Christopher M. Barr

Tracking the evolution of intergranular corrosion through twin-related domains in grain boundary networks

Tailoring the grain boundary network is desired to improve grain boundary-dependent phenomena such as intergranular corrosion. An important grain boundary network descriptor in heavily twinned microstructures is the twin-related domain, a cluster of twin-related grains. We indicate the advantages of using twin-related domains and subsequent statistics to provide new insight into how a grain boundary networks respond to intergranular corrosion in a heavily twinned grain boundary engineered 316L stainless steel. The results highlight that intergranular corrosion is typically arrested inside twin-related domains at coherent twins or low-angle grain boundaries. Isolated scenarios exist, however, where intergranular corrosion propagation persists in the grain boundary network through higher-order twin-related boundaries.Steels: clustered coherent twins stop corrosionClustered twin grain boundaries in stainless steel can stop intergranular corrosion, but only if they are coherent. A team led by Mitra Taheri at Drexel University in the USA analyzed microstructural regions in a 316 stainless steel where all grain boundaries were twinned and found that, when the twins in these clusters were coherent or had a low misorientation angle, they arrested interganular corrosion. They emphasized this effect by engineering more coherent and low-angle grain boundaries with thermomechanical processing, leading to larger twin-related domains. In contrast, twinned clusters with high-angle grain boundaries consistently failed at resisting corrosion, a similar manner to the rest of steel. Twin-related domains may therefore be a good predictor of intergranular corrosion and may help us mitigate metal damage.

Defect Characterization in Irradiated Nanocrystalline Materials via Automated Crystal Orientation Mapping

Nanocrystalline metals and alloys are potential candidates for next generation nuclear reactors due to a high volume fraction of grain boundaries, which can act as efficient sinks for irradiation induced defects [1,2]. Present research is focused on understanding the annihilation and evolution of defects adjacent to grain boundaries and interfaces in irradiated materials [3,4]. Traditionally, transmission electron microscopy (TEM) has played a critical role in the evaluation of irradiation induced vacancy and interstitial dislocation loop size and density [5]. There are conventional methods of imaging defects in crystalline materials via TEM such as two-beam dynamical condition, down-zone imaging, and weak-beam dark-field microscopy which requires tilting of the specimen along desired crystal orientations [5]. However, specimen tilting along a definite crystal orientation is not easily achievable in nanocrystalline metals and alloys with grain sizes smaller than 100 nm. With the advent of automated crystal orientation mapping (ACOM) combined with the routine use of stable field-emission gun electron microscopes, it is now possible to map crystal orientations down to a nanometer spatial resolution without specimen tilting [6]. Here, we describe our efforts to develop new methods of defect characterization in nanocrystalline materials combining the use of ACOM with the conventional TEM defect imaging methods without employing specimen tilting. We explore the various defect imaging techniques and access the suitability of each technique for imaging defects in nanocrystalline materials. We use JEOL 2100F TEM operated at 200 kV having NanoMegas ASTAR precession diffraction system installed on it for all our experiments. We employ irradiated coarse and nanocrystalline grain Ni-5wt%Cr as our face-centered cubic model alloy system. Coarse and nanocrystalline grain specimens were irradiated at 20 MeV and 1.2 MeV, respectively, with Ni ions in Ion Beam Laboratory at Sandia National Laboratories.

Imaging radiation damage in nanoparticles for radiation therapies (Conference Presentation)

Nanomaterials have shown promise for a variety of medical applications due to their unique properties and form factors compared to their bulk counterparts. Several novel medical technologies leveraging these properties are in various stages of development for applications including drug delivery, anti-microbial, diagnostic, or therapy technologies. A subset of these technologies, namely radiation therapy applications, require the nanoparticles to retain their structure and properties in radiation environments. It has been demonstrated that nanoparticle irradiation response can vary greatly from bulk materials response, as damage effects become dominated by sputtering and surface effects. As such, the stability, or rather the resistance of these materials towards radiation-induced degradation needs to be well understood to gauge the efficacy of candidate nanoparticles for these applications. This presentation details ongoing efforts at the In-situ Ion Irradiation Transmission Electron Microscopy (I3TEM) facility at Sandia National Laboratories to study and characterize the structural evolution of nanoparticles utilizing both in-situ and ex-situ ion beam irradiation techniques. Materials systems of interest include CeO2 nanoparticles, used for protecting healthy cells from radiation damage, and Au and HfO2 nanoparticles, used to increase local dose from proton therapies. Observed nanoparticle responses were varied and included stability, coalescence, ablation, cratering, sputtering, and swelling, depending on particle species, morphology, and irradiation condition. This diversity in nanoparticle irradiation response demonstrates the need for additional systematic study to determine the ultimate usefulness of various nanoparticle species for radiation therapy applications.

Investigation of Grain Growth and Deformation in Nanocrystalline Metals Through In-situ TEM Mechanical Testing and Crystallographic Orientation Mapping

Evaluation of the long term mechanical response of nanocrystalline alloys is critical for potential use in structural engineering applications. While nanocrystalline metals have indicated increased fatigue and yield strength in comparison to coarse grain metals, there are a number of limiting factors including undesired mechanical and/or thermal induced grain growth as well as limited strain hardening capacity. To evaluate and correlate microstructural evolution during deformation in nanocrystalline metals, in-situ TEM mechanical testing has provided a number of useful insights on both grain boundary and dislocation based plasticity [1-3]. This paper demonstrates efforts to correlate localized dislocation and grain boundary evolution during in-situ deformation using both qualitative and quantitative mechanical testing approaches in a range of nanocrystalline FCC metals. Specific emphasis is placed on methodologies to correlate localized crystallographic orientations and grain growth during in-situ TEM mechanical testing for improved structure-property relationships.

The Perfect Cut: Focused Ion Beam Preparation for In Situ TEM

In situ TEM techniques have improved considerably in recent years with respect to their ability to understand materials behavior with high temporal and spatial resolution. While significant advances have been made in elucidating atomic-scale mechanisms that control properties of materials for a wide range of applications, geometric compromises made to accommodate in situ TEM experiments could play a detrimental role in the ability to apply data to “real-life” structures or devices.