Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2000"

Recent development in aluminium alloys for the automotive industry

Abstract: The growing demand for more fuel-efficient vehicles to reduce energy consumption and air pollution is a challenge for the automotive industry. The characteristic properties of aluminium, high strength stiffness to weight ratio, good formability, good corrosion resistance, and recycling potential make it the ideal candidate to replace heavier materials (steel or copper) in the car to respond to the weight reduction demand within the automotive industry. This paper summarises the recent developments covering aluminium’s use in castings, extrusions and sheet; two specific examples will be given. The first deals with hang-on parts manufactured by Hoogovens Rolled Products Duffel, for which the weight saving potential can be 50%. Currently, the highly formable 5000 alloys are used mostly for inner panel applications, whilst the heat-treatable 6000 alloys are preferred for outer panel applications. This presentation reviews recent developments in aluminium alloys to improve formability, surface quality in both 5000 and 6000 alloys, and the bake hardening response of 6000 alloys. It also indicates the trend to develop a unialloy system to improve the aluminium scrap recycling. The second area deals with brazing sheet. Over the last 10 years there has been an increasing trend to replace copper heat exchangers with ones manufactured from brazed aluminium. Hoogovens Aluminium Walzprodukte Koblenz is one of the world’s leading supplier of aluminium brazing sheet and is in the forefront of developing alloys with the combination of strength, formability, brazing performance and long life required by its customers. Materials have been development for both vacuum and controlled atmosphere brazing. The current status and future trends in aluminium brazing sheet for automotive applications will be presented. Particular emphasis has been placed on the development of long life alloys with superior corrosion performance over the more conventional materials. Using these two examples the technical and commercial aspects of the manufacturing processes of aluminium automotive components and engineering design support of materials producers are illustrated. The essential feature is the close co-operation at all stages between the material’s supplier and the automotive manufacture.

Recent development in aluminium alloys for aerospace applications

Abstract: Driven by the increasing requirements from aircraft producers, Hoogovens Aluminium Rolled Products GmbH, together with Hoogovens Research & Development, has enhanced the property combinations of their aircraft materials. For these types of material, optimised processing routes as well as new alloy chemistries have been investigated. Whilst retaining the strength levels required by the aerospace industry, new processing routes offer major improvements in ductility, toughness, fatigue performance and in reduction of residual stress in large dimension plate and sheet products. A further goal of investigating new alloy chemistries is the trend towards new joining techniques such as welding and brazing for aircraft structures. These new joining techniques require different property combinations compared to the conventional aerospace alloys. In parallel to these improved processing routes and new alloy developments, new ultrasonic inspection techniques have been developed, which are able to predict fatigue performance and residual stress in thick plate products.

Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS)

Abstract: The graphite die set in spark plasma system (SPS) is heated by a pulse direct current. Weak plasma, discharge impact, electric field and electric current, which are based on this current, induce good effects on materials in the die. The surface films of aluminum and pure WC powders are ruptured by the spark plasma. Pure AlN powder is sintered without sintering additives in the electric field. The spark plasma leaves discharge patterns on insulators. Organic fibers are etched by the spark plasma. Thermosetting polyimide is consolidated by the spark plasma. Insoluble polymonomethylsilane is rearranged into the soluble one by the spark plasma. A single crystal of CoSb3 is grown from the compound powders in the electric field by slow heating. Coupled crystals of eutectic powder are connected with each other in the electric field.

Processing materials with microwave energy

Abstract: Microwave energy (microwave frequency, in this case, includes radio frequencies and ranges from 0.3 MHz to 300 GHz) is being developed as a new tool for high-temperature processing of materials. Examples of the advantages associated with microwave processing include: rapid and uniform heating; decreased sintering temperatures; improved physical and mechanical properties; and, unique properties which are not observed in conventional processes. These advantages observed in materials processed using microwave energy are being attributed to ‘microwave effects’ which are particular to this technology. Researchers at the University of Florida are working to identify and to qualitatively and quantitatively define the mechanisms of microwave–material interactions. A new model has been developed based on the molecular orbital model which predicts the behavior of specific pure materials in a microwave field. Experimental work as well as dielectric property measurements confirm the accuracy of this model in specific cases.

