Improving Soft-Magnetic And Biodegradable Metallic Materials By Means Of Severe Plastic Deformation

The magnetic properties of soft magnetic materials such as Fe-3wt%Si and Fe-6.5wt%Si studied here can be strongly improved by nanocrystallization if the grain size is smaller than the magnetic moment exchange length [1]. Recent top-down techniques like those of Severe Plastic Deformation (SPD), in particular of High Pressure Torsion (HPT), allow for the production of bulk nanocrystalline soft magnetic materials, as they are capable to reach grain and/or subgrain sizes well below 100-10 nm [2], thus underrunning the exchange length of magnetic moments. This fact should lead to a significant decrease of coercive force (Hc) and finally of the hysteresis losses (P/f with f as the frequency), being an inherent goal of soft magnetic materials research. HPT-induced changes of Hc and P/f were found to be in parallel; they could be related to the changes of two most important parameters such as the subgrain size (D), and the dislocation density (N). In both cases, only decreases of D or N led to decreases of Hc. At least the first result confirms that Hc is indeed dominated by exchange coupling within the magnetic domains, their size exceeding that of the subgrains. Since HPT can only achieve decreases of D and concomitant increases of N, thermal treatments were applied after HPT-processing, in order to decrease N while keeping D unchanged. A maximum decrease of Hc could only be reached when a strong increase of D or N during HPT-processing or during thermal treatment could be avoided. Other conditions are that neither the HPT-induced strain is too large nor the processing temperature applied is too low; otherwise nanostructures become resistant to the thermal treatment. Theories from literature [3] which predict a change of the Hc ~ D6law for large-angle misoriented grains, to a law with distinctly lower exponent Hc ~ D3 or D2 in the case of small-angle misoriented subgrains, were confirmed by the experiments [4]. Another example of successful application of SPD to functional nanomaterials is the improvement in functional properties of biodegradable binary and ternary Mg-Zn-Ca alloys achieved by High-Pressure Torsion (HPT)-processing and subsequent heat treatment. These procedures have been applied in order to strengthen the alloy and to limit the corrosion rate [5]. Two paths of HPT-processing fulfilled these conditions; these were (i) low temperature-low strain HPT, or (ii) high temperature-high strain HPT of the solid solution state, both followed by subsequent thermal treatment at 373 K [6]. While treatment (i) yielded strength increases till 250% mainly due to generation of HPT-induced defects including those of vacancies, treatment (ii) produced precipitates with strength increases of only 60% but much higher ductility [6]. In conclusion, SPD-processing achieved different nanostructures with individual extents of strength, ductility and corrosion resistance, thus meeting the specific requirements of different biodegradable implants and prostheses.