Xavier Sauvage

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.

On the origin of the extremely high strength of ultrafine-grained Al alloys produced by severe plastic deformation

Ultrafine-grained Al alloys produced by high-pressure torsion are found to exhibit a very high strength, considerably exceeding the Hall–Petch predictions for ultrafine grains. This phenomenon can be attributed to the unique combination of ultrafine structure and deformation-induced segregations of solute elements along grain boundaries, which may affect the emission and mobility of intragranular dislocations.

Direct evidence of cementite dissolution in drawn pearlitic steels observed by tomographic atom probe

The dissolution of cementite during cold-drawing of pearlitic steels has been directly observed with the tomographic atom probe (TAP). Because of the nanometric interlamellar spacing and size of cementite lamellae, the TAP is shown to be extremely well suited for such a study. Analysis conditions and mass spectra peak deconvolution are shown to lead to quantitative analysis of both ferrite and cementite. It is also shown that specimen preparation always aligns the cementite lamellae habit plane with the analysis direction so that the analysed area is always perpendicular to their habit plane. Local magnification effects are also shown not to affect the carbon concentration measurement in the cementite. The first direct atomic scale quantitative concentration data across a few nanometer-thick cementite lamellae are given, and confirm the dissolution of cementite after cold-drawing. The derived compositions of ferrite, cementite and interfacial areas are obtained, giving information on the cementite dissolution mechanism as well as on its extent.

Solid state amorphization in cold drawn Cu/Nb wires

The microstructure of cold drawn Cu/Nb nanocomposite wires was investigated using a three dimensional atom probe (3D-AP) and transmission electron microscopy (TEM). Although there is no solubility between Nb and Cu in the equilibrium state, atom probe analysis results revealed that intermixing occurs between Nb and Cu filaments as a result of cold drawing with a large strain. High resolution transmission electron microscopy (HRTEM) results revealed that an amorphous layer is formed along some Cu/Nb interfaces. This solid state amorphization is compared with similar reactions observed in Cu-Nb multilayers.

Nanostructure and related mechanical properties of an Al-Mg-Si alloy processed by severe plastic deformation.

Microstructural features and mechanical properties of an Al–Mg–Si alloy processed by high-pressure torsion (HPT) have been investigated using transmission electron microscopy, X-ray diffraction, three-dimensional atom probe, tensile tests and micro-hardness measurements. It is shown that HPT processing of the Al–Mg–Si alloy leads to a much stronger grain size refinement than of pure aluminium (down to 100 nm). Moreover, massive segregation of alloying elements along grain boundaries is observed. This nanostructure exhibits a yield stress even two times higher than that after a standard T6 heat treatment of the coarse-grained alloy.

Grain boundary stability governs hardening and softening in extremely fine nanograined metals

Stabilized nanograin boundaries in nickel-molybdenum alloys result in increased hardness with decreasing grain size. Nanograined metals avoid going soft The Hall-Petch relationship links a metals increasing hardness with decreasing grain size, but it breaks down when grains become very small. This is unfortunate because nanograined metals could otherwise be extremely hard. Hu et al. found a way to circumvent this problem in a set of nickel-molybdenum alloys. They altered the molybdenum composition and annealed the samples at just the right temperature, which stabilized the grain boundaries in their nanograined samples. This allowed hardness to keep increasing with decreasing grain size, which could provide a route for designing superhard coatings. Science, this issue p. 1292 Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni–Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.

Atomic-scale observation and modelling of cementite dissolution in heavily deformed pearlitic steels

Abstract Heavily deformed pearlitic steel wires obtained by cold drawing have been investigated using a three-dimensional atom probe. Concentration profiles reveal the existence of pronounced gradients in the ferrite near ferrite-cementite interfaces. This indicates that cementite lamellae dissolve. Experiments are interpreted and dissolution kinetics are modelled semiquantitatively on the basis of thermodynamics and diffusion arguments. The dramatic increase in interface areas during drawing is considered as the driving force for dissolution through a Gibbs-Thomson effect. The relatively slow cooling of specimen from the temperature of drawing (100–300°C) to room temperature hence leads to the downhill diffusion of carbon from interfaces to the ferrite core. The kinetics equations are numerically solved in order to take into account the nonconstant mobility of carbon during cooling. The model highlights the key parameters which drive dissolution and their influence on kinetics. In comparison with experiments, predicted dissolution rates appear, however, to be underestimated.

Cu2O/ZnO hetero-nanobrush: hierarchical assembly, field emission and photocatalytic properties

Zinc oxide (ZnO) nanorods are grown hierarchically on cuprous oxide (Cu2O) nanoneedles to form a Cu2O/ZnO hetero-nanobrush assembly. This increases the overall aspect ratio, which helps to enhance the field emission properties of the system. Also, the charge separation and transport are facilitated because of the multiple p–n junctions formed at p-Cu2O/n-ZnO interfaces and quasi-1-D structures of both the materials, respectively. This helps to significantly enhance the photocatalytic properties. As compared to only Cu2O nanoneedles, the Cu2O/ZnO hetero-nanobrush shows excellent improvement in both field emission and photocatalytic applications.

Homogeneous Cu–Fe supersaturated solid solutions prepared by severe plastic deformation

A Cu–Fe nanocomposite containing 50 nm thick iron filaments dispersed in a copper matrix was processed by torsion under high pressure at various strain rates and temperatures. The resulting nanostructures were characterized by transmission electron microscopy, atom probe tomography (APT) and Mössbauer spectrometry. It is shown that α-Fe filaments are dissolved during severe plastic deformation leading to the formation of a homogeneous supersaturated solid solution of about 12 at% Fe in fcc Cu. The dissolution rate is proportional to the total plastic strain but is not very sensitive to strain rate. Similar results were found for samples processed at liquid nitrogen temperature. APT data revealed asymmetric composition gradients resulting from deformation-induced intermixing. On the basis of these experimental data, the formation of the supersaturated solid solutions is discussed.

Grain Boundary Segregation in UFG Alloys Processed by Severe Plastic Deformation

Grain boundary (GB) segregations were investigated by atom probe tomography in an Al–Mg alloy, a carbon steel and Armco® Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminum alloy are Mg depleted, but after SPD some local enrichment up to 20 at% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as “non-equilibrum” in ultra fine grained materials processed by SPD. The GB segregation mechanisms are discussed and compared in the case of substitutional (Mg in fcc Al) and interstitial (C in bcc Fe) solute atoms.