Showing papers in "Materials Science and Technology in 2005"
TL;DR: In this article, a finite element model has been established and used to predict the temperature history of direct laser fabricated (DLFed) Ti-6Al-4V thin wall samples, and the effects of laser power and the effect of location within a sample on its temperature history have been modeled and compared with temperatures measured during DLF using thermocouples.
Abstract: A finite element model has been established and used to predict the temperature history of direct laser fabricated (DLFed) Ti–6Al–4V thin wall samples. The effects of laser power and the effect of location within a sample on its temperature history have been modelled and compared with temperatures measured during DLF using thermocouples. The thermal history of the material has been correlated with the observed differences in the microstructures obtained at different locations in a given sample or for a given location in samples obtained at different laser powers.
132 citations
TL;DR: In this article, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the heat affected zones (HAZs) of steels are rationalised along with nucleation mechanisms.
Abstract: Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion de...
128 citations
TL;DR: In this 52nd Hatfield Memorial Lecture, this article described a remarkably beautiful microstructure consisting of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes.
Abstract: Most new materials are introduced by selectively comparing their properties against those of steels. Steels set this standard because iron and its alloys have so much potential that new concepts are discovered and implemented with notorious regularity. In this 52nd Hatfield Memorial Lecture, I describe a remarkably beautiful microstructure consisting of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes. The crystals are generated by the partial transformation of austenite, resulting in an extraordinary combination of strength, hardness and toughness. All this is in bulk steel without the use of expensive alloying elements. We now have a strong alloy of iron, which can be used for making items that are large in all three dimensions, which can be made without the need for mechanical processing or rapid cooling and is cheap to produce and apply.
118 citations
TL;DR: In this article, a bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition, and they were subjected to an aging treatment with and without post-weld solution treatment.
Abstract: Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating micro...
104 citations
TL;DR: In this paper, the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10-50 μm length) fatigue crack formation are reported, along with micro-structural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases.
Abstract: Rail life is controlled by the balance between wear and fatigue damage due to in service loading. To model and optimise rail life, knowledge of the fatigue crack initiation mechanism is required. This paper reports the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10–50 μm length) fatigue crack formation. Micro and nanohardness measurements are reported, along with microstructural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases. It is proposed that the differential straining results in ductility exhaustion in the proeutectoid ferrite and therefore fatigue crack initiation and initial growth in the proeutectoid ferrite phase. Observations of short (<50 μm) cracks in rails taken out of service containing significant amounts of proeutectoid ferrite (≈20%) confirm the proposed mechanism.
78 citations
TL;DR: In this paper, a second interrupted aging cycle was developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature.
Abstract: Earlier work has shown that, if the artificial aging of aluminium alloys is interrupted by a dwell period at lower temperature, higher values of tensile properties and fracture toughness may be achieved than are possible with single stage T6 tempers. A second interrupted aging cycle has now been developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature. Designated a T6I4 (I=interrupted) temper by the authors, this simpler aging cycle may reduce heat treatment costs. Tests on some 30 cast and wrought alloys have resulted in tensile properties close to those for a T6 temper, with higher values of fracture toughness being recorded for some cases. Such an aging treatment can be incorporated into a paint bake cycle to simplify the heat treatment of coated automotive components.
77 citations
TL;DR: The hot deformation of AZ31 magnesium alloy has been studied by compression testing using a Gleeble 1500 machine at temperatures between 250 and 450°C and at strain rates ranging from 0·005 to 5 s−1.
Abstract: The hot deformation of AZ31 magnesium alloy has been studied by compression testing using a Gleeble 1500 machine at temperatures between 250 and 450°C and at strain rates ranging from 0·005 to 5 s−1. Optical microscopy and transmission electron microscopy (TEM) have been used to observe microstructures of the alloy. The experimental results show that the flow stress behaviour can be described by an exponential law at temperatures below 350°C. At higher temperatures a power law of deformation is valid. The hot deformation activation energy Q derived from the experimental data is 112 kJ mol−1 with a stress exponent n=7. Optical microscopy and TEM observations show that dynamic recrystallisation (DRX) takes place during the deformation process and the formation of new grains occurs by conventional DRX nucleation by bulging. The average DRX grain size Drex is sensitively dependent on deformation temperature T and strain rate ϵ and is also a function of the Zener–Hollomon parameter Z. The relationship ...
