CAREER: Using Irradiation to Understand Intergranular Fracture Mechanisms of Anisotropically-Bonded Solute Segregants

非技术描述：在美国，材料断裂造成的经济影响按1983年的美元计算为1190亿美元/年，强调了社会对缓解断裂战略的迫切需求。沿晶断裂是汽车、航空航天、能源、石化等行业结构材料中常见的一种特殊失效模式。这种断裂模式与元素物种偏析到分隔晶体两个取向的区域有关，称为晶界(GB)。这些分离物种形成的键的方向性被认为影响了断裂发生的基本方式。我们将探索平行于国标、垂直于国标或非定向的键，使用铁中的铬、钼或硫等元素的分离。我们可以利用新颖的纳米机械测试技术来证实这一效应。然而，将力学测试样品的体积减少到纳米级，本身就会改变断裂机制。我们可以使用辐射来保持现实和适当的断裂力学。这些结果潜在地确立了辐射作为一种手段来减轻颗粒之间的断裂，并减少材料断裂对许多行业的影响。我们还将与当地一所社区学院建立研究和教育合作伙伴关系，以提高社区学院学生的转校率和保留率，并满足STEM劳动力的需求。我们将在一门未开工的社区大学课程中，就骨折的社会重要性进行授课和互动作业。社区学院的学生还将通过为期11周的暑期研究实习直接参与这项研究。该项目的最佳实践将通过专业协会的服务共享，这种大学-社区学院合作模式可以扩展到其他大学。技术描述：沿晶断裂是结构合金和回收钢中的一种常见失效途径，特别是在许多行业中，那些经受高温操作或循环加载的材料。计算材料科学的最新进展为金属合金的晶界脆化和沿晶断裂提供了巨大的洞察力。然而，实验方法并没有相应地取得进展，阻碍了机制的验证，从而限制了我们缓解这一关键骨折路径的能力。晶间断裂与溶质向晶界(GBs)的偏聚有关，这被认为增加了晶界的表面能，降低了晶界的内聚能，最终导致晶界解聚或解理。但越来越多的理论证据表明，对于在GB处/跨GB处与主体原子表现出高方向性键的偏析溶质，断裂不是简单的键断裂争论，而是位错成核的抑制。这些理论以前没有得到验证，因为在裂纹扩展过程中直接观察位错成核是极其困难的。虽然最近发展的透射电子显微镜(TEM)原位机械测试使这种观察成为可能，但减少的样品体积阻碍了现实的断裂机制。然而，辐照产生了足够的缺陷密度，从而缩短了样品的特征长度，从而实现了块状断裂机制。辐照还会促使溶质偏析到GB。因此，我们利用辐照技术，通过透射电子显微镜原位断口测试，实时地直接观察到晶界的分离和/或位错成核。这些结果将验证内聚力有限元方法(CFEM)模型，以开发基于结合各向异性的沿晶断裂预测能力。该项目的成功可导致采用基于辐射的变革性方法来减轻GB脆化。我们还开发了一个大学-社区学院研究和教育合作伙伴关系的模式，以提高社区学院学生的效率，并建立一支拥有广泛技能、能力和教育水平的多元化劳动力队伍。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL DESCRIPTION:The economic impact of materials' fracture in the United States is $119 billion/year in 1983 dollars, underscoring the critical societal need for fracture mitigation strategies. Intergranular fracture is a specific failure mode common in structural materials throughout the automotive, aerospace, energy, and petrochemical industries. This fracture mode is associated with segregation of elemental species to the area that separates two orientations of crystals, referred to as a grain boundary (GB). The directionality of the bonds formed by these segregated species are believed to influence the fundamental manner in which the fracture occurs. We will explore bonds that are parallel to the GB, perpendicular to the GB, or non-directional, using segregation of elements such as chromium, molybdenum, or sulfur, in iron. We can utilize novel, nano-sized mechanical testing techniques to confirm this effect. However, reducing the mechanical test sample volume to the nano-scale can itself alter the fracture mechanism. We can use irradiation to maintain realistic and appropriate fracture mechanics. These results potentially establish irradiation as a means to alleviate fracture between grains and reduce the impact of materials' fracture across numerous industries. We will also create a research and educational partnership with a local community college in order to boost community college student transfer and retention rates, and fill a STEM workforce need. We will give a lesson and interactive assignment on the societal importance of fracture in a pre-engineering community college course. Community college students will also participate directly in this research through an 11-week summer research internship. Best practices from this program will be shared via the PI's service in professional societies, and this model for a university-community college partnership can be extended to other universities.TECHNICAL DESCRIPTION:Intergranular fracture is a common failure pathway in structural alloys and recycled steels, especially those subject to high temperature operation or cyclic loading, in a multitude of industries. Recent advancements in computational materials science have provided tremendous insight into grain boundary embrittlement and intergranular fracture of metallic alloys. However, experimental approaches have not advanced correspondingly, preventing validation of mechanisms and hence limiting our ability to mitigate this crucial fracture path. Intergranular fracture is associated with solute segregation to grain boundaries (GBs), which is believed to increase the GB surface energy and decrease the GB cohesive energy, culminating in GB decohesion or cleavage. But there is mounting theoretical evidence that for segregating solutes exhibiting highly directional bonds with host atoms at/across the GB, fracture is not attributed to simple bond-breaking arguments, but rather to the inhibition of dislocation nucleation. These theories have not previously been verified because the direct observation of dislocation nucleation during crack propagation is extremely difficult. While the recent development of transmission electron microscopic (TEM) in situ mechanical testing enables such observations, reduced specimen volumes inhibit realistic fracture mechanisms. However, irradiation produces a sufficient defect density so as to reduce the characteristic length of the specimen, enabling bulk-like fracture mechanisms. Irradiation also drives solute segregation to GBs. Hence, we utilize irradiation to enable direct observation of GB separation and/or dislocation nucleation in real time with TEM in situ fracture testing. These results will validate cohesive finite element method (CFEM) models to develop predictive capabilities for intergranular fracture based on bonding anisotropies. Success of this project can lead to transformational irradiation-based approaches for mitigating GB embrittlement. We also develop a model for a university-community college research and education partnership to enhance community college student efficacy and build a diverse workforce having a breadth of skills, abilities, and education levels.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.