Showing papers in "International Materials Reviews in 1997"

Functionally graded metals and metal-ceramic composites: Part 2 Thermomechanical behaviour

Abstract: Following a review of the processing of functionally graded metals and metal-ceramic composites in Part 1; this Part 2 of the two part series focuses on the thermomechanical behaviour. The paper begins with an overview of the fundamentals of thermoelastic and thermoplastic deformation in metal-ceramic composites. Various approaches, including the rule of mixture approximations, mean field theories, crystal plasticity models, discrete dislocation models, and continuum finite element formulations of the constitutive phases of the composites, are discussed, and the significance and limitations of these approaches are highlighted. Issues specific to the thermomechanical analyses of graded materials are then addressed. It is reasoned that the introduction of a new length scale to the problem due to compositional gradients inevitably calls for detailed micromechanical analyses of the size, shape, continuity, and spatial dispersions of the constituent phases of graded metal-ceramic composites. Models for...

Structure of non-graphitising carbons

Abstract: Despite many years of research, the detailed atomic structure of many important carbon materials remains poorly understood. In particular, the structure of those carbons which can not be transformed into graphite by high temperature heat treatment has never been clearly established. These non-graphitising carbons are of considerable commercial importance in a variety of fields, and a better understanding of their structure is clearly needed. Recently, it has been suggested that non-graphitising carbons may have a microstructure which is related to that of fullerenes. In the present paper, the evidence for this will be considered in detail and the advantages of the new model over previous models of non-graphitising carbons will be discussed. As well as microporous nongraphitising carbons, other forms of carbon including glassy carbon and carbon fibres will be considered.

Mechanical properties of FeAl

Abstract: The B2 compound FeAl has attracted considerable attention in the past 20 years as a candidate material for structural applications both in hostile environments and at intermediate temperatures. However, until lately, the mechanical behaviour of FeAl was not well understood because the effects of both vacancy retention following elevated temperature heat treatments and environmental embrittlement were not taken into account. More recently, testing performed under conditions where these effects are controlled has taken place, allowing a more complete understanding of the compound's mechanical properties. The aim of the present paper is to provide an overview of the mechanical behaviour of FeAl, with particular emphasis on recent studies, together with, where appropriate, reinterpretation of older data, in light of our present understanding.

Fracture toughness of type 304 and 316 stainless steels and their welds

Abstract: Fracture toughness data for type 304 and 316 stainless steels and their welds are reviewed. The material and operational parameters evaluated in this paper include: heat to heat variability; weld process variations; welding induced, heat affected zones; crack orientation; cold work; monotonic and cyclic prestrain; long term thermal aging; neutron irradiation; temperature; and loading rates. Statistical analyses of literature data are provided to establish minimum expected fracture toughness values for use in fracture mechanics design evaluations. Guidance is also provided concerning the potential for component failure and when fracture mechanics assessments are required to guard against unstable fracture. Macroscopic fracture toughness properties are correlated with key microstructural features and operative fracture mechanisms.

Grain boundary diffusion: fundamentals to recent developments

Abstract: Diffusion along grain boundaries (GBs) often controls the evolution of structure and properties of engineering materials at elevated temperatures. A knowledge of diffusion characteristics of GBs and deep fundamental understanding of this phenomenon are critical to many materials applications. The purpose of the present review is twofold. First, a comprehensive overview of boundary diffusion theory with emphasis on the interpretation of concentration profiles measured in diffusion experiments is given. The most important situations encountered in boundary diffusion experiments are considered, such as diffusion in the B and C regimes, diffusion in the presence of segregation, diffusion in multiple GBs, diffusion in moving GBs, etc. The discussion of such situations is accompanied by easy to use recipes for the determination of boundary diffusion characteristics from the concentration profiles. Second, the most recent advances in the basic understanding of GB diffusion are reviewed. The focus is plac...

Fatigue behaviour of iron based sintered material: a review

Abstract: During the last decades, the demand for lower production costs and the reduction of weight resulted in the use of PM iron and PM steel for highly stressed fatigue loaded components. The fatigue behaviour of such PM materials has been described in numerous papers. However, the studies were mostly limited to investigations of the influence of various processing, sintering parameters, and alloying techniques. Relatively few authors applied more basic scientific considerations to understand the fatigue behaviour of sintered iron based material. Thus, a main objective of this review is to consider the characteristic porous microstructure, which may consist of singular isolated pores in a solid structure up to an interconnected three dimensional porous configuration resembling an almost different material. The response of such structures to various stages of the fatigue process, starting from the damaging criteria such as the cyclic plastic strain, to crack initiation and crack propagation will be prese...

Development of substrate-coating adhesion in thermal spraying

Abstract: Different mechanisms for the development of substrate-coating adhesion during thermal spraying are considered. The most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed on the droplet impact, and solidification of the lower part of the splat. Possible deformation of the substrate surface, rebounding of the impinging droplets, and influence of spraying at off-normal angles on the development of adhesion are also considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region are shown to be effective in creation of adhesive bonds. The roles of the diffusion processes and the influence of the splat morphology on adhesion are also discussed. The theoretical results agree well with the observed tendencies of behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in good agreement with the experimental data...

Mechanisms of liquid metal induced embrittlement

Abstract: Liquid metal induced embrittlement (LMIE) refers to the loss of ductility of normally ductile metals and alloys when stressed while in contact with a liquid metal. Various models have been proposed to describe the mechanisms by which LMIE failure takes place. In the present study the most promising of these are discussed in detail and analysed critically in the light of experimental results. The advantages and limitations of each are highlighted and, where possible, comparisons between the characteristics of each are presented. The diverse results obtained in previous investigations and the difficulties encountered in LMIE testing have resulted in numerous and often contradictory models.

Synthesis and processing of superhard carbon nitride solids

Abstract: Carbon nitride solids, predicted to exhibit bulk moduli comparable with or exceeding that of diamond, are reviewed for both theoretical and experimental developments. Progress in the theory and preparation of these solids is described detailing the structure, composition, thermodynamic aspects, and physical and mechanical properties. The synthesis and characterisation techniques involved are described from the viewpoint of their potential usefulness. Such issues as assessing the quality of synthesised films by revealing the nature of the chemical bonds between carbon and nitrogen and their chemical composition in each bonding state are discussed. Based on assessment of the experimental work to date, future directions for investigation of both amorphous and crystalline carbon nitrides are suggested. Some lattice matched compounds as suitable intermediate layers to promote the epitaxy of metastable C3N4 have been proposed and superlattice geometries with these lattice matched compounds are suggeste...

Stress induced microstructural evolution via discrete atom method

Abstract: The shapes, sizes, and distribution of second phase precipitates are the principal factors determining the mechanical, electrical, and magnetic properties of a wide variety of high technology alloys and ceramics. In the initial processing, and also in high temperature applications, such as jet engine superalloys, the precipitate morphologies evolve in time and the properties change. An understanding of the elastic interactions between matrix and precipitates, among the precipitates, and between precipitates and dislocations is crucial for predicting and manipulating the properties of the crystalline materials. There has been a need for a computational technique, therefore, by which one can analyse the elastic state associated with arbitrarily shaped precipitates whose elastic constants are different from those of the matrix phase. This overview presents a new technique, termed the discrete atom method, which is predicated on the combination of statistical mechanics and linear elasticity. The probl...