77 citations
TL;DR: In this paper, the role of the chemical composition of the substrate on the characteristics of the coatings was evaluated for two sets of processing parameters, targeting two extreme dilution levels.
Abstract: Components from equipment for different processing industries are exposed to severe service conditions. Parts are therefore manufactured so that specific demands can be met through adequate selection of surface processing techniques and hardfacing alloys. Whenever surface welding techniques are used to improve a component's performance, quality requirements such as controlled dilution of the deposited alloy by the substrate are a priority. This work has evaluated this interaction between a hardfacing Co based alloy and the substrate steel. A high carbon cobalt based alloy was 'plasma transferred arc' (PTA) deposited on three different substrate steels. The role of the chemical composition of the substrate on the characteristics of the coatings was evaluated for two sets of processing parameters, targeting two extreme dilution levels. Coatings characterisation was assessed through microstructure analysis, microhardness measurements, dilution levels, and wear rate measurements. Results showed that t...
63 citations
TL;DR: In this article, the meaning of the X-ray penetration depth for very thin films and the important role of refraction in the generalised non-symmetric case of grazing incidence diffraction are discussed and illustrated by practical examples from different fields in thin film diffraction analysis.
Abstract: Residual stress fields in polycrystalline materials induced by mechanical surface processing or coating often are highly inhomogeneous with respect to depth z below the sample surface. This even applies for thin films, the thickness of which is usually much smaller than the penetration depth t of the X-rays. Therefore, X-ray residual stress analysis in thin films is advantageously performed under grazing diffraction conditions. To avoid erroneous averaging over the film thickness, both the radiation used and the methods applied to analyse the residual stress distributions have to be adapted carefully to the particular situation. In this paper some fundamental aspects concerning the meaning of the X-ray penetration depth for very thin films and the important role of refraction in the generalised non-symmetric case of grazing incidence diffraction are discussed and illustrated by practical examples from different fields in thin film diffraction analysis.
58 citations
TL;DR: In this article, a magnesium composites reinforced with different volume fraction of submicron size Al2O3 particulates were synthesized using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique.
Abstract: In the present study, magnesium composites reinforced with different volume fraction of submicron size Al2O3 particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al2O3 particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al2O3 composites was found to decrease with increasing amount of submicron Al2O3 particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the...
57 citations
TL;DR: In this paper, the microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction.
Abstract: The microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction. The deformation imparted amounts to a true strain of 6·3. Data from previous studies on strain induced transformation of this steel have been combined with new results to show that true strains >2 are required in order to observe mechanical stabilisation, i.e. the cessation of martensitic transformation when the martensite/austenite interfaces are unable to propagate through the dislocation debris created in the austenite.
TL;DR: In this article, the effect of different zirconium contents on the grain size of Mg-9Gd-4Y alloys and the grain refinement mechanism of ZIRconium have been studied.
Abstract: The effect of different zirconium contents on the grain size of Mg–9Gd–4Y alloys and the grain refinement mechanism of zirconium have been studied. The results reveal that zirconium can refine the grains of the alloys to a large extent, and the grains become finer with an increase of zirconium content. Microstructural analysis shows that there is at least one zirconium rich core in almost each grain in alloys with a high zirconium content, whereas the characteristic zirconium rich cores are not found in the alloy with a low zirconium content. It is suggested that the grain refinement mechanism of zirconium in the low zirconium alloy is different from that in the high zirconium alloys: the zirconium works mainly by restricting grain growth in the low zirconium alloy, and by generating nucleants in the high zirconium alloys.
TL;DR: In this paper, the nitriding behavior of the Fe-7Cr alloy was studied at 580°C in a gas mixture of ammonia and hydrogen, and the potential for nitrides was varied from 0.03 to 0.818 atm−1/2.
Abstract: The nitriding behaviour of the Fe–7Cr alloy was studied at 580°C in a gas mixture of ammonia and hydrogen. The nitriding potential was varied from 0.03 to 0.818 atm−1/2. Microstructural analysis of the nitrided specimens was performed by applying light microscopy, hardness measurements, X-ray diffraction, and electron probe microanalysis. The nitrided zone is composed of both regions with finely dispersed small chromium nitride (CrN) precipitates (continuous precipitates) in ferrite (α–Fe) grains and, mainly near the surface, regions where the precipitates have discontinuously coarsened leading to a lamellar CrN/α–Fe morphology. The nitrogen content within the nitrided zone is larger than expected on the basis of the chromium content and the solubility of nitrogen in (stress free) ferrite: excess nitrogen occurs. The hardness maximum in the nitrided zone and the nitriding depth increases with increasing nitriding potential as long as no iron nitride layer develops at the surface of the specimens. ...
