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

非技术描述：在美国，材料断裂的经济影响以1983年美元计算为1190亿美元/年，强调了对断裂缓解策略的关键社会需求。晶间断裂是在整个汽车、航空航天、能源和石化工业中的结构材料中常见的特定失效模式。这种断裂模式与元素物质向将晶体的两个取向分开的区域（称为晶界（GB））的偏析相关联。由这些分离的物种形成的键的方向性被认为影响断裂发生的基本方式。我们将探讨债券是平行于GB，垂直于GB，或非定向，使用元素，如铬，钼，或硫，在铁的偏析。我们可以利用新的纳米级机械测试技术来证实这种效应。然而，将机械测试样品体积减小到纳米级本身可以改变断裂机制。我们可以使用辐照来保持真实和适当的断裂力学。这些结果潜在地确立了辐照作为减轻晶粒之间断裂和减少材料断裂对许多行业的影响的手段。我们还将与当地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学率和保留率，并满足STEM劳动力的需求。我们将在社区大学工程预科课程中，就断裂的社会重要性进行一堂课和互动作业。社区学院的学生也将通过为期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PI的服务在专业协会中分享，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：晶间断裂是结构合金和回收钢的常见失效途径，特别是那些受到高温操作或循环载荷的材料，在许多行业中。计算材料科学的最新进展为研究金属合金的晶界脆化和沿晶断裂提供了重要的理论依据。然而，实验方法并没有相应的进步，防止验证的机制，从而限制了我们的能力，以减轻这一关键fracturepath. Intergrainic断裂与溶质偏析晶界（GBs），这被认为是增加GB表面能和降低GB内聚能，最终在GB脱粘或解理。但是，有越来越多的理论证据表明，对于分离溶质表现出高度定向键与主机原子在/跨越GB，断裂不是由于简单的键断裂的参数，而是抑制位错成核。这些理论以前没有得到证实，因为在裂纹扩展过程中位错成核的直接观察是极其困难的。虽然最近发展的透射电子显微镜（TEM）原位力学测试，使这样的观察，减少试样体积抑制现实的断裂机制。然而，照射产生足够的缺陷密度，以减少试样的特征长度，使散装样的断裂机制。辐照还促使溶质偏析成GB。因此，我们利用辐射，使直接观察GB分离和/或位错成核的真实的时间与TEM原位断裂测试。这些结果将验证凝聚力有限元法（CFEM）模型开发预测能力的晶间断裂的基础上，粘结各向异性。该项目的成功可以导致基于辐照的方法的转型，以减轻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能，并建立一个具有广泛技能，能力和教育水平的多元化劳动力。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值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.