The early stage of chromium precipitation in copper was analyzed at the atomic scale by atom probe tomography (APT). Quantitative data about the precipitate size, three-dimensional shape, density, ...composition and volume fraction were obtained in a Cu–1Cr–0.1Zr (wt.%) commercial alloy aged at 713K. Surprisingly, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction to the equilibrium Cu–Cr phase diagram. APT data also show that some impurities (Fe) may segregate along Cu/Cr interfaces. The concomitant evolution of the precipitate shape and composition as a function of the aging time is discussed. Special emphasis is given to the competition between interfacial and elastic energy, and to the role of Fe segregation.
Using both experimental and modeling approaches, we give some clarifications regarding the mechanisms of interaction between ferrite recrystallization and austenite formation in cold-rolled ...high-strength steels. Using different thermal paths, we show that ferrite recrystallization and austenite formation can be strongly interdependent. The nature of the interaction (weak or strong) affects significantly the austenite formation and the resulting microstructure. We show that the kinetics of austenite formation depends intrinsically on both heating rates and the extent of ferrite recrystallization. An unexpected behavior of austenite growth was also seen at high heating rates. A possible explanation is presented based on the nature of the local equilibrium at the ferrite–austenite interface. The microstructure is more heterogeneous and anisotropic when both austenite formation and ferrite recrystallization overlap. A mechanism of microstructural formation is proposed, and this is supported by 2D simulations’ images.
The effect of cold rolling on the early-stage precipitation mechanisms of Cr in fcc Cu has been investigated using transmission electron microscopy and atom-probe tomography. The results clearly show ...that the deformation leads to an acceleration of both the precipitation and coarsening kinetics. However, cold rolling does not change the precipitation sequence. As reported for the non-deformed state, three kinds of precipitate were observed: spheres, ellipsoid, and plates associated to various crystallographic structures and orientation relationships. If the rolling strain is too high, then intensive recrystallization occurs during aging. Recrystallization is also associated with a fast growth of Cr precipitates and a significant drop in mechanical strength.
The present work investigates the interactions between ferrite recrystallization and austenite formation in dual-phase steels by experiments performed at high heating rate (100 °C/s). It was shown ...that both ferrite recrystallization and austenite formation are strongly coupled and interdependent. The kinetics of ferrite recrystallization is strongly affected by the formation of austenite and can be even inhibited in some cases. The microstructure is more heterogeneous and anisotropic when both the austenite formation and the ferrite recrystallization overlap. It was highlighted that the degree of anisotropy depends on the volume fraction of austenite at a given temperature. Furthermore, an unusual behavior for austenite growth was highlighted. It is characterized by a much higher volume fraction than those obtained under OrthoEquilibrium and ParaEquilibrium. The results, especially those at 715 °C close to the eutectoid plateau, at which the driving force for austenite growth is classically low, suggest a diffusionless transformation for austenite.
The purpose of this work is to study the effect of prior plastic deformation on the precipitation mechanisms of Mg
x
Si
y
in an AA6061 alloy. Differential scanning calorimetry (DSC), tensile testing ...and transmission electron microscopy (TEM) were used to characterize the precipitation sequence in samples that were isothermally aged at 180 °C with and without prior plastic deformation. Compressively deforming the AA6061 alloy by 4% caused a shift in the exothermal precipitation peaks to lower temperatures (DSC) and enabled the peak strength condition to be reached after a shorter aging period, revealing that plastic deformation accelerated the precipitation kinetics. TEM analysis determined that the accelerated precipitation kinetics in the deformed material was due to heterogeneous precipitation of the Q′ phase along dislocation lines and a modification of the precipitation sequence with the L, C and Q′ phases dominating over the β′′ phase (which is dominated in the non-deformed material). Additionally, the formation of β′ was largely suppressed by plastic deformation.
Nuclear stopping has been investigated in central nuclear collisions at intermediate energies by analyzing kinematically complete events recorded with the help of the 4π multidetector INDRA for a ...large variety of symmetric systems. It is found that the mean isotropy ratio defined as the ratio of transverse to parallel momenta (energies) reaches a minimum near the Fermi energy, saturates or slowly increases depending on the mass of the system as the beam energy increases, and then stays lower than unity, showing that significant stopping is not achieved even for the heavier systems. Close to and above the Fermi energy, experimental data show no effect of the isospin content of the interacting system. A comparison with transport model calculations reveals that the latter overestimates the stopping power at low energies.
We report on first experimental observations of nuclear fermionic and bosonic components displaying different behaviours in the decay of hot Ca projectile-like sources produced in mid-peripheral ...collisions at sub-Fermi energies. The experimental setup, constituted by the coupling of the INDRA 4π detector array to the forward angle VAMOS magnetic spectrometer, allowed to reconstruct the mass, charge and excitation energy of the decaying hot projectile-like sources. By means of quantum-fluctuation analysis techniques, temperatures and local partial densities of bosons and fermions could be correlated to the excitation energy of the reconstructed system. The results are consistent with the production of dilute mixed systems of bosons and fermions, where bosons experience higher phase-space and energy density as compared to the surrounding fermionic gas. Our findings recall phenomena observed in the study of Bose condensates and Fermi gases in atomic traps despite the different scales.
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