An experimental study of the recrystallization mechanism during hot deformation of aluminium

Abstract: Discontinuous dynamic recrystallization (involving nucleation and grain growth) is rarely observed in metals with high stacking fault energies, such as aluminium. In this metal, two other types of recrystallization have been observed: continuous dynamic recrystallization (CDRX, i.e. the transformation of subgrains into grains); and geometric dynamic recrystallization (due to the evolution of the initial grains). The main purpose of this work was to bring clearly into evidence and to better characterize CDRX. Uniaxial compression tests were carried out at 0.7 T m and 10 −2 s −1 on three types of polycrystalline aluminium: a pure aluminium (1199), a commercial purity aluminium (1200) and an Al-2.5wt.%Mg alloy (5052), and also on single crystals of pure aluminium. In addition, 1200 aluminium specimens were strained in torsion. The deformed microstructures were investigated at various strains using X-ray diffraction, optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Observations of the single crystalline samples confirm that subgrain boundaries can effectively transform into grain boundaries, especially when the initial orientation is unstable. In the case of polycrystalline specimens, after separating the effects of the initial and new grain boundaries, it turns out that CDRX operates faster in the 1200 aluminium compared to the two other grades. Moreover, it appears that the strain path does not alter noticeably the CDRX kinetics.

Surface nanocrystallization of 316L stainless steel induced by ultrasonic shot peening

Abstract: A new technique was introduced to realize surface nanocrystallization (i.e. generation of a surface layer of nanostructures) on a 316L stainless steel by means of ultrasonic shot peening treatment. The microstructural evolution of the 316L stainless steel was characterized by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. After the ultrasonic shot peening treatment, obvious grain refinement was observed and a nanocrystalline surface layer was found. The thickness of the nanocrystalline surface layer varies from a few to about 30 μm depending upon the treatment duration and the grain size increases from about 10 nm at the top surface layer, gradually to more than 100 nm at a depth of ≈30 μm. The nanocrystallization mechanism was analyzed in terms of the deformation behavior and TEM observations of the microstructural evolution of the treated samples.

Sintering activation by external electrical field

Abstract: Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

Abstract: The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.

Effects of electric current on solid state phase transformations in metals

Abstract: The influence of an electric current on the following solid state transformations in metals are considered: (1) intermetallic compound formation and growth in diffusion couples, (2) precipitation, (3) crystallization of amorphous alloys and (4) recrystallization and grain growth of cold worked metals. The formation and growth of intermetallic compounds were in qualitative accord with electromigration theory. Regarding precipitation, an electric current can either enhance or retard the precipitation rate, depending on the alloy, the current density and its frequency. Important factors appear to be the effect of current on the quenched-in vacancies and the presence of an internal stress. Both a continuous d.c. current and high current density electropulsing enhanced the crystallization rate of amorphous alloys. The effects are greater than can be explained by simple electromigration theory and suggest the cooperative motion of a larger number of atoms. Electropulsing enhanced the recrystallization rate of cold worked metals, but retarded subsequent grain growth. Enhancement of the recrystallization rate resulted mainly from an increase in the pre-exponential factor of the Arrhenius rate equation, which is considered to refer to the nucleation rate. Retardation of subsequent grain growth resulted from a lower residual dislocation density within the newly-formed grains.

Electroplasticity in metals and ceramics

Abstract: The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Observations and issues on mechanisms of grain refinement during ECAP process

Abstract: The equal channel angular pressing route, defined by rotating schemes between adjacent passes, significantly affects effectiveness of grain refinement. It is of interest to study the mechanisms of grain refinement. Previous work has considered the accumulative strain and the effects of shear strain plane in the interpretation of certain experimental observations. However, they are not sufficiently general, and contradict each other in some cases. In this paper, we analyze experimental results available in the literature, and investigate the fundamental mechanisms of grain refinement. We believe that the interaction of shear plane with texture and crystal structure plays a primary role in grain refinement, while the accumulative strain plays a secondary role. Our model can explain the experimental results in the literature very well. Issues on the grain refinement are discussed and further research to solve these issues is suggested.

Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

Abstract: Coarse-grained copper was subject to high-pressure torsion (HPT) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50°C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280°C, caused by recrystallization. Annealing twins were also formed during recrystallization.

