The microstructure and the texture of extruded 6061 Al alloy processed by powder metallurgy (PM), using three different initial powder particle sizes, have been studied after torsion deformation at 300°C at strain rate of 6s−1. The initial extruded microstructure of the three alloys consisted of elongated grains confining substructure with a typical 〈111〉+〈100〉 fiber texture. After torsion deformation, the torque (Γ) as a function of the equivalent strain (ɛeq) showed softening and the microstructure exhibited a gradient across the section so that the inner and outer zones contained sheared-elongated and small-equiaxed subgrains, respectively. It was observed that the equivalent strain to failure for the material processed using powder particles size of less than 45μm was of about ɛeq∼12, compared to ɛeq∼1 for material processed using powder particles size of less than 25μm. The microstructural changes during hot torsion deformation of PM 6061 Al alloy should involve continuous dynamic processes.

A procedure to calculate the spatial fiber length in discontinuously reinforced composites from data of conventional metallographical images has been developed. The method is based on the following three assumptions: first, the measured length of a reinforcing fiber in a metallographical picture is a projection of the actual fiber length; second, the alignment of short fibers is axially symmetrical (for example, extruded composites in which the extrusion direction is a symmetry axis for the orientation of reinforcing fibers); third, for a representative number of fibers, the dependence of the projected length with the corresponding projected angle, along a given sample direction, is known. Either analytical or numerical solution can be found depending on the specific dependence of the projected length with the projected angle. This procedure has been applied to real 6061Al–15vol%SiCw composites obtained by a powder metallurgical route in which the final step for materials consolidation was extrusion of powder blends at different temperatures.

The relative difference between the average dislocation density of 6061Al-15vol%SiC composites investigated by two independent methods (namely hardness, H, and differential scanning calorimetry, DSC), is discussed. They are interpreted in terms of the “actual” temperature at which the measurements are made with each technique. The present analysis accounts for the high microstructural stability of these composites at elevated temperatures.

The determination of residual stresses, RS, in 6061Al-15 vol% SiCw composites was carried out to correlate them with whisker orientation, distribution and aspect ratio. The composites were obtained by a powder metallurgical, PM, route and consolidated by extrusion at four different temperatures, T-extr. This is the main parameter affecting the whisker orientation/distribution; an increase in T-extr leads to an increasing whisker alignment with extrusion axis. The results of the sin (2)psi plots account for a non-hydrostatic stress state in all composites. This is expected because of the deviatoric component of the RS tensor generated by the whisker geometry. The RS can explain the strength differential effect, SDE, observed in uniaxial testing (tensile versus compressive behaviour) of these composites.

. The determination of residual stresses, RS, in 6061Al-15 vol % SiC w composites was carried out to correlate them with whisker orientation, distribution and aspect ratio. The composites were obtained by a powder metallurgical, PM, route and consolidated by extrusion at four different temperatures, T extr . This is the main parameter affecting the whisker orientation/ distribution; an increase in T extr leads to an increasing whisker alignment with extrusion axis. The results of the sin 2 ψ plots account for a non-hydrostatic stress state in all composites. This is expected because of the deviatoric component of the RS tensor generated by the whisker geometry. The RS can explain the strength differential effect, SDE, observed in uniaxial testing (tensile versus compressive behaviour) of these composites.

A systematic study of the microstructure and the texture of powder metallurgy, PM, 6061Al metal matrix composites, MMCs, as influenced by the extrusion temperature, has been conducted. For this purpose, a containerless PM route has been developed to controlling the parameters involved during processing of materials. A strong fiber texture with two components: <111> and <100>, (with the fiber axis parallel to the long extrusion direction) is developed in the matrix of all materials. A limited particle stimulated nucleation process, PSN, occurs during extrusion of the composites when the extrusion temperature, Textr, is low. The SiC in the composites is divided in two populations; whiskers oriented with the extrusion axis (oriented whiskers) and randomly oriented whiskers (random whiskers) plus particles. Whereas the average whisker length does not change significantly with Textr, the “degree” of alignment of oriented whiskers (which follows a Gaussian distribution function) is clearly accentuated in the composites extruded at high Textr with respect the composites extruded at low Textr.

