Properties of Nanoscaled Multiphase Structures and Non-Equilibrium Solid Solutions Obtained by Severe Plastic Deformatio
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Properties of Nanoscaled Multiphase Structures and Non-Equilibrium Solid Solutions Obtained by Severe Plastic Deformation Xavier Sauvage1, Xavier Quelennec1, Peter Jessner1,2, Florian Wetscher3, and Reinhard Pippan3 1 CNRS - Université de Rouen, Groupe de Physique des Matériaux UMR 6634, Avenue de l'Université, Saint-Etienne du Rouvray, 76 801, France 2 CD-Laboratory for Local Analysis of Deformation and Fracture, Erich Schmid Institute of Material Science, Jahnstraße 12, Leoben, 8700, Austria 3 CD-Laboratory for Local Analysis of Deformation and Fracture, Erich Schmid Institute of Material Sciences, Jahnstraße 12, Leoben, 8700, Austria Keywords Severe plastic deformation, Cu-Cr, Cu-Fe, Nanostructure, Three-dimensional atom probe, Supersaturated solid solution, Intermixing
Introduction Cu-Nb nanocomposite wires are widely used to make magnet coils for high pulsed magnetic fields because they exhibit an excellent combination of mechanical, electrical and thermal properties (typically, UTS close to 2GPa and resistivity lower than one mohm.cm at 77K ) [1]. These composite materials are usually made by cold drawing. The large strains involve in this process leads to a strong reduction of the Nb filaments diameter down to the nanoscale which gives rise to these excellent mechanical properties. However, in a previous study we have shown that severe plastic deformation (SPD) could lead to non equilibrium Cu-Nb intermixing and solid state amorphization [2]. The question is: what are the physical mechanisms involved and does this feature affects the properties of the wires ? On the other hand, it is well known that SPD processes such as ECAP (Equal Channel Angular Pressing) or HPT (High Pressure Torsion) give rise to a strong reduction of the grain size in metallic alloys [3]. So, another question is: could one use some SPD processes to make an equi-axed multiphase nanoscaled structure ? To answer these questions, we have investigated two kinds of Cu based composite materials (Cu-Cr and Cu-Fe) processed
by HPT. Atomic scale characterization was performed by three dimensional atom probe (3D-AP) [4].
Cu-Cr nanocomposite Dilute Cu-Cr alloys are widely used as heat transfer elements because they exhibit an excellent combination of thermal conductivity and mechanical strength [5]. They are age hardenable alloys, and the hardening is limited by the Cr precipitate density, which is limited by the low solubility of Cr in Cu at high temperature. Thus, this system was chosen to evaluate if the solubility could be enlarged by SPD. The initial material contains 43wt.% Cr and has a grain size of about 30 microns, and after 25 turns of torsion by HPT, the grain size is less than 20 nm (at 3mm from the sample center). 3D-AP data clearly show also that Cu diffused into the bcc Cr phase during SPD which is attributed to a size effect. Moreover, one should note that this feature leads to a significant decrease of the hardness.
3D reconstruction (10x10x50 nm3 ) of an analysed volume in the Cu-Cr composite processed 25 turns
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