Showing papers in "Materials Science and Technology in 2004"

Hydrogen related failure of steels – a new aspect

Abstract: Recent studies of the characteristics and mechanism of hydrogen related failure in steels are overviewed. Based on an analysis of the states of hydrogen in steels, the role of hydrogen in reducing ductile crack growth resistance is attributed to the increased creation of vacancies on straining. Cases showing the involvement of strain induced vacancies in susceptibility to fracture are presented. The function of hydrogen is ascribed to an increase in the density of vacancies and their agglomeration, rather than hydrogen itself, through interactions between vacancies and hydrogen. The newly proposed mechanism of hydrogen related failure is supported by a recent finding of amorphisation associated with crack growth.

Comparison of different methods for measuring strain induced α-martensite content in austenitic steels

Abstract: Several techniques for measuring the strain induced α-martensite content in EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) austenitic stainless steels were compared in order to determine a correlation curve between Ferritescope measurement results and actual α-martensite contents. The studied methods involved Satmagan measurement, magnetic balance measurement, X-ray diffraction, density measurement and quantitative optical metallography. Satmagan, magnetic balance and density measurements were found to give equal α-martensite contents. X-ray diffraction results were affected by the texture but averaging of several diffraction peaks improved the reliability of the results. It was shown that α-martensite can be detected by means of optical metallography, but quantitative analysis is time consuming and inaccurate. The relationship between the Ferritescope results and actual α-martensite contents measured with the other techniques was found to be linear. According to the results, the Ferritescope rea...

Structural characteristics defining high barrier properties in polymeric materials

Abstract: The packaging industry has dramatically increased the number of packaging systems and designs made of plastics over recent decades. Plastics, in contrast with more traditional packaging materials such as glass and metals, are permeable systems that permit the exchange of low molecular weight compounds, e.g. gases and vapours, between the inner and the outer atmosphere. Despite this drawback, the availability of shapes and forms in which they can be manufactured, their ease of processing and handling, low price, excellent chemical resistance, etc., have made them very attractive in many packaging fields. Consequently, much industrial and academic research has been devoted to understanding the mechanisms of mass transport in polymers, to enable design of materials with improved barrier properties. The present paper reviews some of these developments, and highlights the structural factors that cause polymers to behave as high barrier materials, taking as benchmark the properties of one of the most wi...

Understanding mechanisms of grain refinement of aluminium alloys by inoculation

Abstract: For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al – Ti – B, Al – Ti – C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al – Ti – B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain ref...

Classification and quantification of microstructures in steels

Abstract: The International Institute of Welding (IIW) microstructure classification scheme for ferrous weld metals has been investigated as a basis for the quantification of complex microstructures in steels. The aim has been to cover the full range of microstructures observed in plain carbon and low alloy steel products, as well as ferritic weld metals and parent plate heat affected zones. The mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials have been briefly reviewed. The classification and terminology used for intragranular as well as austenite grain boundary microstructural constituents have been considered, and also the way in which transformation products are orientated in space. Problems encountered in relating microstructural constituents to principal structures have been discussed in detail and solutions proposed. The microstructure classification and terminology...

Tempering of hard mixture of bainitic ferrite and austenite

Abstract: Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV30, tensile strength ~2.3 GPa and toughness ~30 MPa m1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out in relation to the tempering resistance of steel over the temperature range 350 – 750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from decomposition of the carbon enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery dur...

Stability of retained austenite in TRIP-assisted steels

Abstract: A theory is developed for the quantitative representation of the strain induced transformation of retained austenite in low alloy, TRIP-assisted steels of the type developed for the automobile industry. It is possible, therefore, to calculate the fraction of austenite as a function of the plastic strain, chemical composition, deformation temperature and the starting amount of austenite. The effect of composition and temperature is expressed through the free energy available for transformation. Good agreement has been obtained with published experimental data. The model can be used to investigate the stability of the austenite during plastic deformation.

