Showing papers in "Materials Science and Technology in 2000"

Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures

Abstract: This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In...

Development of constitutive equations for modelling of hot rolling

Abstract: Constitutive equations which describe the flow stress behaviour of materials during hot deformation are used to model forming processes. Since the flow stress depends on both temperature and strain rate, the Zener–Hollomon parameter which combines these factors, is frequently used to describe the shape of such curves. For materials which dynamically recover, (only) the flow stress reaches a steady state value at high strains and methodologies which enable such behaviour to be modelled have previously been presented. Beyond the onset of dynamic recrystallisation, the flow stress of materials, such as steels, reach a peak in flow stress before gradual softening. The relative position of the peak in the flow stress shifts as a function of the Zener–Hollomon parameter further complicating such analyses. The present paper describes the development and application of a methodology for modelling the flow stress of microalloyed steels: materials which exhibit dynamic recrystallisation behaviour.

Grain refinement of high purity aluminium by asymmetric rolling

Abstract: For the purpose of grain refinement, development of the microstructure of coarse grain, high purity aluminium during cold asymmetric rolling has been studied by electron backscatter pattern (EBSP) ...

Effect of grain size on tensile behaviour of a submicron grained Al–3 wt-%Mg alloy produced by severe deformation

Abstract: Equal channel angular extrusion has been used to deform an Al–3 wt-%Mg alloy to an effective strain of 10, resulting in a 0.2 µm grain size. In the as deformed condition the yield strength was incr...

Development of fine grained structures using severe plastic deformation

Abstract: Severe plastic deformation provides a processing tool for introducing fine grain sizes into polycrystalline materials. This paper describes the principles of equal channel angular pressing (ECAP) in which a material is pressed through a die where two channels form an L shaped configuration. The process of ECAP imposes a severe strain on the sample but there is no concomitant change in the cross-sectional dimensions so that repetitive pressings may be undertaken to achieve very high total strains. Several factors influence the nature of the microstructures attained in ECAP including the processing route by which the sample is rotated between consecutive pressings, the angle subtended by the two channels within the die, and the speed and temperature associated with the pressing. These various factors are described for a series of experiments conducted on samples of pure aluminium.

Modelling microstructural and mechanical properties of ferritic ductile cast iron

Abstract: It is well known that the mechanical properties of ductile cast iron (DCI) depend on its microstructure, and that the microstructure depends on the properties of the melt and the cooling conditions during casting. There have been many studies of the individual elements of the process of casting DCI, but as yet there have been very few examples of modelling the entire process to predict cooling rates, microstructure, and mechanical properties, particularly for large castings. The present paper describes a method of modelling the microstructural and mechanical properties of ferritic DCI, and applies the methods to the case of a large (13 t) thick walled (300 mm thickness) casting. The microstructure calculated includes nodule count, nodularity, ferrite grain size, and percentage ferrite. The mechanical properties calculated include yield stress, tensile strength, elongation, and static upper shelf fracture toughness (J 1C and K JC). The calculated results compare well with those of a test casting.

Metal matrix composites for aeroengines

Abstract: Metal matrix composites (MMCs) will play a significant role in the future of gas turbine aeroengine development. This paper outlines the benefits and some of the potential applications for Al and Ti MMCs and discusses issues involved in the introduction of this relatively new class of composite materials into engine components. The potential for cost savings and performance improvements which may be achieved by the introduction of silicon carbide particulate reinforced Al alloys (Al–SiCp MMC)are discussed, where the properties of the composite can be tailored to meet the requirements of a specific application. Some of the processing implications for Ti matrix composites (Ti MMCs) are explained for a range of component applications. Illustrations are given of how the manufacturing process can be controlled for a complex component. Finally, the influence of raw material, manufacturing, and component costs on the successful introduction of MMCs into aeroengines is discussed.