TL;DR: In this article, the authors describe the precipitation behavior in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb using analytical transmission electron microscopy.
Abstract: The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the p...
TL;DR: In this paper, the diffusion activation energies for both fcc and bcc γ-Fe matrices were derived from available data such as vacancy migration energies, solute diffusion activation activation energies and vacancy-solute binding energies.
Abstract: Some non-equilibrium kinetic processes, such as thermal non-equilibrium segregation and deformation induced non-equilibrium segregation, require the presence of vacancy–solute complexes to be part of the underlying mechanism. To theoretically evaluate these processes, knowledge of the diffusivity for the complexes is required. Because it is difficult to experimentally measure this diffusivity, mechanisms for the complex diffusion are theoretically described for both fcc and bcc crystals. The activation energies involved in the complex diffusion are in turn estimated for the fcc γ-Fe and bcc α-Fe matrices on the basis of these mechanisms and some available data such as vacancy migration energies, solute diffusion activation energies and vacancy–solute binding energies.
TL;DR: In this article, the microstructural changes occurring in this region during long term creep tests at 600°C and 70 MPa were examined using optical microscopy and scanning and transmission electron microscopy for comparison with the microstructure of this region after post-weld heat treatment.
Abstract: The martensitic 10CrMoVNb 91 (P91) steel has been developed for applications under high ultra supercritical pressure for steam power plants The creep rupture strength of welded joints of this steel is limited by the fine grained region of its heat affected zone (HAZ) The microstructural changes occurring in this region during long term creep tests at 600°C and 70 MPa were examined using optical microscopy and scanning and transmission electron microscopy for comparison with the microstructure of this region after post-weld heat treatment The factors which result in the reduction of creep rupture strength of the fine grained region of HAZ are discussed It was found that the most effective factors reducing the creep rupture strength of the fine grained region of the HAZ of a welded joint in P91 steel, in comparison with the coarse grained region of the HAZ and the base metal of the welded joint, are the finer prior austenite grain size of the fine grained region, which accelerates the rate of g
TL;DR: In this paper, the effect of fly ash addition on the mechanical properties of aluminium casting alloy 535 (A535) was investigated by means of hardness measurements, tensile testing, Charpy impact testing, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and Xray fluorescence spectroscopy (XFS).
Abstract: Fly ash is a lightweight coal combustion byproduct (CCB) which is separated from the exhaust gases of power generating plants using suspension fired furnaces in which pulverised coal is used as the fuel. Its physical and chemical properties make it a useful construction and industrial material, especially in cement manufacturing, concrete, liquid waste stabilisation, and hydraulic mine backfill. The addition of fly ash into aluminium alloys has the potential to reduce the cost and density of aluminium castings while improving other physical and mechanical properties of the resulting metal matrix composites (MMCs). In the present study, the effect of fly ash addition on the mechanical properties of aluminium casting alloy 535 (A535) was investigated by means of hardness measurements, tensile testing, Charpy impact testing, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray fluorescence spectroscopy (XFS). The unreinforced A535 and its MMCs containing a mixture of 5 wt-% fly ash and 5 wt-% silicon carbide, 10 wt-% fly ash and 15 wt-% fly ash were investigated in the as cast and heat treated conditions. The results show that increasing the fly ash content of the melt increased the porosity of the castings, which ultimately affected the tensile and impact properties of the MMCs. The microhardness, tensile strength and Charpy impact energy of the composites decreased with increasing fly ash content. The loss in mechanical properties and impact resistance of the MMCs is attributed partly to the depletion of solid solution strengthening magnesium atoms from the matrix and partly to porosity.
Abstract: A comparative evaluation of the precipitation behaviour in V–Nb–Ti and V steels using transmission electron microscopy was undertaken to study the effective cumulative role of the microalloying add...
TL;DR: In this paper, it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s−1 under four-point bending conditions at 650°C in air.
Abstract: Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s−1 under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is ...
TL;DR: In this paper, an Al-Li-Mg-Sc-Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400°C.
