Steven Van Petegem

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.

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.

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):

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.

Size Effect in the Plasticity of Multiscale Nanofilamentary Cu/Nb Composite Wires During in-situ Tensile Tests Under Neutron Beam

Copper-based high strength nanofilamentary wires reinforced by bcc nanofilaments (Nb or Ta) are prepared by severe plastic deformation for the winding of high pulsed magnets. In-situ tensile tests under neutron beam were performed on a Cu/Nb nanocomposite composed of a multiscale Cu matrix embedding 55 4 Nb filaments with a diameter of 267 nm and spacing of 45 nm. The evolution of elastic strains for individual lattice plane in each phase and peak profiles in the copper matrix versus applied stress evidenced the co-deformation behavior with different elastic-plastic regimes and load sharing: the Cu matrix exhibits size effect in the finest channels while the Nb nanowhiskers remain elastic up to the macroscopic failure, with a strong load transfer from the copper matrix onto zones that are still in the elastic regime. Taking into account results from residual lattice strains also determined by neutron diffraction, the yield stress in the finest Cu channels is in agreement with calculations based on a single dislocation regime.

Microstructure and Defect Characterization of Nanostructured Ni3Al

Nanostructured Ni3Al was produced by the inert gas condensation and in situ compaction technique and characterized by means of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and density measurements. The defect structure was investigated using positron annihilation lifetime spectroscopy (PALS). It is shown that in some samples besides the cubic also the martensitic phase can be present. The defect structure can be divided into three major components: vacancy-like defects in the grain boundaries and nano-voids with a size of 1 nm as seen with PALS, and large pores with sizes up to 8 nm as seen with HRTEM. Furthermore, it is shown that an increasing compaction temperature leads to significantly smaller nano-voids.