Showing papers in "Materials Science and Technology in 1998"
TL;DR: In this paper, the final grain size depends on the kinetics of both α aluminium, TiB2, and Al3 Ti, and is determined by adding Al-Ti-B master alloys.
Abstract: Grain refinement in commercial aluminium alloys can be achieved by addition of Al–Ti–B master alloys, containing α aluminium, TiB2, and Al3 Ti. The final grain size depends on the kinetics of both ...
203 citations
TL;DR: Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe alloys with additions of Al and Si have been investigated in this paper, where tensile tests were carrie...
Abstract: Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carrie...
160 citations
TL;DR: In this article, the authors studied the kinetics of the γ → α phase transformation in Fe-Mn alloys with low Mn contents using differential scanning calorimetry.
Abstract: The kinetics of the γ → α phase transformation in Fe-Mn alloys with low Mn contents was studied using differential scanning calorimetry. Analysis of the measured heat capacity yielded both the austenite fractions as a function of temperature as well as the enthalpy difference between ferrite and austenite which decreases with increases in both the temperature and the Mn concentration. The transformation was modelled using an interface controlled growth model. It was assumed that the interface velocity is proportional to the chemical potential difference of the Fe lattice. The interface mobility has a simple exponential temperature dependence. Taking into account the austenite grain size distribution, the activation energy and the pre-exponential factor for the interface mobility were estimated from the heat effects at 140 kJ mol−1 and 58 mm mol J−1 S−1 respectively.
100 citations
TL;DR: In this article, the effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied.
Abstract: Effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied. It was observed that the structure in the as solidified weld was predominantly columnar in the case of the conventional (i.e. continuous current) gas tungsten arc welding process. The grain structure became finer and more equiaxed with the introduction of current pulsation. Moreover, there was an optimum frequency range over which the grain refinement was a maximum. The same optimum frequency range corresponded with maxima in hardness, ultimate tensile strength, and percentage elongation. Tensile strength increased, in general, after solution treatment and aging (STA). The best combination of tensile properties was achieved for welds deposited under a 6 Hz pulse frequency in the STA condition.
90 citations
TL;DR: In this article, the authors explored the issues involved in characterising the spatial distribution of reinforcement particles in MMCs and concluded that tessellation of the microstructure into cells, each of which contains a single particle, is necessary if comprehensive characterisation is required.
Abstract: The issues involved in characterising the spatial distribution of reinforcement particles in MMCs are briefly explored. It is concluded that tessellation of the microstructure into cells, each of which contains a single particle, is necessary if comprehensive characterisation is required. Experimental results are presented from microstructural analysis and mechanical testing of a number of AI-Si/20 vol.-%SiCp MMCs produced by different processing routes. The most useful parameter for characterising the severity of clustering is found to be the ratio of the variance of the distribution of the cell areas to the corresponding variance obtainedfrom a random particle distribution with the same average areal density of particles. Either Dirichlet or particle growth tessellation procedures can be used. High values of the clustering parameter were found to correlate with low ductilities. The transition from random to significantly regularised microstructures correlates with a sharp increase in ductility f...
80 citations
TL;DR: In this paper, the feasibility of double compression and stress relaxation for measuring the static recrystallisation rate in hot deformed austenite was studied, and the results showed that double compression is more accurate than stress relaxation.
Abstract: The feasibility of two mechanical testing methods, double compression and stress relaxation, for measuring the static recrystallisation rate in hot deformed austenite was studied. The results were ...
75 citations
TL;DR: In this paper, the influence of aging on the microstructure, tensile properties, fracture toughness, and fractographic features of a cobalt free 18%Ni 250 grade maraging steel has been investigated.
Abstract: The influence of aging on the microstructure, tensile properties, fracture toughness, and fractographic features of a cobalt free 18%Ni 250 grade maraging steel has been investigated. This steel, designated as T250 steel, with 17.1%Ni, 2.25%Mo, and 1.39%Ti as the main alloying elements, contains no cobalt and less molybdenum than the widely used 18Ni-8Co-5Mo type 250 maraging steel (M250). The present investigation shows that tensile properties as well as plane strain fracture toughness values comparable with those of M250 can be obtained in the T250 steel with marginal change in aging cycle. The Ni3 Ti phase precipitating in the present steel has been found to have excellent resistance to coarsening at the normal aging temperatures. The condition for maximum yield and tensile strengths is also the one that gives peak values of toughness. While a low value of toughness in the underaged condition is attributed to the planar slip mode of deformation, the formation of reverted austenite and coarsenin...
