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Sliding Wear of Nanocrystalline Ni-W: Structural Evolution and the Apparent Breakdown of Archard Scaling
No full textSliding wear of nanocrystalline Ni–W alloys with grain sizes of 3–47 nm, a range which spans the transition in deformation mechanisms from intra- to inter-granular, has been studied through pin-on-disk wear testing. The extreme conditions produced during sliding wear are found to result in structural evolution and a deviation from Archard scaling for the finest grain sizes; in the finest nanocrystalline materials wear resistance is higher than would be expected based on hardness alone. The repetitive sliding load is found to lead to a modest amount of grain growth and grain boundary relaxation, which in turn leads to local hardening in the wear track. Analysis of the dynamic microstructure suggests that it is produced primarily as a result of local plasticity and is not principally due to frictional heating.United States. Army Research Office (contract W911NF-09-1-0422
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Nanocrystalline Al-Mg With Extreme Strength Due to Grain Boundary Doping
Get PDFA major difference between coarse-grained materials and nanocrystalline materials is the volume fraction of grain boundaries, which is the factor that alters nanocrystalline materials’ physical and mechanical properties. Mechanical properties are also changed due to the fact that dislocation movements are impeded by small grains and other enhanced stabilizing mechanisms (dislocation locking, Zener pinning, etc.), reducing the driving force for deformation. The understanding of physical phenomena that takes place in such microscopic level poses an extremely interesting area of study where grain boundaries can be manipulated to alter physical and mechanical properties of a material. This thesis focuses on grain boundary doping as a mean of manipulation. Nanocrystalline Al and Al-Mg alloys with an average grain size of 24 nm were used to isolate the effect of grain boundary doping on strength. To begin, Mg was added to the Al lattice using mechanical milling, a process that produces materials in nanocrystalline form. This was followed by annealing treatments to induce segregation of solute (Mg) to the grain boundary, therefore changing the overall energy state that affects material properties. A combination of energy dispersive spectroscopy and X-ray diffraction were used to quantify the composition of the grain interiors and grain boundaries, then the relative contributions of solid solution strengthening and grain boundary segregation were extracted. Our results showed that nanocrystalline Al -7 at. % Mg had a maximum hardness of 4.56 GPa, approximately three times the hardness of pure nanocrystalline Al with the same grain size. Microcompression experiments on the strongest powder alloys indicated a yield strength value of 865 MPa and a specific strength value of 329 kN⋅m/kg, making these materials among the strongest Al alloys ever made. This work confirms that grain boundary segregation lowers the overall energy state and increases strength
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Quantification of Grain Boundary Mediated Plasticity Mechanisms in Nanocrystalline Metals
Get PDFNanocrystalline metals have been a topic of great discussion over recent years due to their exceptional strengths and novel grain boundary-mediated deformation mechanisms. Their microstructures are known to evolve through dynamic processes such as grain boundary migration and grain rotation, but how the collective interaction of these mechanisms alter the microstructure on a larger scale is not completely understood. In this thesis, we present coupled atomistic modeling and experimental tasks that aim to understand how the grain structure, grain boundaries, and associated grain boundary network change during nanocrystalline plasticity. Due to the complex three-dimensional nature of these mechanisms and the limited spatial and temporal resolution of current in-situ experimental techniques, we turn to atomistic modeling to help understand the dynamics by which these mechanisms unfold. In order to provide a quantitative analysis of this behavior, we develop a tool which fully characterizes nanocrystalline microstructures in atomistic models and subsequently tracks their evolution during molecular dynamics simulations. We then use this algorithm to quantitatively track grain structure and boundary network evolution in plastically deformed nanocrystalline Al, finding that higher testing temperature and smaller average grain size results in increased evolution of grain structure with evidence of larger scale changes to the grain boundary network also taking place. This prompts us to extend our analysis technique to include full characterization of grain boundary networks and rigorous topographical feature identification. We then employ this tool on simulations of Al subject to monotonic tension, cycling loading, and simple annealing, and find that each case results in different evolution of the grain boundary network. Finally, our computational work is complemented synergistically by experimental analyses which track surface microstructure evolution during sliding wear of nanocrystalline Ni-W thin films. These experiments track the development of a surface grain growth layer which evolves through grain boundary mediated plasticity and we are able to make direct connections between this evolution and that which was observed in our simulation work. All of the findings of this thesis are a direct result of the dynamic and collective nature by which nanocrystalline materials deform