Processing and properties of mild interstitial free steels

Abstract: Over recent years, new interstitial free (IF) steel grades containing controlled low levels of carbon and nitrogen have been adopted to fabricate car body panels. The processing and properties of these steels are reviewed. They can provide very high levels of formability, as indicated by width to thickness strain ratios during forming r of 1.8 (r values of traditional aluminium killed steels rarely exceed 1.8). Typically total interstitial contents are in the region <0.0030 wt-%C and <0.0040 wt-%N. Elimination of these solutes damaging to formability is achieved through careful control of the melt chemistry during steelmaking and by the hyperstoichiometric addition of titanium and/or niobium to react with carbon and nitrogen to form precipitates. Precipitation control during hot mill processing is instrumental in determining the grain size of the hot rolled material, which must be as small as possible, and grain size in the final cold rolled and annealed product, which must be optimised for forma...

Investigation into refractories as crucible and mould materials for melting and casting γ-TiAl alloys

Abstract: An assessment has been made of the suitability of magnesia, calcia, alumina, and yttria for manufacturing melting crucibles and investment casting moulds for γ-TiAl alloys. Small refractory crucibles have been prepared by pressing or plasma spraying techniques and used to melt a small quantity of a Ti–48Al–2Nb–2Mn (at.-%) alloy in a carbon resistance furnace. The effects of the refractory type and melting time on the oxygen content, penetration, and microstructure have been established. The depth of penetration versus the holding time at 1550°C can be expressed by a power law. Based on these small­scale tests, it can be concluded that magnesia and silica containing alumina are unsuitable, whereas both pure calcia, yttria, and yttria­coated magnesia show promise for melting and casting γ-TiAl alloys.

Application of superplasticity in commercial magnesium alloy for fabrication of structural components

Abstract: An investigation of the superplastic characteristics of magnesium alloys with several grain sizes revealed that grain boundary sliding took place more easily with grain refinement. The required grain size for high strain rate superplastic forming was estimated to be ∼2 μm. The required grain structure could be obtained by several procedures, hot extrusion with a high extrusion ratio, severe plastic deformation via equal channel angular extrusion, consolidation of machined chip, and/or powder metallurgy processing of rapidly solidified powders, on a laboratory scale. The processing route of hot extrusion was selected in this study. An experimental study of superplastic press forming was conducted for a commercially extruded ZK60 alloy. The fabricated product did not essentially contain macroscopic defects, i.e. cracks or cavities. From an examination of tensile characteristics, it was found that the post-formed alloy exhibited higher strength and higher ductility compared with some conventional cast magnesium alloys, aluminium alloys, and steels. The experimental results support the possibility of using superplastically formed magnesium to produce structural components.

Effect of aging treatments on microstructure and impact properties of tungsten substituted 2205 duplex stainless steel

Abstract: The effects of partial substitution of tungsten for molybdenum on the microstructure and impact properties in 22Cr–5Ni–3Mo (wt-%) duplex stainless steel (DSS) have been investigated following aging heat treatments in the temperature range 600–1000°C. During aging the intermetallic σ and χ phases were precipitated, and the impact toughness was significantly decreased with an increase in the σ phase content. The χ phase had been precipitated on the α/γ boundary in the early stages of aging. Ferrite and χ phases in tungsten substituted duplex stainless steel contain a large amount of tungsten, and their decomposition rates are much lower compared with those in steel containing only molybdenum. Consequently, the precipitation of the σ phase is retarded in tungsten substituted DSSs, which results in high impact toughness. However, after aging for a long time, the α and χ phases transformed to the σ and austenite phases in the tungsten substituted steels, and the steels showed low impact toughness.

