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.

Solute segregation is used to limit grain growth in nanocrystalline metals, but this stabilization often breaks down at high temperatures. Amorphous intergranular films can form in certain alloys at sufficiently high temperatures, providing a possible alternative route to lower grain boundary energy and therefore limit grain growth. In this study, nanocrystalline Ni-W that is annealed at temperatures of 1000 °C and above, then rapidly quenched, is found to contain amorphous intergranular films. These complexions lead to a new, unexpected region of nanocrystalline stability at elevated temperatures.

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showing that annealing provides relaxation and hardens the alloys. The dash line represents strengthening from grain size reduction, which is much less significant than that from doping..22

Phone/Fax: þ86 10 623 32743Though carbon nanotube (CNT) arrays have tremendous potential due to their attractive mechanical, electrical, and thermal properties, the growth kinetics of CNTs are still not fully understood. Thus, we report on the effect of synthesis parameters, such as growth temperature, on the resulting arrays. In this work, CNT arrays were synthesized using catalytic chemical vapor deposition (CCVD) with furnace temperatures varying from 680 to 900 8C. Microscopy was used to investigate the effect of growth temperature on the structural properties, such as tube diameter, array length, and the amount of amorphous carbon produced at the top of the canopy as a growth by-product. Additionally, Raman spectroscopy was used to elucidate the effect growth temperature has on the resulting purity of the CNTs. It was then revealed that crystalline inhomogeneity exists along the length of the tubes with respect to crystallinity. Transmission electron microscopy (TEM) further determines the degree of tube crystallinity as well as the thickness of amorphous carbon coating around the nanotubes. Through both microscopy and spectroscopy, we found two distinct temperature regimes within the range of 680-900 8C. Below 800 8C, the growth of tube length and diameter remained relatively stagnant followed by a rapid growth rate above 800 8C with the highest tube crystallinity obtained within the regime of 800-840 8C. This indicates the presence of an important transitional temperature for CNT CCVD growth. Additionally, growth temperature was determined to play an important role in the amount of the resulting amorphous carbon by-product.

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability.In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance.

Chemical segregation and structural transitions at interfaces are important nanoscale phenomena, making them natural targets for atomistic modeling, yet interatomic potentials must be fit to secondary physical properties. To isolate the important factors that interatomic potentials must capture in order to accurately model such behavior, the performance of four interatomic potentials was evaluated for the Cu-Zr system, with experimental observations used to provide validation. While experimental results show strong Zr segregation to grain boundary regions and the formation of nanoscale amorphous complexions at high temperatures and/or dopant compositions, a variety of disparate behaviors can be observed in hybrid Monte Carlo/molecular dynamics simulations of doping, depending on the chosen potential. The potentials that are able to recreate the correct behavior accurately reproduce the enthalpy of mixing as well as the bond energies, providing a roadmap for the exploration of interfacial phenomena with atomistic modeling. Finally, we use our observations to find a reliable potential for the Ni-Zr system and show that this alloy should also be able to sustain amorphous complexions.

Nanocrystalline metals typically have high fatigue strengths, but low resistance to crack propagation. Amorphous intergranular films are disordered grain boundary complexions that have been shown to delay crack nucleation and slow crack propagation during monotonic loading by diffusing grain boundary strain concentrations, suggesting they may also be beneficial for fatigue properties. To probe this hypothesis, in situ transmission electron microscopy fatigue cycling is performed on Cu-1 at.% Zr thin films thermally treated to have either only ordered grain boundaries or to contain amorphous intergranular films. The sample with only ordered grain boundaries experienced grain coarsening at crack initiation followed by unsteady crack propagation and extensive nanocracking, whereas the sample containing amorphous intergranular films had no grain coarsening at crack initiation followed by steady crack propagation and distributed plastic activity. Microstructural design for control of these behaviors through simple thermal treatments can allow for the improvement of nanocrystalline metal fatigue toughness.