Effects of Point Defects on Dislocation Nucleation in Metals

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TECHNICAL SUMMARYTraditional methods of strengthening materials often focus on increasing the resistance to plastic deformation by decreasing dislocation mobility. In many of the rapidly developing metallic materials that exhibit enhanced strengths, such as multilayered metals and ultrafine grained metals, the limiting factor is likely the creation and multiplication of dislocations, rather than the motion of dislocations. While there is substantial work on this topic by many groups around the world, most studies focus on pristine or extremely controlled structures, whereas most engineering alloys are complex, multicomponent systems that have prevalent defect structures. Exposure to radiation and existing non-equilibrium processing is particularly likely to generate excess vacancies and other point defects. This project will provide a fundamental study of the nucleation of and operation of sources dislocations with a wide range defect structures in metals using both experimental and computational studies. The team will experimentally quantify the impact of vacancies, impurities, and grain boundaries on dislocation nucleation and plasticity at extreme stresses in a variety of metallic systems using indentation techniques to probe the onset of plasticity. Point defect concentration will be assessed by positron porosimetry. Nanoindentation studies will also be used to demonstrate the impact of existing defects on the propagation of dislocations at the nm scale at high stresses, and these results will be compared to computational simulations using both molecular statics and dynamics. This coupling will develop stronger relationships between computational models of incipient plasticity and experimental studies through the development of multi-scale modeling techniques addressing both length and time scales.NON-TECHNICAL SUMMARYOf the many methods used by engineers and scientists to strengthen metallic materials there is increased emphasis on developing nanoscale structures that exhibit the -smaller is stronger- paradigm, where having a smaller length scale in the material provides more resistance to deformation. This project will focus on determining the fundamental effects of existing defects in metals on the onset of plasticity. There will be an experimental component, wherein the onset of plasticity is measured in a variety of metallic materials to quantify the ultimate strength of the material. These results will be compared to computational simulations developed to address both time and length scale issues in modeling the onset of permanent deformation. The graduate students supported will be partnered with a group of materials science and engineering undergraduates that have developed an outreach kit of materials for junior high students, and will gain experience in organizing teams of engineering students and distributing the kits to dozens of underserved classrooms around the region.