Nanocrystalline material

A nanocrystalline (NC) material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any long range order and conventional coarse-grained materials. Definitions vary, but nanocrystalline material is commonly defined as a crystallite (grain) size below 100 nm. Grain sizes from 100–500 nm are typically considered "ultrafine" grains. The grain size of a NC sample can be estimated using x-ray diffraction. In materials with very small grain sizes, the diffraction peaks will be broadened. This broadening can be related to a crystallite size using the Scherrer equation (applicable up to ~50 nm), a Williamson-Hall plot, or more sophisticated methods such as the Warren-Averbach method or computer modeling of the diffraction pattern. The crystallite size can be measured directly using transmission electron microscopy. Synthesis Nanocrystalline material

Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction

Stefan Brandstetter

Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unique physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, considerable effort has been put into uncovering the new deformation mechanisms that give rise to this outstanding response of nanocrystalline materials. Moreover, the production of nanocrystalline materials of reasonable sizes for structural applications remains a challenge. However, the size limitation is of no issue for their present application in the growing field of MEMS and NEMS devices. Ultimately, the reliability and lifetime prediction of these devices will depend on the accurate knowledge of their mechanical response. This dissertation addresses experimental and simulation procedures used to understand the fundamental deformation mechanisms operating in bulk nanocrystalline Nickel. Recent results from simulations suggested dislocations as a dominant carrier of plasticity in nanocrystalline materials. In contrast to coarse-grained materials, these dislocations are nucleated at grain boundaries and, after propagating through the nano grains, they are absorbed there as well. Deformation experiments during in-situ X-ray diffraction strengthened the predicted outcome from simulation but many open questions remained. Within this thesis a more extensive range of in-situ testing experiments are performed that aim to systematically investigate the nanocrystalline deformation mechanism in terms of both temperature and external loading conditions. The development of a low temperature tensile test set-up allowed to study temperature dependent behavior and revealed that dislocation activity in nanocrystalline Nickel is a strongly thermally activated process where propagation of dislocation seems to be as important as nucleation from dislocations at the grain boundary. This finding is further supported by strain-dip tests, which revealed that pinning points strongly influence dislocation propagation. Nanocrystalline Nickel exhibits, in its as prepared state, large internal stress variations. These stress variations and the small grain size are most likely responsible for the microplastic regime characterized by an extended macroscopic strain, making the usage of the classical definition of yield questionable. Furthermore it could be shown that upon annealing, which reduces the samples' internal stress, this extended microplastic regime was observed to be less pronounced. To study the structural stability of nanocrystalline Nickel at large strains, the material was investigated by compression experiments, revealing no changes in mean grain size. Furthermore, the three dimensional atomic probe technique was utilized for localizing impurity concentrations. The program was rounded by calculating diffraction peaks from simulated nanocrystalline structures with a single type of defect. This allowed investigating the characteristics of the diffraction pattern of nanocrystalline systems in a bottom up approach. Finally, the results of the thesis are discussed in terms of a thermally activated deformation mechanism that involves the nucleation, propagation and absorption of dislocation within the nanocrystalline environment.

EPFL2008

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

EPFL2016

Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction

Stefan Brandstetter

EPFL2008

Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction

Microstructure and mechanical properties of nanocrystalline Ni produced by three different synthesis techniques

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Microstructure and mechanical properties of nanocrystalline Ni produced by three different synthesis techniques

Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction