Jean-Philippe Monchoux

Morphological changes of graphite spheroids during heat treatment of ductile cast irons

Abstract The partial dissolution of graphite spheroids in ductile cast iron during two heat treatments was investigated. The dissolution takes place along crystalline defects of the spheroid regularly spaced out around it. A growing matrix intrusion forms at these locations, spheroidizes and then detaches itself from the matrix, creating an enclosed matrix particle in the graphite. An estimation made with an existing model (Rayleigh instability) of globulization kinetics is in good agreement with the observations. The role of the silicon present in the matrix on the dissolution rate and on the resulting graphite–matrix interface is discussed at the end.

Microstructural properties of Nb-Si based alloys manufactured by powder metallurgy

An NbTiHfCrAlSi niobium silicide based atomized powder has been compacted by a conventional technique (hot extrusion) and by spark plasma sintering to nearly fully dense alloys. Both materials exhibit a metastable fine micrometer-sized microstructure that has been coarsened by a subsequent heat treatment. The densification of the SPS sample takes place between ca. 800°C and 1300°C.

High Creep Resistance of Titanium Aluminides Sintered by SPS

Reducing fuel consumption, noise, and greenhouse gas emission of airplanes engines requires to use lighter materials. Titanium Aluminides (TiAl) are of great interest to be employed for high temperature applications like turbine blades as they are twice lighter than superalloys currently used. A few years ago, two engines produced by GENERAL ELECTRIC and SNECMA-SAFRAN including TiAl turbine blades have been certified. However, TiAl alloys still suffer from a poor ductility at room temperature, a difficult and expensive manufacturing process, and a limited creep resistance at working temperature. We adapted the Spark Plasma Sintering, a powder metallurgy technique, to produce near-net shape turbine blades with an optimized TiAl alloy containing heavy elements to enhance the creep resistance. In this paper, we will present a study of creep properties under extreme conditions such as 700 °C/300 MPa of TiAl alloys, sintered by SPS, able to resist more than 4000 h with a minimum creep rate of 3.5 × 10−9 s−1. These outstanding properties will be correlated with microstructure features, chemistry, and deformation mechanisms.