Strength by deformation induced Vacancies in SPD Nanometals

By the end of the 1990ies, nanocrystalline materials have gained attention because of their outstanding physical properties i.e. mechanical, magnetic, thermoelectric ones and others. Since they can be processed in bulk shape, they have become important also in view of applications and commercial use. The most convenient method which achieves bulk nanomaterials with 100% density is Severe Plastic Deformation (SPD) which also provides additional advances in mechanical properties not being available with other processing routes. The reason for these advances lies in the presence and arrangement of deformation induced lattice defects such as dislocations, planar defects, but also point defects. The current project focuses on the deformation induced point defects from which â according to current plasticity research - vacancies and/or vacancy agglomerates play the dominant role. These vacancies are important for the diffusion properties of nanomaterials but also seem to significantly affect or even increase the mechanical properties of SPD nanometals, as they are produced in very high concentrations. The current project lays special emphasis in the study and quantification of the direct influence of vacancy agglomerates to the macroscopic strength which can be increased by up to 100% and which has never been systematically investigated so far in the literature of nanomaterials. SPD processing will be achieved by means of High Pressure Torsion (HPT) because this technique allows a controlled variation of all deformation parameters such as temperature, strain rate and hydrostatic pressure over a wide range. Studies will comprise not only fcc nanometals but also hcp and bcc materials where the effects of vacancy agglomeration to strength is expected to be increased. In order to maintain thermal stability of the vacancy type defects and the properties related, representative experiments with hydrogen are planned which acts as an alloying element favouring the formation and stability of the defects. Available models for void hardening will be tested and improved by comparison with specific experiments.

The aim of this project was the investigation of the influence of point defects, especially vacancies, on the hardening of highly deformed metals. It could be successfully proven, that vacancies increase the hardness additionally to about 35% by forming vacancy clusters. Nanocrystalline materials are stronger, more ductile and often change their electrical or magnetic properties due to their manufacturing process. The increased strength with increasing deformation plays a particularly important role. Since these materials can also be produced in massive form, they are also gaining in importance for commercial applications.An important manufacturing method is "High Pressure Torsion" (HPT). In the case of conventional deformation, such as cold rolling, cracks develop in the material with increasing processing parameters, which subsequently lead to the breaking of the workpiece. With HPT the deformation has no limits. Due to the high pressure in the process (up to 100 tons on a coin-sized sample), cracks are suppressed and even brittle materials such as glass or ceramics can be deformed. The high degrees of deformation primarily affect the strength of the materials. The hardness of copper, for example, is almost five times higher after HPT (from 0.3 GPa to 1.4 GPa)! The cause of these improvements is the existence and arrangement of the deformation-induced lattice defects, which include dislocations, planar defects, but also point defects (vacancies). The idea of this project was, to examine whether it is possible, to further improve this extremely high strengthening. For this purpose, the very high vacancy concentration, which is produced during the HPT deformation, was used. If the materials are slighly heated, vacancies can form agglomerates and thus lead to additional hardening of the sample. Although this effect is known in conventional metals, it was not clear if it can be used in such highly modified materials. For investigation various metals and alloys were deformed with HPT, and subsequently heated investigating the hardness. It turned out that this idea was a success. In deformed copper, an additional hardening of up to 8% can be achieved, in the case of nickel even up to 20%. The results in Mg with a hardening of up to 17% were especially interesting, as this led to the idea to investigate a biocompatible Mg alloy. The strength of medically usable metals plays an important role. The possibility of increasing the hardness without having to add additional alloy components is of great interest in medicine. In the course of this project, the hardness of the medically used Mg-Zn-Ca alloy was increased up to 35% by additional hardening. This development for biologically usable metals is very promising, as the alloying in biocompatible materials is limited due to health aspects.