Showing papers in "International Materials Reviews in 1998"

Modelling of hot deformation for microstructural control

Abstract: Bulk metal working processes are carried out at elevated temperatures where the occurrence of simultaneous softening processes would enable the imposition of large strains in a single step or multi...

Novel materials synthesis by mechanical alloying/milling

Abstract: An account is given of the research that has been carried out on mechanical alloying/milling (MA/MM) during the past 25 years. Mechanical alloying, a high energy ball milling process, has established itself as a viable solid state processing route for the synthesis of a variety of equilibrium and non-equilibrium phases and phase mixtures. The process was initially invented for the production of oxide dispersion strengthened (ODS) Ni-base superalloys and later extended to other ODS alloys. The success of MA in producing ODS alloys with better high temperature capabilities in comparison with other processing routes is highlighted. Mechanical alloying has also been successfully used for extending terminal solid solubilities in many commercially important metallic systems. Many high melting intermetallics that are difficult to prepare by conventional processing techniques could be easily synthesised with homogeneous structure and composition by MA. It has also, over the years, proved itself to be superior to rapid solidification processing as a non-equilibrium processing tool. The considerable literature on the synthesis of amorphous, quasicrystalline, and nanocrystalline materials by MA is critically reviewed. The possibility of achieving solid solubility in liquid immiscible systems has made MA a unique process. Reactive milling has opened new avenues for the solid state metallothermic reduction and for the synthesis of nanocrystalline intermetallics and intermetallic matrix composites. Despite numerous efforts, understanding of the process of MA, being far from equilibrium, is far from complete, leaving large scope for further research in this exciting field.

Progress in joining of advanced materials

Abstract: Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure-mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...

Effects of hydrostatic pressure on mechanical behaviour and deformation processing of materials

Abstract: The processing and subsequent mechanical behaviour of a variety of commercially important materials are affected by the imposed stress state. In this review, the experimentally documented effects of superimposed pressure on deformation under quasistatic conditions are summarised, followed by a presentation of the effects of superimposed pressure on the fracture behaviour of a variety of materials including both ductile and brittle systems. It is shown that the pressure responses of a variety of materials show distinct differences and the potential reasons for these differences are presented. Finally, in the light of all of these observations, the effects of changes in stress state on deformation processing are reviewed. In particular, the evolution of hydrostatic stresses during various forming operations is covered followed by a review of published work and the potential benefits of superimposing pressure during processing of a variety of materials.

Microwave dielectric solid–solution phase in system BaO–Ln203–Ti02 (Ln = lanthanide cation)

Abstract: Microwave resonators are used extensively in telecommunications equipment, including cellular telephones and satellite links, and are at the heart of a multibillion pound market. Many microwave materials are based on or related to the perovskite crystal structure. One such family of microwave materials is Ba6–3xLn8+2xTi18O54 (Ln = lanthanide cation), which has been produced commercially for over 10 years. The extensive, and often contradictory, literature on Ba6–3xLn8+2xTi18O54 is here critically reviewed, with special attention given to its historical development, crystal structure, and the many attempts to tune its properties via doping and variations in processing parameters. Emphasis is placed on the commercially important Nd analogue, although other systems are also considered; and the technologically significant BaO–TiO2 subsystem, which comprises several other electrical ceramics, is revisited to correct many historical inaccuracies.

Ferritic power plant steels: remanent life assessment and approach to equilibrium

Abstract: The steels used in the power generation industry are almost always given a severe tempering heat treatment before they enter service. This might be expected to give them a highly stable microstructure which is close to equilibrium. In fact, they undergo many changes over long periods of time. This paper is a review of some of the methods which exploit the changes in order to estimate the life that remains in alloys which are only partly exhausted.

Analytical electron microscopy of discontinuous solid state reactions

Abstract: The discontinuous solid state reactions represent a group of diffusional solid state phase transformations during which the solute redistribution accompanying the formation of a new phase occurs on a nanometer scale just at the moving reaction front. Such solute changes can be sufficiently revealed by means of analytical electron microscopy (AEM). A number of systematic investigations made using AEM during the last decade in discontinuous precipitation, dissolution, and coarsening are reviewed and critically analysed. Measurements of the solute concentration profiles in the direction parallel to the reaction front are strongly emphasised, as they allow verification of theoretical models of discontinuous reactions and consideration of discontinuous reactions as local phenomena in the form of a number of events occurring in individual cells. Such an attempt removes the divergence between the tracer diffusivity values obtained by the sectioning technique and the chemical diffusivity data obtained fro...

Creep analysis and life assessment using cross-weld specimens

Abstract: Cross-weld creep specimens have been widely studied in order to aid in the understanding of the creep behaviour of real, welded structures. The studies published to date have been of an analytical, numerical, or experimental nature. This review describes the analytical methods used to predict the creep behaviour of cross-weld specimens with simple geometries. For both simple and more complex geometries, use of the finite element (FE) method to obtain stationary state stresses and strains has been made, and the solutions contrasted with those of the simpler analytical methods. Use of the creep continuum damage constitutive equations in FE analyses to predict the failure times of, and rupture positions in, cross-weld specimens has been described. The effect of various features, defined by the analysis methods, is highlighted. Problems with the experimental validation of these effects are considered and selected experimental data from creep tests on cross-weld creep specimens for various materials an...

Laser surface hardening of structural ferrous alloys

Abstract: Laser surface alloying is a technique for altering the surface properties of a substrate without affecting those of the bulk, by simultaneously melting the surface of the substrate and adding alloying elements to it. By applying the proper combination of processing parameters and alloying elements, a surface can be modified to accommodate a wide variety of properties. Alloying with Cr and Ni improves the resistance to oxidation and corrosion and increases the hardness. Boronising with the aid of a laser results in a brittle layer, exhibiting very high hardness and low wear rates but a low corrosion resistance due to the cracked surface. The addition of high melting point carbides to the surface leads to increased hardness and wear resistance, except when the matrix is very soft. In this review the microstructure and properties are presented in detail.