials, directly from FeAl intermetallic powder. Based on a series of single line scans, the processing window was first explored, paying attention to the melting mechanisms. Then, FeAl samples were produced using various selected parameters according to the previously established processing map. These samples were investigated in terms of microstructure, surface roughness, densification and microhardness. Observations revealed that these characteristics are strongly dependent on the processing parameters. Finally, interesting FeAl specimens with a relatively high density (98% of the theoretical value), high microhardness and smooth surface were obtained using a laser power of 90 W and a scanning speed of 0.2 m/s, corresponding to a continuous melting mechanism of the powder bed.

Fabrication and microstructure characterization of selective laser‐melted FeAl intermetallic parts

Multiscale mechanical performance and corrosion behaviour of plasma sprayed AlCoCrFeNi high-entropy alloy coatings

This work provides a tutorial overview of recent research efforts in modeling and control of the high-velocity oxygen-fuel (HVOF) thermal spray process. Initially, the modeling of the HVOF thermal spray, including combustion, gas dynamics, particle in-flight behavior, and coating microstructure evolution is reviewed. The influence of the process operating conditions as predicted by the fundamental models on particle characteristics and coating microstructure is then discussed and compared with experimental observations. Finally, the issues of measurement and automatic control are discussed and comments on potential future research efforts are made.

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Modeling and Control of High-Velocity Oxygen-Fuel (HVOF) Thermal Spray: A Tutorial Review

Residual stresses were calculated from the curvature of coating-substrate coupons using three different models: a simple two-beam elastic model, the Tsui–Clyne progressive deposition model, and the Tsui–Clyne progressive deposition model with substrate plasticity. The coatings studied were metallic and prepared by high-velocity oxy-fuel (HVOF) thermal spraying. The calculated stresses were compared to those measured on the same coupons using X-ray diffraction (XRD) techniques. Coating surface stresses calculated using the two-beam elastic model disagreed with those measured using XRD for coupons with significant curvature. Trends in residual stresses (with varying coating and substrate thickness, substrate material, and HVOF spray particle velocity) predicted by the elastic and elastic-plastic versions of the Tsui–Clyne progressive deposition model agreed with the trends measured by XRD. The magnitudes of stresses calculated using the Tsui–Clyne model agreed with the XRD measurements for coatings sprayed at low particle velocities but were significantly more compressive for coatings sprayed at higher velocities. Accounting for substrate plasticity in the Tsui–Clyne model improved the agreement with the XRD results, but only slightly.

Residual stress determination in thermally sprayed coatings—a comparison of curvature models and X-ray techniques

The microstructure and state of stress present in Fe3Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.

Microstructure and stresses in HVOF sprayed iron aluminide coatings

X-ray based residual stress measurements were made on type 316 stainless steel and Fe3Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500 µm. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, near-surface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe3Al coatings. Deposition efficiency for both materials is maximized at an intermediate (∼600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.

Residual stresses in high-velocity oxy-fuel metallic coatings

Tensile tests and thermal-expansion measurements were performed on free-standing, high-velocity oxy-fuel (HVOF)-sprayed Fe3Al coatings produced at spray-particle velocities of 390, 560, and 620 m/s. To examine the relationship between properties and spray conditions, the microstructures of the coatings were characterized in terms of the fractions of unmelted particles, porosity, and oxide inclusions, as well as the dislocation density assessed by X-ray diffraction (XRD) line-broadening analysis. Residual coating stresses were determined as a function of coating thickness using curvature measurements. The tensile behavior was entirely brittle at room temperature; fracture strengths increased with spray-particle velocity; and the increase in fracture-strength results from decreasing fractions of microstructural defects and better interparticle bonding. The mean thermal-expansion coefficients for the coatings were lower than those for an equivalent wrought material; the differences were attributed to a 7 to 15 vol pct fraction of oxide inclusions.

Mechanical and physical properties of high-velocity oxy-fuel-sprayed iron aluminide coatings

The microstructure and corrosion behavior of Hastelloy C-22 coatings produced using the high velocity oxygen fuel (HVOF) method have been determined and related to in-flight measurements of the particle velocity and temperature. Average particle temperatures ranged from 1280–1450 °C and velocities ranged from 565–640 ms−1. All of the coatings were greater than 98% of theoretical density and exhibited passivating behavior in 0.1 M HCl during cyclic potentiodynamic polarization testing. The passive current density was somewhat higher compared with wrought C-22 alloy and an active-passive peak attributed to the formation of a Cr-rich surface layer was observed. Resistance of corrosion and deposition efficiency improved as the particle temperature decreased. There was little effect of particle velocity on the corrosion behavior over the range of deposition conditions examined. Our results suggest that feedback control based on measurement of the particle temperature can be used to process coatings with optimum properties.

Corrosion of thermal spray hastelloy C-22 coatings in dilute HCl

Thermally sprayed coating characteristics and mechanical properties are in part a result of the residual stress developed during the fabrication process. The total stress state in a coating/substrate is comprised of the quench stress and the coefficient of thermal expansion (CTE) mismatch stress. The quench stress is developed when molten particles impact the substrate and rapidly cool and solidify. The CTE mismatch stress results from a large difference in the thermal expansion coefficients of the coating and substrate material. It comes into effect when the substrate/coating combination cools from the equilibrated deposit temperature to room temperature. This paper describes a laser-based technique for measuring the curvature of a coated substrate and the analysis required to determine residual stress from curvature measurements. Quench stresses were determined by heating the specimen back to the deposit temperature thus removing the CTE mismatch stress. By subtracting the quench stress from the total residual stress at room temperature, the CTE mismatch stress was estimated. Residual stress measurements for thick (>1mm) spinel coatings with a Ni-Al bond coat on 304 stainless steel substrates were made. It was determined that a significant portion of the residual stress results from the quenching stress of the bond coat andmore » that the spinel coating produces a larger CTE mismatch stress than quench stress.« less

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William David Swank

Papers

Microstructure and stresses in HVOF sprayed iron aluminide coatings

Residual stresses in high-velocity oxy-fuel metallic coatings

Mechanical and physical properties of high-velocity oxy-fuel-sprayed iron aluminide coatings

Corrosion of thermal spray hastelloy C-22 coatings in dilute HCl

Residual Stress Determination from a Laser-Based Curvature Measurement