Smart fluids: current and future developments

Abstract: A smart fluid comprises a suspension of micrometre-sized particles in a dielectric carrier liquid. The application of a suitable stimulus – either electrical or magnetic – causes a significant increase in the resistance to flow of the fluid. Current and future developments in smart fluids are reviewed, paying particular attention to electro-rheological (ER) and magneto-rheological (MR) fluids. ER fluids have been the focus of intense academic interest over several decades. However, not until the re-discovery of MR fluids did commercial exploitation finally occur. Both microscopic and macroscopic models of smart fluids will be described. The link with industrial applications is established by introducing the three modes of operation through which smart fluids are harnessed. Insight into the behaviour of smart fluids is obtained through the development of relatively simple quasi-steady and dynamic models. A concise summary of the latest industrial applications is provided. Future trends are then con...

Development of high performance magnesium nanocomposites using solidification processing route

Abstract: In the present study, elemental and nanoAl2O3 particulate reinforced magnesium materials were synthesised using an innovative disintegrated melt deposition technique followed by hot extrusion. Microstructural characterisation of the composite samples showed retention and uniform distribution of reinforcement, grain refinement of magnesium matrix, and the presence of minimal porosity. Physical properties characterisation revealed that the addition of nanoAl2O3 particulates as reinforcement improves the dimensional stability of pure magnesium. Mechanical properties characterisation revealed that the presence of nanoAl2O3 reinforcement led to significant improvement in hardness, elastic modulus, 0·2% yield strength, UTS and ductility. The results further revealed that the combination of 0·2% yield strength, UTS, and ductility exhibited by nanoAl2O3 reinforced magnesium remained much superior even when compared to magnesium reinforced with a much higher volume percentage of micrometre size SiCp. An at...

Reduction of titanium and other metal oxides using electrodeoxidation

Abstract: Many reactive and refractory metals are currently produced industrially by reducing their compounds, including oxides, using a more reactive metal. In some cases, where there is substantial oxygen solubility in the metal, the oxygen is first removed by carbochlorination followed by reduction. Titanium and zirconium are made by reduction of the volatile tetrachlorides by magnesium. The processes consist essentially of two reduction steps: reducing magnesium chloride to magnesium metal and then reduction of the metal compound; this makes the overall reduction process relatively expensive. Electrodeoxidation is very simple in that the oxide to be reduced is rendered cathodic in molten alkaline earth chloride. By applying a voltage below the decomposition potential of the salt, it has been found that ionisation of oxygen is the dominant cathode reaction, rather than alkaline earth metal deposition. In the laboratory, this technique has been applied to reduce a large number of metal oxides to the metal...

Modified titania nanomaterials for sunscreen applications – reducing free radical generation and DNA damage

Abstract: Inorganic UV absorbers such as titanium oxide and zinc oxide are in common use in formulations of sunscreens and cosmetics. These materials strongly absorb UV, and show no UV induced degradation over time. However, the dissipation mechanism of the UV energy is not often considered. It is demonstrated that the primary de-excitation mechanism of inorganic sunscreen components is via the surface of the particle and leads to the creation of free radicals. Substantial free radical generation is observed under simulated solar energy with all sunscreen grades of titanium oxide and zinc oxide. Free radicals are implicated in a number of potential health issues such as skin aging. Doping trap centres into titanium oxide and zinc oxide results in the total elimination of free radical generation under simulated solar energy. The trap centres provide sites within the oxide via which it is energetically favourable for photogenerated charge carriers to de-excite. Thus the charge is confined on sites within the...

