Fatigue and corrosion performance of model Mg-Zn-Zr alloys subjected to the High Pressure Torsion Extrusion

• Applicants: Dr. Julia Ivanisenko; Professor Dr. Eberhard Kerscher
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 446067715

Application of Mg alloys as implant materials is limited due to their low strength, which is often lower than the human bone strength. Furthermore, they demonstrate low corrosion resistance in the presence of strengthening precipitates. To avoid those disadvantages, High Pressure Torsion Extrusion (HPTE) will be used within the project to increase the strength of a solution annealed alloy due to (sub)grain boundary strengthening without forming precipitates which might be critical in corrosive environments. Therefore, the main aim of the project is to study the microstructure and texture evolution in a commercial biocompatible Mg alloy during different HPTE routes and subsequent thermal treatment, and the effect of the developed microstructure and texture on the monotonic and cyclic mechanical properties, even in corrosive environments. In particular, the following efforts will be undertaken: - Establishing the optimal HPTE processing conditions (extrusion and rotation rates, temperature) for manufacturing of bulk flawless specimen out of commercial biocompatible Mg alloy. - Study of the precipitation process kinetics in the gradient structure resulting from HPTE.- Evaluation of the corrosion resistance and fatigue properties of most promising specimens in air and in liquids mimicking the blood revealing the potential of HPTE for microstructure modification of implant material.- Correlation of the macroscopic fatigue performance of HPTE specimens with their microstructural state.It is expected that the HPTE processing will be beneficial for the enhancement of the Mg alloy performance under cyclic loading not only thanks to the microstructure refinement. More important is the formation of gradient microstructure with a finer grain size at the periphery of the extruded rod due to the strain gradient at HPTE. The influence of the typical HPTE texture, which will modify the activation of slip systems involved in the deformation, will be also elucidated. The nature, presence, and distribution of intermetallic precipitates, affecting the recrystallization events during the HPTE processing, might be detrimental in terms of corrosion resistance, what will be carefully evaluated.These project goals shall be reached by the use of innovative approaches: we are going to use a novel method of severe plastic deformation by high pressure torsion extrusion to obtain large scale bulk ultrafine grained specimens. In addition to classical diffraction and metallographic analysis, we will use EBSD and ACOM-STEM for the grain structure analysis of UFG Mg alloy to quantify grain size, twin density, and GB character. Finally, we will study the fatigue properties of UFG specimens in different media and systematically explore variations of the following microstructural parameters: grain size and shape, presence of gradient structure, volume fraction, size, and spatial distribution of intermetallic particles, dislocation and twin density.

• DFG Programme: Research Grants