Coupled simulations of low temperature microstructural evolution in nanocrystalline metals

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Technical Abstract: From the earliest days of metalworking, it has been understood that quenching freezes in the desirable attributes of the finished object. However, the advent of nanocrystalline metals has offered significant counterexamples to this generalization, with substantial grain growth occurring in days, hours, even minutes at temperatures as low as 77K. The common denominator in these instances of low temperature evolution is abnormal grain growth (AGG), where a few grains grow very large at the expense of the more slowly evolving matrix grains. As yet, low temperature microstructural evolution has not been satisfactorily explained. Grain boundary motion is widely accepted to occur by thermally activated atomic processes, so that the rate of boundary motion should decrease exponentially as temperature decreases. However, a recent computational survey of grain boundary mobilities identified three new mobility categories that permit fast motion at low temperatures. In this project, a synthetic driving force molecular dynamics method will be applied to explore and characterize these fast, cold grain boundaries to confirm whether high mobility persists to low temperature; to discover the atomic mechanisms of low temperature boundary motion; and to determine the occurrence of these boundaries in real microstructures. This atomistic study will provide the first fundamental understanding of this newly revealed grain boundary motion regime. High mobility boundaries alone are not sufficient to cause the abnormal grain growth observed in nanocrystalline metals. AGG requires the concerted motion of most or all of the boundaries surrounding the abnormal grain, and as such is fundamentally a collective process within the grain boundary network. In this project, a mesoscale model of microstructural evolution, incorporating full grain and grain boundary structure and crystallography, will be coupled with the results of atomistic simulations of grain boundary motion to explore and characterize AGG in systems containing fast, cold boundaries. Initial simulations will address which microstructural features are required for AGG. Subsequent simulations will incorporate additional factors that enhance the frequency and rate of AGG. Finally the results will be used to develop analytical models for low temperature AGG. Low temperature grain growth in nanocrystalline metals is not just an academic problem. Implementation of nanocrystalline materials requires that the microstructure remains stable during service; microstructural evolution changes, and usually degrades, the desirable properties of these materials. Understanding low temperature evolution is the key to controlling it. Non-technical Abstract: We have all seen an image of a sword maker plunging a red-hot blade into cold water to freeze in its final structure. The same principle applies to many materials that we use in our everyday lives. Metal objects from a soup spoon to a car axle are manufactured using a series of heating and cooling operations, with the final structure frozen in by cooling to room temperature. Once the material has been quenched, we expect it not to change anymore, so it is an important scientific and technological issue when it does. Nanocrystalline metals are made from familiar ingredients - such as copper, nickel, or iron - processed into tiny crystallites one-millionth the size of a grain of sand that are packed closely together to form a solid substance. Because these new materials are harder and stronger than conventional metals, they may enable lighter airplanes and more reliable cars. But there is one major problem: scientists have observed that the structure of nanocrystalline metals changes over hours, days, or years at room temperature. In this project, we investigate how the structure of nanocrystalline metals evolves at low temperatures. We first use computer simulations of atomic motion to understand how individual interfaces between certain crystallites can move even at low temperatures. We then simulate the motion of large groups of interconnected interfaces to reveal how fast-moving interfaces can change the overall structure of the material. Understanding low temperature structural evolution is the key to controlling it. The goal of this project is to use computer simulations to develop that understanding.