Colonies Of Grains Research Articles

Spatial Uniformity of the Rate of Grain Coarsening in a Submicron Al-Sc Alloy Produced by Severe Plastic Deformation

The uniformity of grain coarsening throughout the microstructure of a submicron grained particle-containing aluminium alloy has been investigated using high resolution EBSD. The alloy was processed by equal channel angular pressing (ECAP) and low temperature ageing to generate ~0.8 􀁐m diameter grain structure containing a relatively uniform dispersion of nanosized Al3Sc particles. While the initial processing route generated a uniform grain size distribution, the distribution of grain orientations was rather inhomogeneous with certain regions of microstructure containing colonies of grains consisting predominantly of either HAGBs or LAGBs. It was found that, despite the difference in grain boundary character between these regions, the fine particles produced by pre-ageing promote uniform coarsening throughout the microstructure despite the marked differences in boundary character between these regions. An analytical model is outlined which predicts the uniformity of grain coarsening in particle-containing alloys despite the presence of orientation gradients in the microstructure.

An EBSD Analysis of the Origins of Ridging in AISI 430 Steel Sheets

In the present work the characterisation of both the microstructure and the microtexture of two ferritic stainless steel sheets - AISI 430 (Fe17% Cr) and AISI 434 (17% Cr, 1% Mo) - known to display different ridging behaviours, was carried out by optical metallography, EBSD and OIM techniques. It was concluded that severe ridging is a consequence of: (a) presence of {111}<uvw> and {001}<uvw> colonies of grains alternating throughout the sheet plane; (b) through thickness texture inhomogeneity with layers of {001}<110> oriented large grains of low Rvalues sandwiched between layers of higher R-value textured material.

Uniformity of Grain Coarsening in Submicron Grained Al-Sc Alloy Containing Local Variations in Texture

The effect of fine particles on the uniformity of grain coarsening in a submicron grained Al-Sc alloy containing significant local variations in texture has been investigated using high resolution EBSD. The alloy was processed by severe plastic deformation and low temperature ageing to generate a fine-grained (0.8 µm diameter) microstructure containing either a dispersion of nanosized Al3Sc particles or a particle-free matrix. The initial processing generated a uniform grain size distribution, but the distribution of grain orientations was inhomogeneous with the microstructure containing colonies of grains consisting predominantly of either HAGBs or LAGBs with the latter possessing orientation gradients of up to 10 o/µm. Despite the marked differences in boundary character between these regions, the alloy undergoes slow and uniform grain coarsening during annealing at temperatures up to 500 oC with no marked change in the grain size distribution, boundary distribution and texture. A model of grain coarsening that takes into account the influence of fine particles on the kinetics of grain growth within an orientation gradient is outlined. The model predicts that a large volume fraction of fine particles (large f/r-value) tends to homogenize the overall rate of grain coarsening despite the presence of orientation gradients in the microstructure.

Direct experimental analysis of the relation between the grainstructure and the distribution of critical current density inYBa2Cu3Ox coated conductors

We present a relatively simple experimental method to correlate the grain structure ofYBa2Cu3Ox coated conductors with spatial variations of the critical current density on a macroscopicscale. Variations of the current density on a micrometre scale are visualized withmagneto-optical imaging, using the flux trapped by colonies of grains to quantify thedegree of connectivity. Integrating these trapped flux profiles over larger distances yieldsdirect information of the critical current distribution on a millimetre scale, providedthat contributions from negative return flux in colony boundaries are properlyeliminated. Flux polarity is determined using the wavelength dependence of the Verdetconstant. The validity of this analysis is demonstrated by comparing the resultswith direct transport data on the lateral current distribution, measured with themagnetic knife technique. The combination of both experiments shows that thecritical current density is suppressed over several millimetres of conductor length atareas where a large number of high-angle grain boundaries or defects are present.

Superplastic properties of a δ/γ stainless steel

The high temperature deformation behavior of a duplex stainless steel has been studied by tensile tests at temperatures ranging from 850 to 1100°C. The microstructure of the as-received material consists of elongated islands of austenitic grains in a more or less continuous ferrite matrix. Metallographic studies after tensile tests showed that the material recrystallizes during heating at testing temperature and the austenite transforms into fine colonies of grains of 10–15 μm in size. The results of the mechanical tests in the temperature range 850–1100°C show a high elongation to failure (up to 300%) and a low value of n, 2–3, for strain rates up to 10 −3 s −1. This result suggests that the controlling deformation mechanism is grain boundary sliding, GBS. Microstructure and texture evolution studies of deformed samples confirmed the importance of GBS. The colonies of austenitic grains decrease in size during superplastic deformation and, at the same time, only minor changes in the texture was observed in the deformed region. Finally, at high strain rates an increase of stress exponent to 5 is observed, which suggests a transition to a slip-creep controlling mechanism.

A Dislocation-Model of Limit Strain Prediction in Aluminium Sheet Metals under Biaxial Deformation

The development of strain inhomogeneity and fracture events are influenced to a great extent by the local properties of the microstructures. The effect of dislocation hardening is incorporated into a rate sensitive crystal plasticity model for the prediction of limit strain in biaxial deformation. The assumption of a pre-existing surface groove defect in the Marciniak-Kuczynski method of limit strain prediction is relaxed, and the surface defect is seen to arise from the differential deformation between two colonies of grains with different crystallographic orientations. With the use of a modified localized necking criterion which incorporates the changes in the dislocation density, the limit strain of a sheet metal with two ideal orientations is predicted.

