Hydrogen-assisted fracture at notches

Abstract

Although notched specimens have been much used in studies of hydrogen embrittlement, precracked–specimen testing has become more widespread in the last 15 years or so. However, recent work has revived interest in the use of notched specimens for fundamental studies. The background for, and recent results from, the use of notched specimens is described, and current problems in the interpretation of fracture behaviour are discussed. Of particular interest are the questions relating to the appropriateness of assuming a critical, local hydrogen concentration for fracture and to the degree of fracture control by stress, strain, or some combination. An encouraging development is the recent convergence in interests between micromechanism studies by materials scientists and work on the parameters of microfracture by the mechanics community.

MST/74     

Magneto-Optical Imaging of Superconductors for Liquid Hydrogen Applications

Restricted deposits of fossil fuels and ecological problems created by their extensive use require a transition to renewable energy resources and clean fuel free from emissions of CO₂. This fuel is likely to be liquid hydrogen. An important feature of liquid hydrogen is that it allows wide use of superconductivity. Superconductors provide compactness, high efficiency, savings in energy and a range of new applications not possible with other materials. The benefits of superconductivity justify use of low temperatures and facilitate development of fossil-free energy economy. The widespread use of superconductors requires a simple and reliable technique to monitor their properties. Magneto-optical imaging (MOI) is currently the only direct technique allowing visualization of the superconducting properties of materials. We report the application of this technique to key superconducting materials suitable for the hydrogen economy: MgB₂ and high temperature superconductors (HTS) in bulk and thin-film form. The study shows that the MOI technique is well suited to the study of these materials. It demonstrates the advantage of HTS at liquid hydrogen temperatures and emphasizes the benefits of MgB₂, in particular.     

Development of intermetallic materials for aerospace systems

Abstract

Certain intermetallic materials have undergone an evolutionary process whereby application of some of them could provide major benefits in aerospace systems. The realisation of the potential of intermetallic alloys based on Ti₃Al has provided significant hope for making still greater advances in turbine performance through further developments in other intermetallic materials. However, examination of the past four years of progress toward this goal has highlighted the problem that much of the fundamental understanding of process–structure–property relationships in these materials, which is the technology base upon which their application relies, has simply not been developed and suggests that widespread implementation of these materials lies in the distant future. This paper briefly discusses the problems of employing intermetallics in aerospace systems, reviews recent research progress on selected intermetallic alloys currently under investigation as high temperature structural materials, and assesses the status of intermetallics as turbine engine materials. Specifically, advances and findings from studies carried out during the past few years on alloys of the titanium and nickel aluminides and on intermetallics that are intended for service at temperatures above 1000°C are discussed. Technical challenges and factors affecting the pace of development are highlighted throughout.

MST/1562     

Perovskite-type catalytic materials for environmental applications

Abstract

Perovskites are mixed-metal oxides that are attracting much scientific and application interest owing to their low price, adaptability, and thermal stability, which often depend on bulk and surface characteristics. These materials have been extensively explored for their catalytic, electrical, magnetic, and optical properties. They are promising candidates for the photocatalytic splitting of water and have also been extensively studied for environmental catalysis applications. Oxygen and cation non-stoichiometry can be tailored in a large number of perovskite compositions to achieve the desired catalytic activity, including multifunctional catalytic properties. Despite the extensive uses, the commercial success for this class of perovskite-based catalytic materials has not been achieved for vehicle exhaust emission control or for many other environmental applications. With recent advances in synthesis techniques, including the preparation of supported perovskites, and increasing understanding of promoted substitute perovskite-type materials, there is a growing interest in applied studies of perovskite-type catalytic materials. We have studied a number of perovskites based on Co, Mn, Ru, and Fe and their substituted compositions for their catalytic activity in terms of diesel soot oxidation, three-way catalysis, N₂O decomposition, low-temperature CO oxidation, oxidation of volatile organic compounds, etc. The enhanced catalytic activity of these materials is attributed mainly to their altered redox properties, the promotional effect of co-ions, and the increased exposure of catalytically active transition metals in certain preparations. The recent lowering of sulfur content in fuel and concerns over the cost and availability of precious metals are responsible for renewed interest in perovskite-type catalysts for environmental applications.     

Robust and exploratory data analysis techniques for modelling materials and process variables in welding

Abstract

The modelling and quantification of materials data using numerically optimised mathematical functions is an important and widespread practice. In the present paper the development of robust and exploratory data analysis strategies, based on exploratory data analysis procedures and robust statistics, is described. Application of this new PC based data analysis system to two sets of established data which relate materials information to process variables demonstrates the advantages of a comprehensive and robust approach to analysis. The advantages of techniques that use both ‘degree off it’ and also predictive statistical criteria for mathematical/statistical models are described.

