No abstract

A recently developed gel casting system based on water soluble epoxy resin was successfully applied to sialon ceramics. The solids loading could reach 48.9% without any pH adjustment, and all slurries exhibited shear-thinning behaviour. Flexural strengths of sialon ceramic obtained from 44.5 vol% slurry were 295.2 MPa at room temperature and 281.4 MPa at 700 1C; the microhardness was 16.5 GPa. The net-shape manufacturing of sialon ceramics could benefit from this investigation.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author -deposited version published in: http://oatao. • -400• C, the alloy embrittles through the decomposition of the martensite into iron-rich and chromium-rich domains. Depending on the ageing time, these domains are either interconnected or unconnected with each other. The embrittlement results in a drastic drop of the impact strength-energy and an increase of the ductile-to-brittle transition temperature. The initial microstructure and mechanical properties can be recovered through a re-homogenization of the distribution of chromium and iron atoms in the material in the case where the decomposition of the matrix is not too pronounced. The application of a stress higher than 60 per cent of the yield strength further enhances the ageing kinetics in the case where the combined effect of temperature and time results in the spinodal decomposition of the martensite.

Alloy 15-5 PH is a stainless steel with 15 wt.% Cr and 5 wt.% Ni that is precipitation hardened by addition of Cu. In its semi-finished state, this alloy consists in Cu-supersaturated soft martensite; its high specific properties come from a final tempering consisting in a heating to 550-600°C, holding for 4 hours, and then air cooling. This treatment leads to nanometric Cu precipitation that hardens the material and to transformation of some martensite to reverted austenite which is then stable and provides ductility. While a' embrittlement of such steels is known to occur at temperature in the range 450-520°C, it has been reported that they can be sensitive to the same phenomenon after long term ageing at temperature as low as 300°C, with a significant loss of ductility and an increase of the ductile-to-brittle transition temperature. Atom probe studies showed that this degradation is related to demixtion of martensite into Fe-rich and Cr-rich phases. Depending on the ageing temperature, demixtion can proceed through a nucleation and growth precipitation or by spinodal decomposition of the martensitic matrix. The present study reports differential thermal analyses (DTA) performed upon heating samples of material held at various temperatures (290-525°C) for various times (410 h to 8500 h) that have been characterized by atom probe. A clear DTA signal is obtained upon the reverse spinodal transformation that is further found to depend on ageing conditions.

Gel-casting of ceramics is a promising technique that can be used to manufacture complex structures in a netshape approach, with the density and mechanical properties of the green and sintered structures being strongly influenced by the composition of the gel formulation and the casting approach. In this study, a parametric study was performed to assess the influence of the gel formulation (solid loading and resin content), the casting approach (centrifugal casting versus gravity casting), and the sintering treatment on the microstructural and mechanical properties development in gel-cast sialon ceramics. Optimum levels of solid loading and resin content were identified for the gel formulation. The study showed that a significant improvement in the high temperature flexural strength (~1.5-2 folds) can be achieved by employing a centrifugal gel-casting approach, due to the reduction of the porosity caused by the entrapped gas that gets entrapped during the standard casting process.