63 citations
TL;DR: In this paper, the transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strength.
Abstract: The transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strengths. In this preliminary study, the TLP bonding of a 2124 aluminium alloy MMC is investigated using nickel interlayers, and the initial results show that good bonds are produced with no effect on the SiC dispersion in the matrix. The absence of segregation is attributed to the high diffusivity of the nickel in the aluminium MMC, which produces rapid isothermal solidification at the bonding temperature. Bond shear tests show that near parent metal strengths are possible when thin nickel interlayers are used, and failure occurs at the MMC/bond interface. When thick interlayers are used, failure is predominately through the centre of the bondline.
57 citations
TL;DR: In this article, an extension of this model by taking into account the effect of friction, the angle of impingement, and the hardness ratio is presented, which shows a good agreement with the residual stresses obtained by X-ray diffraction analysis.
Abstract: A model has been developed by Guechichi et al. and improved by Khabou et al. to predict residual stresses and plastic deformation induced by shot peening in metallic parts. The major controlling factors were taken into account. The effect of the tangential friction between the shot and the treated material, the shot hardness to material hardness ratio, and the angle of impingement were not taken into account. This paper presents an extension of this model by taking into account the effect of friction, the angle of impingement, and the hardness ratio. The material is characterised by using the model of Khabou et al. as extended by Hamdane in the case of triaxial loading. Application on a plate made of Udimet 720 shows a good agreement with the residual stresses obtained by X-ray diffraction analysis. Calculations made using different coefficients of friction and angles of impingement show a good correlation with the experimental data available in the literature.
57 citations
TL;DR: In this article, a laser Raman microscope was used to collect spectra from individual phase regions visible in a polished cross-section of a thick oxide scale grown on a steel at high temperature.
Abstract: A laser Raman microscope was used to collect spectra from individual phase regions visible in a polished cross-section of a thick oxide scale grown on a steel at high temperature. The technique was shown to be capable of unambiguously distinguishing the three iron oxides, FeO, Fe3O4,and Fe2O3. The standard spectra were then used to analyse the phases within thin (<10 μm) millscales developed on the steel during the hot rolling stage of steel strip production. Poor quality surface appearance was demonstrated to be associated with the development of fragmentary Fe2O3 regions.
53 citations
TL;DR: In this article, the factors controlling the reaction kinetics have been investigated using X-ray photoelectron spectroscopy and differential thermal analysis, and a minimum adiabatic temperature of 1800 K is necessary for the occurrence of combustion during the mechanosynthesis process.
Abstract: During ball milling of a powder mixture of elemental titanium and graphite, TiC is synthesised by a combustion reaction. The factors controlling the reaction kinetics have been investigated using X-ray photoelectron spectroscopy and differential thermal analysis. In the incubation period before a combustion reaction, the carbon atoms diffuse along the grain boundaries of Ti, resulting in the mixing of the reactants on a nanometre scale. A transitional bonded state (Ti … C) is formed, reducing the ignition temperature for a combustion reaction. In addition, a minimum adiabatic temperature of 1800 K is necessary for the occurrence of combustion during the mechanosynthesis process.
TL;DR: A detailed overview of the state of the art and newly developed manufacturing routes and materials, metallic and nonmetalli materials, can be found in this paper, where the authors also present several prototype disc brakes being manufactured by various casting processes.
Abstract: Disc brake manufacturing for railway vehicles has become a popular fieldfor assessing the possibilities of MMC application during recent years. The substitution of conventional ferrous materials by light alloys allows for a significant reduction in the rotating masses in a vehicle bogie which leads to a decrease in energy consumption. The use of conventional aluminium casting alloys is not suitable to solve this. Castable MMC materials are a highly promising alternative. Under economic pressure MMC solutions are required to be not only mechanically superior but also cost effective, and due to the low ductility of these particle reinforced materials there is a needfor new concepts. Locally optimised discs consist of a ductile carrier (body) with wear resistant rubbing surfaces. This led to several prototype disc brakes being manufactured by various casting processes. This paper gives an overview of the state of the art and newly developed manufacturing routes and materials, metallic and non-metalli...
TL;DR: In this article, Alloy Alloy Alloy was bonded to 304 stainless steel by a friction welding procedure, and some intermetal1ic compounds, Fe2Al5, FeAl, and Fe3Al, were formed at the weld interface.
