Showing papers in "Materials Science and Technology in 2006"

Surface modification of nanoscale fillers for improving properties of polymer nanocomposites: a review

Abstract: Direct incorporation of inorganic nanoscale building blocks into polymers represents a typical way for preparing polymeric nanocomposites. The most important aspect in preparation of nanocomposites through dispersive blending is surface modification of the nanofillers. It is able to increase hydrophobicity of the fillers, enhance interfacial adhesion via chain entanglement or chemical bonding and eliminate the loosen structure of filler agglomerates. The present paper reviews the state of the art of nanoparticles/polymer composites, including the specific surface pretreatment techniques and their applications. Especially, the role of treated nanoparticles and the mechanisms involved in the improvement of mechanical properties and wear resistance of the composites are highlighted.

Type IV cracking in ferritic power plant steels

Abstract: There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.

High resolution mapping of strains and rotations using electron backscatter diffraction

Abstract: The angular resolution of electron backscatter diffraction (EBSD) measurements can be significantly improved using an analysis based on determination of small shifts in features from one pattern to the next using cross-correlation functions. Using pattern shift measurements at many regions of the pattern, errors in the best fit strain and rotation tensors can be reduced. The authors show that elements of the strain tensor and small misorientations can be measured to ± 10−4 and ±0·006° for rotations. We apply the technique to two quite different materials systems. First, we determine the elastic strain distribution near the interface in a cross-sectioned SiGe epilayer, Si substrate semiconductor heterostructure. The plane stress boundary conditions at the sample surface are used to separate every term in the strain tensor. Second, the applicability to structural materials is illustrated by determining the lattice curvature caused by dislocations within the plastic zone associated with the wake and ...

Mechanical stabilisation of austenite

Abstract: A theory has been developed for the mechanical stabilisation of plastically deformed austenite by balancing the force which drives the transformation interface against the resistance from dislocation debris in the austenite. The work has been used to explain why very large strains are required to mechanically stabilise certain stainless steels, and also to interpret the subunit mechanism of bainite growth.

Comparative study on sigma phase precipitation of three types of stainless steels: austenitic, superferritic and duplex

Abstract: The present work studies, in a comparative manner, the sigma phase precipitation of three stainless steels: austenitic type 316L (17Cr–12Ni–2·5Mo, wt-%), superferritic type DIN W. Nr. 1·4575 (Nb–28Cr–4Ni–2Mo, wt-%) and duplex type DIN W. Nr. 1·4462 or UNS S31803 (22Cr–5·5Ni–3Mo–0·14N, wt-%). In austenitic stainless steel, the formation of sigma phase occurred both at austenite grain boundaries and inside delta ferrite islands. In superferritic stainless steel, sigma phase occurred both at grain boundaries and in the grain interior. In the ferrites, both in the duplex and in the austenitic steel, sigma phase formation occurred by eutectoid reaction ferrite→sigma + austenite. The tendency towards precipitation of the sigma phase in the three types of steel investigated can be placed in the following sequence: duplex>superferritic>austenitic.

Extent of back diffusion during solidification of experimental nickel based single crystal superalloy

Abstract: The microsegregation inherited from the directional solidification and quenching (DSQ) of an experimental third generation single crystal superalloy is characterised using electron probe microanalysis (EPMA). A statistical treatment of the data is used to estimate the composition of the solid in the mushy zone. Comparison of the experimental data with the predictions from a numerical model indicates that the results cannot be rationalised without acknowledging that back diffusion is occurring; it is demonstrated that the magnitudes of the diffusion coefficients which need to be invoked are consistent with recent experimental measurements of the coefficients deduced from diffusion couples.

Nanocomposites of polymers and inorganic particles: preparation, structure and properties

Abstract: Although composites with, e.g.nanosized SiO2, TiO2, carbon black or gold are known for a long time, an increasing number of polymer composites comprising inorganic nanoparticles have been described only in the last 15 years. Frequently employed inorganic materials include metals (e.g. gold, silver or copper), semiconductors (e.g. PbS or CdS) or clay minerals (e.g.montmorillonite or vermiculite). In most cases, nanocomposites with spheric or plate like particles have been prepared so far but materials with nanorods (rod like particles, including nanotubes) also attracted attention. The structure of nanocomposites is essentially established by the arrangement of the particles in the polymer matrix. The particles may be dispersed as individual primary particles or as agglomerated particles (secondary particles), and the primary or secondary particles can be arranged randomly or in an ordered or oriented state, depending on the method of nanocomposite preparation and processing (including, e.g. coprec...

