Gilles L'Espérance

Interfacial reactions and age hardening in AlMgSi metal matrix composites reinforced with SiC particles

Abstract AlSiMg matrix composites have been fabricated by the compocasting technique using as-received and oxidized particles. Interfacial reactions occurring during fabrication and during subsequent remelting of the composites were characterized by detailed analytical transmission electron microscopy. The effects of these reactions on the age hardening response of the materials are also characterized and discussed in detail. The results show that it is possible to control the interfacial reactions either to totally suppress age hardening or to give rise to hardening in the case of a normally nonhardenable matrix alloy.

Effects of chemistry, density and size distribution of inclusions on the nucleation of acicular ferrite of CMn steel shielded-metal-arc-welding weldments

Abstract The influence of inclusions on the nucleation of acicular ferrite of six as-deposited CMnTi welds prepared by the shielded-metal-arc-welding process was studied. The microstructure of the welds, the inclusion size distribution, the density and the average chemical composition were characterized. It was found that a titanium-rich phase which partially covers the inclusions is responsible for the nucleation of acicular ferrite. It is estimated that a titanium content of about 39 wt.ppm in the weld metal is necessary for the formation of this titanium-rich phase. However, even if the presence of the titanium-rich phase in the inclusions is necessary, it is not the only factor affecting the nucleation of acicular ferrite (AF). After correcting for the effect of austenite grain size, it was found that the efficiency of the inclusions for the nucleation of AF is related to the total external surface area of the titanium-rich phase covering the inclusions per unit area of weld metal. This is in turn related to the density, the size distribution and the titanium level of the inclusions.

Effect of the nature of grain boundary regions on cavitation of a superplastically deformed aluminium alloy

Abstract Superplastic deformation of aluminium alloys induces cavity formation throughout the material, so that superplastic forming usually requires to be carried out under superimposed gas pressure to minimize strain-induced damage. This paper deals with the beneficial effects of heat treatment at high temperature for several hours before deformation on cavitation behaviour of a superplastically deformed 7475 alloy. Transmission electron microscopy observations show that several microstructural transformations are induced by superplastic deformation and affected by the heat treatment. At first, the generation of dispersoid free zones at the periophery of the grains is observed, the composition of which depends on the prior history of the specimen. Secondly, the formation of long thin fibres extending in the cavities in the as received specimens, these fibres being no longer present in the heat-treated conditions. A TEM characterization of the fibres is presented and a mechanism of their formation is discussed. Such a reduction of the cavitation level for a given strain is interesting in view of superplastic forming of aluminium alloys under atmospheric pressure.

Geometrical factors influencing the orientations of dipolar dislocation structures produced by cyclic deformation of f.c.c. metals

Abstract A recently proposed geometrical model to explain the orientations of labyrinth dipolar walls produced by cyclic deformation of f.c.c. metals is extended to explain the orientations of loop patches, channels within the matrix structure, and dipolar walls within persistent slip bands. The model is based on determining favourable stacking arrangements for dipolar loops involving one or two Burgers vectors and slip systems and on determining the directions in which stacking should be limited. The stacking of dipolar loops into regular networks results evidently in low energy dislocation structures. The dipolar loops are assumed to be swept into the dipolar dislocation arrangements by edge dislocations. When excess dipolar loops of one slip system are present, the ability of the dipolar arrangements to accommodate these becomes important. Good agreement is found between the predictions of this model and the available experimental evidence.

The identification of labyrinth wall orientations in cyclically deformed AISI-SAE 316 stainless steel

Abstract Labyrinth walls observed in a grain of cyclically deformed AISI-SAE 316 stainless steel were identified as having orientations intermediate between (001) and (012) and between (100) and (210). The principal Burgers vectors were identified as [101] and [101]. Labyrinth walls of these {100} and {210} orientations can be predicted for this pair of Burgers vectors by considering a Taylors network arrangement of the dipole loops in the walls. Walls of intermediate orientations can be considered to consist of segments of a pair of {100} and {210} orientations.