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Radiation Tolerant Interface Design and Complexion Dynamics via Atomistic Modeling
Get PDFTraditionally, grain boundary character in nanoscale materials has been tailored to maximize different types of mechanical behavior, whether it be exhibiting near-theoretical strengths or prolonging fracture by dramatically increasing a material’s ductility. As more complex systems develop for nuclear and other extreme environment applications, the need for these types of materials is quickly identified. Specifically, materials in a nuclear reactor need to be amended to extend their longevity to promote safety and reliable usage. One strategy for improving radiation tolerance is the design and control of internal interfaces in a material. Atomistic simulations can give insight into the foundational principles of grain boundary structure and formation.Two comprehensive simulation models are developed to bridge this gap with respect to radiation tolerant interfaces and structural transitions in binary alloy systems. Firstly, the radiation damage of an ordered grain boundary is compared to a disordered amorphous intergranular film, to investigate how interface thickness and free volume impacts point defect recombination. Collision cascades are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. Secondly, hybridMonte Carlo/molecular dynamics simulations are used to study segregation-induced intergranular film formation in Cu-Zr and Cu-Nb alloys. While Cu-Zr alloys form structurally disordered or amorphous films, second phases precipitate at the interfaces of Cu-Nb. Finally, the effect of free surfaces on dopant segregation and complexion formation is investigated for both alloys
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Grain boundary structure and interfacial complexions for the creation of tough, stable nanostructured metals
Get PDFNanocrystalline metals have been the focus of current literature due to their interesting mechanical properties. This is a result of having nanometer sized grains and high volume fraction of grain boundaries. While these materials have high strength, the large number of boundaries is also responsible for the limited ductility and thermal instability often observed for nanocrystalline systems. Despite the current efforts in the literature, these challenges still prevent widespread use of nanocrystalline metals in real engineering applications. In this thesis, we study these problems by focusing on tailoring the grain boundary structure and chemistry and propose a methodology that can be used to mitigate those challenges. First, we study the plastic flow and failure as a function of grain boundary volume fraction (i.e., grain size) using microcompression in a nanocrystalline Ni-W. Since grain boundary physics are extremely important here, we also study how the relaxation of nonequilibrium grain boundaries affects failure. We show that nanocrystalline metals with larger grain boundary volume fractions and relaxed boundary structures are stronger, but also more likely to fail prematurely through catastrophic shear banding. We also show that shear banding can create a dynamic microstructure leading to grain coarsening. A major take-away from this work is that disordered grain boundaries can actually be beneficial. Therefore, in the next study we introduce amorphous complexions, highly disordered grain boundaries, through grain boundary doping as an all-in-one solution to design against failure and thermal instability. We use nanocrystalline Cu with the addition of Zr as our model system to explore complexion engineering in these materials. High resolution transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy demonstrates segregation of Zr to the boundaries of Cu-Zr alloys created with mechanical alloying. This provided evidence for the formation of amorphous grain boundaries complexions under certain conditions. Microcompression and in-situ bending experiments are then used to quantify the effect of doping on mechanical behavior. Finally, our results show that strength, strain-to-failure, failure mode, and thermal stability can be controlled with boundary doping. The proposed methodology described here is rather general and can be applied to other material systems to make bulk nanocrystalline metals with improved mechanical properties
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Local Crystallographic Orientation Correlation Measurements Connecting the Processing and Properties of Face-Centered Cubic Metals
