Helena Van Swygenhoven

Deformation in nanocrystalline metals

I t i s n o w p o s si bl e t o s y n t h e si z e p ol y c r y s t alli n e m e t al s m a d e u p of g r ai ns that average less than 100 nm in size. Such nanocrystalline metals contain a significant volume fraction of interfacial regions separated by nearly perfect crystals. The small sizes involved limit the conventional operation of dislocation sources and thus a fundamental question arises: how do these materials deform plastically? We review the current views on deformation mechanisms in nanocrystalline, face-centered cubic metals based on insights gained by atomistic computer simulations. These insights are discussed with reference to recent striking experimental observations that can be compared with predictions made by the simulations.

On the Microstructure of Nanoporous Gold: An X-ray Diffraction Study

The evolution of the grain structure, internal strain, and the lattice misorientations of nanoporous gold during dealloying of bulk (3D) Ag-Au alloy samples was studied by various in situ and ex situ X-ray diffraction techniques including powder and Laue diffraction. The experiments reveal that the dealloying process preserves the original crystallographic structure but leads to a small spread in orientations within individual grains. Initially, most grains develop in-plane tensile stresses, which are partly released during further dealloying. Simultaneously, the feature size of the developing nanoporous structure increases with increasing dealloying time. Finally, microdiffraction experiments on dealloyed micron-sized nanoporous pillars reveal significant surface damage introduced by focused ion beam milling.

{110} Slip with {112} slip traces in bcc Tungsten

While propagation of dislocations in body centered cubic metals at low temperature is understood in terms of elementary steps on {110} planes, slip traces correspond often with other crystallographic or non-crystallographic planes. In the past, characterization of slip was limited to post-mortem electron microscopy and slip trace analysis on the sample surface. Here with in-situ Laue diffraction experiments during micro-compression we demonstrate that when two {110} planes containing the same slip direction experience the same resolved shear stress, sharp slip traces are observed on a {112} plane. When however the {110} planes are slightly differently stressed, macroscopic strain is measured on the individual planes and collective cross-slip is used to fulfill mechanical boundary conditions, resulting in a zig-zag or broad slip trace on the sample surface. We anticipate that such dynamics can occur in polycrystalline metals due to local inhomogeneous stress distributions and can cause unusual slip transfer among grains.

Strain rates in molecular dynamics simulations of nanocrystalline metals

To study deformation mechanisms using molecular dynamics, very high strain rates have to be applied. The effect of lowering the strain rate by two orders of magnitude on the deformation characteristics of nanocrystalline Al was investigated. For the highest strain rate, the onset of dislocation propagation is delayed, resulting in a stress overshoot. With decreasing strain rate the grain-averaged resolved shear stress reduces and cross-slip occurs more frequently. However, even at the lowest applied strain rate the grain boundary network can not accommodate all arriving dislocations, illustrating the challenge to determine the rate-limiting deformation mechanisms for experimental conditions.

Plastic Deformation in Metals with Nanosized Grains: Atomistic Simulations and Experiments

Molecular dynamics (MD) computer simulations of fully 3D-nanocrystalline (nc) metals with mean grain sizes up to 30 nm highlight the role of the structure and stress state of grain boundaries (GB) in the deformation mechanism by means of the atomic activity that plays a key role in GB sliding and the dislocations emitted from GBs. This paper discusses how the applied stress is accommodated by the nc-structure.

In Situ Biaxial Mechanical Testing at the Neutron Time-of-Flight Diffractometer POLDI

Complex strain paths are often applied to materials during production processes. This paper shows the first successful in-situ biaxial mechanical tests during neutron diffraction performed on a cruciform steel sample and reports on the differences compared to uniaxial deformation. Digital image correlation is demonstrated to be an appropriate tool to monitor spatially resolved the macroscopic straining. The new, modular biaxial machine that will be installed at the neutron diffractometer POLDI is presented.

Size Effects in Plasticity: Experiments and Simulations

Large scale computer simulations suggest that in nanocrystalline metals grain boundaries act as source and sink for dislocations. This suggestion has been the motivation for developing a new in-situ X-ray diffraction technique that allow peak profile analysis of several Bragg diffraction peaks during tensile deformation. Synergies between simulations and experiments are discussed including new applications of the in-situ technique.

POLDI: Materials Science and Engineering Instrument at SINQ

The time-of-flight (TOF) diffractometer POLDI (Pulse Overlap DIffractometer) operational at the continuous spallation source SINQ was specifically designed and optimised for the study of residual stresses and mechanical behaviour in engineering materials. The novel feature of POLDI compared to typical TOF instruments is the use of pulse overlap, whereby the faster neutrons of a pulse emitted from the slits of a chopper can catch up the slower neutrons from the previous slit. In this case, the flight time cannot be simply calculated by the arrival time of the neutron, but by recording the angular dependence of the neutron arrival time and the flight time of the neutron, the lattice spacing d can be determined [1]. In a plot of intensity vs. arrival time and scattering angle, each Bragg reflection is represented by a Bragg line (see Fig. 1). The slopes of these Bragg lines are given by Eq. (1):

Athermal critical stresses for dislocation propagation in nanocrystalline aluminium

Molecular dynamics simulations investigating the mechanical properties of nanocrystalline Al have revealed the importance of both dislocation nucleation at grain boundaries and subsequent propagation as possible rate-limiting microscopic plastic process. Here, we present an approach to determine the athermal stress–strain behaviour of selected dislocations as they propagate within the nanocrystalline environment. In the first case study, the critical resolved shear stress to depin resulting into a planar propagation is found to be 735 MPa. In a second case study, a pinned dislocation is found to cross-slip at a critical resolved shear stress of 372 MPa to circumvent a near unfavoured grain boundary region.

IN SITU TIME RESOLVED LAUE DIFFRACTION DURING MICRO-COMPRESSION EXPERIMENTS

We present in situ and ex situ Laue micro-diffraction experiments on micron-sized single crystal pillars. We show that the focused ion beam technique introduces measurable damage in Si pillars. The dynamics of the Laue patterns of Au pillars demonstrate the occurrence of crystal rotation and strengthening is explained by plasticity starting on a slip system that is geometrically not predicted but selected because of the character of the pre-existing strain gradient.