Characterisation of precipitation microstructures in aluminium alloys 7040 and 7050 and their relationship to mechanical behaviour

Abstract: This paper investigates the precipitation microstructures in aluminium alloys 7040 and 7050 (Al - Zn - Mg - Cu variants) as a function of quench rate and aging treatment, and the associated compromise between yield strength and fracture toughness The precipitate microstructures are quantitatively characterised by a combination of techniques covering the different scales involved: transmission electron microscopy (TEM), field emission gun scanning electron microscopy (FEG-SEM) and small angle X-ray scattering (SAXS), and fracture toughness is estimated using the Kahn tear test Yield strength and strain hardening behaviour are determined by conventional tensile tests It is shown that the composition modification from alloy 7040 to alloy 7050 results in a better compromise between yield strength and toughness The strength decrease in alloy 7040 following a slow quench is much reduced compared to alloy 7050 because of a lower sensitivity to quench-induced precipitation on dispersoids Both alloys

Bainite formation influenced by large stress

Abstract: By using an iron alloy that transforms slowly to bainite at a low temperature, it has been possible to study the development of the upper bainite under the influence of exceptionally large stresses, which are nevertheless below the yield strength of the parent austenite. It is found that a uniaxial stress whose magnitude is below the elastic limit, strongly favours the growth of compliant variants, leading to an organised microstructure. It also accelerates the overall rate of reaction. A comparison between samples transformed with and without an applied stress revealed significant changes in the crystallographic texture, consistent with the observed microstructures. Stress assisted transformation resulted in large blocks of bainite in identical orientation.

Laser drilling of cooling holes in aeroengines: state of the art and future challenges

Abstract: Advances in the efficiency of the aeroengine have led to an increase in combustion and exhaust gas temperatures. Although superalloys have been developed to withstand these high temperatures, additional cooling of the components is often necessary. This is achieved through thousands of closely spaced cooling holes drilled into the components. Laser drilling offers economical advantages over other non-conventional drilling techniques (e.g. electrical discharge machining). The key limitations and the areas requiring further investigation to develop the laser drilling process to meet future aeroengine requirements have been identified. Principal areas include the development and investigation of laser barrier methods when drilling through increasingly complicated and smaller cavities, the successful drilling of lower angled effusion holes and the increased use of laser drilling to produce the cooling holes in rotating components. More recent designs of the aeroengine have incorporated thermal barrier...

Short term precipitation kinetics of delta phase in strain free Inconel* 718 alloy

Abstract: Understanding and control of short term precipitation of delta phase in Inconel 718 alloy is of importance as it is used to control grain growth during forging. This precipitation has been ...

Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract: In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous...

Precipitates, zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract: Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn2, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.

Influence of oxide film defects generated in filling on mechanical strength of aluminium alloy castings

Abstract: The influence of oxide film defects generated from the filling process on the mechanical strength of aluminium alloy castings has been investigated. Using numerical simulation and experimental validation, the investigation aims to reveal the relationships among the liquid aluminium flow behaviour in the filling, the likely oxide film defect distribution caused by surface turbulence and the final mechanical strength of the castings. CFD modelling was used to investigate the liquid metal flow behaviour and the likely oxide film defect distribution in the filling at different ingate velocities. In particular, a numerical algorithm - Oxide Film Entrainment Tracking (OFET, 2-D) has been proposed and developed for predicting such oxide film defect distribution in the liquid aluminium throughout the filling. Also, light microscopic and SEM techniques were used to identify the microstructures of oxide film casting defects. The Weibull statistics method was employed to quantify the effects of oxide film distributions on the mechanical strength and reliability of the acquired aluminium alloy castings for different runner systems. It was found from the numerical simulations that the ingate velocities acquired using different runner systems have significant influence on the distribution of oxide film defects generated by surface turbulence in the filling process, which results in the disparities of the final mechanical strength of the castings. The results of the mechanical property test and the SEM micro-structural analysis of the castings are consistent with the numerical simulations.

Mechanical properties of nanocrystalline copper and nickel

Abstract: The present review paper traces the development of nanocrystalline copper (Nc-Cu) and nanocrystalline nickel (Nc-Ni) and their mechanical properties. The objective is to summarise the various results available in the literature. The mechanical properties discussed are elastic modulus, Poisson's ratio, hardness, yield stress, ultimate tensile stress, strain/elongation to failure, superplasticity, creep, fatigue and fracture properties. The review is limited to bulk nanocrystalline materials. Shortcomings and limitations of the various studies that have been conducted are also highlighted. The present compilation is expected to be useful for researchers engaged in experimental work and computer modelling in this area.

