On the remarkable thermal stability of nanocrystalline cobalt via alloying

Nanostructured Co materials are produced by severe plastic deformation via alloying with small amounts of C and larger amounts of Cu. The thermal stability of the different nanostructured Co materials is studied through isothermal annealing at different temperatures for various times and compared to the stability of severe plastically deformed high-purity nanocrystalline Co. The microstructural changes taking place during annealing are evaluated by scanning electron microscopy, transmission electron microscopy and microhardness measurements. In the present work it is shown that the least stable nanostructured material is the single-phase high purity Co. Alloying with C improves the thermal stability to a certain extent. A remarkable thermal stability is achieved by alloying Co with Cu resulting in stabilized nanostructures even after annealing for long times at high temperatures. The essential reason for the enhanced thermal stability is to be found in the immiscibility of both components of the alloy

Phase decomposition and nano structure evolution of metastable nanocrystalline Cu-Co solid solutions during thermal treatment

Nanocrystalline and ultrafine-grained Cu100-xCox (x = 26 and 76) solid solutions have been prepared by severe plastic deformation (SPD) of elemental powder mixtures. For both concentrations a supersaturated solid solution fcc phase was identified after the deformation process with grain sizes of less than 50 nm for Co rich solutions and around 100 nm for Cu rich solutions. Additionally, synthesis of nanocrystalline materials in the Cu-Co alloy system by electrodeposition has been conducted. Microstructural characterization by scanning and transmission electron microscopy, differential scanning calorimetry, and microhardness measurements are used to investigate the structural evolution, the thermal stability and mechanical properties of the different nanocrystalline Cu-Co alloy materials during isothermal and non-isothermal annealing. In this study it is shown that the phase decomposition of the metastable Cu-Co solid solutions has a significant influence on their thermal stability, which can be linked to the underlying microstructure that forms during annealing

Friction and Tribo-Chemical Behavior of SPD-Processed CNT-Reinforced Composites

Nickel (Ni) and carbon nanotube (CNT)-reinforced Ni-matrix composites were manufactured by solid state processing and severely deformed by high-pressure torsion (HPT). Micro-tribological testing was performed by reciprocating sliding and the frictional behavior was investigated. Tribo-chemical and microstructural changes were investigated using energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and focused ion beam (FIB). The CNT lubricity was hindered due to the continuous formation of a stable oxide layer promoted by a large grain boundary area and by irreversible damage introduced to the reinforcement during HPT, which controlled the frictional behavior of the studied samples. The presence of CNT reduced, to some extent, the tribo-oxidation activity on the contact zone and reduced the wear by significant hardening and stabilization of the microstructure

Metastable nanomaterials and nanocomposites obtained by high-pressure torsion powder consolidation

Nanostructuring can dramatically improve the mechanical and functional properties of metallic materials and composites and to achieve the goal of a nanostructured bulk material innovative techniques have to be used. High Pressure Torsion (HPT), a method of Severe Plastic Deformation (SPD) is a novel processing route for powder consolidation, featuring a complete absence of a sintering treatment – bulk samples are the direct result of the SPD process. HPT further shows two advantages: First, the starting material (powder mixtures) can be processed at any concentration. Even immiscible compositions were successfully processed for different systems (e.g. Cu-Fe, Cu-Co). Second, the severe deformation gives rise to supersaturated solid solutions (“far from equilibrium”) yielding interesting material properties, different from the pure or alloyed elements’ ones. These supersaturated solid solutions, upon adequate annealing, show phase separations yielding nanoscaled composites. This gives a tool in one’s hands to systematically tune certain material properties. The nanostructures that were formed in such a way show high strength and ductility but also interesting functional properties. To give an example, annealing of above mentioned binary systems changes their magnetic properties regarding coercivity, remanence and magnetoresistance. Thus, combining the right choice of processed powders (composition, size, shape), HPT-processing parameters (applied strain, processing temperatures) and subsequent annealing treatment (time, temperature) results in desired, tailored microstructures of optimized properties. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 757333

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

Bulk nanocrystalline materials of small and medium ferromagnetic content were produced using severe plastic deformation by high-pressure torsion at roomtemperature. Giant magnetoresistive behavior was found for as deformed materials, which was further improved by adjusting the microstructure with thermal treatments. The adequate range of annealing temperatures was assessed with in-situ synchrotron diffraction measurements. Thermally treated CuCo materials show larger giant magnetoresistance after annealing for 1 h at 300C, while for CuFe this annealing temperature is too high and decreases the magnetoresistive properties. The improvement of magnetoresistivity by thermal treatments is discussed with respect to the microstructural evolution as observed by electron microscopy and ex situ synchrotron diffraction measurements

Microstructural evolution during heating of CNT/Metal Matrix Composites processed by Severe Plastic Deformation

Carbon nanotube reinforced nickel matrix composites (Ni/CNT) with different CNT compositions were fabricated by solid state processing and subjected to severe plastic deformation (SPD) by means of high pressure torsion (HPT). A thorough study on the microstructural changes during heating and on the thermal stability was performed using differential scanning calorimetry (DSC), high temperature X-ray diffraction (HT-XRD) and electron backscattered diffraction (EBSD). Furthermore, the formation and dissolution of the metastable nickel carbide Ni3C phase was evidenced by DSC and HT-XRD in composites, where sufficient carbon atoms are available, as a consequence of irreversible damage on the CNT introduced by HPT. Finally, it was shown that the composites exhibited an improved thermal stability with respect to nickel samples processed under the same conditions, with a final grain size dependent on the CNT volume fraction according to a VCNT-1/3 relationship and that lied within the ultrafine grained range