Surface modifications induced in 316L steel by laser peening and shot-peening. Influence on pitting corrosion resistance

Abstract: The influence of laser peening (LP) on the electrochemical behavior of AISI type 316L stainless steel in a saline environment was evaluated. Surface modifications were investigated as they might have beneficial effects on the corrosion behaviour. Low residual stress and work hardening levels were found, when compared with a conventional shot-peening (SP) treatment, mainly because of the absence of martensite transformation in the case of LP. Surface changes were accompanied by small roughening effects and a global preservation of the surface chemistry after treatment. Therefore, electrochemical tests performed on samples after LP and SP treatments showed increases in rest potentials, reductions of passive current densities and anodic shifts of the pitting potentials evidenced by a stochastic approach of pitting. The better pitting resistance was observed after LP treatment, which seems to reflect a reduction or an elimination of active sites for pitting at lower potentials. Even though the deleterious surface state of shot peened surfaces possibly counterbalances the beneficial influence of residual stresses, a beneficial influence of mechanical surface treatments has been demonstrated regarding the localized corrosion properties.

Strain rate sensitivity of a closed-cell aluminum foam

Abstract: An experimental investigation into the strain rate sensitivity of a closed-cell aluminum foam at room temperature and under compression loading is conducted. The nominal strain rates are varied by four orders of magnitude, from 3.33×10−5 to 1.6×10−1 s−1. Within this range, experimental results show that the plastic strength and the energy absorbed increase (by 31 and 52.5%, respectively) with increasing strain rate. However, the plastic strength was found to increase bilinearly with the logarithm of strain rate, whereas dense metals tend to show only a linear response. As is the case with dense metals, the strain rate sensitivity of the foam was not a constant value, but found to be dependent on the strain and incremental change in strain rate. These results are explained with the aid of suitable micromechanical models such as microinertial effects against the bucking of cell walls at high strain rates that are unique to foams.

Scientific principles of making an alloying addition of scandium to aluminium alloys

Abstract: Research works concerning making an alloying addition of scandium to aluminium alloys are carried out for nearly 30 years in Russia. As a result of these studies, main regularities characterising the effect of scandium on a structure and properties of aluminium alloys have been found and, based on this, principles of making an alloying addition of scandium to aluminium alloys and foundations of a technology for production of wrought semiproducts have been formulated.

Sonochemical synthesis and characterization of pure nanometer-sized Fe3O4 particles

Abstract: Sonochemical synthesis of pure nanometer-size Fe3O4 powder with particle size of ca 10 nm is reported in this article. Fe3O4 can be simply synthesized by sonication of iron(II)acetate in water under an argon atmosphere. The properties of pure nanometer-size Fe3O4 particles were characterized by X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) with an external magnetic field, and quantum design SQUID magnetization measurements. The prepared Fe3O4 nanoparticles are superparamagnetic and its magnetization at room temperature is very low (

High strain rate compression of closed-cell aluminium foams

Abstract: The compressive deformation behavior of open- and closed-cell aluminum foams was assessed under static and dynamic loading conditions. High strain rate experiments were conducted in our laboratory using a split Hopkinson pressure bar system at strain rates ranging from 400 to 2500 s−1. A strain rate effect was demonstrated for Alporas, a closed-cell aluminum. foam. The strain-rate effect was more significant for a higher density (i.e. 15% relative density) Alporas foam and is attributed to the kinetics of gas flow through the cell structure. The experimental results are discussed in reference to recent findings by other investigators on the dynamic behavior of similar open- and closed-cell aluminum foams.

On the rule of mixtures for the hardness of particle reinforced composites

Abstract: The hardness variations with the volume fraction of the hard particle in the particle reinforced composite have been analysed using several models for a rule of mixtures and finite element method. The stress states are found to be nearly iso-strain at high volume fraction of the hard particle and iso-stress at low volume fraction of the particle.

Effects of rare earths on the microstructure, properties and fracture behavior of Mg–Al alloys

Abstract: AZ91–xRE and Mg–6Al–xRE magnesium alloys were studied, where x is 0, 1, 2 and 3% (in weight percent, wt.%), respectively. Influence of rear earths (RE) on the microstructure was investigated. Fine morphology could be achieved by high cooling rate. By casting fluidity spiral specimens, fluidities of the alloys were achieved. The hardness and microhardness of the alloys was tested. RE improved fluidity and hardness. By casting specimens in permanent mold, tensile properties of the alloys with different RE additions at ambient and elevated temperatures were studied. RE had little effect on ambient temperature tensile strength of AZ91 alloy but greatly improved that of Mg–6Al alloy and high temperature tensile properties of both alloys. The fracture behavior of the alloys, which was changed by RE and high temperature, was examined by scanning electron microscopy (SEM) and optical microscopy. Fracture of the alloys is predominantly brittle cleavage or/and quasi-cleavage failure.

