W foil: Toughness - Identification of the mechanisms of the evolution of the activation energy of the brittle-to-ductile transition caused by cold rolling

到目前为止，钨(W)仅作为功能材料使用，因为其在室温下的低断裂韧性和高脆延性转变温度（BDTT）使其无法作为结构材料使用。那么问题来了，如何使W具有延展性。作者评估的方法是用W箔合成W层压板。冷轧W箔在延展性和韧性方面具有非凡的性能。通过合成W层压板，作者成功地将箔的性能转移到体上。此外，作者还生产了具有令人信服的热机械性能的W层压板管，并讨论了将其用作创新的高温能量转换系统的结构部件。基于作者对W层压板材料的工作，出现了几个科学问题。在本提案的框架内，将确定冷轧引起的脆性到韧性转变（HBDT）的活化能演变的机制。ROBERTS和GUMBSCH关于W单晶BDT的结果不一致，没有明确的科学模型。这种差异在多晶钨材料中继续存在。在这里，ROBERTS说晶界对BDT没有影响，而HARTMAIER的模拟结果表明，非常细粒度的W材料具有降低的速率依赖性，从而增加了HBDT。造成这种现象的原因是晶界限制了塑性区。这种约束导致位错在晶界处堆积，从而降低了迁移率。HARTMAIER现在假设HBDT可能是一种位错-晶界相互作用能（邻近晶粒中滑移系统的激活）。但根据该模型，晶粒细化会降低断裂韧性，晶粒细化会增加BDTT。但这与PIPPAN的实验结果和本提案的作者相矛盾。该项目的目的是确定冷轧过程中脆性到延性转变活化能演变的机制，并支持对多晶w脆性到延性转变的理解和认识。在此过程中，通过电子显微镜分析(EBSD, HR-EBSD， KAM， ECCI，TEM)和间接方式通过确定脆性到韧性转变的活化能。最后，通过将电镜分析结果与脆性-延性转变活化能的演化进行比较，解决了HARTMAIER、ROBERTS、PIPPAN以及本文作者模型中的矛盾，提出了一种修正的基于机理的W脆性-延性转变模型。

Up to now, tungsten (W) has only been used as a functional material, as its low fracture toughness at room temperature and its high brittle-to-ductile transition temperature (BDTT) exclude W from being used as a structural material. So here the question of how to make W ductile arises. The approach assessed by the author of this proposal is the synthesis of a W laminate made of W foil. Cold-rolled W foil has extraordinary properties in terms of ductility and toughness. Through the synthesis of a W laminate the author succeeded in transferring the properties of the foil to the bulk. Furthermore, the author produced W laminate pipes that are convincing due to their thermo-mechanical properties and are discussed with a view to being used as structural parts for innovative high-temperature energy conversion systems.Based on the work of the author on W laminate materials, several scientific questions arise. Within the framework of this proposal, the mechanism of the evolution of the activation energy of the brittle-to-ductile transition, HBDT, caused by cold rolling will be identified.Results on the BDT of W single crystals from ROBERTS and GUMBSCH are inconsistent and show no clear scientific model. This discrepancy continues for polycrystalline W materials. Here ROBERTS says that grain boundaries have no influence on the BDT, while simulation results from HARTMAIER show that very fine-grained W materials have a reduced rate dependence and thus an increased HBDT. The reason for this behaviour is the confinement of the plastic zone by grain boundaries. This confinement leads to dislocation pile-ups at the grain boundaries and thus reduced mobility. HARTMAIER now assumes that the HBDT might be a kind of dislocation-grain-boundary interaction energy (activation of slip systems in the neighbouring grains). However, according to this model, the fracture toughness would decrease through grain refinement and the BDTT would increase through grain refinement. But this is in conflict with the experimental results of PIPPAN and the author of this proposal.The aim of the project is to identify the mechanisms of the evolution of the activation energy of the brittle-to-ductile transition through cold rolling and to support the understanding and knowledge of the brittle-to-ductile transition of polycrystalline W. In doing this, the mechanisms are identified both in a direct manner through electron microscopy analyses (EBSD, HR-EBSD, KAM, ECCI, TEM) and in an indirect manner through the determination of the activation energy of the brittle-to-ductile transition. Finally, by comparing the results of the electron microscopy analyses with the evolution of the activation energy of the brittle-to-ductile transition, the conflicts in the models of HARTMAIER, ROBERTS, PIPPAN, as well as the author of this proposal, will be solved and a modified and mechanism-based model of the brittle-to-ductile transition of W will be presented.