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

镁合金的强度低，通常低于人体骨骼的强度，因此限制了镁合金作为植入材料的应用。此外，当存在强化沉淀时，它们表现出较低的耐腐蚀性。为了避免这些缺点，高压扭转挤压（HPTE）将在项目中使用，以提高溶液退火合金的强度，因为（亚）晶界强化而不会形成沉淀，这在腐蚀性环境中可能是至关重要的。因此，本项目的主要目的是研究商用生物相容性镁合金在不同HPTE路径及后续热处理过程中的微观组织和织构演变，以及即使在腐蚀性环境下，已形成的微观组织和织构对单调和循环力学性能的影响。特别是，将进行以下工作：-建立最佳的HPTE加工条件（挤压和旋转速率，温度），用于制造商业生物相容性镁合金的大块完美样品。-研究由HPTE产生的梯度结构中的沉淀过程动力学。-评估最有希望的样品在空气和模拟血液的液体中的耐腐蚀性和疲劳性能，揭示HPTE在植入材料微观结构改性方面的潜力。- HPTE试件宏观疲劳性能与其微观组织状态的相关性。期望HPTE处理有利于镁合金在循环载荷下性能的提高，而不仅仅是组织的细化。更重要的是，由于HPTE的应变梯度，在挤压杆的外围形成了晶粒尺寸更细的梯度组织。本文还将阐明典型HPTE织构对变形中滑移系统激活的影响。影响HPTE加工过程中再结晶事件的金属间相的性质、存在和分布可能对耐腐蚀性有害，对此将仔细评估。这些项目目标需要采用创新的方法来实现：我们将采用高压扭转挤压剧烈塑性变形的新方法来获得大规模的大块超细晶试样。除了经典的衍射和金相分析外，我们将使用EBSD和ACOM-STEM对UFG镁合金进行晶粒结构分析，以量化晶粒尺寸，孪晶密度和GB特性。最后，我们将研究UFG试样在不同介质中的疲劳性能，并系统地探索以下微观组织参数的变化：晶粒尺寸和形状、梯度结构的存在、体积分数、金属间颗粒的尺寸和空间分布、位错和孪晶密度。

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.