The relation between the polycrystal deformation and single crystal deformation has been studied for pure polycrystalline copper deformed in tension. The dislocation microstructure has been analyzed for grains of different orientation by transmission electron microscopy (TEM) and three types of microstructures have been identified. A correlation is found between microstructure and grain orientation, which agrees well with earlier observations in tensile deformed aluminum polycrystals and copper single crystals. The stress–strain curve of the copper polycrystal is calculated with good accuracy from single crystal data, which are weighted according to the volume fractions of the three different types based on a quantitative texture measurement of the polycrystal.

The different interpretation derived from two kinetic models to fit β′′ and β′ reactions in PM 6061Al and 6061Al-15vol.%SiCw composites, as measured by DSC, is discussed. It will be seen that the decrease of the Avrami exponent with temperature can also be understood in terms of a fittable exponent for impingement.

The different interpretation derived from two kinetic models to fit beta " and beta' reactions in PM 6061Al and 6061Al-15vol.%SiCw composites, as measured by DSC, is discussed. It will be seen that the decrease of the Avrami exponent with temperature can also be understood in terms of a fittable exponent for impingement. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

The tensile and compressive properties at room temperature of powder metallurgy Ni3Al synthesised using rapidly solidified powder particles, which were milled for up to 20 h, have been investigated. Furthermore, the mechanical behaviour at high temperatures has been studied by strain rate change tests in compression at strain rates ranging from 5 x 10(-6) to 2 x 10(-3) s(-1) and temperatures from 1073 to 1373 K. For comparative purposes, tensile tests at room temperature were carried out in materials consolidated from particles in both the as rapidly solidified condition (RS) and after 2 h of milling time. Both materials showed a very similar microstructure composed of equiaxed grains. However, an increase in the yield strength of close to 40% was exhibited by the 2 h milled particle material. The intermetallic made of 20 h milled particles presented a bimodal microstructure and showed a dramatic increase in yield strength at room temperature to 740 MPa, more than twice the value reached by the material consolidated with RS particles. The analysis of the stress-strain rate data of tests performed at elevated temperatures revealed a transition from a stress exponent n approximate to 5 to an exponent n approximate to 3.5 at low stresses and/or high temperatures of testing. An activation energy for plastic flow of 370 kJ mol(-1) was deduced in the low stress regime.

The precipitation kinetics of 6061-Al–15 vol.% SiCw PM composites consolidated by hot extrusion at three different temperatures have been studied by means of differential scanning calorimetry (DSC). The results have been analyzed on the basis of the Kissinger method and the Johnson–Mehl–Avrami model. The differences in the precipitation behavior result from the change in the effective activation energy for diffusion of Si in Al, if it is considered that this is the controlling process for the growth of precipitates. These differences are interpreted in terms of different dislocation densities. The contribution of Si diffusion along dislocations decreases the effective activation energy as the dislocation density increases. In this manner, values of the dislocation density for all the materials have been calculated.

Evolution of nano-precipitates after Cryorolling (CR) followed by warm rolling (WR) at rolling temperatures of 100 °C, 145 °C and 175 °C on the microstructure and strengthening of Al-6061 alloy has been investigated. The enhanced yield (401 MPa) and tensile strength (415 MPa) with 6% ductility was achieved after CR plus 80% WR. The evolution of various precipitates such as G.P. zones, Cluster/co-cluster, β' and β" after CR plus WR was investigated, by studying the thermal behavior using Differential scanning calorimetry studies (DSC). Time of the optimized artificial ageing temperature (125 °C for 45 Hrs) was obtained using hardness testing and DSC study. Significant improvement in ductility (9%) of CR plus 80% WR Al 6061 alloy after peak ageing showing 450 MPa tensile strength was obtained, due to the evolution of highly dense coherent nano-precipitates (β") responsible for uniform flow localization. A graphical model is proposed explaining the effect of solution treatment, deformation temperature, dislocations and peak ageing on evolution of precipitates.