Deformation mechanisms of nanoporous hexagonal metals

In spite of the technical relevance of hexagonally close-packed (hcp) metals such as titanium (Ti) and magnesium (Mg), they have not been as widely used as ubiquitous face-centered cubic (fcc) and body-centered cubic (bcc) metals. The poor ductility of hcp metals due to the limited number of slip systems coupled with a medium range of tensile strength has been regarded as not a useful combination. Since worldwide energy-saving efforts were initiated, there has been continuous demand for the replacement of existing materials to light-weight materials in many applications. In addition, extension of the human lifetime requires the development of biocompatible polymers and metal alloys to replace the human organs and bones. As most of these requests can be fulfilled by hcp metals, these materials recently gained renewed research interests. However, the development of new materials with tailored mechanical properties suitable for retaining biocompatibility is still very challenging, as there are many scientifically unsolved problems. The aim of this joint project is to create a light-weight alloy with low stiffness well suited for application in the human body. Light weight and biocompatibility can be achieved by use of hcp metals such as Mg or Ti. Low elastic stiffness can be designed by creating a nanoporous structure. With the reduction of the ligament size in a porous metal, however, the mechanical properties are subject to significant change. Therefore, it is absolutely necessary to understand the size-dependent deformation mechanisms governing the mechanical properties of such nanoporous Mg and Ti alloys of different porosity in comparison to their massive counterparts. To properly address this, size-dependent micromechanical compression and tensile experiments will be conducted in-situ in a scanning electron microscope (SEM) to examine the size effects concerning the yield and hardening behavior of single crystal samples with sizes ranging from several m down to ~100 nm. These experiments will be complemented by in-situ transmission electron microscopy (TEM) experiments to determine a change in deformation mechanism, i.e. from twinning to dislocation plasticity. High resolution TEM of the initial material and the deformed samples will serve to determine the twinning mechanisms. Once this knowledge is established, the focus of the investigations will be shifted to nanoporous Mg and Ti alloys. Different porosity, pore size distribution, and accordingly different ligaments, will be determined by 3D scanning transmission electron microscopy (STEM). After a detailed TEM characterization of the nanoporous material, micromechanical in-situ testing will be carried out in TEM as well as SEM to measure the change of material stiffness and hardening behavior depending on the amount of porosity and investigate possible size dependent behavior and changes in deformation mechanisms compared to the massive bulk material. The last phase of the project is to hybridize the advantages of the two materials by infiltrating Mg into nanoporous Ti. The resultant optimized nanocomposite is again subject to a detailed analytical and in-situ SEM/TEM characterization to determine the mechanical properties and the governing deformation processes. A successful implementation of these tasks to unveil the nanoscale deformation mechanisms of nanoporous hcp materials requires collaboration between the Austrian (Montanuniversity Leoben) and Korean (POSTECH) groups. Both are highly specialized and gained distinguished expertise in the fields of in-situ SEM micromechanical testing and in-situ TEM deformation experiments, respectively, and a successful collaboration between the principal investigators over the last six years is documented by joint publications in high profile journals.

A better understanding of the size dependent deformation processes of miniaturized hexagonal metals, investigated as single crystals, porous foams, and composites, was in the focus of this project. Using highly resolved in situ testing methods in electron microscopes, we were able to gain new insights into the nature of the competing processes. As a prerequisite, we were successful in developing novel heat treatment techniques that allow us to fabricate defect free single crystal samples for later studies. Also, we succeeded in creating porous materials with adapted elastic properties using a space holder technique, as well as exceptionally strong nanocomposites using a severe plastic deformation route. The deformation behavior of these hexagonal materials in small dimensions turned out to be quite complex, as it depends not only on sample size, but also on crystal orientation and deformation rate, which was not realized before. Due to these interplays, basal slip proceeds by classic dislocations comparable to the behavior of face centered metals. The harder deformation by prismatic dislocations exhibits a significant thermally activated component to the flow stress, comparably to body centered metals. This mode is competing in critical resolved shear stress with twinning mediated plasticity. All three mechanisms exhibit size dependent strength, but with different scaling behavior. Moreover, while prismatic slip is strongly thermally activated, twinning is an essentially athermal process. Our results show that for hexagonal metals such as Magnesium or Titanium, by a designed reduction of the microstructure and a controlled adjustment of the deformation rate, the deformation behavior can be modified from the unwanted directional deformation by twinning to a more bulk-like homogenous deformation by prismatic dislocations. These novel insights are of importance for future applications, for example in the fields of micro forming or nanoarchitecture.