Modelling the evolution of particle size distribution during nucleation, growth and coarsening

Abstract: A model, based on the numerical framework of Kampmann and Wagner, has been developed to predict the evolution of particle size distribution (PSD) during the decomposition of supersaturated solid solutions by nucleation, growth, and coarsening. During the early stages of transformation, where nucleation and growth are dominant, the PSD shape is predicted to be constantly evolving. Only during the latter stages of transformation, when coarsening becomes dominant, does the PSD tend towards a steady state shape, which closely matches that expected from classical coarsening theory. It is also predicted that as the PSD evolves, a transient double peak forms and then decays. The physical basis of this double peak has been investigated and the effect of supersaturation on its formation has been predicted.

Study of mechanisms of cold roll welding of aluminium alloy to steel strip

Abstract: The mechanisms of cold roll welding of aluminium alloy to steel strip has been assessed based on a study of interfacial behaviour at deformations of up to 60% together with the surface characteristics of peeled Al clad steel strip. The proposed mechanisms are applicable where the contact surfaces are degreased and then scratch brushed before the rolling deformation. According to this mechanisms the scratch-brushed layers are fractured shortly after entering the roll gap at a reduction of approximately 10%, which is about the threshold deformation for the roll bonding of aluminium to steel. The transfer cracks formed then become wider so that the distance between each coherent block increases as the rolling deformation increases through the mill. This process allows the bond to be established between the underlying areas, termed as virgin metal, of base metal, which are extruded between the cracks at the scratch brushed regions.

Microstructure and mechanical properties of friction stir welded joints of AC4A cast aluminium alloy

Abstract: AC4A cast aluminium alloy was friction stir welded, and the microstructure and mechanical properties of the joint and its component parts were examined and analysed. In the weld nugget of the weld, a large number of small silicon particles were homogeneously distributed in a continuous aluminium matrix, and the original casting voids were completely eliminated from the weld nugget, which therefore exhibited much better mechanical properties than the base material. In the thermomechanically affected zone (TMAZ) of the weld, there were short silicon plates and no void flaws, accordingly the weld composed of weld nugget and TMAZ exhibited better mechanical properties than the base material. The global joint fractured in the base material, and thus possessed the same tensile strength as the base material. These results clearly show that friction stir welding is an optimum welding process for cast aluminium alloys.

Biocompatible gold nanoparticles

Abstract: Thiolalkylated polyethylene glycol (PEG) ligands have been designed and synthesised for use in the manufacturing of stable water soluble gold MPCs. In addition to ongoing work on the synthesis of size and shape selective aq. (aqueous) gold nanoparticles, the effect of avidin gold nanoparticle (AGNP) ratio on biotinylated peptide gold nanoparticles (BPGNP) has also been investigated.

Effect of particulate reinforcement on wear behaviour of aluminium matrix composites

Abstract: The friction and wear behaviour of aluminum matrix composites reinforced with particulates of M7C3, SiC, SiFe, Al2O3 and SiFe - Al2O3 was investigated under various loads and temperatures. Wear tests were conducted under dry sliding conditions using a pin on disc apparatus. Results of the investigations indicated that the coefficient of friction and wear rate of the composites were lower than those of pure aluminium, and the difference between the friction coefficient and wear rate of the composites and those of pure aluminium was enhanced with an increase in load and in contact temperature. The type of reinforcement did not significantly influence the wear and friction properties of the composites. However, the volume fraction of reinforcement affected the wear rate excessively. Nevertheless, it was seen that both coefficient of friction and wear rate behaviours of the composites were approximately similar.

Development of Ni/Y2 O3 – ZrO2 cermet anodes for solid oxide fuel cells

Abstract: Ni/Y2 O3 – ZrO2 (Ni/YSZ) cermets are the most common anode materials for high temperature solid oxide fuel cells (SOFC). Electrode performance of Ni/YSZ cermets is critically dependent on the microstructure and distribution of Ni and YSZ phases in the cermets. This in turn is closely related to the characteristics of the NiO and YSZ powders, and the fabrication process. The objective of this paper is to discuss some important issues associated with the fabrication and development of Ni/Y2 O3 – ZrO2 anode materials for SOFC from the viewpoint of materials science and technology. Results indicate that the optimisation of Ni/YSZ cermet anodes is a complicated process and understanding of the H2 oxidation reaction is also very important in the development of high performance Ni/YSZ cermet anodes.