Abstract: Type 1050 aluminium was bonded to type 304 stainless steel by a friction welding procedure. The aluminium was greatly deformed, and the grains were elongated and refined near the weld interface. The stainless steel was slightly deformed and partly transformed at the faying surface from austenite to martensite owing to hard friction. As a result, the hardness of both materials in the vicinity of the weld interface was higher than that of the base metals. Constituent elements of both materials had interdiffused through the weld interface, and some intermetal1ic compounds, Fe2Al5, FeAl, and Fe3Al, were formed at the weld interface. It was presumed by estimating with metallographic observation that the welding temperature was lower than the lowest eutectic temperature in the metal system. So the friction welding mechanism of this system was based on interdiffusion of each constituent in the solid state.
TL;DR: In this paper, high temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a gene.
Abstract: High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a gene...
TL;DR: In this article, the volume of solvent and N2 prepressure are adjusted in such a way that the solvent/vapour phase boundary should not be crossed before reaching the critical point of the solvent (Me0H), to obtain crack free transparent (≲93% at wavelength 900 nm for 10 mm thickness sample) silica aerogels.
Abstract: Silica aerogels have been prepared by hydrolysis and polymerisation of methanol (MeOH) diluted tetramethoxysilane (TMOS) in the presence of a basic catalyst (ammonia) at a pH of ∼7. To obtain high optical quality silica aerogels for window applications, various supercritical drying parameters such as volume of solvent (MeOH), nitrogen (N2) prepressure in the autoclave, alcogel heating rate, stabilisation period, and solvent evacuation rate have been varied. It has been found that the volume of solvent and N2 prepressure are to be adjusted in such a way that the solvent/vapour phase boundary should not be crossed before reaching the critical point of the solvent (Me0H), to obtain crack free transparent (≲93% at wavelength 900 nm for 10 mm thickness sample) silica aerogels. Higher autoclave heating (>40 K h-1 ) and evacuation rates (> 70 bar h-1) resulted in cracked and less transparent (83% at 900 nm) aerogels. It has been observed that a stabilisation period of <0.5 h is required to avoid shrinka...
TL;DR: The morphology of β-AlFeSi phase was investigated in permanent mold castings of an Al-7Si (wt-%) base alloy at the three iron levels of 0.3, 0.6, and 1.0 wt-% and at two mould temperatures of 25 and 400°C as discussed by the authors.
Abstract: The morphology of β-AlFeSi phase was investigated in permanent mould castings of an Al-7Si (wt-%) base alloy at the three iron levels of 0.3, 0.6, and 1.0 wt-% and at two mould temperatures of 25 and 400°C. At high cooling rates and low iron levels, the β platelets were found to be at the interface between flake silicon and aluminium. Increasing the iron level and decreasing the cooling rate encouraged the β platelets to crystallise independently of silicon. The pre-eutectic β platelets which form at high iron levels were efficient nucleants for the eutectic silicon in unmodified and strontium modified alloys. The size of the β platelets increased with increase in iron level and with decrease in cooling rate. It is postulated that the silicon content of the alloy will also influence the size of β platelets at high iron levels; the platelet size in this case will increase with increase in silicon content.
TL;DR: In this paper, the microstructural features which control the behavior of crack deflecting structures are quantitatively described and issues associated with the manufacture of such structures and the most appropriate types of applications are discussed, as well as the implications for desirable microstructures and the means of obtaining them.
Abstract: It has been shown previously that laminated ceramic structures of layers (or fibres) separated by crack deflecting structures give a simple, easy, and general method of enhancing the properties of ceramics, as determined in both the laboratory and under service conditions, particularly in environments of extreme thermal loading. In this paper the microstructural features which control the behaviour are quantitatively described. Issues associated with the manufacture of such structures and the most appropriate types of applications are also discussed, as well as the implications for desirable microstructures and the means of obtaining them.
TL;DR: In this article, a method is described for calculating changes in the surface area and edge per unit volume as a function of common deformations encountered in the production of steels, where each grain in the undeformed material is represented by a tetrakaidecahedron, a shape which is a realistic representation of equiaxed grains.
Abstract: The deformation of a polycrystalline material leads to changes in the amount of grain surface and grain edge per unit volume. These parameters are of importance in kinetic theory since both surfaces and edges are heterogeneous nucleation sites. In the present study a method is described for calculating changes in the surface area and edge per unit volume as afunction of common deformations encountered in the production of steels. Unlike previous analyses, each grain in the undeformed material is represented by a tetrakaidecahedron, a shape which is a realistic representation of equiaxed grains. There are some interesting results which are compared with previous work.
