Metal matrix composites – From science to technological significance

Polypropylene (PP)−clay hybrids (PPCH) have been prepared by simple melt-mixing of three components, i.e., PP, maleic anhydride modified polypropylene oligomers (PP-MA), and clays intercalated with stearylammonium. The dispersibility of 10-A-thick silicate layers of the clays in the hybrids was investigated by using a transmission electron microscope and X-ray diffractometer. It is found that there are two important factors to achieve the exfoliated and homogeneous dispersion of the layers in the hybrids:  (1) the intercalation capability of the oligomers in the layers and (2) the miscibility of the oligomers with PP. Almost complete hybrids were obtained in the case where the PP-MA has both intercalation capability and miscibility. The hybrids exhibit higher storage moduli compared to those of PP especially in the temperature range from Tg to 90 °C. The highest relative storage modulus at 80 °C of the hybrid based on a mica and the miscible PP-MA is as high as 2.0 to that of PP and is 2.4 to that of the ...

Preparation and Mechanical Properties of Polypropylene−Clay Hybrids

Fundamentals and advances in magnesium alloy corrosion

Mechanically alloyed nanocomposites

Our safety, comfort and peace of mind are heavily dependent upon our capability to prevent, predict or postpone the failure of components and structures on the basis of sound physical principles While the external loadings acting on a material or component are clearly important, There are other contributory factors including unfavourable materials microstructure, pre-existing defects and residual stresses Residual stresses can add to, or subtract from, the applied stresses and so when unexpected failure occurs it is often because residual stresses have combined critically with the applied stresses, or because together with the presence of undetected defects they have dangerously lowered the applied stress at which failure will occur Consequently it is important that the origins of residual stress are understood, opportunities for removing harmful or introducing beneficial residual stresses recognized, their evolution in-service predicted, their influence on failure processes understood and safe structural integrity assessments made, so as to either remove the part prior to failure, or to take corrective action to extend life This paper reviews the progress in these aspects in the light of the basic failure mechanisms

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Residual stress and its role in failure

Metal Matrix Composites

Oxide fiber/oxide matrix composites form an important and very attractive subpart of ceramic matrix composites because of their inherent stability in oxidizing atmospheres at high temperature. In particular, mullite fiber/mullite matrix composites have the potential of high temperature usage in oxidizing atmospheres. The interface in mullite fiber/mullite matrix was engineered by using thick BN (1 μm) or BN/SiC double coating on mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the room temperature mechanical properties of these mullite fiber/mullite matrix composites could be achieved by incorporation of these interfacial coatings and by using a colloidal processing route to make dense mullite matrix.

Processing, structure, and properties of mullite fiber/mullite matrix composites

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al2O3+28 wt.-% SiO2+2 wt.-% B2O3). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.

Effect of BN Coating on the Strength of a Mullite Type Fiber.

Characterization of Ceramics. Edited by Ronald E. Lochman, Materials Characterization Series—Surfaces, Interfaces, Thin films, Butterworth–Heinemann, Oxford 1993, 295 pp., £ 47.50, ISBN 0‐7506‐9253‐7

The thermal degradation behavior of single-layer BN and of double-layer BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single-layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°-1200°C. The single-layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double-layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double-layer coating system improved the oxidation resistance of BN in the 1200°-1300°C temperature range, despite a partial oxidation of SiC to SiO2.

K.K. Chawla

Papers

Metal Matrix Composites

Processing, structure, and properties of mullite fiber/mullite matrix composites

Effect of BN Coating on the Strength of a Mullite Type Fiber.

Characterization of Ceramics. Edited by Ronald E. Lochman, Materials Characterization Series—Surfaces, Interfaces, Thin films, Butterworth–Heinemann, Oxford 1993, 295 pp., £ 47.50, ISBN 0‐7506‐9253‐7

Thermal degradation of fiber coatings in mullite-fiber-reinforced mullite composites