New Al–Mg–Sc alloys

Abstract: Data on new 01515, 01523, 01535, 01545, 01570 and 01571 wrought weldable alloys based on the Al–Mg–Sc system are presented. These alloys differ from each other, mainly, by magnesium content and belong to the family of non-heat-treatable alloys. Wrought Al–Mg–Sc alloy semiproducts as-hot worked or as-annealed show much higher properties (especially yield strength) than those made from conventional Al–Mg alloys with the same magnesium content. Strengthening relates to both direct strengthening effect of the secondary Al3Sc particles and development of a stable nonrecrystallized structure. Of Al–Mg–Sc alloys, 01570 alloy containing about 6% Mg has found the widest application. Yield strength of hot worked or annealed 01570 alloy semiproducts is at a level of 300 MPa, while yield strength of semiproducts made from a similar scandium-free alloy is about 180 MPa. Prospects of the usage of the new alloys for production of various structures are discussed.

Fabrication and characterisation of pure magnesium-30 vol.% SiCP particle composite

Abstract: Pure magnesium-30 Vol.% SiC particle composite are fabricated by melt stir technique without the use of a flux or protective inert gas atmosphere. After hot extrusion with an extrusion ratio of 13, Mg-30 vol.% $SiC_P$ composites have been evaluated for their tensile properties at room and elevated temperatures (up to 400°C). Composites in the as-cast conditions do not show any change in dendrite arm spacing:cell size compared to unreinforced pure magnesium. However, in the extruded conditions average grain size of the composites is 20 mm compared to 50 mm in the pure magnesium. Microstructure shows no evidence of reaction product at particle:matrix interface. At room temperature, stiffness and UTS of the extruded composites are 40 and 30% higher compared to unreinforced pure magnesium, signifying significant strengthening due to the presence of the SiC particles. Further, up to temperatures of 400°C, composites exhibit higher UTS compared to pure magnesium. Mg composites show a wear rate lower by two orders of magnitude compared to pure Mg, when tested against steel disc using pin-on disc machine.

On the die corner gap formation in equal channel angular pressing

Abstract: A die internal corner gap is usually found during equal channel angular pressing (ECAP) of materials. Finite element analysis using the DEFORM2D code is carried out in order to investigate the corner gap formation between the die and workpiece during the plane strain ECAP process. The comparison of the deformation and the corner gap formation behaviour between the strain hardening material and the quasi-perfect plastic material was made. The mechanism of the corner gap formation is described in conjunction with the strain hardening behaviour and local flow velocity of the workpiece in the deforming zone. The adjustment of the corner angle from the die corner angle to the arc curvature of the workpiece is necessary for a better prediction of the strain during ECAP.

Development of a multi-pass facility for equal-channel angular pressing to high total strains

Abstract: A multi-pass facility was fabricated for equal-channel angular (ECA) pressing which gave a total strain of approximately 5 on a single passage through the die. Experiments on high purity aluminum show that, when comparisons are made at the same total strains, both the hardness and the evolution of the microstructure are identical when using the multi-pass facility or when repetitively pressing samples through a standard die containing a single shearing plane.

Deformation induced martensite and superelasticity in a β-metastable titanium alloy

Abstract: This work investigates the formation of deformation induced martensite and shape-memory effects of water quenched β-rich microstructures of a β metastable titanium alloy denominated β-Cez (developed by the Cezus company). In highly β metastable microstructures — solution treatments for 1 h at 920 (β domain) or 860°C (α+β domain) — the β phase transforms to a martensite that is stress-assisted in nature. Solution treatment at a slightly lower temperature (850°C) increases the stability of the β phase and the martensite thus becomes a strain-induced type. Decreasing the solution treatment temperature further (800°C) leads to a sufficiently stable β phase so that no martensite is formed under tensile deformation and only slip is activated. These different ways of accommodating deformation result in various mechanical responses. In particular, the stress-assisted martensite in this alloy is partly reversible under unloading, which produces significant superelasticity.

