Brittle-to-ductile transition in tungsten single and polycrystals: Microsturcture and failure mechanisms

微观组织对所有脆性材料，特别是脆性金属的破坏模式和断裂韧性起决定性作用。晶界是容易断裂的路径，同时也是位错的障碍和来源。同样，预先存在的位错组织也会对裂纹尖端塑性和断裂韧性产生显著影响。因此，需要详细了解所有这些导致变形和断裂的微观机制，以便更好地控制材料对断裂和BDT的响应。由于对单晶钨的详细研究是可行的，所以选择这种材料作为我们的模型材料。在提出的研究项目中，钨的脆性和延性破坏机制将通过实验和建模相结合的方式进行研究。该项目从对钨单晶断裂的深刻理解开始，这在过去几十年的文献中得到了发展，申请人对此做出了重大贡献。新的研究计划旨在研究多晶材料和具有预先存在的位错微观结构的材料，这是朝着将在单晶上获得的知识应用于技术框架迈出的一步。与此同时，还有一些具有重大根本利益的未解决问题。例如：在没有脆性夹杂物的情况下，裂纹是如何产生的；在这一过程中，晶界或预先存在的错位林起什么作用？起裂过程是否决定主要是发生晶间断裂还是穿晶断裂？该项目的主要目标是对钨（单晶和）多晶的变形和损伤机制进行定性理解和定量描述。为了理解微观结构对脆性破坏向延性破坏转变的影响，需要对原子、介观和宏观长度和时间尺度进行理论和实验研究。该项目依赖于多尺度建模和实验工作的结合，因为只有两种方法在设计新的实验和模拟时提供相互指导，才能保证一个成功的研究项目。此外，模拟结果的实验验证和经验模型的关键输入参数的验证对于建模的进展至关重要。对钨多晶的断裂韧性和脆性到韧性转变进行物理上的定量描述是一个从基础和应用方面都很有吸引力的目标。根据文献中的知识状态和我们团队的经验，这个目标现在似乎触手可及。

The microstructure plays a decisive role for the failure mode and the resulting fracture toughness of all brittle materials and particularly for brittle metals. Grain boundaries are easy fracture paths and serve as obstacles and sources for dislocations at the same time. In a similar way a pre-existing dislocation microstructure will have a pronounced influence on crack-tip plasticity and fracture toughness. A detailed understanding of all these micro-mechanisms contributing to deformation and fracture is therefore required to better control materials response with respect to fracture and the BDT. Because of the availability of detailed studies of single crystalline tungsten this material is chosen as out model material. In the proposed research project the brittle and ductile failure mechanisms of tungsten will be investigated in a combined experimental and modeling effort. The projects starts from a solid understanding of fracture in tungsten single crystals that has been developed in the literature in the last decades, and to which the applicants contributed significantly. The new research initiative to investigate polycrystalline materials and materials with a pre-existing dislocation microstr-ucture is a Konsequent step towards making the knowledge gained on single crystals applicable in a technological framework. At the same time there is a number of unsolved questions of great fundamental interest. For example: How are cracks initiated in the absence of brittle inclusions; Which role do grain boundaries or pre-existing dislocation forests play in this process; Does the initiation process decide whether predominantly interor transgranular fracture occurs? The main objektive of this project is the development of qualitative understanding and a quantitative description of the deformation and damage mechanisms in tungsten (single and) polycrystals. Theoretical and experimental investigations on atomic, mesoscopic and macroscopic length and time scales are required for an understanding of the influence of microstructure on the transition from brittle to ductile failure. The project relies on a combination of multiscale modeling and experimental work, because only the mutual guidance both approaches provide in designing new experiments and simulations holds the promise for a successful research project. Moreover experimental verification of simulation results and validation of critical input pararneters for empirical models are essential for the progress in modeling. The development of a physically motivated quantitative description of fracture toughness and the brittle-to-ductile transition in tungsten polycrystals is a goal that is attraktive from fundamental as well as from applied aspects. Based on the state of knowledge in the literature and the experience of our team this goal seems now to be within reach.