Continuous twin screw rheo-extrusion of an AZ91D magnesium alloy

© The Minerals, Metals & Materials Society and ASM International 2012The twin screw rheo-extrusion (TSRE) is designed to take advantage of the nondendritc microstructure and thixotropic characterization of semisolid-metal slurries and produce simple metal profiles directly from melts. The extrusion equipment consists of a rotor-stator high shear slurry maker, a twin screw extruder, and a die assembly. The process is continuous and has a potential for significantly saving energy, manufacturing cost, and enhancing efficiency. The present investigation was carried out to study the process performance for processing rods of an AZ91D magnesium alloy and the microstructure evolution during processing. The semisolid slurry prepared by the process was characterized by uniformly distributed nondendritic granular primary phase particles. AZ91D rods with uniform and fine microstructures and moderate mechanical properties were produced. For the given slurry making parameters, decreasing extrusion temperature was found to improve microstructures and properties. The mechanisms of particle granulation and refinement and the effect of processing parameters on process performance and thermal management are discussed. © 2012 The Minerals, Metals & Materials Society and ASM International.EPSRC (UK) and Rautomead Lt

In situ study of granular micromechanics in semi-solid carbon steels

The granular micromechanics of semi-solid steel at ∼80% solid are studied by synchrotron radiography. A particulate soil mechanics approach to image analysis shows that deformation occurs by the translation and rotation of quasi-rigid grains under the action of contact forces, and that the changes in directional fabric and grain–grain contacts occur by mechanisms similar to those of highly compacted soils including “locked sands”. Grain-scale phenomena are then linked to the macroscopic displacement and strain fields and it is shown that shear-induced dilation is a fundamental response at both the grain and macro scales. Based on this, recommendations are made on future rheology experiments

The Effect of Convection on Disorder in Primary Cellular and Dendritic Arrays

Directional solidification studies have been carried out to characterize the spatial disorder in the arrays of cells and dendrites. Different factors that cause array disorder are investigated experimentally and analyzed numerically. In addition to the disorder resulting from the fundamental selection of a range of primary spacings under given experimental conditions, a significant variation in primary spacings is shown to occur in bulk samples due to convection effects, especially at low growth velocities. The effect of convection on array disorder is examined through directional solidification studies in two different alloy systems, Pb-Sn and Al-Cu. A detailed analysis of the spacing distribution is carried out, which shows that the disorder in the spacing distribution is greater in the Al-Cu system than in Pb-Sn system. Numerical models are developed which show that fluid motion can occur in both these systems due to the negative axial density gradient or due the radial temperature gradient which is always present in Bridgman growth. The modes of convection have been found to be significantly different in these systems, due to the solute being heavier than the solvent in the Al-Cu system and lighter than it in the Pb-Sn system. The results of the model have been shown to explain experimental observations of higher disorder and greater solute segregation in a weakly convective Al-Cu system than those in a highly convective Pb-Sn system

NSC32258

Expedition 308 of the Integrated Ocean Drilling Program (IODP) was the fi rst phase of a two-component project dedicated to studying overpressure and fl uid fl ow on the continental slope of the Gulf of Mexico. We examined how sedimentation, overpressure, fl uid fl ow, and deformation are coupled in a passive margin setting and investigated how extremely rapid deposition of fi ne-grained mud might lead to a rapid build-up of pore pressure in excess of hydrostatic (overpressure), underconsolidation, and sedimentary masswasting. Our tests within the Ursa region, where sediment accumulated rapidly in the late Pleistocene, included the first-ever in situ measurements of how physical properties, pressure, temperature,and pore fluid compositions vary within low-permeability mudstones that overlie a permeable, overpressured aquifer, and we documented severe overpressure in the mudstones overlying the aquifer. We also drilled and logged three references sites in the Brazos-Trinity Basin IV and documented hydrostatic pressure conditions and normalconsolidation. Post-expedition studies will address how the generation and timing of overpressure control slope stability, seafl oor seeps, and large-scale crustal fluid fl ow. The operations ofExpedition 308 provide a foundation for future long-term in situ monitoring experiments in the aquifer and bounding mudstones

Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occured on the continental slope, offshore Louisiana, Gulf of Mexico

International audienc

On the Characterization of Directionally Solidified Dendritic Microstructures

A novel method for the stereological assessment of arrays of directionally solidified dendrites is presented. It is suitable particularly for the evaluation of the distribution of primary dendrite arm spacings and the dendrite coordination number. The method involves (1) determination of the center of gravity of each dendrite in the array under consideration, (2) calculation of the distances to all neighboring dendrites in the data set (3) sorting of the distances determined into ascending order (4) under the assumption of a minimum number of nearest neighbors-typically three-calculation of the average distance to these and associated standard deviation, (5) an assessment of whether the remaining neighbors are within the range of proposed nearest-neighbor distances and (6) repetition of the previous to determine a set of values for the entire data set and, thus, the distributions of dendrite arm spacings and coordination numbers. By application of the method to experimental microstructures, it is demonstrated that a detailed statistical assessment of directionally solidified dendritic arrays can be accomplished. © 2010 The Minerals, Metals and Materials Society and ASM International