Recent advances in polymer/layered silicate nanocomposites: an overview from science to technology

Abstract: Polymer/layered silicate nanocomposites (PLSNCs) offer remarkably improved mechanical and other properties with low inorganic filler loading. The major development in this field has been carried out over the last one and a half decades. However, the authors are far from the goal in terms of understanding the mechanisms of the enhancement effect in the nanocomposites. Continued progress in nanoscale controlling and an improved understanding of the physicochemical phenomena at the nanometre scale have contributed to the rapid development of novel PLSNCs. The present paper describes recent advances in PLSNCs with the primary focus on these advances from basic science to technology.

Thinking big (and small) about energetic materials

Abstract: A significant recent development in the field of energetic materials has been the introduction of nanotechnology. Nanoparticle based materials have the potential of releasing more than twice as much energy as the best molecular explosives. The possibility of developing nanoenergetic materials that are optimised for specific applications by controlling the spatial organisation on length scales ranging from nanometres to metres is discussed. To do so a fundamental understanding of the relationships between structure and performance that does not yet exist is required. Experimental measurements using molecular spectroscopy with high time and space resolution are presented that help clarify the fundamental mechanisms of reaction between fuel nanoparticles and surrounding oxidisers, and the relationships between nanoenergetic structure and performance properties such as the energy release rate and the reaction propagation rate.

Application of EBSD to analysis of microstructures in commercial steels

Abstract: Electron backscatter diffraction (EBSD) is a very powerful technique for microstructural characterisation and analysis of crystalline materials. Many of the structural parameters that control the properties and performance of the material can be derived from EBSD data, e.g. grain size, phase constituents, mechanical anisotropy and residual strain. This should make EBSD a valuable tool to control and develop microstructures of commercial metallic materials but technique has mainly been used in basic research at universities and not to the same extent for research and development in industry. The development in scanning electron microscopes and EBSD equipment in recent years makes it possible to measure 'difficult' structures, e.g. with higher dislocation content, but can it manage the complexity of the structures of commercial materials and achieve reliable data? During 2004 an EBSD round robin test on industrial metallic materials was coordinated by KIMAB to test the current status of the techniqu...

X-ray microtomographic observation of intergranular stress corrosion cracking in sensitised austenitic stainless steel

Abstract: Intergranular stress corrosion cracking in a sensitised type 302 stainless steel wire has been observed in situ using high resolution X-ray microtomography. Tomography enables the development and failure of crack bridging ligaments to be studied in detail in three dimensions. Direct comparison of these features has been made with scanning electron microscopy fractography. The crack bridges failed in a ductile manner, with a morphology that is consistent with non-sensitised low energy grain boundaries.

Crystal sensitivities of energetic materials

Abstract: Organic and inorganic explosives were first developed and put into service in the 19th century, before there was much understanding of how the energy release mechanisms differed from those of the long established gunpowder. Theoretical advances in the understanding of shock waves combined with improvements in photographic and electronic techniques led to the hypothesis that a detonation is a shock wave maintained by the rapid release of chemical energy. Studies of accidental ignitions/initiations showed that explosive events can occur even when the energy input is much less than that required to heat the bulk explosive to the deflagration temperature. Hence, the highly fruitful idea of the localised hot spot was conceived. Apart from electrical stimuli, the main hot spot mechanisms are currently accepted as being adiabatic asymmetric collapse of gas spaces (producing gas heating, jetting, viscoplastic work) and the rubbing together of surfaces as in friction or adiabatic shear. Initiation mechanis...

Material dynamics under extreme conditions of pressure and strain rate

Abstract: Solid state experiments at extreme pressures (102100 GPa) and strain rates (10 6 –10 8 s 21 )a re being developed on high energy laser facilities, and offer the possibility for exploring new regimes of materials science. These extreme solid state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth from the Rayleigh–Taylor instability in solid state samples. Lattice compression, phase and temperature are deduced from extended X-ray absorption fine structure (EXAFS) measurements, from which the shock induced a2v phase transition in Ti and the a2e phase transition in Fe, are inferred to occur on subnanosec time scales. Time resolved lattice response and phase can also be measured with dynamic X-ray diffraction measurements, where the elastic– plastic (1D–3D) lattice relaxation in shocked Cu is shown to occur promptly (,1 ns). Subsequent large scale molecular dynamics (MD) simulations elucidate the microscopic dislocation dynamics that underlies this 1D–3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single crystal Cu shocked along the [001] direction is shown to occur at shock strengths of ,20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P.1000 GPa, in the solid state on the National Ignition Facility (NIF) laser.