TL;DR: In this paper, a doubly extruded magnesium-based composite, ZK60 reinforced with 17 vol.-%SiC particles, was investigated and the influences of strain rate on the flow stress and elongation in the composite were characterised.
Abstract: Superplasticity in a doubly extruded magnesium based composite, ZK60 reinforced with 17 vol.-%SiC particles, was investigated. The influences of strain rate on the flow stress and elongation in the composite were characterised. It was found that the composite exhibited superplasticity (elongation > 400%) at high strain rates (∼ 0.1 s−1). In addition, the material showed a high strain rate sensitivity of 0.5 and an activation energy of 81 kJ mol−1 in the superplastic region. Based upon these results, it was proposed that the dominant deformation mechanism is grain boundary sliding accommodated by a grain boundary diffusion process. A comparison of the superplastic behaviour was also made between the doubly extruded and singly extruded composites.
TL;DR: In this paper, a neural network technique trained within a Bayesian framework was applied to the analysis of the yield strength, ultimate tensile strength, and percentage elongation of mechanically alloyed oxide dispersion strengthened ferritic steels.
Abstract: A neural network technique trained within a Bayesian framework has been applied to the analysis of the yield strength, ultimate tensile strength, and percentage elongation of mechanically alloyed oxide dispersion strengthened ferritic steels. The database used was compiled using information from the published literature, consisting of variables known to be important in influencing mechanical properties. The analysis has produced patterns which are metallurgically reasonable, and which permit the quantitative estimation of mechanical properties together with an indication of confidence limits.
TL;DR: In this article, the influence of cooling rate on the hot ductility of plain C-Mn steels containing 0.1-0.7%C has been examined for tensile specimens tested in the temperature range 1000-550°C.
Abstract: The influence of cooling rate on the hot ductility of plain C-Mn steels containing 0.1–0.7%C has been examined for tensile specimens tested in the temperature range 1000–550°C. Increasing the cooling rate from 10 to 60 K min−1 in all cases caused the ductility to deteriorate. This deterioration was found to be associated with the presence of a finer sulphide distribution in the ferrite surrounding the austenite grains, as well as a reduction in the thickness of the ferrite covering. Decreasing the strain rate from 3 x 10−3 to 5.3 × 10−4 S−1 led to a smaller influence of cooling rate. However, ductility was always poorer at the lower strain rate as grain boundary sliding was enhanced. Evidence is also submitted to indicate that MnS inclusions can encourage the production of deformation induced ferrite, leading to an earlier recovery in ductility at the low temperature end of the trough in the hot ductility curve.
TL;DR: In this paper, metal matrix composites were produced with modified pressure liquid infiltration equipment using short δ-A1203 (Saffil) fibre preforms, containing 10, 15, 20, 25, and 30 vol.-% fibres as reinforcements.
Abstract: Metal matrix composites were produced with modified pressure liquid infiltration equipment using short δ-A1203 (Saffil) fibre preforms, containing 10, 15, 20, 25, and 30 vol.-% fibres as reinforcem...
TL;DR: In this article, the effect of heat treatment and composition on the toughness and hardness of mar aging alloys based on Fe-Ni-Mn is first summarised, and the results show that the increase in hardness follows the relationship ∆H = (Kt)n, where t is the isothermal aging time and K is a temperature dependent rate constant.
Abstract: The effect of heat treatment and composition on the toughness and hardness of mar aging alloys based on Fe-Ni-Mn is first summarised. Age hardening data for an Fe-12Ni-6Mn alloy have been subjected to quantification. In the early stages of aging, the increase in hardness (∆H) has been found, both practically and theoretically, to follow the relationship ∆H = (Kt)n, where t is the isothermal aging time and K a temperature dependent rate constant, giving an activation energy of 79 ± 1.3 kJ mol−1 for the process. This is reasonablefor pipe diffusion of Ni and Mn, as is the value of for the effective diffusion coefficient Do for the temperature range 350–425°C.
TL;DR: In this paper, the authors investigated particle redistribution and matrix microstructure development during hot extrusion of aluminium matrix composites and the influence of process parameters on these two key microstructural features.
Abstract: The aim of the present investigation is to understand particle redistribution and matrix microstructure development during hot extrusion of aluminium matrix composites and the influence of process parameters on these two key microstructural features. Cast composite billets have been partially extruded to study the sequence of microstructure development. Strain and strain rate distribution in the partial extrudates have been measured using a ‘visioplasticity’ technique. Results indicate that strain and strain rate distribution are more homogeneous in the composites than in the unreinforced alloy. Extrudate microstructures reveal the presence of particle bands and deformation bands aligned in the extrusion direction. It is noted that shearing of SiC particle clusters leads to particle banding, and an increase in extrusion ratio and/or a decrease in particle size enhances particle band formation. This phenomenon is attributed to increasing strain partitioning between the cluster and the matrix with i...