Low temperature processing of NiZn ferrite by citrate precursor method and study of properties

Abstract: Nickel zinc ferrite of composition Ni0·5 Zn0·5 Fe2O4 was prepared by the citrate precursor method at temperatures as low as 100°C This is much lower than those used in the conventional ceramic method for the preparation of ferrites (~1000°C) The dc resistivity of the sintered specimens was observed to be ~108 Ω cm which is greater, by at least two orders of magnitude, than that for specimens prepared by the conventional method Although the initial permeability values are similar to those for conventionally prepared specimens, the losses are lower and the operational frequency range is larger The significance of the citrate method lies, therefore, in producing ferrites with better properties at reduced processing temperatures

Processing of steel for ultrafine ferrite grain structures

Abstract: The attainment of ultrafine ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very fine grain sizes. This paper presents a summary of research aimed at producing ultrafine ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain refinement (TGR), in which extrafine austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of <1 µm in a surface layer, and 1.5 µm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1.5 µm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previou...

Microstructural evolution during formation of ultrafine grain structures by severe deformation

Abstract: High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al–0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500°C, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2–5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments. After an effective strain of 10 the microstructures for both materials co...

Precipitation of chromium containing dispersoids in Al–Mg–Si alloys

Abstract: The aim of the present investigation was to study precipitation of Cr in Al–Mg–Si alloys during high temperature annealing. It has been shown that Cr containing dispersoids precipitate during heating to the homogenisation temperature. During the heating small α-Al(CrMn)Si particles nucleate at the surface of the β-AlFeSi particles and after additional heating to a higher temperature the α-Al(CrFe)Si particles grow and the β-AlFeSi particles dissolve. If the content of Cr is high (~0·3 wt-%Cr) both α-Al(CrFe)Si and α~-AlCrSi dispersoids are present in the matrix during heating while only α-Al(CrFe)Si dispersoids are present when the content is low (~0·15 wt-%Cr). The nucleation mechanisms for the dispersoids are not fully understood but most likely the Cr containing dispersoids nucleate by means of an intermediate phase. This intermediate phase is found to be a ‘u phase’ precipitate.

Ultrafine grained steels: industrial prospects

Abstract: Ultrafine grained (∼ 1 μm) steels are currently the subject of a great deal of research worldwide. They offer the prospect of very high strength with very high toughness from standard steel compositions, and recently it has been shown that such fine grains can be obtained from the hot rolling mill rather than merely from specialist, small scale techniques. If such material could indeed be produced economically in bulk, it offers a step advance in ongoing issues for the industry, such as light weighting of structures and components, rationalising the large number of current steel grades by extending the property combinations achievable from select compositions, aiding weldability and recyclability, and reducing the alloy costs by meeting high specifications from more dilute grades and promoting a high tech image from offering an apparently exotic product. However, it has quickly become apparent that this material has its 'Achilles' heel' in that it tends to exhibit unstable plasticity upon yielding, severely restricting its potential applications. Possible ways around this problem are being explored, but at present the industrial prospects remain unclear. This paper explores these issues, and suggests that ultrafine grained steels are not going to sweep through all the major steel markets, but will find their place for certain applications.

Development of low cost metal matrix composites for commercial applications

Abstract: Metal matrix composites are considered as a possible material to replace the materials actually used in some automotive and railway industrial components. Two different ways of taking advantage of the properties of MMCs at low cost are presented: locally reinforced aluminium components and new castable aluminium MMCs developed by the London & Scandinavian Metallurgy Co. (LSM).

Plastic flow of aluminium extruded under complex conditions

Abstract: This paper describes the extrusion process of aluminium in which cyclic changes of the strain path are induced in the natural deformation zone of the extruded piece of metal, through reversible rotation of the die (patented KOBO metal forming process). Such a procedure allows significant reduction in the extrusion force, as well as in the global mechanical work needed for the extrusion of a rod when compared with the conventional process. Macro- and microstructural observations of the products and microhardness and tensile tests show that cyclic changes in the strain path cause a reduction in the differences in the rate of plastic flow in the particular microvolumes of the metal, and lead to structural homogenisation. This process also causes a significant improvement in the plasticity of the extruded rod and a reduction in its work hardening.