Get PDFOne of the important characteristics of crystalline microstructures is how the crystal orientation changes in space, often abruptly to form grain boundaries. Quantifying crystalline microstructure can be accomplished in many ways, ranging from measurements of grain size, to long range network analyses. For example, studies of how the density of grain boundaries affects strength have led to the discovery of both the Hall-Petch relation and its breakdown in nanocrystalline metals. In another case, measurements of grain boundary character have been central to the success of grain boundary engineering in improving corrosion resistance. Where the first field focuses on the quantity of grain boundaries, the second emphasizes their qualities. This sort of analytical division can be very productive because it defines focused research problems, but it also prescribes limits around the possible findings. This thesis bridges some of these inevitable gaps by applying more expansive local orientation correlation metrics to situations where they were not, or could not, be used in the past. Starting with the analytically simplest case, we measured the types of grain boundaries found in nanocrystalline metals prepared by different processing routines. This is of special interest because the extreme density of grain boundaries in nanocrystalline metals exaggerates the importance of their character. Despite its importance, practical limits on microscopy previously prevented most prior research from analyzing boundary character. The development of the grain boundary character distributions have been examined to provide insight into the mechanisms responsible for their formation. Next, we use longer range metrics to unravel how the complicated topology of grain boundary engineered microstructures is formed. Studying the grain boundary network topology of these materials helps to rationalize their processing and clarify several prior studies that relied on two-point metrics. Similar grain boundary engineered materials will then be used to explore how boundary type affects grain size strengthening. The result is a new measure of how much twin boundaries contribute to yield strength, which is important to understanding the strength of advanced materials with high twin fractions. The next chapter describes a thermomechanical method for grain boundary engineering nanocrystalline metals, where conventional techniques cannot be applied. Grain boundary network measurements of these materials are then applied to understand the mechanisms at work, revealing new information about the response of nanocrystalline metals to cyclic deformation. This has value for developing new processing methods and understanding changes that may occur during service. In each section, new insights are gained by applying different local orientation correlations than have been typical in prior inquiries
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Probing solute-grain boundary interactions in alloys
Get PDFGrain boundaries are important planar defects which influence a variety of bulk properties, while the structures and behaviors of grain boundaries can be altered by solute atoms to enable the design of beneficial material performance. Different interactions between solutes and grain boundaries exist. One example is that under certain conditions, solutes travel to grain boundaries and change grain boundary chemistry and structure, to vary the so-called “grain boundary complexion” state. Current studies on grain boundary complexions have been primarily limited to binary alloys. While in ternary or quaternary systems, multi-element segregation might be utilized to create thicker amorphous intergranular films (AIFs), a structural feature that helps toughen nanocrystalline alloys. Another example is the migration of grain boundaries at the presence of solutes, a situation in which there are both segregated dopants on boundaries and dopants as solid solution additions in the surrounding crystal. A promising dopant concentration might be found at which grain boundary motion in alloys can be changed and therefore leads to optimal microstructure beneficial for material performance. To provide guidance to experiments as well as explaining the physical mechanisms responsible for the experimental observations, atomistic modeling is chosen to address the challenges mentioned above. In this thesis, certain criteria for selecting appropriate interatomic potentials for modeling are first established. Then, the interfacial segregation and structural transition behavior in ternary alloys have been investigated and the possibility of forming thicker AIFs in certain ternary alloys by controlling the ratio of different dopant elements has been demonstrated, which could open up the opportunity of tuning the structure and properties of materials by co-doping. In the second part of the thesis, the migration of twin boundaries, basal-prismatic interfaces and conjugate twin boundaries (also referred to as twin tips in some cases) in pure Mg and Mg alloys has been studied. By understanding the effect of segregated dopants as well as randomly distributed dopants in the matrix on the motion of a group of special boundaries, it is able to alter the microstructure evolution by alloying and further tune the mechanical properties of Mg alloys
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Amorphous Intergranular Film Design Criteria and Application as Damage Tolerant Features in Nanocrystalline Alloys