MST/1945     

Effect of hydrogen on crack tip blunting characteristics of 304L stainless steel

Abstract

J integral measurements have been conducted on various samples of 304L stainless steel having different levels of austenite stability as a result of variations in composition. In certain materials, significant deviations from the theoretical blunting line (?a=J/2σ_f, where ?a is the effective crack length for a particular J integral and σ_f is the mean of the 0·2% proof stress and the ultimate tensile strength) were shown in the experimental results. These deviations are discussed and a method is proposed for rationalising the results to allow a more meaningful assessment to be made of the effects of hydrogen charging on the various materials. In this way, a significant reduction in the value of J_Ic is clearly revealed in materials in which martensite may be produced by deformation. Hydrogen charging also produces a significant decrease in the slope of the experimental blunting line for these materials that is not consistent with the observed increase in proof stress. It is suggested that this decrease is associated with the localisation of plastic deformation by the presence of hydrogen.

MST/775     

Mechanical property requirements for aero gas turbine materials

Abstract

The outstanding performance of current military and civil aero gas turbine engines is linked closely to the way in which modern design and manufacturing techniques have become totally integrated with materials designed specifically for operation within the hostile environment of a gas turbine. Advanced titanium alloys are used extensively throughout the compressor and nickel-base superalloys dominate materials application in the turbine. In spite of current achievements, the engine designer is still under severe competitive pressure to improve engine performance still further and this will inevitably lead to even more demanding material requirements. The present paper outlines the continuing trends in engine development and describes the impact these are having on materials technology in general and the mechanical property requirements of nickel-base superalloys in particular.

MST/512     

Hydrogen storage of a mechanically milled carbon material fabricated by plasma chemical vapor deposition

Abstract

Carbon materials were prepared by plasma chemical vapor deposition at different pressures without catalyst, and the structure and hydrogen storage characteristics of milled and unmilled samples of the materials were evaluated. Using this approach, we were able to fabricate graphite microcrystals with a crystallite size of several nanometers, and the crystallite size and surface area could be controlled by changing the pressure during plasma chemical vapor deposition. The hydrogen storage capacity of the unmilled materials was 0.3?wt%, but milling increased this value to 1.0?wt% by reducing the crystallite size and increasing the crystallite surface area.     

Forgotten phenomenon of materials selection and use in geothermal energy applications

Abstract

Pioneering activities of experimental testing and application of engineering experience identified a range of materials and process design guidelines for early geothermal energy plant developments. The success of these guidelines is evidenced by energy plant operation periods extended well beyond design lifetimes, not only in New Zealand but around the world. The efficacy of these guidelines has frequently been tested and in many cases failures have been encountered where the specific conditions of environment-material combinations have not been reliably defined. New R&D continues to build on these guidelines with opportunity for development for more aggressive environments. The historical and evolving “rules of thumb” for materials selection for geothermal energy applications are reviewed and illustrated by experienced successes and failures, the majority of the failures being attributable to readily identifiable forgotten phenomenon.     

Functionalization of [60]Fullerene Using a Novel Chemical Vapor Modification Method

Abstract

Gas phase reactions of [60]fullerene with hydrogen atoms and/or methyl radicals have been performed by hot filament chemical vapor modification (CVM) to produce hydrogenated and/or methylated [60]fullerene. The CVM process parameters such as the time of reaction, pressure, and percentage of methane diluted in hydrogen have been varied so as to obtain the addended fullerene reaction products. Mass spectrometry and ¹H NMR techniques were used to characterize the reaction products obtained.     

Materials aspects of copper interconnection technology for semiconductor applications

Abstract

Metallisations, which form key components on semiconductor devices and circuits, have gone through many changes since the first discoveries of these devices. Metallisation applications include contacts to semiconductor surfaces, gate on metal oxide semiconductor devices, and interconnections, each defining its own criteria for the application of a given metal or metallisation scheme. This paper focusses only on the interconnection application. A brief examination of the desired properties of an interconnection metal, especially Cu, and a discussion of the materials aspect of this application has been carried out, including whether copper satisfies the needs of today and will satisfy them in the future.     

Global gene expression analysis for evaluation and design of biomaterials

Abstract

Comprehensive gene expression analysis using DNA microarrays has become a widespread technique in molecular biological research. In the biomaterials field, it is used to evaluate the biocompatibility or cellular toxicity of metals, polymers and ceramics. Studies in this field have extracted differentially expressed genes in the context of differences in cellular responses among multiple materials. Based on these genes, the effects of materials on cells at the molecular level have been examined. Expression data ranging from several to tens of thousands of genes can be obtained from DNA microarrays. For this reason, several tens or hundreds of differentially expressed genes are often present in different materials. In this review, we outline the principles of DNA microarrays, and provide an introduction to methods of extracting information which is useful for evaluating and designing biomaterials from comprehensive gene expression data.     

