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The creep behaviour of the PM NR3 nickel-based superalloy has been studied at 700°C in a wide stress range as a function of its microstructure. It was first confirmed that at this critical temperature a coarse-grained microstructure was preferable to a fine-grained one to obtain the highest creep resistance. The role of the tertiary γ ′ phase precipitates in the creep behaviour of coarsegrained NR3 was investigated at two stress levels typical of both low stress and high stress creep regimes. Careful identification of dislocation mechanisms by transmission electron microscopy was performed in creep strained specimens in order to correlate the macroscopic behaviour with the creep controlling mechanisms. Prior elimination of the tertiary γ ′ precipitates using an adequate overageing heat treatment promotes easy glide of a/2<110> perfect matrix dislocations between the secondary γ ′ precipitates and therefore confers to the alloy a poor creep resistance. On the contrary the presence of fine tertiary γ ′ particles impedes propagation of the perfect matrix dislocations which are forced to cut the γ ′ precipitates through two different modes of dissociation depending on the local precipitate distribution. Reduction of the γ phase channel width promotes the decorrelation in the matrix of the two a/6<112> partial dislocations constituting an a/2<110> perfect dislocation. This mechanism which leads to stacking fault configurations extending through both γ and γ ′ phases evidences the strong resistance of the microstructure against creep deformation. Coupled analysis of the creep behaviour and of the dislocation structures in the low stress creep regime indicates a growing involvement of the grain boundary areas in the macroscopic creep deformation.

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The effect of thermal cycling creep on the dislocation networks at the c/c 0 interfaces in the MC2 superalloy is investigated. Tensile creep tests were performed under thermal cycling and isothermal conditions at low stress (80 MPa) and high temperature (1150°C). In these conditions c 0 rafts may dissolve and reprecipitate during thermal cycling creep. The difference between the effects of isothermal and thermal cycling conditions on the c/c 0 interface dislocation networks, characterized by transmission electron microscopy, is exposed, as well as their evolution during the cycle.

Aluminium was added to a 0.2% C-2.5% Cr-1.4% Mo-11% Ni steel to modify the precipitation sequence during tempering treatment. The main goal was to obtain fine co-precipitation of an intermetallic phase and M 2 C carbides (where M is a combination of Cr, Mo and small amounts of Fe). Small angle neutron scattering, synchrotron X-ray diffraction, transmission electron microscopy and atom probe tomography were performed to characterize the nanometric precipitation. The tempering response of samples austenitized at 900 °C revealed a strong interaction between the two types of precipitation, leading to a significant modification of both the precipitation sequence of carbides and the arrangement of carbide nucleation sites compared with these sites in a single precipitation steel. Indeed, a microstructural investigation clearly showed that iron carbide precipitation was either delayed or did not occur during the tempering process, depending of the alloying elements added. Moreover, double precipitation directly influenced the mechanical resistance, as well as the toughness, leading to an ultrahigh-strength, high toughness steel.

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The mobile dislocation pileups observed during in situ straining experiments have been used as probes to describe the short-range order (SRO) present in the c-phase of nickel-base superalloys. An experimental approach based on the measurement of the dislocation positions to characterise the SRO has been developed. This paper focuses on the main results we have obtained on this topic. A quantitative analysis allows to get the chemical forces associated with SRO and also the SRO degree. For the first time the energies associated with SRO is evaluated along a dislocation pileup as a function of temperature. More generally, a description of the SRO is proposed: SRO is seen as a heterogeneous distribution at the micrometer scale of nanometric areas. Another characteristic has been evidenced: the reversible character of the SRO creation.

International audienceLocal order is evidenced in nodules of the duplex microstructure of a Ti-6Al-4V alloy using in situ straining experiments in a transmission electron microscope (TEM). This local order is identified to be short range order (SRO) because of the absence of superlattice diffraction spots, which are associated with alpha(2) (Ti3Al) precipitates and because of the formation of single pairs of mobile dislocations, which are a signature of SRO. The strengthening effect of this SRO is quantitatively evaluated. Qualitatively, the presence of SRO inhibits strongly the cross-slip in nodules in comparison with dislocations gliding in lamellar colonies where no SRO is present. The well-known strengthening effect of the core structure of dislocation in Ti-alloy is revisited here in the presence of SRO to determine its possible influence