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A stray grain exposed on the surface of a Re-containing single crystal Ni-based superalloy turbine blade was examined by high-resolution analytic electron microscopy. An intermediate layer with the size range of 3 to 4 μm, composed of elongated γ′ phase, was clearly detected forming a boundary between a normal matrix grain and a stray grain. Beyond the intermediate layer, the {100} direction of the γ′ phase in the matrix changed slightly, and a stray grain was formed. In each γ′ grain in the intermediate boundary, a Re-rich region of the size range of 100 to 200 nm was detected, suggesting a role for rhenium in the formation of stray grains.

An aluminum alloy was cast by a laboratory scale horizontal twin roll caster with or without melt conditioning by the intensive shearing prior to solidification and then examined by high-resolution electron microscopy. The combined twin roll casting process with solidification formed channels and induced centerline segregation without the conditioning. In comparison, the melt conditioning minimized the severe segregation on the surface as well as at the centerline. Furthermore, large amounts of solute elements were uniformly distributed along grain boundaries or interdendritic regions. Analytical electron microscopy detected a fine oxide particle or a fragmented aluminum particle particularly at the center region of one nucleated aluminum grain. In addition, large oxide particles of about 1 to 5 μm nucleated aluminum grains easily due to low undercooling necessary for the heterogeneous nucleation, whereas small oxides with the size of about 100 to 200 nm requiring large undercooling were pushed along the grain boundaries instead of contributing to the nucleation. The enhanced nucleation of aluminum grains and well-distributed solute atoms in the melt by the melt conditioning resulted in the minimization of macro- and micro-segregations and the formation of a uniform microstructure.

High-temperature oxidation of molten aluminum was investigated by high-resolution electron microscopes in order to determine the possibility of heterogeneous nucleation of aluminum grains on oxide for the grain refinement and structural uniformity of intensively melt-sheared aluminum alloys. High-resolution observations detect initial amorphous phase and gamma-alumina phase and show fine clusters with size of about 150 to 200 nm composed of extremely fine aluminum grains and gamma-alumina or amorphous aluminum oxide. Furthermore, high-resolution lattice images and diffraction patterns show no orientation relationship, although there is a specific orientation between gamma-alumina and aluminum along (111)[110] with high potency of heterogeneous nucleation. The volumetric shrinkage by the transformation of gamma- into alpha-alumina causes the surface oxide films to repeatedly rupture and leads to the creation of channels to the base melt surface for further oxidation of fresh metal. Based on the observations, the mechanism of high-temperature oxidation of molten aluminum and formation of the fine clusters as well as the possibility of the heterogeneous nucleation of aluminum grains are discussed.

Thick titanium coatings were prepared by the warm spraying (WS) and cold spraying (CS) processes to investigate the oxidation and microstructure of the coating layers. Prior to the coating formations, the temperature and velocity of in-flight titanium powder particles were numerically calculated. Significant oxidation occurred in the WS process using higher gas temperature conditions with low nitrogen flow rate, which is mixed to the flame jet of a high velocity oxy-fuel (HVOF) spray gun in order to control the temperature of the propellant gas. Oxidation, however, decreased strikingly as the nitrogen flow rate increased. In the CS process using nitrogen or helium as a propellant gas, little oxidation was observed. Even when scanning electron microscopy or an x-ray diffraction method did not detect oxides in the coating layers produced by WS using a high nitrogen flow rate or by CS using helium, the inert gas fusion method revealed minor increases of oxygen content from 0.01 to 0.2 wt.%. Most of the cross-sections of the coating layers prepared by conventional mechanical polishing looked dense. However, the cross-sections prepared by an ion-milling method revealed the actual microstructures containing small pores and unbounded interfaces between deposited particles.