Bond coat oxidation and its significance for life expectancy of thermal barrier coating systems

Abstract: The time and temperature dependent evolution of the microstructure of thermal barrier coating systems under isothermal conditions between 950 and 1100 ° C up to 5000 h are investigated for two APS - TBC systems and two EB-PVD systems. Kinetics for the thermally grown oxide thickness values, the β phase depletion underneath the oxide scale and the physical defects in and around the thermally grown oxide are determined by extensive SEM and subsequent interactive image analysis. In the case of physical defects, the size of pores, interacting pore populations and maximum crack lengths are measured. Additionally, the latter are classified with respect to their local orientation in the thermally grown oxide or its vicinity. Finally, the results are discussed with regard to their significance in lifetime modelling of gas turbine components.

Effect of solid carbide particle size on deposition behaviour, microstructure and wear performance of HVOF cermet coatings

Abstract: The deposition process of WC – Co and Cr3 C2 – NiCr cermet coatings through a high velocity oxyfuel (HVOF) spray process was examined using powders of different carbide particle sizes that ranged f...

Multistep induction heating regimes for thixoforming 7075 aluminium alloy

Abstract: Thixoforming involves processing alloys with a spheroidal microstructure in the semisolid state. Commercially it is applied to conventional casting alloys, and one of the scientific challenges is to extend its application to high performance aluminium alloys such as 7075. Aluminium alloy 7075 is readily available in extruded form, and one route to a spheroidal microstructure is to reheat extruded material into the semisolid state to obtain recrystallisation, with the liquid penetrating the recrystallised boundaries. Here this route has been followed, but it has been found that the presence of pinning particles in the microstructure inhibits recrystallisation. To overcome this, a multistep induction heating regime has been developed consisting of a 1 min hold at 475-500 °C, a 1 min hold at 575-600 °C and a shorter 20 s hold for the final step at 620 °C.

Effect of bonding time on joint properties of vacuum brazed WC-Co hard metal/carbon steel using stacked Cu and Ni alloy as insert metal

Abstract: Hard metal WC – Co and carbon steel were successfully joined using double layered Cu alloy and amorphous Ni alloy as inert metal and an oil cooling method after brazing. Defects such as cracks and voids were not formed near the bonded zone. This result means that double layered insert metals and oil cooling minimised the residual stress near the bonded zone after brazing. The shear strength of the joints decreased with increasing bond time. The reasons why the shear strength decreased as bond time increased could be many, including shape of the interface, formation and growth of brittle intermetallic compounds, and coarsening of WC particles near the bond zone. The maximum shear strength of the joints was 310 MPa under conditions 0·6 ks bond time and 8 wt-%Co content in the WC hard alloy.

Preparation, characterisation and computational study of poly(ϵ-caprolactone) based nanocomposites

Abstract: In recent years, scientific and industrial interest has been focused on the preparation of organic/inorganic nanocomposites because of their unique hybrid properties correlated with the enormous interfacial adhesion region that is a characteristic of nanoparticles. The objective of the whole research was to improve poly-caprolactone (PCL) mechanical and barrier performances by using silica spherical nanoparticles for filling. In particular, in order to improve the polymeric matrix/inorganic nanofiller's interfacial adhesion and consequently to achieve a fine nanometric dispersion, silica nanoparticles have been chemically modified by grafting onto them a hydroxyl endcapped PCL, after wheich they have been melt blended with high molecular weight PCL. In the present paper, details on the synthesis, morphology and mechanical properties of the prepared nanocomposites are reported. Moreover, a numerical tool has been used to predict the mechanical properties of the nanocomposite, starting from the morp...