X-ray diffraction as a promising tool to characterize bone nanocomposites

Abstract

To understand the characteristics of bone at the tissue level, the structure, organization and mechanical properties of the underlying levels down to the nanoscale as well as their mutual interactions need to be investigated. Such information would help understand changes in the bone properties including stiffness, strength and toughness and provide ways to assess the aged and diseased bones and the development of next generation of bio-inspired materials. X-ray diffraction techniques have gained increased interest in recent years as useful non-destructive tools for investigating the nanostructure of bone. This review provides an overview on the recent progress in this field and briefly introduces the related experimental approach. The application of x-ray diffraction to elucidating the structural and mechanical properties of mineral crystals in bone is reviewed in terms of characterization of in situ strain, residual stress–strain and crystal orientation.     

Growth and characterisation of nitride nanostructures

Abstract

The group III nitrides are fascinating materials, which, due to their large, direct bandgaps, have found widespread application in optoelectronics devices. The self-assembled growth of nanostructures has the potential to improve device characteristics and to stimulate the development of new concepts. Some exciting research on nitride nanostructures has already been carried out, but to truly understand their formation and control their properties many aspects of materials science must be brought together. It is necessary to understand nitride crystal structures and surface properties, to identify the growth modes in both homo- and heteroepitaxy, and to explore the thermodynamic and chemical factors that control them. To elucidate the properties of nanostructures also requires understanding of the unusual band structure of the nitrides, and knowledge of its effects on the luminescent characteristics of nanostructure arrays and individual quantum dots. In other materials systems, particularly Ge/Si and InAs/GaAs, a wealth of research has already been done to aid understanding of the formation and properties of self-assembled nanostructures. This will be reviewed in exploring the behaviour of the nitrides, and discoveries about nitride nanostructures will be used in turn to expand the overall picture of nanostructure growth.     

Recent progress in microstructural hydrogen mapping in steels: quantification,kinetic analysis,and multi-scale characterisation

ABSTRACT

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

CHARACTERISATION OF THE STRUCTURE OF CONCENTRATED,FLOCCULATED SUSPENSIONS

ABSTRACT

The structure and properties or concentrated, flocculated suspensions are of widespread importance with respect to separation, handling and related processes in the chemical and allied industries. To improve understanding of such systems it is essential that these materials be studied using methods which directly probe structure and structure-dependent characteristics. In this paper we describe investigations of floc structure using techniques which are suitable for examining morphology in concentrated dispersions, avoiding dilution, or other gross perturbation of the system, before or during measurement. From studies or electrolyte-coagulated and, in particular, polymer-flocculated suspensions a number of general principles are mapped out concerning the influence of basic parameters on the structure and properties of aggregated dispersions.     

Ceramic coating technology

Modern technology calls for systems performing satisfactorily under extreme and often adverse operating conditions; hence it has become a technical and economic necessity to protect structural materials from the hostile environment. Thus ceramic coating technology (CCT) has found widespread applications in many diverse industries for the protection of structural materials. Ceramic coatings are used to minimise effects such as high temperature degradation, corrosion, erosion and wear. Two techniques, viz. thermal spray and chemical vapour deposition, are well-developed and commercially established for preparing coatings of almost all ceramic materials. Each process has its own advantages and disadvantages and the choice of any particular method depends on the application, economics etc. CCT is both technically and commercially well-established abroad. Commercial equipment suitable for general application in coating shops as well as custom-engineered ones to suit specific requirements are readily available. Raw materials such as powders, wires, rods, gases, chemicals etc. are readily available and are moderately priced. R & D is continuously in progress adding new materials, techniques and applications every year. In India, the full potential of the technology has not been tapped as yet. A few governmental organisations such as BARC, BHEL, DMRL, HAL, Air India etc., have exploited the application potential of the technology, while the private industry has begun to adopt the technology only recently. Such a situation is due to a variety of reasons including limited awareness of these new techniques amongst our user industries, nonavailability of raw materials and equipment indigenously etc. This article is based on a report co-authored with three other scientists and submitted to the Department of Science and Technology, New Delhi.     

‘Beautiful’ unconventional synthesis and processing technologies of superconductors and some other materials

Abstract

Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of ‘beautiful’ technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches.     

First-principles electronic-band calculations on organic conductors

Abstract

Predicting electronic-band structures is a key issue in understanding the properties of materials or in materials design. In this review article, application examples of first-principles calculations, which are not based on adjustable empirical parameters, to study electronic structures of organic conductors are described.     

Monitoring elastic strain and damage by neutron and synchrotron X-ray beams

Abstract

Large scale neutron and synchrotron X-ray facilities have been providing important information for physicists and chemists for many decades. Increasingly, materials engineers are finding that these facilities can also provide important information non-destructively. Highly penetrating neutron and X-ray synchrotron beams provide the materials engineer with a means of obtaining information about the state of stress and damage deep within materials. The principles underlying the elastic strain measurement and damage characterisation techniques are introduced. The capabilities of the methods are illustrated through a number of practical applications including: mapping damage and stress transfer fibre by fibre in continuous fibre reinforced composites during loading, measurement of residual stresses in welding, the use of measurements to refine finite element models, and creep cavitation cracking in power plant steels.