Warm spraying has been developed by NIMS, in which powder particles are accelerated and simultaneously heated, and deposited onto a suitable substrate in thermally softened solid state. In this study, commercially available titanium powder was sprayed onto steel substrate by the spraying process. Microstructural developments and deposition behaviors from a deposited single particle to a thick coating layer were observed by high resolution electron microscopes. A single titanium particle sprayed onto the substrate was severely deformed and grain-refined mainly along the interfacial boundary of particle/substrate by the impact of the sprayed particle. A successive impact by another particle further deformed the previously deposited particle and induced additional grain refinement of the remaining part. In a thick coating layer, the severe deformation and grain refinement were also observed. The results have demonstrated the complex deposition behavior of sprayed particles in the warm spraying using thermally softened metallic powder particles.

High-resolution observations by electron microscopy have revealed the existence of nanoscale amorphous oxide film along the interfacial regions of kinetically sprayed titanium particles. The oxide was formed as the surface of feedstock particles was dynamically oxidized during the flight of several milliseconds. In spite of the severe plastic deformation associated with the impacts of these particles onto the substrate, as well as onto the previously deposited particles, some portion of the oxide remained, providing a bond between the particles.

Thermally softened titanium powder particles were impacted at about 760 m/s onto three substrate materials, i.e., titanium, aluminum, and zirconia, at the same time. In spite of the very different physicochemical properties of these substrate materials, most of particles were bonded to the substrate. However, their microstructures suggested different dynamic deformation behaviors and interface features. On titanium or zirconia substrate, particles showed shear instability and direct metallurgical bonding or metal-ceramic bonding that resulted from the removal of oxide covered on them, while on aluminum, although the particle was not heavily deformed and its oxide remained it was also bonded to the substrate due to the instability of substrate and the removal of aluminum oxide. The results have demonstrated the complex and material dependent nature of bonding formation in kinetic spraying using thermally softened metallic powder particles.

Titanium powder particles accelerated and simultaneously heated by the supersonic gas flow were deposited onto steel substrate by the warm spraying process. The sprayed particles were heavily deformed and bonded to the substrate in solid state. Especially, all the deposited particles showed jetting-out of materials out of the particle-substrate interface triggered by the adiabatic shear instability known to occur under such shock impact conditions. High-magnified images showed that grain refinement occurred in the jetting-out region by dynamic recrystallization. Furthermore, the elemental analysis using the electron energy loss spectrum showed jetting-outs of the substrate as well as the particle. Numerical simulation based on the Johnson-Cook plastic deformation model showed that the jetting-out phenomenon commences about 10 ns after the initial contact of the particle with the substrate and at a position away from the center bottom of particle, where the highest compressive stress is experienced.

Micron-sized titanium particles were deposited on steel substrates by the warm spraying, which is a modified high velocity oxy-fuel (HVOF) spraying technique. In the process, nitrogen gas is mixed with the HVOF flame jet to lower the temperature of injected powder particles. Detailed observations of splats formed on polished substrates by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to investigate the effects of particle temperature on the bonding of splats with the substrate and the microstructure within the splats. At lower nitrogen flow rates, the particles observed were heavily deformed and exhibited diverse splat morphologies and microstructures. At higher nitrogen flow rates, most of the particles were impacted in the solid state and the oxidation of particles was remarkably less. The TEM observation revealed distinctively different microstructures within the splats as well as the splat/substrate interfaces depending on whether the particle was molten or solid before the impact.

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.

The boundary region formed on the surface of nickel-based single-crystal turbine blades was investigated by high-resolution microscopy observation. There was a distinguishable intermediate layer with the size of about 2 to 5 μm between the matrix and surface defect grains such as stray grains, multiple grains, freckle grains, and even low-angle grain boundaries which were formed during the solidification of turbine blades. The intermediate layer was composed of many elongated γ′ as well as γ phases. In addition, only one side of the intermediate layer was coherent to the matrix grain or defect grain due to good orientation match. At the coherent interface, the γ′ (as well as γ) phase started to extend from the parent grain and coincidently, rhenium-rich particles were detected. Furthermore, the particles existed within both elongated gamma prime and gamma phases, and even at their boundary. Based on experimental observations, the formation mechanism of this intermediate layer was discussed.