Joaquín Ibáñez

Mechanical properties of some PM aluminide and silicide reinforced 2124 aluminium matrix composites

Tensile properties of PM 2124 aluminium composites reinforced with Ni3Al, NiAl, Cr3Si and MoSi2 intermetallic powder particles have been investigated in tempering conditions T1 and T4. Intermetallics were produced by selfpropagated high-temperature synthesis and composite powders consolidated by extrusion. 2124/MoSi2 showed the highest thermal stability, and the best mechanical properties even after comparison with those of 2124/SiC composite after processing through the same route. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

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.

The Role of Carbon Black in LiMn2O4-Based Composites as Cathodes for Rechargeable Lithium Batteries

LiMn 2 O 4 -based composites have been prepared by cold pressing physical mixtures of LiMn 2 O 4 , carbon black (CB), and polyvinylidenefluoride (PVDF). The PVDF content has been kept constant (ca. 14.5% by volume or 10% by weight). The LiMn 2 O 4 and CB contents have been changed in an opposite way, i.e., from 59,6 vol % (or 90,6 wt %) to 19.2 vol % (or 49 wt %) for LiMn 2 O 4 , and from 0 vol % (or 0 wt %) to 32.2 vol % (or 38 wt %) for CB. The microstructure of the composites shows clusters formed of aggregated particles of either LiMn 2 O 4 or CB. The electrical conductivity follows a percolating process in which the conductivity changes by five orders of magnitude, the percolation threshold being 3 vol % of CB. The first discharge capacity changes from 0 to 135 mAhg -1 for increasing the CB content. The change in capacity also seems to follow a percolation process in which the sharp increase in capacity is observed for CB contents close to 3 vol %. The discharge capacity of the LiMn 2 O 4 -based composites is controlled by electronic transport within the composite, the electronic transport depending on the microstructure of the composite.

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.

Effect of the phase transition of on the ion conduction in - Teflon composites

A composite - 25% vol Teflon was prepared to prevent breakages in powder pellets due to the phase transition of and, thus, to study the effect of the phase transition on conduction. The composite microstructure, as followed by scanning electron microscopy, shows aggregated particles surrounded by Teflon regions which operate as a skeleton. The imaginary part of the electric modulus shows two peaks at high and low frequency which are ascribed to grain-interior and grain-boundary response, respectively. In the two phases the ionic conductivity inside the grains is about three orders of magnitude larger than that found for the grain boundary. The phase transition affects the activation energy in different ways. It decreases for the grain-interior response from 0.68 eV for the low-temperature phase to 0.34 eV for the high-temperature phase, while it increases from 0.45 to 0.56 eV for the movement through the grain boundaries. An anomalous increase in activation energy for the grain-interior response in the high-temperature phase before it is transformed into the low-temperature phase has been found.

On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening

Flat fatigue specimens of biomedical Ti6Al4V ELI alloy were surface-processed by high pressure waterjet peening (WJP) without abrasive particles using moderate to severe conditions that yield roughness values in the range of those obtained by commercial grit blasting (BL) with alumina particles. Fatigue behavior of WJP and BL specimens was characterized under cyclical uniaxial tension tests (R=0.1). The emphasis was put on a comparative analysis of the surface and subsurface induced effects and in their relevance on fatigue behavior. Within the experimental setup of this investigation it resulted that blasting with alumina particles was less harmful for fatigue resistance than abrasiveless WJP. BL specimens resulted in higher subsurface hardening and compressive residual stresses. Specimens treated with more severe WJP parameters presented much higher mass loss and lower compressive residual stresses. From the analysis performed in this work, it follows that, in addition to roughness, waviness emerges as another important topographic parameter to be taken into account to try to predict fatigue behavior. It is envisaged that optimization of WJP parameters with the aim of reducing waviness and mass loss should lead to an improvement of fatigue resistance.

Lüders bands formation in a rapidly solidified Ni3al alloy ribbon

The phenomenology of Luders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Luders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Luders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures ( 470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σeff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Luders front motion and responsible for serration appearance.

Activated Carbon Fiber Monoliths as Supercapacitor Electrodes

Activated carbon fibers (ACF) are interesting candidates for electrodes in electrochemical energy storage devices; however, one major drawback for practical application is their low density. In the present work, monoliths were synthesized from two different ACFs, reaching 3 times higher densities than the original ACFs’ apparent densities. The porosity of the monoliths was only slightly decreased with respect to the pristine ACFs, the employed PVDC binder developing additional porosity upon carbonization. The ACF monoliths are essentially microporous and reach BET surface areas of up to 1838 m2 g−1. SEM analysis reveals that the ACFs are well embedded into the monolith structure and that their length was significantly reduced due to the monolith preparation process. The carbonized monoliths were studied as supercapacitor electrodes in two- and three-electrode cells having 2 M H2SO4 as electrolyte. Maximum capacitances of around 200 F g−1 were reached. The results confirm that the capacitance of the bisulfate anions essentially originates from the double layer, while hydronium cations contribute with a mixture of both, double layer capacitance and pseudocapacitance.