Gaspar González-Doncel

High temperature creep behavior of metal matrix AluminumSiC composites

Abstract The tensile creep behavior of AlSiC metal matrix composites has been investigated and analyzed over the temperature range from 230 to 525°C. It is shown that plastic flow in these materials is lattice-diffusion controlled dislocation creep in the aluminum matrix. All data on AlSiC have been assessed by a creep relation developed for creep of metals at constant structure with the added contribution of a threshold stress. The threshold stress for creep in AlSiC composites is not a thermally-activated process and is shown to have a linear dependence with temperature becoming zero at 470°C. The threshold stress is higher for the whisker composites than for the particulate composites. The origin of the threshold stress is not well understood and cannot be explained by contemporary dislocation models involving dislocation bowing or unpinning around particles sites. The observed interparticle-interwhisker spacing is shown to influence the creep rate in the same way as observed for mechanical alloyed (MA) Al base materials.

Influence of extrusion temperature on the microstructure and the texture of 6061Al–15 vol.% SiCw PM composites

Abstract 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: and , (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.

Calorimetric study of 6061-Al-15 vol.% SiCw PM composites extruded at different temperatures

Abstract The precipitation kinetics of 6061-Al–15 vol.% SiC w 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.

Deformation of whisker-reinforced metal-matrix composites under changing temperature conditions

A numerical technique for simulating the plastic response of whisker-reinforced metal-matrix composites under conditions of changing temperature and applied stress is developed. The model simulates an elastic-plastic (diffusion-controlled power-law creep) matrix and elastic whiskers, with variable whisker length and spacing. To test this model, the mechanical behavior of a metal-matrix composite of 6061 aluminum, reinforced with 20 vol pct discontinuous, oriented silicon carbide whiskers was studied under conditions of repeated temperature cycling and isothermal creep. The results of the thermal-cycling experiments are compared to those of the model. Both the experiments and the model demonstrate that the composite flw stress may be significantly reduced by thermal cycling (relative to isothermal, elevated temperature behavior) and that under appropriate conditions, the composite strain rate is proportional to the applied stress. Also, agreement between the experimental results and the first-principles model is very good in terms of both magnitude and trends, despite simplifications in the model.

Influence of extrusion temperature on the aging behavior of 6061Al-15vol%SiCw composites

It is known that some discontinuously reinforced metal matrix composites, MMCs, with precipitation hardenable matrices, like 6061Al with SiC whiskers or particles, show faster aging behavior than the unreinforced alloys. Regarding the influence of processing techniques on the aging behavior, some investigations have found acceleration on aging of materials consolidated by powder metallurgy (PM) with respect to materials consolidated by ingot metallurgy (IM). This was attributed to the higher presence of oxide inclusions in the PM than in the IM materials. For a given composite preparation technique, however, the influence of the processing variables on the aging response has been barely studied. In this work, the effect of extrusion temperature on the aging of 6061Al-15vol%SiC{sub w} composites processed by a powder metallurgy route is investigated. The results obtained in this research go deep into the influence of the dislocation density on both the accelerated aging and the increase in strength of composites. Whereas the effect of dislocation density on accelerated aging is well documented, its effect on the strength of MMCs is not yet clear.

Extrudability of PM 2124/SiCp aluminium matrix composite

Abstracts are not published in this journal

Effects of the alumina scale on the room-temperature tensile behavior of preoxidized MA 956

This article deals with the effects of theα-Al2O3 scale (∼5µm) developed during preoxidation (1100 °C/100 hours) of MA 956 on its room-temperature tensile behavior. The tensile tests were made in the strain-rate range of 10−5 to 10−1 s−1. It is shown that the scale, fine and tightly adherent to the substrate, affects the tensile behavior in two relevant ways. First, the yield strength and the tensile strength are lowered with respect to those of the scale-free material. This is explained in terms of the residual stresses generated in the scale during preoxidation. From the analysis of the differences in the yield strength of preoxidized MA 956 with respect to the scale-free material, residual compression stresses in the scale of about 5500 MPa were obtained. These high stresses account for the surprisingly high tensile strain achieved (1.4 pct) before scale spallation occurs. Second, a ductile to brittle transition (DBT), which is not observed in the scale-free samples, occurs at intermediate strain rates (10−3 s−1). The brittle fracture is related to the increase of the triaxiality state in the substrate near the scale/metal interface.

Tension versus compression superplastic behavior of a Mg-9 wt% Li-5 wt% B4C composite

Abstract A fine-grained (2 μm) Mg-9 wt% Li-5 wt% B 4 C particulate composite was superplastic in the temperature range 150–200°C. The flow stress in compression was about two to three times higher than in tension in the superplastic region. This difference is attributed to a greater ease of grain-boundary sliding in tension than in compression.

Thermal oxidation of medical Ti6Al4V blasted with ceramic particles: Effects on the microstructure, residual stresses and mechanical properties

Roughening of Ti6Al4V by blasting with alumina or zirconia particles improves the mechanical fixation of implants by increasing the surface area available for bone/implant apposition. Additional thermal oxidation treatments of the blasted alloy have already shown to be a complementary low-cost solution to enhancing the in vitro biocompatibility and corrosion resistance of the alloy. In this work, the effects of oxidation treatment on a grit blasted Ti6Al4V biomedical alloy have been analysed in order to understand the net effect of the combined treatments on the alloy fatigue properties. Synchrotron radiation diffraction experiments have been performed to measure residual stresses before and after the treatments and microstructural and hardness changes have been determined. Although blasting of Ti6Al4V with small spherical zirconia particles increases the alloy fatigue resistance with respect to unblasted specimens, fatigue strength after oxidation decreases below the unblasted value, irrespective of the type of particle used for blasting. Moreover, at 700°C the as-blasted compressive residual stresses (700MPa) are not only fully relaxed but even moderate tensile residual stresses, of about 120MPa, are found beneath the blasted surfaces. Contrary to expectations, a moderate increase in hardness occurs towards the blasted surface after oxidation treatments. This can be attributed to the fact that grit blasting modifies the crystallographic texture of the Ti6Al4V shifting it to a random texture, which affects the hardness values as shown by additional experiments on cold rolled samples. The results indicate that the oxidation treatment performed to improve biocompatibility and corrosion resistance of grit blasted Ti6Al4V should be carried out with caution since the alloy fatigue strength can be critically diminished below the value required for high load-bearing components.

Texture Stability of a Rapidly Solidified Dispersion Strengthened Al-Fe-V-Si Material

The authors gratefully acknowledge the support of the Comision Interministerial de Ciencia y Tecnologia (CICYT) under Grant No. MAT94/0888.