Hydrogen as a temporary alloying element for the formation of specific microstructural gradients in the (alpha+beta) titanium alloy Ti-6Al-4V

工程部件在耐久性和可靠性方面的要求越来越高。同时，它们的设计、生产和应用应满足可持续、生态和经济的要求。为了满足这些要求，发展热化学或热机械工艺是必要的。由于钛合金的气体溶解度相对较高，暂时充氢作为所谓的热氢处理（THT）的一部分是可能的。由于氢引起弹性或（通过氢化物形成）塑性晶格变形，并且已知是一种强β稳定元素，因此与传统生成的钛合金微观结构相比，建立改进的微观结构有可能改善其机械性能。此外，THT适用于无法通过传统机械表面处理工艺进行表面处理的复杂几何形状。本研究旨在通过有目的的局部改变β晶粒尺寸和强化相的分布和形态（与表面距离的函数）来进行微观结构调整，特别是改善材料的疲劳性能。为了能够设计THT工艺，需要了解与初始微观结构相关的相稳定性、氢扩散系数和氢溶解度数据。根据这些数据，通过模拟计算设计THT工艺步骤，并通过系统的实验研究进行校核和调整。在此基础上，对单调加载和循环加载下的力学性能改善进行了实验研究。在研究项目的最后阶段，将使用代表真实组件的样品部件记录已开发的最佳THT工艺对复杂几何形状的可转移性。先前关于应用THT建立迄今为止无梯度的微观组织的研究表明，有目的地调整THT参数应该允许建立微观组织梯度，从而提高抗疲劳性。在这个关键时刻，两个概念似乎很有希望，即建立临时的氢浓度梯度和形成氢化物沉淀区。这两个概念将在计划的研究项目中进行调查。

Engineering components are subject to increasing demands with respect to durability and reliability. Simultaneously, their design, production and application should fulfil sustainable, ecological and economic demands. In order to meet these requirements, the development of thermochemical or thermomechanical processes is necessary. Because of the comparatively high gas solubility of titanium alloys, a temporary hydrogen charging as part of a so-called thermo-hydrogen treatment (THT) is possible. Since hydrogen causes elastic or (via hydride formation) plastic lattice deformation and is known to be a strong β-stabilizing element, the establishment of a modified microstructure is possible improving the mechanical properties as compared to conventionally generated microstructures of titanium alloys. Moreover, THT is applicable to complex geometries which cannot be surface-treated via conventional mechanical surface treatment processes. This study aims at a microstructural adjustment via a purposeful local change of the β grain size and the distribution and morphology of strengthening precipitates as a function of the distance to surface (microstructural gradient) causing in particular an improvement of the material’s fatigue properties.In order to be able to design the THT process, data on the phase stabilities, the hydrogen diffusion coefficient and the hydrogen solubility in dependence of the initial microstructure needs to be known. Based on this data, the THT process steps will be designed by means of simulation calculations and checked and adjusted by means of systematic experimental investigations. Subsequently, the improvement of the mechanical properties under monotonic and cyclic loading will be evaluated experimentally. In the final stage of the research project, the transferability of the best of the developed THT processes to complex geometries will be documented using a sample part, which represents a real component.Previous studies on the application of THT to establish a hitherto gradient-free microstructure show that a purposeful adjustment of the THT parameters should allow for an establishment of microstructure gradients, which cause an improvement of the fatigue resistance. In this juncture, two concepts seem to be promising, namely the establishment of a temporary hydrogen concentration gradient and the formation of a hydride precipitation zone. These two concepts shall be investigated in the research project planned.