TECHNICAL: Changes in the local composition of metals and alloys are key to many fundamental aspects of materials science and engineering (MS&E). These variations may be large in compositional difference, but they are usually small in spatial extent, often occurring over a nanometer or less. When composition fluctuations are associated with defects in polycrystalline materials, they may induce deleterious property changes, such as the catastrophic brittle failure of metals and alloys via, for example, hydrogen-, temper-, or irradiation-enhanced embrittlement. Until recently, it has not been possible to measure these composition changes directly and relate them to the mechanical behavior, the crystallography, the electronic structure, and the defect structure of the material. However, such measurements are now possible. In a previous NSF grant, the PI has developed methods for mechanical testing of small volumes of material, including samples that can contain a single grain boundary. In two prior NSF grants, the co-PI has participated in the acquisition of two aberration-corrected transmission electron microscopes (TEMs) and has developed computer-processing methods to detect and quantify sub-nanometer scale composition changes at large numbers of grain boundaries (GBs) and precipitates. Initial data have been obtained in model systems and commercial alloys. The objective of this project is to explore and accurately quantify nanoscale composition changes associated with GB segregation in metals and alloys, and to establish the connections between GB character and mechanical behavior through direct testing and through modeling. The assembled team of international collaborators has combined skills that will enhance understanding of the complex interplay between GB crystallography and local chemistry via a unique combination of the most advanced electron microscopy/spectroscopy and 3D atom-probe methods with the latest first-principles simulations and in-situ mechanical testing techniques. The research will generate hitherto-unobtainable composition and mechanical behavior data associated with grain-boundary segregation, selecting specific interfaces from hundreds or even thousands that can now be analyzed for the first time. NON-TECHNICAL: The broader impact of this research will arise through the potential to revise the basic understanding of the role of composition variations on segregation and mechanical properties, which are central to the education and training of all MS&E students. These latest advances in materials characterization will enhance the research and education infrastructure and will be taught to many classes of Lehigh students and hundreds of industrial attendees at Lehigh?s Annual Microscopy School, now in its 38th year. The results will be disseminated to the materials community through technical presentations, publications and a new textbook. Improvements in the measurement of nanoscale elemental changes in metals and alloys have the potential to modify basic theories of segregation, and will lay the groundwork for similar studies of precipitation in the future. In turn, this knowledge may permit the re-design of standard fabrication and processing methods that control the properties of materials. Thus the long-term result may affect society in the broadest sense through improved metals and alloys for the physical infrastructure, the hydrogen economy, aerospace and automobiles.
The specific objectives of this program are: (1) to develop novel micro-scale mechanical testing techniques to shed light on the fracture behavior of pure and contaminated interfaces in metals and alloys, (2) to develop and refine scanning transmission electron microscope (STEM) analysis techniques that reveal the local orientation, structure, and chemistry at a grain boundary (GB), and (3) to use the mechanical test and grain boundary characterization techniques to explore and quantify the effects of nanoscale composition changes associated with grain boundary segregation in metals and alloys on mechanical behavior. The specific material system investigated was copper (Cu) with and without bismuth (Bi) impurities. Intellectual Merit A new method was developed and used for measuring mechanical properties such as the fracture toughness of model GBs inside a scanning electron microscope (SEM). Figure 1 compares close-up images of Cu microtensile specimens: (a) an intact beam, (b) a fractured clean GB (without Bi doping), and (c) a fractured Bi-doped GB. The microtensile specimen with the clean GB fractured after significant plastic deformation. In contrast, the Bi-doped specimen fractured in a brittle manner straight along the weakened GB. Two key outcomes of the mechanical testing portion of this project are: (1) Micrometer-scale notched tensile bar tests can be used to measure fracture toughness in specimens exhibiting ductile fracture. This is not true of most other micron-scale tests, suggesting that the tensile techniques developed here have a significantly broader range of potential applications even though they are difficult to perform. (2) Even in the case of the most brittle boundaries, some plastic deformation preceded fracture, indicating that twist boundaries may be inherently more fracture resistant than pure tilt boundaries or more general boundaries. This is a fundamental scientific finding that will inform future attempts to model boundary fracture. Several new analysis techniques have been developed for the electron microscopy characterization of grain boundary chemistry and structure. Pure and doped GBs were characterized by (i) direct observation of Bi atom distributions at the atomic scale in the vicinity of the GBs, (ii) quantitative measurement of Bi segregation at GB’s by X-ray nanoanalysis, (iii) characterization of the valence state of atoms at such GBs by using electron energy-loss spectrometry (EELS) in collaboration with Dr. Herzing (NIST), and (iv) local orientation measurement including boundary plane determination. With the exception of (iii), these groundbreaking characterization steps were all enabled by the use of a newly installed state-of-the-art aberration-corrected scanning transmission electron microscope (ac-STEM) JEOL JEM-ARM200CF instrument that was purchased through a previous NSF MRI grant (DMR-1040229, PI: M. Watanabe). Figure 2 shows an atomic-resolution high-angle annular dark-field (HAADF) STEM image in the vicinity of the Bi-doped GB. In this imaging mode, the Bi-atom distribution (the bright spots) can be directly seen in the vicinity of the GB. Three key outcomes for the electron microscopy portion of this project are: (1) Detection and resolution of single Bi atoms on Cu GBs is possible in the JEM-ARM200CF using two common imaging modes. (2) The confocal depth-sectioning STEM technique that has been developed has shown the ability to determine atomic resolution information in 3 dimensions. (3) By EELS analysis, certain spectral features of the Bi-doped boundary are slightly different from those from the Cu matrix, which implies that that electronic structure of Cu at the doped boundary could be modified by the presence of Bi atoms. Broader Impact The results of this study have been disseminated to the scientific community through 8 graduate student presentations, 62 presentations by the Co-PI and PI, 29 journal and conference publications, and 4 software products. The project has fully or partially supported the training of 3 graduate students in the areas of metallurgy, in-situ nanomechanical testing, and aberration corrected analytical electron microscopy. The first of these is a well-established and vital part of the U.S. economy, while the second and third are on the cutting edge of several materials fields. Three undergraduate students, one of whom is a female from an underrepresented ethnic group, performed independent research programs under the auspices of this program. During the early stages of this program the Lehigh group collaborated with a U.S. corporation, Hysitron, Inc., on development of in-situ test equipment and techniques. Faculty and graduate students, as well as undergraduate volunteers, participated in three K-12 outreach activities (CHOICES, NanoDays, and MatCamp). The CHOICES and NanoDays outreach activities both included a number of young students who are members of underrepresented groups. The CHOICES program is all female, whereas NanoDays draws from the general population of the Lehigh Valley, thereby including girls and boys from many different ethnic groups. NanoDays impacted over 1000 Lehigh Valley residents per year. Professional training was provided through the Lehigh Microscopy School, a summer course that extends knowledge of electron microscopy techniques to representatives from universities, national laboratories, and private companies.