Microstructural development of Al-15wt.%Mg2Si in situ composite with mischmetal addition

Abstract: In situ Al-Mg2Si composites, as a new class of ultralight materials for aerospace and other advanced engineering applications, are attracting more and more attention of both material scientists and design engineers. It has been demonstrated in our previous work that with proper microstructural control, in situ Al-Mg2Si composites can offer very promising combinations of strength, ductility and fracture toughness. In the present work, the effects of mischmetal (MM, a mixture of rare-earth, RE elements) additions on the microstructural development of an in situ Al-15wt.%Mg2Si composite were investigated. It was found that with increasing MM additions in the composites. the size of primary Mg2Si particles was considerably reduced, and that the pseudo-eutectic Mg2Si was changed from a fibrous morphology to a flake-like morphology, showing a divorced character. A small amount of RE-containing compounds in the form of Al11RE3 were formed as a result of MM additions. It is believed that MM additions have a strong influence on the nucleation process of primary Mg2Si particles and the subsequent formation of the alpha-Al phase and the pseudo-eutectic matrix. (C) 2000 Elsevier Science S.A. All rights reserved.

Activation energy and crystallization kinetics of untreated and treated oil palm fibre reinforced phenol formaldehyde composites

Abstract: In this paper, oil palm fiber reinforced phenol formaldehyde (PF) treated, as well as untreated, composites have been taken for the study. The untreated sample (sample 1) contains oil palm fiber reinforced in the PF matrix, and the same fiber is treated with silane (sample 2) and with alkali (sample 3) to produce two types of treated fibers. These treated fibers were then reinforced in the matrix to produce two treated samples. Differential scanning calorimetry has been employed to study the crystallization kinetics and the energy of crystallization for all the samples. All the samples show the well-defined peaks of crystallization. In the case of silane-treated sample, double crystallization is observed. The crystallization data are analyzed in terms of a modified Kissinger’s equation to determine the activation energy. The activation energy and other crystallization parameters have also been determined using Matusita’s equation and are compared with the values obtained from other equations. It has also been found that various treatments have improved the thermal stability of the composites to different extents.

Effect of cell morphology on the compressive properties of open-cell aluminum foams

Abstract: The mechanical properties of open-cell 6101 aluminum foams with different densities (∼5–10%) and morphologies (4–16 cells cm−1) were characterized in compression. It was found that density is the primary variable controlling the modulus and yield strength of foams. The effects of other variables such as cell size and shape were also studied. Whereas the cell size appears to have a negligible effect on the strength of foams, at a fixed density, the cell shape was shown to effect the strength of foams. In the present paper, theoretical models are offered to explain the differences in modulus and strength caused by the differences in cell shape and size.

Evolution of near-equiaxed microstructure in the semisolid state

Abstract: The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

On the size-dependent phase transformation in nanoparticulate zirconia

Abstract: We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and undergo a phase transformation when the particle size is of the order of 18 nm, for reasons associated with the surface energy, and the occurrence of this phase transformation produces a sudden change in the driving force for coarsening. Grain size distributions below the critical size for the transforrnation are log-normal, but as the transformation occurs, the size distribution changes to a markedly less skewed form. The development of this distribution is followed to establish whether it grows self-similarly, or returns to log-normality once normal driving forces are restored after the phase transformation is complete.

Precipitation of dispersoids containing Mn and/or Cr in Al-Mg-Si-alloys

Abstract: The formation of dispersoids containing Mn and/or Cr in Al–Mg–Si alloys was studied metallographically and by means of electrical resistivity measurements. Special emphasis was placed on the nucleation mechanisms. For alloys containing Mn and/or Cr it was shown that during the heating of the as-cast alloys to 580°C (which is a typical homogenization temperature) and at a heating rate of 3 K min −1 , an intermediate phase, referred to as the ‘u-phase’ nucleated on the β′-Mg 2 Si needles. The phase has a hexagonal unit cell with a =0.670 nm, c =0.808 nm. The precipitates were rod-shaped and were lined up in the [100] directions in the Al lattice. The phase was found to be rich in Mn and/or Cr. With continued annealing, dispersoids containing Mn and Mn+Cr nucleated heterogeneously on the ‘u-phase’ precipitates before these precipitates dissolved. The nucleation sites for the dispersoids with Cr were not conclusively identified.