S. Lay

Segregation of Fe during the sintering of doped W–Cu Alloys

In Fe dilute W–Cu alloys, the Fe distribution in W and liquid Cu, and the Fe interfacial segregation are studied by a thermodynamic approach and by energy dispersive analysis in transmission electron microscopy. The results show the preferred Fe distribution in liquid Cu and some Fe segregation in W grain boundaries. � 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Interface structure in a WC–Co alloy co-doped with VC and Cr 3 C 2

The effect of carbide additions is investigated at the atom scale by studying the interface between the cubic carbide phase and the WC prismatic facet in a VC Cr 3 C 2 co-doped WC Co alloy using high-resolution transmission electron microscopy. The alloy was prepared from WC powder about 0.5 lm in size and sintered at 1400°C for 2 h. The orientation relationship between WC and the MC carbide at the prismatic facet of WC is the same orientation as the one found at a WC/(Cr, W)C interface in Cr 3 C 2 -doped alloys. the image of WC shows triangular patterns indicating the position of the C atomic columns in this crystal. In MC crystal, some elongation of the bright areas parallel to the interface is observed on the simulation. The presence of dislocations at interfaces can be related to the parametric mismatch between the two neighboring crystals, to the presence of different stable structural units, or to an angular deviation between the crystals.

Elaboration of (Steel/Cemented Carbide) Multimaterial by Powder Metallurgy

A steel/cemented carbide couple is selected to generate a tough/hard two layers material. The sintering temperature and composition are chosen according to phase equilibria data. The choice of optimal sintering conditions needs experimental studies. First results evidence liquid migration from the hard layer to the tough one, leading to porosity in the hard region. The study of microstructure evolution during sintering of the tough material (TEM, SEM, image analysis) evidences the coupled mechanisms of pore reduction and WC dissolution, and leads to temperature and time ranges suitable to limit liquid migration. The sintering of the two layer material is then shown to need further compromises to avoid interface crack formation due to differential densification.

Evidence for disordered character of grain boundaries in a Ni3Al-based alloy during reordering

Metastable disordered polymorphs of L1{sub 2}-type Ni{sub 3}Al + Fe intermetallics obtained by heavy deformation or rapid solidification reorder upon annealing. In order to understand the mechanism of such reordering, the authors have obtained TEM images of the reordering process during in situ annealing of mostly disordered melt-spun Ni{sub 75}Al{sub 12}Fe{sub 13} which is slightly off-stoichiometric with excess nickel. The results indicate that the grain boundaries in such L1{sub 2}-type Ni{sub 3}Al + Fe alloys are incapable of nucleating chemical long-range order and appear to maintain a disordered f.c.c. character. Reordering starts vis one-dimensional growth of lamellar structures from pre-existing nanometric ordered nuclei behind moving grain boundaries and is consistent with evacuation of excess nickel first to anti-phase domain boundaries and then to grain boundaries. This interpretation is confirmed by a kinetic analysis of magnetization variations during reordering which yields a Johnson-Mehl-Avrami exponent n = 1/2.