Atomistic simulations are used to study segregation-induced intergranular film formation in CuZr and CuNb alloys. While CuZr forms structurally disordered or amorphous films, ordered films comprised of a second phase usually precipitate in CuNb, with a critical nucleation size of ~ 1 nm below which the ordered phase cannot form. While the ordered film is retained at high temperature for a low energy Ʃ11 (113) boundary, a disordering transition is observed for a high energy Σ5 (310) boundary at low dopant concentrations. Finally, the effect of free surfaces on dopant segregation and intergranular film formation is investigated for both alloys.Download high-res image (156KB)Download full-size image

Nanocrystalline Al-Mg alloys are used to isolate the effect of grain boundary doping on the strength of nanostructured metals. Mg is added during mechanical milling, followed by low homologous temperature annealing treatments to induce segregation without grain growth. Nanocrystalline Al -7 at% Mg that is annealed for 1 h at 200 °C is the strongest alloy fabricated, with a hardness of 4.56 GPa or approximately three times that of pure nanocrystalline Al. Micropillar compression experiments indicate a yield strength of 865 MPa and a specific strength of 329 kN m/kg, making this one of the strongest lightweight metals reported to date.

Complexions are phase-like interfacial features that can influence a wide variety of properties, but the ability to predict which material systems can sustain these features remains limited. Amorphous complexions are of particular interest due to their ability to enhance diffusion and damage tolerance mechanisms, as a result of the excess free volume present in these structures. In this paper, we propose a set of materials selection rules aimed at predicting the formation of amorphous complexions, with an emphasis on (1) encouraging the segregation of dopants to the interfaces and (2) lowering the formation energy for a glassy structure. To validate these predictions, binary Cu-rich metallic alloys encompassing a range of thermodynamic parameter values were created using sputter deposition and subsequently heat treated to allow for segregation and transformation of the boundary structure. All of the alloys studied here experienced dopant segregation to the grain boundary, but exhibited different interfacial structures. Cu-Zr and Cu-Hf formed nanoscale amorphous intergranular complexions while Cu-Nb and Cu-Mo retained crystalline order at their grain boundaries, which can mainly be attributed to differences in the enthalpy of mixing. Finally, using our newly formed materials selection rules, we extend our scope to a Ni-based alloy to further validate our hypothesis, as well as make predictions for a wide variety of transition metal alloys.Download high-res image (144KB)Download full-size image

•Grain boundary state controls the thermal stability and deformation of nano-grains.•Complexion theory can describe boundary structure and dopant segregation patterns.•Concurrent increases to strength and ductility may be possible with boundary design.Nanocrystalline metals have excellent strength due to the high density of grain boundaries inside. However, these same boundaries lead to limited thermal stability and a tendency to fail in a brittle manner, issues which limit the practical usage of these materials. Most strategies for stabilization of nano-grains against coarsening rely on the idea of using segregating dopants to lower excess boundary energy. The theory of interface complexions is a useful tool for describing the thermodynamics behind segregation as well as identifying distinct segregation patterns. Some of these same complexions can also dramatically alter mechanical behavior. Unlike past strategies, which always result in a trade-off between strength and ductility, the addition of complexions can potentially increase ductility while retaining or even increasing strength. In this paper, we discuss how complexions offer a unique opportunity to address these limitations simultaneously. In addition to reviewing the current-state-of-the-art, important areas where innovation is needed are also identified.

Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were created with mechanical alloying and their thermal stability was studied from 550–950°C. Annealing drove Zr segregation to the grain boundaries, which led to the formation of amorphous intergranular complexions at higher temperatures. Grain growth was retarded significantly, with 1 week of annealing at 950°C, or 98% of the solidus temperature, only leading to coarsening of the average grain size to 54 nm. The enhanced thermal stability can be connected to both a reduction in grain boundary energy with doping as well as the precipitation of ZrC particles. High mechanical strength is retained even after these aggressive heat treatments, showing that complexion engineering may be a viable path toward the fabrication of bulk nanostructured materials with excellent properties.

Solid solution effects on the strength of the finest nanocrystalline grain sizes are studied with molecular dynamics simulations of different Cu-based alloys. We find evidence of both solid solution strengthening and softening, with trends in strength controlled by how alloying affects the elastic modulus of the material. This behavior is consistent with a shift to collective grain boundary deformation physics, and provides a link between the mechanical behavior of very fine-grained nanocrystalline metals and metallic glasses.

Atomistic simulations have become a powerful tool in materials research due to the extremely fine spatial and temporal resolution provided by such techniques. To understand the fundamental principles that govern material behavior at the atomic scale and directly connect to experimental works, it is necessary to quantify the microstructure of materials simulated with atomistics. Specifically, quantitative tools for identifying crystallites, their crystallographic orientation, and overall sample texture do not currently exist. Here, we develop a post-processing algorithm capable of characterizing such features, while also documenting their evolution during a simulation. In addition, the data is presented in a way that parallels the visualization methods used in traditional experimental techniques. The utility of this algorithm is illustrated by analyzing several types of simulation cells that are commonly found in the atomistic modeling literature but could also be applied to a variety of other atomistic studies that require precise identification and tracking of microstructure.

The abrasive wear of nanocrystalline Ni–W alloys with grain sizes of 5–105 nm has been studied using Taber abrasion testing. The wear resistance of the finest grain size specimen is found to be higher than would be predicted based on hardness alone. This deviation from Archard scaling is traced to mechanically-driven structural evolution, consisting of grain growth and grain boundary relaxation, which occurs during wear. Comparison of these observations with previous wear studies suggests that the extent of structural evolution during wear depends on contact stresses and material removalrates.Highlights► Abrasive wear was studied in nanocrystalline Ni–W alloys with grain sizes of 5–105 nm. ► The finest grain size (5 nm) was found to wear less than predicted by Archard scaling. ► Structural evolution was observed near the surface due to local plasticity. ► Structural changes during Taber abrasion and pin-on-disk sliding are compared.

•The short-range order of amorphous intergranular complexions is studied.•Three distinct structural regions are found, depending on distance from the crystal.•Higher temperatures reduce the residual order from the crystal into the films.Amorphous materials lack long-range order but short-range order can still persist through the recurrence of similar local packing motifs. While the short-range order in bulk amorphous phases has been well studied and identified as an intrinsic factor determining the material properties, these features have not been studied in disordered intergranular complexions. In this work, the short-range order in two types of amorphous complexions is studied with a Voronoi tessellation method. Amorphous complexions can have three distinct regions: amorphous-crystalline interfaces, regions deep inside the films that have short-range order identical to a bulk amorphous phase, and transition regions that connect the first two regions. However, thin amorphous films contain only the amorphous-crystalline interface and the transition region, providing further evidence of the constraints imposed by the abutting crystals. The thickness of the transition region depends on film thickness at low temperatures but becomes thickness-independent at high temperatures. Similarly, the complexion short-range order is dependent on the interfacing crystal plane at low temperatures, but this effect is lost at high temperatures. Our findings show that amorphous complexions contain spatial gradients in short-range order, meaning they are both structurally and chemically different from bulk metallic glasses.Figure optionsDownload full-size imageDownload high-quality image (393 K)Download as PowerPoint slide