O. Beffort

Microstructure and mechanical properties of squeeze cast and semi-solid cast Mg–Al alloys

Magnesium–aluminium castings produced by means of squeeze casting, new rheocasting and thixocasting have been investigated. These casting processes provide very different microstructures consisting of α-Mg and β-Mg17Al12. The shape and distribution of the brittle β-Mg17Al12 has a large influence on mechanical properties. Isolated particles of β-phase in squeeze cast components are less detrimental to ductility than the continuous β-phase network found in semi-solid processed parts. A heat treatment results in complete dissolution of β-Mg17Al12 and accounts for significant improvements of ductility and fracture toughness. Crack propagation in solution heat treated Mg–Al castings is associated with extensive twinning.

Study of microstructure and interfaces in an aluminium–C60 composite material

Abstract This work deals with the study of microstructure and interfacial reactions in an aluminium–fullerene (Al/C 60 ) composite produced by pressurized liquid metal infiltration (squeeze casting) of a tap-packed C 60 powder preform. The obtained composite shows a homogeneous distribution of C 60 crystals in the Al matrix, indicating that the original aggregated crystalline structure of the C 60 fullerenes was not disrupted during the composite fabrication process, even though there is evidence of partial sublimation of C 60 . The C 60 crystals are well integrated in the matrix and there is no hint either of porosity or of weak interfacial bonding. At the interface between Al and C 60 rod-shaped and hexagonal plate-like phases are occasionally observed by TEM. These phases were identified as Al 4 C 3 by XRD and Raman spectroscopy and were shown to nucleate on very thin layers of amorphous carbon at the surface of the C 60 crystals.

Site-specific specimen preparation by focused ion beam milling for transmission electron microscopy of metal matrix composites.

This work describes the application and usefulness of the focused ion beam (FIB) technique for the preparation of transmission electron microscopy (TEM) samples from metal matrix composite materials. Results on an Aldiamond composite, manufactured by the squeeze casting infiltration process, were chosen for demonstration. It is almost impossible to prepare TEM specimens of this material by any other conventional method owing to the presence of highly inhomogeneous phases and reinforcement diamond particles. The present article gives a detailed account of the salient features of the FIB technique and its operation. One of the big advantages is the possibility to prepare site-specific TEM specimens with high spatial resolution. The artifacts occurring during the specimen preparation, for example, Ga-ion implantation, curtain effects, amorphous layers, bending of the lamella, or different milling behaviors of the materials have been discussed. Furthermore, TEM examination of the specimens prepared revealed an ultrafine amorphous layer of graphite formed at the interface between the Al and diamond particles that may affect the interfacial properties of the composite materials. This may not have been feasible without the successful application of the FIB technique for production of good quality site-specific TEM specimens.