Emission rates and physico-chemical characteristics of Mn particles emitted by vehicles using Methylcyclopentadienyl Manganese Tricarbonyl (MMT) as an octane improver

Since 1990, methylcyclopentadienyl manganese tricarbonyl (MMT) has been added to all gasoline in Canada as an antiknock agent. The objective of this study is to determine the percentage of manganese emitted by different types of automobiles and to evaluate the size and chemical characteristics of the Mn-containing particles. Nine vehicles with different mileage and engine capacity were tested using standard procedures for urban and highway driving cycles. One ran on gasoline without MMT and served as a control. The particles were collected using two separate systems: a trapping device consisting of a water tank connected to the tailpipe and a pumping device linked to a cassette containing Teflon filters. Water samples were analyzed by neutron activation to determine the amount of Mn emitted at the tailpipe for each test. Teflon filters were analyzed by electron microscopy to determine the size and the chemical characteristics of the particles. The amount of manganese emitted from the tailpipe varied from 4 to 41% of the manganese consumed, depending on the driving cycle and the vehicle. For the urban cycle, the emission rate was positively correlated with previous mileage. Almost all particles found on teflon filters had a size less than 5 μm. They appeared to be mainly Mn oxides but other elements could be masked by the gold and paladium peaks.

Effect of thermomechanical processing on the microstructure and mechanical properties of AlMg (5083)/SiCp and AlMg (5083)/Al2O3p composites. Part 1: Dynamic recrystallization of the composites

Abstract Three types of composites, reinforced by as-received SiC, artificially oxidized SiC or Al 2 O 3 particles, were fabricated by a compocasting method and extruded at 480 °C with an extrusion ratio of 16:1. Extrusion of these composites causes large scale dynamic recrystallization resulting in a fine matrix microstructure. The extrusion increases not only the ultimate tensile strength, but also the elongation to fracture of the composites. In the first part of our series of three articles, the microstructure of as-cast composites, extruded composites and extruded monolithic 5083 aluminum alloy is studied. The effects of reinforcing particles and other parameters on the dynamic recrystallization behaviour of the composites are discussed.

A Monte Carlo code to simulate the effect of fast secondary electrons on κAB factors and spatial resolution in the TEM

Fast secondary electrons (FSE), which result from inelastic scattering of incident electrons, are known to generate a significant number of X‐rays for light elements, and also to degrade the spatial resolution of X‐ray microanalysis in thin foils. A Monte Carlo program simulating the generation and diffusion of FSE in binary systems has been developed to study the effect of composition on κAB factors and spatial resolution. The effect of accelerating voltage and thickness is also presented.

Effect of volume fraction of ain particle on superplasticity of AlN/6061 aluminum alloy composite

Ceramic whisker or particulate reinforced aluminum alloy composites which exhibit higher elastic modulus and tensile strength, excellent heat resistance and wear resistance, high thermal conductivity and can produce high strain rate superplasticity (HSRS), are expected to have application in engine and wear resistant components and even in semi-conductor packaging for the automobile and aerospace industries. The purpose of this study is to examine the effect of volume fraction of AlN on HSRS behavior of an AlN particulate reinforced 6061 aluminum alloy composite and the maximum volume fraction by which the HSRS could produce in order to understand the deformation mechanism of the HSRS.

Development, quantitative performance and applications of a parallel electron energy‐loss spectrum imaging system

The development of a parallel electron energy‐loss spectrum imaging system is presented. The analytical performance of the imaging technique was investigated and the system applied to materials science problems. The system, which allows acquisition and storage of a parallel electron energy‐loss spectrum at each pixel of an image, was developed by interfacing a multichannel analyser and a microscope to a computer workstation. In the experimental conditions used for imaging, detection limits and quantification errors were large and varied as a function of spatial resolution and the range of chemical elements of interest in the image. Applications of this imaging technique in materials science showed that quantitative chemical information is provided by the system and that the use of relative thickness maps and detailed statistical analysis of the spectrum‐image allowed an unbiased interpretation of the images. As energy‐loss spectra are available after processing, spectroscopic information about the analysed material can be used to provide supplementary information.