Microstructural evolution of Inconel* 718 during ingot breakdown: process modelling and validation

Abstract: A computer model is presented that describes microstructural evolution during the ingot breakdown of nickel base superalloy Inconel 718 via the open die cogging operation. To support the development of the model, a compression testing programme has been carried out which covers the ranges of temperatures, strains, and strain rates experienced during thermomechanical processing. Analysis of the flow curves has allowed the identification of the regimes in which the various deformation mechanisms take place. Logic based rules have been incorporated into the model, and this has allowed predictions of the microstructural evolution to be made. Where possible, the results have been compared with the available experimental data and it is shown that theory and experiment are in reasonable agreement. A number of computational experiments have been carried out, to study the effects of changing the forging procedure.

Influence of processing conditions on morphology of metal foams produced from metal powder

Abstract: The influence of the processing conditions on the evolution of the cell morphology of aluminium foams is investigated. The precursor material, a compacted mixture of aluminium and titanium hydride powder, is foamed to different degrees by heating it to different maximum temperatures. The pressure of the surrounding argon atmosphere is varied from low pressure to overpressure. The cell morphology is characterised by the mean cell diameter, its circularity, and the gas/metal interface length per unit area. The cell morphology is found to be primarily determined by the density and cannot be influenced by the outside pressure. The morphology is governed by one simple evolution law: the mean cell wall thickness is constant during expansion.

Directional solidification of Ni base superalloy IN738LC to improve creep properties

Abstract: The high Cr, Ni base superalloy IN738LC has been directionally solidified on both laboratory and industrial scales using Bridgman and liquid metal cooling (LMC) methods respectively. In the Bridgman experiments, cylindrical rods were grown using a graphite chill with temperature gradient G = 5·0 K mm-1 and a water cooled copper chill with G = 8·5 K mm-1, and a wide range of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h-1. In the LMC rigs, several turbine blades were grown using a wide range of withdrawal rates of R = 120, 225, 330, 420, and 630 mm h-1. Grain and dendritic structures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions. Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 650°C and creep tests at 152 MPa and 982°C, 340 MPa and 850°C, and 586 MPa and 760°C. It was shown that at R = 600 mm h-1 ...

Diffusion of chromium in ferritic and austenitic 9–20 wt-%chromium steels

Abstract: Radiotracer 51Cr diffusion experiments were conducted on 9–20 wt-% chromium ferritic and austenitic steels. Volume diffusion coefficients have been determined in the temperature range 881–1281 K, a...

Evaluation of induction hardened case depth through microstructural characterisation using magnetic Barkhausen emission technique

Abstract: The influence on the hysteresis loop and the magnetic Barkhausen emission (MBE) of microstructure within the case of different induction hardened carbon steel shafts has been studied The hysteresis loop shows a distortion with a sudden reduction in the rate of magnetisation (dB/dH ) before approaching the maximum magnetic flux density indicating surface hardening The systematic changes in the MBE profile for different voltages applied during induction heating indicate the microstructural variations within the case A single peak MBE profile for a fully martensitic structure gradually changes into two peaks on reducing the induction hardening voltage indicating the formation of an additional soft ferrite phase within the case The systematic changes in the two MBE peak heights indicate the synergistic decrease in the volume fraction of martensite and the increase in the volume fraction of ferrite phase within the case due to reduction in the induction hardening voltage The changes in the MBE pro

Influence of heat treatments on microstructural parameters and mechanical properties of P92 steel

Abstract: The microstructural parameters (dislocation density, martensite lath width, precipitate diameters, and volume fractions) have been measured for the 9%Cr steel P92 (NF616) after different heat treatments. The austenitising temperatures were 970, 1070, and 1145°C and the tempering temperatures 715, 775, and 835°C. Increasing the austenitising temperature led to an increase in the austenite grain size and in the martensite lath width, but no significant effect on the tensile properties at 20, 600, and 650°C was observed. The creep strength was, however, reduced by tempering at 835°C due to rapid recovery of the martensitic structure with a sharp decrease in dislocation density. The lowest creep strength was found for the P92 steel subjected to a heat treatment that produced a fully ferritic microstructure; the secondary creep rate was four orders of magnitude higher than that of the steel in the usual martensitic condition.