Artefacts in X-ray microtomography of materials

Abstract: X-ray microtomography is becoming an increasingly popular tool in the study of microstructure and failure mechanisms in biological and engineering materials, producing three-dimensional (3D) maps of the X-ray linear attenuation coefficient. Limitations of the technique are due to, for example, limited X-ray flux, use of polychromatic radiation (in laboratory systems), finite resolution, discrete sampling and X-ray scatter. These give rise to artefacts in the reconstructed image. Knowledge of these artefacts helps to distinguish them from real features and is important for optimising experimental design so as to minimise their effect on the results. To aid identification, artefacts were simulated computationally using an analytical phantom projection generator. Streak, ring, motion and beam hardening artefacts are considered as well as the results of errors in the centre of rotation and missing information in cone beam geometry. Examples of tomographic images of real materials specimens with motion...

Influence of pin geometry on material flow in friction stir welding process

Abstract: The material flow in friction stir welded 2014 Al alloy has been investigated using a marker insert technique (MIT). Results of the flow visualisation show that the material flow is asymmetrical during the friction stir welding (FSW) process and there are also significant differences in the flow patterns observed on advancing and retreating sides. Combining the data from all the markers, a three-dimensional (3D) flow visualisation similar to the 3D image reconstruction technique was obtained. The 3D plot gives the tendency chart of material flow in FSW process and from the plot it can be seen that there is a vertical and circular motion around the longitudinal axis of the weld. The influence of the pin geometry on material flow in FSW was studied. When the column and taper pin are used, there is a 'hole' defect in the vertical mixing plot. When the taper with screw thread pin is used, the vertical material flow is more obvious and there is no distinct 'hole'. Viewing from the advancing side, the p...

Three-dimensional visualisation of fatigue cracks in metals using high resolution synchrotron X-ray micro-tomography

Abstract: The present paper reviews recent developments in high resolution synchrotron X-ray micro-tomography for the study of fatigue cracks in metals. The possibilities and limitations of the tomography technique are described. A fatigue machine used for in situ cycling of samples is presented and three-dimensional images of the growth of fatigue cracks initiated in various Al alloys on natural or artificial defects are shown. Examples of quantitative use of such 3D images are given.

Modelling the 'universal' dielectric response in heterogeneous materials using microstructural electrical networks

Abstract: The frequency dependent conductivity and permittivity of a ceramic composite are modelled using electrical networks consisting of randomly positioned resistors and capacitors. The electrical network represents a heterogeneous microstructure that contains both insulating (the capacitor) and conductive regions (the resistor). To validate model results, a model ceramic conductor–insulator composite was designed consisting of a porous lead zirconate titanate impregnated with different concentrations of water. Excellent agreement between experimental and model data was achieved with a strong correlation with many other ceramics, glasses and composites. It is proposed that the 'universal' dielectric response of many materials is a consequence of microstructural heterogeneity. The modelling approach could be used as a simple and effective method for microstructural design of ceramics and other materials with tailored dielectric properties.

X-ray microtomography studies of localised corrosion and transitions to stress corrosion cracking

Abstract: Two forms of high resolution X-ray tomographic experiments (i.e. synchrotron based X-ray microtomography and desktop microfocus computed X-ray tomography) are demonstrated in the present paper to illustrate the wide application of these techniques for qualitative and quantitative studies of localised corrosion and environmentally assisted cracking. Specifically, synchrotron based X-ray tomography was used to investigate the localised corrosion morphology within aluminium specimens when exposed in situ to a chloride environment while microfocus computed X-ray tomography was used to investigate the morphology and quantify the transition from localised corrosion to stress corrosion cracking in steel specimens exposed ex situ to a simulated corrosive condensate environment.