A Dislocation-Model of Limit Strain Prediction in Aluminium Sheet Metals under Biaxial Deformation

The development of strain inhomogeneity and fracture events are influenced to a great extent by the local properties of the microstructures. The effect of dislocation hardening is incorporated into a rate sensitive crystal plasticity model for the prediction of limit strain in biaxial deformation. The assumption of a pre-existing surface groove defect in the Marciniak-Kuczynski method of limit strain prediction is relaxed, and the surface defect is seen to arise from the differential deformation between two colonies of grains with different crystallographic orientations. With the use of a modified localized necking criterion which incorporates the changes in the dislocation density, the limit strain of a sheet metal with two ideal orientations is predicted.

Development of physical vapour deposited Mg–Zr alloys Part 1 – Characterisation of as deposited alloys

Alloys of Mg–Zr produced via physical vapour deposition (PVD) were studied in the as deposited condition using bulk characterisation and surface analysis techniques. The solid solubility of Zr in Mg was extended up to ∼3·07 at.-%. The Mg–Zr deposits showed a preferred growth orientation, and microstructural studies revealed typical PVD features such as domed colonies of grains, porosity, compositional inhomogeneity, and a high dislocation density. Zirconium oxide was found to coexist with magnesium oxide/hydroxide in the surface film.

Synthesis and stability of HgRe1212 and HgRe1223 superconductors

Synthesis, phase stability, and superconducting properties of Re-doped Hg1212 and 1223 superconductors are reported. HgRe1212 and HgRe1223 were synthesized using a common synthesis protocol, using commercial BaCaCuO precursor powders with CaHgO/sub 2/ as the external Hg-source. Both starting composition and reaction temperature affect the phase stability and microstructure of the final product. The resulting superconducting phase in both Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/ and Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ compositions was either HgRe1212 or HgRe1223, depending on the reaction temperature. The microstructure and superconducting properties of HgRe1212 and HgRe1223 samples, however, depended to a large extent on the starting composition. Both the HgRe1212 and HgRe1223 samples synthesized from a nominal composition of Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/ had larger and aligned colonies of grains whereas the samples synthesized a nominal composition Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ had smaller and randomly oriented grains. Measured T/sub c/ values for HgRe1212 and Hg1223 phases were 124 K and 134 K, respectively. Both microstructure and magnetization hysteresis measurements suggest that HgRe1212 is a better candidate for applications.

Studies of the morphology of the Al-rich interfacial layer formed during the hot dip galvanizing of steel sheet

A small addition of Al in the liquid Zn bath inhibits the Fe/Zn reactions during hot dip galvanizing of steel sheets. Although it is well known that Al and Fe are tied up in an layer located at the interface between Fe and Zn, the effect of the galvanizing parameters on the formation of this Al-rich interfacial layer is not. This study has been carried out to determine the effect of the galvanizing parameters on the formation of this Al-rich layer. Five zinc baths containing 0.10, 0.12, 0.13, 0.16, and 0.18 wt pct Al were used to produce galvanized coatings of commercial ultra-low-carbon (ULC) steel substrate. Full inhibition of the Fe/Zn reactions was achieved in baths with Al content above 0.15 wt pct. The Al-rich layer is isomorphous to Fe2Al5 crystals and the Fe/Al ratio is close to Fe2Al5 with large amounts of Zn present (22 to 28 wt pct). The morphology of the Al-rich layer is strongly related to the Al content of the bath. Indeed, bath contents above 0.15 wt pct are associated with two sublayers of Fe2Al5 crystals, making up colonies of grains of similar crystallographic orientation and directly associated to the crystallographic orientation of the steel grains. In baths with contents below 0.15 wt pct, the Al-rich layer has only one sublayer of crystals and shows colonies of grains with similar orientation. Finally, the Fe2Al5 crystals have a strong crystallographic texture. X-ray diffraction (XRD) reveals that the (200) planes are parallel to the surface of the substrate.

The influence of metallic interfaces on the properties of superconductors

The influence of AgPd and Au interfaces on the formation of the (HgBi-1223) superconductor was investigated. Variations in the BaCaCuO precursor phase assemblage and the use of as the Bi source were examined in an attempt to lower the reaction temperatures suitable for the growth of HgBi-1223 in conjunction with AgPd and Au substrates. Phase pure samples of HgBi-1223 were synthesized at temperatures below . Use of as a Bi source combined with variations of the BaCaCuO precursor phase assemblage allowed the formation of some liquid phase during the reaction and resulted in dense HgBi-1223 samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. Comparison of the microstructural features of bulk samples and the metal-superconductor interface showed that the interface has a positive influence on grain growth and alignment. The metallic interfaces, however, were found to have a detrimental effect on the purity of the superconducting phase due to the formation of amalgams.

Texture Characterization of High-Temperature Superconductor Wires

Texture measurements have been made for BSCCO tapes produced by dip coating and by powder-in-tube techniques. The texture of the Bi2212 grains was characterized for both c axis texture using (001) pole figures from 008 reflections and a–b axis texture using (115) pole figures. The critical current densities of these tapes were also measured and correlated with texture parameters obtained from contours of the pole figures. For the tapes with good current transport, the arrangement of grains in a crystallographic sense was inferred from the pole figures and was found to be consistent with a model based on global alignment of the c axes and the presence of colonies of grains differing mostly in c axis twist.

Synthesis of (Hg,X)Ba2Ca2Cu3Ox superconductors

Synthesis and processing parameters for (Hg1−xBix)Ba2Ca2Cu3.1Oy were optimized for growth at temperatures suitable for growth on Ag and Au substrates (<850° C). Use of Bi2CuO4 as a Bi source combined with variations of BaCaCuO precursor phase assemblages allowed the formation of some liquid phase during the reaction and resulted in dense (Hg1−xBix)Ba2Ca2Cu3.1Oy samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. The results of the Bi doped samples are compared with those of previously reported Re doped samples.