Iron-phosphorus-carbon system part 1 : Mechanical properties of low carbon iron-phosphorus alloys

Abstract: As part of a wider archeometallurgical investigation the present paper records the results of mechanical tests on iron–phosphorus alloys, which were important in early iron usage. Phosphorus increases the yield and ultimate tensile strengths with a corresponding reduction in ductilities as measured by elongation and reduction in area, to a point where brittle failure occurs. However, the work hardening exponent is only slightly decreased and some cold working, as in drawing, can be achieved. The presence of carbon complicates these effects.

Mechanisms of inclusion formation in low alloy steels deoxidised with titanium

Abstract: In the present investigation, the conditions for inclusion formation in two Ti deoxidised steels and one Al–Ca deoxidised steel have been examined by means of optical and electron microscopy, in co...

Effect of instability of TiC particles ongrain refinement of Al and Al–Mg alloysby addition of Al–Ti–C inoculants

Abstract: Thermodynamic modelling shows that while TiC is a stable phase in Al–Ti–C grain refiners, it should decompose on addition of the refiners to typical Al alloys. The present work assesses the impact of melt composition on TiC stability and on grain refiner performance, which has been tested at various addition levels in Al and Al–Mg alloys. In melts with compositions that stabilise TiC, there is effective grain refinement with little fading of performance on holding the refiner in the melt. In melts with compositions in which TiC is not stable, grain refinement is impaired even at short holding times, and becomes markedly worse at longer holding times. It is concluded that in typical cases progressive decomposition of TiC does occur, and that this can have a significant effect on grain refinement by inoculation with Al–Ti–C master alloys.

Preparation and corresponding structure of nickel foam

Abstract: A nickel foam with a three-dimensional reticulated structure has been prepared by an electrodeposition technique. The microcrystalline structure of the nickel layer was maintained following treatment in air at 600°C for 4 min. Sintering in a reductive decomposed NH3 atmosphere for 40 min, at either 980 or 850°C, transformed the product into a compact nickel crystal with a flat surface and large crystal grains.

Precipitation characteristics of Cu–15Cr–0·15Zr in situ composite

Abstract: The precipitation characteristics of Cu–15Cr–0·15Zr in situ composite made by cold drawing have been studied using analytical TEM and hardness measurement. It has been observed that the in situcomposite reaches peak hardness at a lower aging temperature than the corresponding alloy which is not subjected to cold working prior to aging treatment. The Cr rich precipitates in the peak aged composite are much smaller than those in the peak aged alloy. Under peak aging conditions, precipitation free zones adjacent to the grain boundaries of the Cu matrix are also observed in both the alloy and the composite. The addition of Zr enhances the precipitation hardening effect in the alloy and the composite. The major mechanism of this enhancement may be through increasing the Cr rich precipitate nucleation rate and precipitate/matrix interfacial energy.

Interactions between precipitation and recrystallisation in an Al–Mg–Si alloy

Abstract: Work has been carried out to study a broad range of possible behaviours in a simple model, high-purity, ternary Al–Mg–Si alloy. The starting conditions investigated, before deformation, included a supersaturated solid solution, a very coarse β (Mg2Si)particle distribution, and a range of preaged conditions with a combination of β′ rods and β platelets, obtained by isothermal treatments at 300°C. On annealing at the same temperature of 300°C, after cold rolling to a 70% reduction, the recrystallisation times were found to vary from 10 min to 1600 h, depending on the initial material condition. The fastest recrystallisation time was observed with the coarse β particles, where nucleation of recrystallisation took place rapidly by particle stimulated nucleation. In the solution treated sample, recrystallisation took the intermediate time of 10 h and the recrystallisation rate was found to be dominated by the transformation of the metastable β′ phase, to the equilibrium β phase at the migrating grain b...