Showing papers in "International Materials Reviews in 2000"

Instability criteria for hot deformation of materials

Abstract: Forming and forging processes are among the oldest and most important materials related technologies. New materials technologies centre on the development and widespread use of thermomechan...

Fundamentals of inclusion removal from liquid steel by bubble flotation

Abstract: Attempting to remove the maximum number of inclusions from liquid steel by bubble flotation is an important means of improving steel cleanliness. There has, however, been no complete theoretical fundamental study on this issue, and the present review represents a tentative step towards this objective. An attempt is made to apply results from the field of mineral processing to the liquid steel–inclusion–bubble system for the case of silica inclusions and argon bubbling. However, it should be noted that this approach has limitations: the size of the bubbles and particles involved and the degree of turbulence, the surface tension of the liquid, and the contact angle between inclusion and bubble are quite different in the two systems. The mechanism of bubble–inclusion interaction, bubble size and rising velocity, liquid film formation and rupture between bubble and inclusion, collision time, film drainage time, sliding time, and collision and adhesion probabilities are discussed. A simple mathematical...

Silicon carbide for microelectromechanical systems

Abstract: Silicon carbide (SiC) has recently attracted attention as a wide bandgap semiconductor with great potential for microelectromechanical systems (MEMS). SiC exhibits excellent electrical, mechanical, and chemical properties, making it well suited for harsh environment applications where traditional MEMS are constrained by the physical limitations of silicon (Si). This paper reviews the material properties, deposition techniques, micromachining processes, and other issues regarding the fabrication of SiC-based sensors and actuators. Special emphasis is placed on the properties that make SiC attractive for MEMS, and the Si-based processing techniques that have been adapted to realise SiC MEMS structures and devices. An introduction to micromachining is provided for readers not familiar with MEMS fabrication techniques.

Superhard diamondlike carbon: preparation, theory, and properties

Abstract: One of the many forms of carbon, diamondlike carbon (DLC) or tetrahedral amorphous carbon (ta-C) consists mainly of sp3 bonded carbon atoms. If properly prepared, DLC can have properties that rival those of crystalline diamond. The beneficial properties of DLC stem from the continuous rigid random networks of sp3 carbon atoms, and the properties can essentially be tailored by controlling the sp3/sp2 ratio. Techniques that have been successfully used to prepare high quality DLC coatings or thin films include pulsed laser ablation, filtered cathodic vacuum arc deposition, and mass selected ion beam deposition. Diamondlike carbon coatings that possess properties close to diamond in terms of hardness, atomic smoothness, infrared transparency, and chemical inertness can be processed easily with these techniques. In the past decade, tremendous progress has been made in experimental and theoretical investigations of hydrogen free DLC. Experimental and commercial applications in areas including microelect...

Phase transformations in γ based titanium aluminides

Abstract: There have been many recent advances in the development of γ based titanium aluminides for high temperature load bearing applications. Several improvements in the properties of these materials have...

Effect of minor elements on weldability of nickel base superalloys

Abstract: The present state of knowledge on the effects of C, B, S, and P on the weldability of Ni base superalloys is reviewed. Consideration is also given to the effects of Mg and the rare earths on weldability, though the data are sparse and the manufacturing route often not defined. From the analysis, B is considered to be the most detrimental element to the weldability, with lesser effects from C and S. The data on P are limited, but recent results show that P has a variable effect on weldability, being most detrimental in low C alloys (0·01 wt-%), and less of a problem in C levels >0·03 wt-% and above. Consideration is also given to the effects of heat treatment in influencing solute segregation to the grain boundary and on grain growth. The potential effects of two or more elements on grain boundary microchemistry and its effects on weldability are also discussed.

Use of phase diagrams in studies of refractories corrosion

Abstract: Typical applications of phase diagrams to specific problems of refractories corrosion are highlighted. Saturation solubilities of refractories components in molten slags can be estimated using existing phase diagrams. These can then be used to predict the corrosion behaviour, and qualitatively compare the corrosion resistance, of a refractory in different slags or different refractories in the same slag. If the slag is not saturated with refractories components, using relevant phase diagrams, the conditions under which solid reaction product phases form at refractories/slag interfaces can be predicted, which assists understanding of direct and indirect dissolution. By checking the compatibility between impurities or additives and refractories components at high temperatures, their influence on corrosion resistance can be predicted, aiding their selection. Phase diagrams also reveal that atmosphere affects refractories corrosion resistance by altering the valence of some components (in particular, ...

