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

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.

Microstructure and deformation mechanisms in nanocrystalline Ni-Fe. Part I. Microstructure

Steven Van Petegem, Helena Van Swygenhoven

We investigate various microstructural features of nanocrystalline electrodeposited Ni-Fe with a nominal iron content of 50% and compare them with the well-known nanocrystalline Ni system. In-depth electron microscopy analysis reveals the existence of large strain variations in the grain interior. Three-dimensional atom probe studies demonstrate no significant segregation to grain boundaries. However, significant deviations from a perfect chemically disordered system are found, which can be related to compositional variations of 5% over length scales of only a few nanometres. Finally, X-ray diffraction measurements suggest higher dislocation and twin density in Ni-Fe compared to Ni. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2013

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

Deformation mechanism in nanocrystalline FCC metals studied by atomistic simulations

Christian Brandl

Polycrystalline materials with crystallite diameters below hundred nanometer exhibit extraordinary strength which goes along with a decrease in ductility. In order to tailor tough materials, which combine strength and ductility, the underlying deformation mechanisms have to be understood. Molecular dynamics suggested partial dislocation mediated plasticity in nanocrystalline face-centred cubic (FCC) metals in terms of 1) dislocation nucleation at the grain boundaries, 2) dislocation propagation through the nano-grain, and 3) eventual dislocation absorption in the neighbouring grain boundary network. The partial dislocation nucleation sites were seen to exhibit stress concentrations such as in triple junctions. Moreover, it has been shown that the partial dislocation nucleation is energetically related to the so-called general stacking fault energies (GSFEs). Transmission electron microscopy, in-situ X-ray diffraction, and deformation experiments can be interpreted in terms of the aforementioned dislocation processes. On basis of this knowledge this thesis utilizes atomistic simulation methods to extend the understanding of the fundamental deformation mechanisms which mediate the plasticity in bulk nanocrystalline FCC metals. In the first part of this thesis, ab-initio simulations are exploited to investigate the effect of the superimposed strains in Ni, Al, and Cu on the GSFEs, which describe the energy penalty for rigidly shearing of the crystal. The energy curves are discussed in terms of a) the second and third order elastic constants b) the energy difference between hexagonal closed packed and face centred cubic structures, c) the GSFE / unstable-twinning-fault-energy interdependence and c) the implication on dislocation nucleation. The second part explores the strain rate, temperature, applied stress, and grain size dependence of nanocrystalline Al utilizing classical molecular dynamics simulations. The deformation behaviour is examined on the basis of the underlying deformation mechanism. The higher the strain rate the more the onset of dislocation mediated plasticity is delayed. This can manifest itself into a stress overshoot that diminishes as the strain rate is lowered. The associated critical resolved shear stresses are seen to cause the strain rate sensitivity in the flow stress. In conjunction with the rate dependence of the critical resolved shear stresses, the temperature dependence of the flow stresses supports the picture of thermally activated dislocation processes which control the deformation of nanocrystalline Al in molecular dynamics. Moreover, the strain rate simulations evidence that a subset of dislocation processes operate at the athermal limit which indicates, once more, the difficulty to extrapolate from the high strain rate regime of molecular dynamics towards common experimental conditions. Furthermore, using an iterative strain/relaxation method, athermal critical stresses are calculated for an isolated nano-grain with a nucleated dislocation therein. Dislocation cross-slip is seen to enable a dislocation to by-pass stress concentration, which can hinder the dislocation propagation through the grain, and allows the dislocation to be deposited in preferred grain regions as e.g. triple junctions. This rationalized the observed lack of strain hardening in the molecular dynamics study. Additionally dislocation cross-slip is more pronounced at lower strain rates which indicate the growing importance of the grain boundary structure for dislocation motion as the stresses are lowered. Slip transfer through a high angle grain boundary is observed and is understood in terms of the grain boundary structure, which exhibits regions of preferred dislocation transmission sites. Such structural features are discussed by means of the implication on mesoscopic models of dislocation transmission. Moreover, the grain size study shows discrete stress-drops, which are associated with dislocation mediated processes, in the so-called "inverse" Hall-Petch regime. The deformation behaviour of nanocrystalline Al in molecular dynamics is rationalized in terms of thermally activated processes, and therein the implications of the grain size dependencies are qualitatively discussed.