Modelling the precipitation of NbC on dislocations in a-Fe

A model has been developed for describing the precipitation of NbC on dislocations in ferrite in an Fe–C–Nb steel. This model is a continuous description of the classical laws for nucleation growth and coarsening, which are adapted to the specific case where precipitates only form on dislocations. This model is successfully applied on an extensive data set obtained by small-angle neutron scattering for a wide temperature range and two alloy contents. Using this model, it is possible to estimate the effects of process parameters on the final microstructure and, notably, it is shown that the initial dislocation density has a pronounced influence on the maximum precipitate density

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

The heterogeneous precipitate microstructure of a AA2050 Al–Li–Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction and size of both T1 platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure

Clustering kinetics during natural ageing of Al-Cu based alloys with (Mg, Li) additions

Room temperature solute clustering in aluminium alloys, or natural ageing, despite its industrial relevance, is still subject to debate, mostly due to its experimentally challenging nature. To better understand the complex multi-constituents' interactions at play, we have studied ternary and quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and Al-Cu-Li-Mg. We used a recently introduced correlative technique using small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the chemically resolved kinetics of room temperature clustering in these alloys, which we completed with differential scanning calorimetry (DSC) and micro-hardness measurements. The comparison of the clustering behaviours of each subsystem allowed us to highlight the paramount role of Mg as a trigger for diffusion and clustering. Indeed, while a strong natural ageing was observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very slight addition of Mg (0.4%) to this system, however, drastically changed the situation to a rapid formation of essentially Cu-rich hardening clusters, Mg only joining them later in the reaction. This diffusion enabling effect of Mg is discussed in terms of diffusion mechanism and complex interactions with the quenched-in vacancies

Development of compositional-gradient metallic alloys for combinatorial investigation of microstructures

International audienc

Use of space-resolved in-situ high energy X-ray diffraction for the characterization of the compositional dependence of the austenite-to-ferrite transformation kinetics in steels

In-situ high energy X-Ray diffraction (HEXRD) was used on compositionally graded steels to study the effect of substitutional elements on ferrite growth kinetics in Fe-C-X and Fe-C-X-Y systems. Two systems have been selected to illustrate the applicability of the combinatorial approach in studying such transformations, Fe-C-Mn and Fe-C-Mn-Mo. Comparison between the measured ferrite growth kinetics using HEXRD and the predicted ones using Para-Equilibrium (PE) and Local Equilibrium with Negligible Partitioning (LENP) models indicates that the fractions reached at the stasis of transformation are lower than the predicted ones. Experiments indicated a deviation of measured kinetics from both PE and LENP models when increasing Mn and decreasing Mo (in Fe-C-Mn-Mo system). The large amount of data that can be obtained using this approach can be used for validating existing models describing ferrite growth kinetics

Low temperature precipitation kinetics of niobium nitride platelets in Fe

International audienceSingle plane platelets of niobium nitride have been observed to form in a Fe-Nb-N alloy during ageing at 600 degrees C, using High Resolution Electron Microscopy, Field Ion Microscopy and Atom Probe Tomography. Small-angle neutron scattering has been used to investigate the kinetics of formation of these platelets. They are shown to nucleate in less than 5 min at this ageing temperature, and subsequently to grow in-plane to a size of about 10 nm without experiencing any change in thickness

Microstructure distribution in an AA2050 T34 friction stir weld and its evolution during post-welding heat treatment

This paper presents a systematic study where the distribution of precipitate microstructures is mapped in the cross-section of a friction stir weld made with an AA2050 Al–Cu–Li alloy in the naturally aged temper, as well as the evolution of this microstructure during subsequent post-welding heat treatment (PWHT). This study is carried out using spatially resolved small-angle X-ray scattering, supported by transmission electron microscopy, differential scanning calorimetry and microhardness mapping. The as-welded microstructure is dominated by solute clusters, while very little precipitation has taken place during the welding operation. During PWHT, the precipitation kinetics in the different zones of the weld is mainly controlled by the local dislocation density inherited from welding, and by the amount of solute available for precipitation, which depends on the volume fraction of welding-induced intermetallics. Pre-deforming the weld before the PWHT results in a very effective strength recovery and a nearly homogeneous distribution of hardness

Hydrogen trapping by VC precipitates and structural defects in a high strength Fe-Mn-C steel studied by small-angle neutron scattering

The trapping of hydrogen by VC precipitates and structural defects in high strength Fe-Mn-C steel was studied by small angle neutron scattering. No interaction between H and V in solid solution has been detected but a significant interaction between H and structural defects introduced by plastic deformation has been measured. This last effect was reversible upon outgassing of the H. Moreover a significant interaction between H and VC precipitates has been measured; 5 ppm wt. of H could be trapped in the precipitates. This is consistent with the homogeneous trapping of H within the precipitates rather than at the precipitate/matrix interface