Development of fracture toughness of ultrahigh strength, medium carbon, low alloy steels for aerospace applications

Abstract: Ultrahigh strength, low alloy, steels with a medium carbon (0·25–0·50 wt-%) content and various amounts of chromium, molybdenum, nickel, silicon, and vanadium have been needed for high performance aerospace parts. The steels can be successfully employed at yield strengths of ≥1400 MPa, but their commercial use is often limited in practice by their poor fracture toughness. The present review surveys studies carried out over about the past 10 years to improve the fracture toughness of ultrahigh strength, medium carbon, low alloy steels, and the present understanding of this property is outlined. Particular emphasis is placed on improving the fracture toughness by microstructural control via thermal and thermomechanical treatments, modification of sulphide inclusions, and new alloying design. The major metallurgical factors controlling the fracture toughness are discussed for each of these techniques.

Interface engineering in oxide fibre/oxide matrix composites

Abstract: Oxide fibre/oxide matrix composites form an important and attractive subpart of ceramic matrix composites because of their inherent stability in oxidising atmospheres at high temperatures An important attribute of such composites, however, is that the interfacial bond between oxide matrix and oxide fibre is generally very strong, and consequently, the toughness and damage tolerance of such composites are low One way to overcome this problem is to tailor the interface such that the energy dissipating phenomena such as debonding and crack deflection at the fibre/ matrix interface, followed by fibre pullout are brought into play In this paper, the salient aspects of control of interface characteristics in oxide fibre/oxide matrix composites, with emphasis on composites consisting of alumina and mullite based oxide fibres in a variety of oxide matrixes, are reviewed

Materials issues in renewable energy power generation

Abstract: Rapid increases in power generation from renewable sources are being advocated as part of the global strategy to reduce CO2 emissions. Renewable energy technologies range from the well established, such as hydropower, to the emergent, such as biomass gasification. Each technology has its own individual materials requirements, to permit reductions in capital costs, to increase longevity in harsh environments, or to enable generating efficiency to be increased. The status of all of the major renewable energy technologies is briefly reviewed with particular emphasis on the structural materials issues and advances pertinent to their further development. The technologies covered include wind, hydro, wave and tidal, waste and biomass combustion and gasification, geothermal, solar thermal, and solar photovoltaic.

Hydrogen transport through metals determined by electrochemical methods

Abstract: The topics related to electrochemical hydrogen absorption reaction into the metal electrode are reviewed, with particular attention to palladium as a model system. Faradaic admittance expressions involving hydrogen transport through the metal in the low hydrogen concentration range are reviewed under the appropriate boundary conditions experimentally accessible. It was shown that direct hydrogen absorption into the Pd foil without adsorbed intermediate state of hydrogen does occur at certain hydrogen overpotentials. The nature of the subsurface hydride just beneath the metal surface is discussed together with its role in hydrogen transport through Pd metal. Transport of hydrogen through the metal in the presence of two coexisting phases of hydrogen-rich and -deficient phases is discussed in terms of stress generation at the interface of two hydride phases as well as the movement of the boundary between two phases. In addition, the effects of hysteresis and microstructure on the hydrogen absorptio...

Micromechanics of defects in transversely isotropic elastic solids with interfaces

Abstract: The response of microstructural defects to internal and external fields is an important subject in materials science and engineering. The knowledge of elastic deformations due to defects is the first step towards understanding the fundamental effects of defects on the mechanical and physical properties of materials. A recently developed method for obtaining the explicit three- dimensional closed form elastic solution in solids with defects in the presence of an interface is reviewed. The interface is a planar boundary between two semi-infinite solids. The solids are either perfectly bonded together or in frictionless contact with each other at the planar interface. The defects are point defects, dislocations, disclinations, inclusions, elastic inhomogeneities, and thermal inhomogeneities. The method is a modified point force method. The elastic solution is expressed in terms of force double Green's functions in a simple form. The force double Green's functions are the partial derivatives of point...