TL;DR: In this article, a new surface treatment procedure was used in which the surfaces to be joined were coated with an organic solution immediately after the removal of surface oxides, and the surface quality was assessed using optical microscopy, tensile strength measurements, and scanning electron microscopy of the surfaces of the fractured tensile specimens.
Abstract: Diffusion bonding studies of hot rolled 7075 aluminium alloy have been carried out in vacuum on a Gleeble 1500 test machine over a range of temperatures, pressures, and times. A new surface treatment procedure was used in which the surfaces to be joined were coated with an organic solution immediately after the removal of surface oxides. Bond quality was asssessed using optical microscopy, tensile strength measurements, and scanning electron microscopy of the surfaces of the fractured tensile specimens. Bonds with parent metal strength and microstructure were obtained and the optimum bonding conditions were identified. The characteristics of the surface treatment method and the bonding mechanism are discussed
TL;DR: In this paper, the authors investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0-7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon.
Abstract: The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transform...
TL;DR: In this paper, tensile samples were cast in situ and cooled to test temperatures in the range 760 to 1100°C, at cooling rates of 25, 60, and 200 K min−1.
Abstract: Tensile samples were cast in situ and cooled to test temperatures in the range 760 to 1100°C, at cooling rates of 25, 60, and 200 K min−1. In all cases, increasing the cooling rate to that undergone during thin slab casting caused the ductility to deteriorate. For C-Mn-Al steels this deterioration resulted from both a finer AIN precipitation as well as a finer dispersion of sulphides. In the case of C-Mn-Nb-Al steels, the inferior ductility arose because increasing the cooling rate caused more Nb to be held in solution before testing so that more of the detrimental strain induced Nb(CN) precipitation could occur during the test.
TL;DR: In this article, a ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively, and a model was developed relating fracture toughness to the yield strength (σ y ) volume fraction of retained Austenite and the carbon content of the retained austenite (C y ).
Abstract: A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised hy y optical microscopy and X-ra y diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (σ y ) volume fraction of retained austenite (X y ) and the carbon content of the retained austenite (C y ). Experimental results showed excellent agreement with the prediction of the model that K 2 1C is proportional to σ y (X γ C γ ) 1/2 .
TL;DR: In this paper, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory has been tested and it has been found that measured JMAK exponents (n = 1.3-2.1) are lower than those predicted theoretically and are strongly dependent on deformation ratio.
Abstract: The kinetics of recrystallisation of cold rolled nickel (99.98% purity) have been investigated by means of in situ heating in a scanning electron microscope and quantitative post-mortem metallographic examinations as well as microhardness measurements. The specimens were deformed over a wide range of deformation ratio (e = 0.25–1.12) and isothermally annealed at 455°C. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory has been tested, and it has beenfound that measured JMAK exponents (n = 1.3–2.1) are lower than those predicted theoretically and are strongly dependent on deformation ratio. Therefore, it is confirmed that the JMAK theory fails to apply to the recrystallisation of plastically deformed nickel. Two hypotheses, competition between recovery and recrystallisation and non-random distribution of nucleation sites, are discussed to explain the low values of the JMAK exponent.
TL;DR: In this paper, thermal and microstructural evolution during hot rolling of low carbon steel in a continuous six stand mill is simulated with a two-dimensional explicit finite difference model in which the cross-sectional area of the strip is divided into small elements of equal volume.
Abstract: Thermal and microstructural evolution during hot rolling of low carbon steel in a continuous six stand mill is simulated with a two-dimensional explicit finite difference model in which the cross-sectional area of the strip is divided into small elements of equal volume. The heat transfer coefficients at the surface of the strip are allowed to change as it is assumed that the strip is in air or is being descaled or deformed. Results of the microstructural modelling indicate that austenite is able to undergo dynamic recrystallisation when the conditions within the roll gap are propitious. This model also allows for the occurrence of metadynamic and static recrystallisation once the material leaves the gap and for grain growth after their completion. From this, it is concluded that the most important controlling mechanism is grain growth. The thermal portion of the model was validated with measurements made on a six stand continuous mill. It was not possible to obtain a direct validation of the micr...