Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

This overview highlights very recent achievements and new trends in one of the most active and developing fields in modern materials science: the production of bulk ultrafine-grained (UFG) materials using severe plastic deformation (SPD). The article also summarizes the chronology of early work in SPD processing and presents clear and definitive descriptions of the terminology currently in use in this research area. Special attention is given to the principles of the various SPD processing techniques as well as the major structural features and unique properties of bulk UFG materials that underlie their prospects for widespread practical utilization.

https://link.springer.com/content/pdf/10.1007%2Fs11837-006-0213-7.pdf

Producing bulk ultrafine-grained materials by severe plastic deformation

The objective of this paper is to demonstrate the versatility of electron beam-physical vapor deposition (EB-PVD) technology in engineering new materials with controlled microstructure and microchemistry in the form of coatings. EB-PVD technology is being explored in forming net-shaped components for many applications including space, turbine, optical, biomedical and auto industry. Coatings are often applied on components to extend their performance and life under severe environmental conditions including thermal, corrosion, wear, and oxidation. In addition, coatings have been used in designing and developing sensors. Performance and properties of the coatings depend upon its composition, microstructure and deposition condition. This paper presents recent results of various materials including ceramic, metallic, and functionally graded coatings produced by EB-PVD. Simultaneous co-evaporation of multiple ingots of different compositions in the high energy EB-PVD chamber has brought considerable interest in the architecture of functional graded coatings, nano-laminated coatings and designing of new structural materials that could not be produced economically by conventional methods. In addition, high evaporation and condensate rate allowed fabricating precision net-shaped components with nanograined microstructure for various applications. This paper will also present the results of various metallic and ceramic coatings including chromium, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), and partially yttria stabilized zirconia (YSZ), and HfO2-based TBC coatings deposited by EB-PVD for various applications.

Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD)

This paper discusses microstructural changes produced by two novel approaches using electron beam-physical vapor deposition (EB-PVD) in which periodic strain fields/microporosity was incorporated within the large columnar grains of ZrO 2 –8 wt.% Y 2 O 3 (8YSZ). The traditional columnar microstructure of partially stabilized zirconia has been slightly modified to produce a lower thermal conductive thermal barrier coating (TBC) by periodically interrupting the condensing vapor resulting in microstructural modifications with diffuse or sharp interfaces and morphological changes on the submicron scale without changing the composition of the TBC. These microstructural modifications resulted in a 20–30% reduction in the thermal conductivity, 28–56% increase in hemispherical reflectance, improved oxidation cyclic life (over 100%), and better strain tolerance as compared to standard ZrO 2 –8 wt.% Y 2 O 3 deposited on platinum–nickel–aluminide and CoNiCrAlY bond-coated MAR-M-247 test samples. The TBC with tailored microstructures were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), steady-state laser heat flux technique, hemispherical reflectance and thermal cyclic oxidation tests.

Tailored microstructure of EB-PVD 8YSZ thermal barrier coatings with low thermal conductivity and high thermal reflectivity for turbine applications

Zirconia and hafnia based thermal barrier coating materials were produced by industrial prototype electron beam-physical vapor deposition (EB-PVD). Columnar microstructure of the thermal barrier coatings were modified with controlled microporosity and diffuse sub-interfaces resulting in lower thermal conductivity (20–30% depending up on microporosity volume fraction), higher thermal reflectance (15–20%) and more strain tolerance as compared with standard thermal barrier coatings (TBC). The novel processed coating systems were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction, thermal conductivity by laser technique, and hemispherical reflectance.

Tailored microstructure of zirconia and hafnia-based thermal barrier coatings with low thermal conductivity and high hemispherical reflectance by EB-PVD

High reflectance thermal barrier coatings consisting of 7% Yittria-Stabilized Zirconia (7YSZ) and Al 2 O 3 were deposited by co-evaporation using electron beam physical vapor deposition (EB-PVD). Multilayer 7YSZ and Al 2 O 3 coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 μm wavelength. The variable 7YSZ and Al 2 O 3 multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 μm. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m-K for the 20-layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations.

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine Applications

High-resolution transmission electron microscopy (HRTEM) was employed to study the nucleation and subsequent growth mechanism of crystalline diamond grown on copper TEM grids by the hot-filament chemical vapour deposition process. The HRTEM revealed direct evidence for the formation of a diamond-like amorphous carbon layer 8–14 nm thick, in which small diamond microcrystallites about 2–5 nm across were embedded. These diamond microcrystallites were formed as a result of direct transformation of the diamond-like carbon into diamond. Large diamond crystallites were observed to grow from these microcrystallites. The diamond surface was found to be non-uniform. It is envisaged that the diamond microcrystallites present in the amorphous, diamond-like carbon layer provide nucleation sites on which the large diamond crystallites grew. A mechanism of diamond growth has been proposed, based on the experimental findings, and is consistent with available theoretical models and numerous experimental observations reported in the literature.

Jogender Singh

Papers

Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD)

Tailored microstructure of EB-PVD 8YSZ thermal barrier coatings with low thermal conductivity and high thermal reflectivity for turbine applications

Tailored microstructure of zirconia and hafnia-based thermal barrier coatings with low thermal conductivity and high hemispherical reflectance by EB-PVD

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine Applications

Nucleation and growth mechanism of diamond during hot-filament chemical vapour deposition