Abstract: An Al–Li–Mg–Sc–Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400°C. The superplastic properties and microstructure evolution of the alloy were examined in tension in the temperature interval 250–500°C at strain rates ranging from 1·4 × 10−5 to 1·4 s−1. Superior superplastic properties (elongation to failure of 3000% with a corresponding strain rate sensitivity coefficient m of ∼0·6) were attained at 450°C and ɛ=1·4 × 10−2 s−1 in samples processed at 400°C to a total strain of ∼16. The alloy in this state had an average grain size of ∼2·6 μm and the recrystallisation fraction was about 90%. It was shown that the highest superplastic ductility appears in samples with more uniform microstructure containing the highest portion of high angle boundaries. It was established that the uniformity of structure and its stability under superplastic deformation is more impo...
TL;DR: In this paper, the effects of Ca/Sr separate and composite additions to AZ91D magnesium alloy on its microstructure and mechanical properties have been investigated, and the results indicate Ca can refine both the grain and eutectic phase of AZ91d magnesium alloy.
Abstract: Weight reduction to improve automobile fuel economy has triggered renewed interest in magnesium. The effects of Ca/Sr separate and composite additions to AZ91D magnesium alloy on its microstructure and mechanical properties have been investigated. The results indicate Ca can refine both the grain and eutectic phase of AZ91D magnesium alloy. Sr hampers microstructure refinement when composite Ca/Sr additions are made. In addition, separate Ca additions to AZ91D magnesium alloy increase yield strength but decrease elongation of this alloy. By adjusting the Ca/Sr composite proportions, additions to AZ91D magnesium alloy are able to improve both microstructure and mechanical properties of the alloy.
TL;DR: In this article, an advanced evaluation scheme is presented to determine the predominant rotation axis within individual grains of a deformed polycrystal and displaying the sign carrying disorientation angle, characteristic features of the deformation structure, as alternating orientation differences or orientation gradients.
Abstract: Orientation information as obtained by electron back scattering diffraction in crystalline material is conventionally visualised in orientation maps with different colours representing different orientations. The possibility of resolving orientation inhomogeneities from such maps depends strongly on the selected colouring scheme. For gaining spatial information on the orientation distribution and retrieving orientation correlations within selected regions of an orientation map, an advanced evaluation scheme is presented. By determining the predominant rotation axis within individual grains of a deformed polycrystal and displaying the sign carrying disorientation angle, characteristic features of the deformation structure, as alternating orientation differences or orientation gradients, are resolved.
TL;DR: In this paper, the time-temperature property of the C curve for aluminium alloys was determined using a large number of quenches and isothermal holds to calibrate a set of constants that describes the shape of the shape.
Abstract: Determination of the time–temperature property C curve for aluminium alloys usually involves a large number of quenches and isothermal holds to calibrate a set of constants that describes the shape...
TL;DR: Orientation mapping based on electron backscatter diffraction (EBSD) technique is applied to reveal microstructural and textural characteristics of flip chip bonded Au at its different processing sites as mentioned in this paper.
Abstract: Orientation mapping based on electron backscatter diffraction (EBSD) technique is applied to reveal microstructural and textural characteristics of flip chip bonded Au at its different processing s...
TL;DR: In this paper, the minimum superheat necessary to solder components on printed circuit boards successfully using 95.5 wt-%Sn, 3.8 wt -%Ag, 0.7 wt −%Cu solder was investigated, using separate solder balls of the type used in ball grid arrays.
Abstract: To investigate the minimum superheat necessary to solder components on printed circuit boards successfully using 95.5 wt-%Sn, 3.8 wt-%Ag, 0.7 wt-%Cu solder, experiments were carried out using separate solder balls of the type used in ball grid arrays. Significant differences in microstructure were observed depending on the peak temperature reached in the liquid on melting. On cooling, substantial undercooling was often observed, with values up to 18 K. Under some freezing conditions, the primary phase formed was Ag3Sn, while under other conditions the primary phase was Sn. The amount and type of eutectic microstructure formed was observed to vary with freezing conditions. The types of microstructure formed are illustrated. Nucleation phenomena and their effect on subsequent growth are discussed.
TL;DR: In this paper, the authors developed a torsion fracture model for notched elements of brittle materials, based on the theory of brittle fracture due to Griffith, and derived a formula for the Torsion notch strength of a brittle material.