Get PDFThe large grain boundary volume fraction of nanocrystalline metals can bestow desirable mechanical behaviors, but the same grain boundaries that impart these beneficial properties challenge their application. Excess surface energy due to the substantial interfacial volume drives grain growth, and thus a loss of the benefits gained by nanocrystalline grain sizes. Also, not all grain boundaries are equal, where material behaviors can fluctuate dramatically depending on the grain boundary structure and composition. New pathways to fundamentally alter the grain boundary structure are needed to stabilize the nanocrystalline grain size and leverage the full potential of nanocrystalline metals. “Complexion” is a term used to describe the phase-like behavior of grain boundaries, where grain boundaries, similar to bulk phases, can undergo discrete transitions in structure and composition based on external factors such as temperature. Amorphous intergranular films, a type of complexion, are of particular interest due to their ability to enhance both mechanical and radiation damage tolerance due to the excess free volume present in these structures. In this thesis, we seek to expand the current materials toolbox of alloys that can form amorphous intergranular films, and then investigate the impact of these unique damage tolerant features in applications where they can be leveraged. First, we propose a set of materials selection rules aimed at predicting the formation of amorphous intergranular films, and then apply these rules to discover new alloys that can form these features. In doing this, we also discover a counterintuitive, ultra-high temperature grain size stabilization regime driven by the formation of amorphous intergranular films. Next, we investigate the impact of these features in applications where damage tolerance is critical, particularly fatigue and radiation. We find that incorporation of amorphous intergranular films throughout the grain boundary network of nanocrystalline alloys increases plasticity preceding a fatigue crack, and dramatically improves radiation tolerance. In summary, through an array of sputtered, electroplated and ball-milled Cu and Ni-based alloys, specialized heat treatments, and electron microscopy techniques, we find that amorphous intergranular films can potentially be found in a large number of alloys, and can dramatically improve nanocrystalline alloy behaviors
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Grain Boundary Structure and Stability in Complex Concentrated Alloys
Get PDFComplex concentrated alloys have emerged as a promising class of materials, offering exceptional properties and unique microstructures enabled by novel design principles. While their chemical complexity is a defining and even tunable feature, it also presents major challenges in predicting defect stability, particularly at grain boundaries. The interfaces in these materials exhibit a dynamic interplay of structural and chemical disorder, leading to site competition, solute interactions, and local structural transitions—factors that collectively determine interfacial stability. Gaining a deeper understanding of these dynamic behaviors is essential for advancing complex concentrated alloys from concept to practical engineering solutions. This dissertation employs atomistic modeling to address this challenge, providing the spatial and chemical resolution necessary to capture the diverse local atomic environments within these materials. The first section investigates multi-component segregation and its impact on grain boundary structural transitions in body-centered cubic NbMoTaW. The results demonstrate that Nb is the dominant segregant, primarily driven by its ability to relieve local stress. However, strong chemical interactions can overcome structural relaxation in specific cases, leading to unexpected Ta and Mo segregation. At elevated temperatures, enthalpy-driven segregation gives way to chemical disorder, disrupting a structural transition that otherwise occurs readily in a pure metal. The investigation then extends to face-centered cubic CrCoNi, a ternary alloy known for its tendency to form local chemical ordering. At a Σ11 grain boundary, the formation and alignment of amplified concentration waves are observed, anchored by solute enrichment, intrinsic elastic distortions, and the alloy’s mixing tendencies. This nanoscale patterning creates a dense, uniform region that facilitates more accurate experimental detection of local chemical ordering and is also expected to induce a drag effect, slowing grain growth kinetics. To further assess this, the impact of this ordered region on radiation resistance is explored, demonstrating that enhanced near-boundary chemical ordering suppresses irradiation-induced boundary migration by restricting point defect mobility and accumulation. As damage accumulates and local chemical order degrades, this stabilizing effect diminishes. Overall, this work reveals how dynamic interactions among solutes and defect structures fundamentally govern grain boundary behavior in complex concentrated alloys, providing critical insights for the design and application of these advanced materials
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