Materials science and technology aspects of energetic (explosive) materials

Abstract: Crystallographic, phase diagram, X-ray diffraction and mechanical property aspects of energetic materials are described in relation, first, to the properties of individual crystals as employed in composite explosive material formulations and then, second, to initiation of chemical decomposition by imposed mechanical forces and deformations. The mechanically induced decomposition properties are tied to the special character of dislocations in the molecularly bonded energetic crystal structures formed by individual covalently bonded molecules. Microindentation fracture mechanics, hardness stress–strain and drop weight impact measurements on energetic crystals are assessed. Of particular interest are the material granular compaction properties associated with a mechanically induced deflagration to detonation transition (DDT) behaviour. Shock induced initiations of detonation involve experimental and theoretical model considerations at nanometric dimensions. The total results point to improved mechani...

TRIP-assisted steels: cracking of high-carbon martensite

Abstract: Modern TRIP assisted steels contain retained austenite with carbon concentrations in excess of 1 wt-%. Some of their mechanical properties, in particular the toughness and ductility, rely on the diffusionless transformation of this austenite into high-carbon martensite, induced by stress and strain. The properties can be excellent in spite of the fact that freshly formed high-carbon martensite is brittle. This contradictory behaviour has yet to be explained. In the present paper, the authors propose and show experimentally that the tendency of the martensite to crack in a mixed microstructure of austenite and martensite depends on its absolute size. It is demonstrated that in these mixtures, it is more difficult to crack fine martensite. It is the fine scale of the retained austenite in TRIP assisted steels that permits the martensite to be tolerated without endangering their mechanical properties.

A review of advances in processing and metallurgy of titanium alloys

Abstract: Titanium and its alloys exhibit excellent mechanical properties, unrivalled corrosion resistance and outstanding biocompatibility; however, annual global titanium production is dwarfed by commodity metals. This is in part due to the current primary production method (Kroll process), which requires a complex and discontinuous reduction route in addition to several costly downstream processing steps to convert titanium sponge to usable product forms. Alternative extraction processes are reviewed in the present paper with emphasis on the electrochemical reduction routes, such as the Fray–Farthing–Chen (FFC) Cambridge process. Improvements in thermomechanical processing including the use of modelling are briefly examined. Alloy development is also discussed with particular regard to superelastic and shape memory alloys.

Joining of Ceramics

Abstract: Overview, M.G.Nicholas. Part 1 Glass-metal seals, A.P.Tomsia and J.A.Pask: classification of glass-metal seals physical factors in making glass-metal seals chemical factors in making glass-metal seals the making of glass-metal seals. Part 2 Joining glass-ceramics to metals, G.Partridge: the glass-ceramic process bonding glass-ceramics to metals bond development bond quality. Part 3 Metallizing ceraimic surfaces, A.Hey: the ceramic the primary metallizing layer the secondary metallizing layer the brazing stage thermal contraction faults testing metallized components. Part 4 Active metal brazing, M.G.Nicholas: brazes brazing brazements. Part 5 Diffusion bonding, B.Derby: mechanisms the use of interlayers bond strengths and toughness. Part 6 Physiochemical aspects of ceramic-metal joining, J.T.Klomp: interface formation interfacial bonding joint stability. Part 7 Bond strength of ceramic-metal joints, G.Elssner: testing methods bond strength measurements appendix - analytical interpretation of fracture mechanics bend tests. Part 8 The non-destructive evaluation of ceramic materials and joints, H.Reiter: ultrasonic testing radiography thermography and thermal wave interferometry. Part 9 Joining of silicon nitride to metals and to itself, K.Suganuma: interfacial reactions and microstructures formed between silicon nitride and metals residual (thermal) stresses in silicon nitride-metal joints other important features joining of silicon nitride to itself. Part 10 Ceramic metal-diffusion bonding - a case history, T.Joy: design objectives primary bond requirements main steps in the development programme process development initial problems The MK I tube the production run.

Development of radiation resistant materials for advanced nuclear power plant

Abstract: A unique feature of the environment of nuclear power reactors is the presence of high energy neutron radiation, which can lead to degradation processes in the materials of critical components. An understanding of the operative elevated temperature radiation damage mechanisms enables these effects to be minimised by appropriate alloy selection together with compositional optimisation. An overview of these aspects relevant to advanced fission and fusion reactor systems is presented. Several key examples are highlighted and include radiation embrittlement of low alloy pressure vessel steels for pressurised water reactors, coating designs for fuel in high temperature pebble bed reactors, and selection and design of low activation materials for proposed tokomak fusion reactor devices. In addition, void swelling, non-equilibrium solute segregation and radiation induced precipitation processes in alloys at high displacement doses and relevant to several reactor systems are covered. Likely trends in futur...

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract: Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the g...