C. Colin

Processing of functional-gradient WC-Co cermets by powder metallurgy

WC-Co cemented carbides are widely used as machining tools, mining tools, and wear-resistant parts. Multilayer graded structures with Co content ranging from 10 to 30 wt.% from one side of the structure to the other have been prepared by using either solid-state or liquid-phase sintering. Post-hot isostatic pressing was used to improve the final density of some of these samples. Special attention was given to the homogenization process that occurs during liquid-phase sintering of the graded structures.

Tensile flow properties of Al-based matrix composites reinforced with a random planar network of continuous metallic fibres

Squeeze casting was used for processing two new types of composites: pure Al matrix composites reinforced with fibres of Inconel 601, and AS13 (Al-12% Si) matrix composites reinforced with fibres of Inconel 601 or stainless steel 316L. The fibres are continuous a with a diameter of 12 mu m and their volume fraction in the composites varied from 20 to 80%. The processing conditions were such that no trace of interfacial reaction compound or of matrix precipitate resulting from the dissolution of elements of the fibres could be detected. The quality of the process was attested by Youngs modulus and electrical conductivity measurements. Tensile tests were carried out from room temperature up to 300 degrees C. The composites with the pure Al matrix present a remarkable tensile ductility. They thus constitute convenient materials for assessing continuum plasticity models for composites. Properties of composites with the AS13 matrix are much affected by interface adhesion strength

Oxidation behaviour in air of thin alloy 601 fibres

Abstract The oxidation behaviour in air of 12 μm diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).

A Phenomenological Model for High Temperature Oxidation of Si3N4-TiN Composites

Based on microstructural and chemical analyses, a phenomenological model is proposed in order to describe the high temperature oxidation of Si 3 N 4 -TiN ceramics. The model consists of three steps in the 1000-1200°C temperature range. In a first step, the oxidation of the TiN phase is controlled by the diffusion of Ti through TiO 2 formed at the outer surface, leading to the formation of a porous sub-layer. Simultaneously, the Si 3 N 4 phase oxidation is controlled by oxygen diffusion through SiO 2 . In a second step, the oxidation of inner TiN phase is controlled by the diffusion of oxygen through TiO 2 . The Si 3 N 4 transformation into SiO 2 leads to a high molar volume increase, SiO 2 , which is vitreous in the considered temperature range, creeps in the sub-layer porosity. In a third step, SiO 2 forms a continuous sub-layer, and oxidation is controlled by the diffusion of O through this layer. The kinetic laws proposed from this phenomenological model are in good agreement with thermogravimetric analyses between 1000°C and 1100°C. For higher temperatures, fluctuations in the experimental curves seem to indicate a succession of fracture and crack healing in the oxide layer.

On the control of interface reaction in squeeze cast aluminium matrix composites

The processing parameters for achieving proper control of the kinetics of interface reaction even in very reactive systems are discussed. The work concentrates on composites consisting of a pure Al matrix reinforced by continuous fibres of Inconel 601. Use is made of a low thermal inertia squeeze casting set-up equipped with monitoring of the sample temperature. Microstructural characterization and DTA analysis allow to elucidate some aspects of the reaction mechanisms. A model based on a parabolic growth law is developed for ranking the severity of processing conditions.