Abstract: In the present paper, attempts are made to develop a torsion fracture model for notched elements of brittle materials. Based on the theory of brittle fracture due to Griffith, a formula for the torsion notch strength of a brittle material is thus derived. Observations of the fracture mode show that torsion fracture of both smooth and notched specimens of the inorganic glass tested is typical brittle fracture. Experimental results for torsion fracture strength determined by testing smooth and notched specimens of the brittle inorganic glass are summarised and applied to check the applicability of the abovementioned model. Moreover, it is noted that the torsion strength of a brittle material is not a material constant but dependent on the specimen geometry and fracture toughness. Expressions of torsion notch strength of brittle materials with given survivability are predicted from the probability distribution parameters of torsion fracture strength and the formula for torsion notch strength, and are...
TL;DR: In this article, several homogenization treatments were applied to direct chill (DC) cast ingots of aluminium alloy 6063, in order to analyse the resulting microstructures developed from these diverse conditions and their effects on the hot ductility of this alloy.
Abstract: Several homogenisation treatments were applied to direct chill (DC) cast ingots of aluminium alloy 6063, in order to analyse the resulting microstructures developed from these diverse conditions and their effects on the hot ductility of this alloy. Imaging was performed using scanning electron microscopy (SEM) and a focused ion beam (FIB) instrument. These techniques identified variations in distribution and morphology of second phase particles (AlFeSi and Mg2Si). FIB results for the various AlFeSi particles correctly identify their shapes in three dimensions (3D). The particles were identified by energy dispersive spectroscopy (EDS) in the SEM, and by X-ray diffraction (XRD) for bulk samples. Hot tensile testing (HTT) was conducted between 470 and 600°C to asses the hot ductility for each condition. The inferior ductility of as cast samples was due to the poor bond strength of the β AlFeSi phase at the grain boundaries. Homogenised samples, which contain α AlFeSi, exhibited improved ductility. Sa...
TL;DR: In this paper, the correlation between low and high strain rate deformation characteristics and the microstructure of novel strip steel products employed by the automotive industry has been assessed and the results obtained indicate that the dual phase steel grades having similar microstructures, although with varying strength levels, displayed the same response to high strain-rate loading conditions.
Abstract: The correlation between low and high strain rate deformation characteristics and the microstructure of novel strip steel products employed by the automotive industry has been assessed. Research has been carried out to establish whether microstructural changes during high speed deformation could explain the relative attributes of mild steel, carbon manganese steel and novel dual phase steel grades of increasing yield and tensile strength, in relation to their crash behaviour. The results obtained indicate that the dual phase steel grades having similar microstructures, although with varying strength levels, displayed the same response to high strain rate loading conditions. They both exhibited a reduction in the level of hardness at high strain rate compared with low strain rate deformation. The strain rate softening effect observed with the dual phase steel was accompanied by an increase in plastic deformation of the ferrite areas around the martensitic constituent regions.
TL;DR: In this article, the Rheo-die casting (RDC) process, a new semi-solid processing technology developed by BCAST at Brunel University, was used to overcome such casting difficulties and to improve mechanical properties.
Abstract: AM50 Mg alloy is usually processed by high-pressure die casting (HPDC) for applications demanding high ductility. However, this alloy has a high tendency of hot tearing, and is difficult to cast because of its large freezing range and low final solidification temperature. In this investigation, the rheo-die casting (RDC) process, a new semi-solid processing technology developed by BCAST at Brunel University, was used to overcome such casting difficulties and to improve mechanical properties. It was found that the RDC AM50 alloy exhibited much improved mechanical properties, especially for the elongation, over those of the same alloy produced by HPDC and other semi-solid processing technologies. The improved mechanical properties can be attributed to the elimination of large gas pores as a result of gas entrapment, substantial reduction of hot tearing, and fine, uniform microstructure, as a consequence of solidification under high shear rate and high intensity of turbulence in the RDC process.
TL;DR: In this paper, the authors quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0·001 s-1 and 100 s- 1 respectively), on the tensile and work hardening properties of a range of commercial dual phase products.
Abstract: As a result of their unique combination of strength and ductility dual phase steels play an important role in reducing weight in automobile components and improving crashworthiness. The purpose of this paper is to quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0·001 s-1 and 100 s-1 respectively), on the tensile and work hardening properties of a range of commercial dual phase products. The objective is to establish whether dual phase steels maintain their desirable mechanical property characteristics of low yield strength, high tensile strength and high work hardening rates during plastic deformation under the application of a high strain rate loading. The results confirmed that the yield/proof strength and tensile strength increased with increasing volume fraction of second phase constituents and increasing strain rate. In particular, a dual phase steel with a microstructure consisting of a significant volume fraction (>1...