Abstract
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.
Highlights
Nanocrystalline materials possess unique mechanical and functional properties, which are often enhanced or even completely different from the ones of their coarse grained counterparts [1]
From the above mentioned Co materials, disk shaped samples with a diameter of 8 mm and a thickness of 0.8 mm are high-pressure torsion (HPT) deformed at room temperature for 10 rotations
The different Co materials exhibit a rather constant microhardness as a function of the radii of the HPT samples and the typical microhardness distribution of HPT samples, which are only partly deformed to saturation, is not observed [27]
Summary
Nanocrystalline (nc) materials possess unique mechanical and functional properties, which are often enhanced or even completely different from the ones of their coarse grained counterparts [1]. Nc Co and Co alloys exhibit, for example, interesting magnetic properties like an extraordinary high saturation magnetization and the giant magneto resistance effect making them to one of the promising candidates for components in microelectromechanical systems (MEMS) [2,3] Because their unique properties are manifested by their small grain size, one major reason restricting potential technological applications of nc structures is their low thermal stability. Motz / Materials Science & Engineering A 624 (2015) 41–51 stability could be improved, significant grain growth yields to micrometer-sized grains for temperatures as low as 773–873 K [12] Another disadvantage emerges from the use of additives during the ED process. These effects are discussed together with the observation of small pores in the nc Co-based Cu alloy samples
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