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

Purdue University, West Lafayette IN

Investigators

Abstract

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.