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

由于钨具有较低的室温断裂韧性和较高的脆塑性转变温度(BDTT)，使其不能作为结构材料使用，因此到目前为止，钨(W)仅被用作功能材料。因此，这里出现了如何使W具有延展性的问题。这项建议的作者评估的方法是合成一种由W箔制成的W层压板。冷轧W箔在延性和韧性方面具有非凡的性能。通过合成一种钨层压板，作者成功地将铝箔的性能转移到了块材上。此外，作者还生产出了因其热机械性能而令人信服的W层压管，并对其作为新型高温能量转换系统的结构部件进行了讨论。在此框架内，将确定冷轧引起的脆韧转变激活能HBDT的演化机制。Roberts和GUMBSCH对W单晶的BDT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，非常细晶的W材料具有较低的速率依赖性，从而增加了HBDT。这种行为的原因是晶界对塑性区的限制。这种限制导致位错在晶界堆积，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(激活邻近颗粒中的滑移系统)。然而，根据该模型，断裂韧性会通过细化晶粒而降低，而BDTT会通过细化晶粒来提高。但这与PIPPAN和作者的实验结果相矛盾。该项目的目的是确定冷轧过程中脆韧转变激活能演变的机制，并支持对多晶W脆韧转变的理解和认识。在此过程中，通过电子显微镜分析(EBSD，HR-EBSD，KAM，ECCI，TEM)直接识别机制，并通过测定脆韧转变激活能间接确定机制。最后，通过将电子显微镜分析的结果与脆韧转变激活能的演化进行比较，将解决HARTMAIER、Roberts、PIPPAN模型以及本文作者所提出的模型中的矛盾，并提出一个修正的、基于机理的W的脆韧转变模型。

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.