Traditional 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 SUMMARY
Of 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.
Metals deform plastically (permanently) through the creation and motion of specific defects called dislocations. The motion of these mis-stacked atomic structures, effectively partial planes of atoms, allow metals to bend rather than break. Conventional strengthening of metals involves making it harder to move dislocations by creating other defects, however this can only work up to a certain maximum strength. Creating ultra-strong metallic materials requires an understanding of both the mechanisms of creating dislocations and moving dislocations in new alloys. It usually takes more energy to make dislocations in a solid than to move them, and so ultra-strong structures are being designed that strive for atomistic perfection with defect free structures (such as metallic whiskers discovered in the 1950’s). However, one major problem with whiskers is while it’s very hard to start dislocations moving, it’s very easy to move them once created, making a material that is fails catastrophically rather than gradually. Almost any atomistic defect can strengthen a material, ranging from impurities to missing atoms in a material (a vacancy). Since atomic defects are stable and prevalent in most metals, they are a target way to achieve additional strength and to ensure strength continues past the initial onset of deformation. Several groups have hypothesized that vacancies play a role in the onset of plastic deformation. In this study we examined this hypothesis with both experiments and computational simulations. Students involved in this project discovered that vacancies do indeed make it easier to nucleate dislocations in otherwise dislocation free solids; simulations showed that a single missing atom can lower the strength of an otherwise perfect solid by over 10%. However, clusters of defects (di-vacancies, or stacking faults) can lower the strength by over 50%. These simulations match the experimental observations using nanoindentation to test dislocation nucleation. Experimentally we observed that nickel, processed to have high vacancy concentrations, exhibits the onset of plastic deformation over a range of stresses on the order of 25 – 50% lower than materials with very low vacancy concentrations. The key point discovered in this research is that we have determined the importance that hard to characterize defects (it’s challenging to find a missing atom, it’s the proverbial needle in a haystack) have on controlling the mechanical response of otherwise perfect solids. Therefore, as we design new metals to achieve ultra-strong alloys we must carefully consider all scales of defects. Concurrently with this study, students involved in the project were part of a program to deliver, free of charge, "materials kits" to junior high classrooms in the state of Washington. These kits, with 10 different materials and tools for measuring density and magnetic properties, were distributed to over 20 classrooms during the life of the grant, with on average 15 kits per classroom. The kits, which are designed to be used by pairs of junior high students in their science class, were made by the Washington State University Material Advantage student chapter, and target specific WA state learning goals so that teachers can "drop in" the materials kit activities in their classroom.
