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This study aimed to test empirically the developmental goal pursuit model of paediatric chronic pain, which draws upon Self-Determination Theory for understanding risks and resources for living wit...

Plastic effects during sheet metal forming can lead to undesirable distortions in formed components. Here, the three-stage work hardening and plastic strain recovery (“springback”) in a cold-rolled, α-phase commercially pure titanium is examined. Interrupted standard tensile tests with in situ x-ray diffraction and quasi-in situ electron backscatter diffraction show that twinning plays a minor role in both of these phenomena. The experiments give evidence that the observed work hardening plateau is the result of an abrupt activation and multiplication of 〈 c + a 〉 slip and a subsequent redistribution of load between grain families. The springback can be attributed to inelastic backwards motion and annihilation of dislocations, driven by backstresses from dislocation-based hardening during loading. The peak broadening behavior, observed by x-ray diffraction, suggests that the internal stress state is highest in the rolling direction, resulting in consistently higher springback magnitude along this direction.

Single-crystalline cast aluminium microwires with a diameter near 15 μm are characterised by Laue microdiffraction. A microwire in the as-cast condition exhibits a misorientation below 1∘ over a le...

This study aimed to test empirically the developmental goal pursuit model of paediatric chronic pain, which draws upon Self-Determination Theory for understanding risks and resources for living with chronic pain. This study examined the relationship between basic psychological need satisfaction (i.e. the satisfaction of the needs for autonomy, relatedness and competence) and the fear-avoidance model of pain in adolescents suffering from chronic pain. Hundred and twenty adolescents (mean age = 14.52, 71.6% female), receiving treatment through paediatric pain centres for chronic pain, were enrolled. Adolescents completed measures of basic psychological need satisfaction, fear and avoidance of pain, and pain-related functional impairment. Path analyses model indicated that higher levels of autonomy and competence satisfaction were associated with lower levels of functional disability, through the mediation of fear and avoidance of pain. Relatedness satisfaction was not significatively related to fear of pain, avoidance, and functional disability. The integration of Self-Determination Theory in the paediatric pain literature may further our understanding of potential resources for decreasing functional disability in children living with chronic pain.

The stress-induced fcc-austenite to hcp-martensite transformation in the iron based shape memory alloy (SMA) Fe-17Mn-5Si-10Cr-4Ni with and without VC precipitates is investigated by in-situ neutron diffraction measurements upon uniaxial loading and unloading. Based on experimentally derived elastic moduli the critical resolved shear stress (CRSS) for the fcc to hcp phase transformation was calculated. VC precipitates promote the martensite transformation by shifting the CRSS from 152 MPa to 85 MPa. A nearly perfect plastic behavior is found for the (220) grains with a high Schmid factor of 0.47. While (220), (111) and (200) oriented grains exhibit a phase transformation, (311) grains plastically deform solely by slip. During plastic deformation a load redistribution from soft behaving (220) grains to hard behaving (200) orientated grains takes place. The presence of VC precipitates leads to a broadening of the stress interval at which a martensite transformation is induced. This is explained by spatially heterogeneously distributed martensite transformation temperatures which are caused by VC precipitates. The microstructural reason for pseudo-elasticity is found to be a combination of back transformation from hcp to fcc and a reversible motion of Shockley partial dislocations.

This work reports on the microstructure and mechanical properties of porous silver films produced with different sintering parameters. The microstructure is investigated by high-resolution ptychographic X-ray computed tomography and scanning electron microscopy. The mechanical properties are obtained from constant strain rate tensile tests. It is found that for this class of samples both grain size and ligament diameter have little effect on the magnitude of the yield stress. Instead, a clear correlation is found between strength and porosity.

Three-dimensional finite element simulations of nano-porous silver structures are performed to understand the correlation between the porous morphology and the mechanical behavior. The nanostructures have been obtained from ptychographic X-ray computed tomography. The simulations allow distinguishing between the interplay and role of the ligament size, the pore morphology and the porosity, and therefore provide a better comprehension of the experimental observations. We show that the proposed model has a predictive character for mechanical behavior of nano-porous silver.

The stress and strain relationship in the gauge region of six cruciform geometries is studied: the ISO standard geometry with slits in arms, two geometries with thinned gauge areas, two geometries with thinned gauge areas and slits in arms, and one modified ISO standard geometry with slits in arms and a thinned gauge area. For all the geometries, finite element simulations are performed under uniaxial loading to compare the plastic strain, the von Mises stress distribution and the in-plane stress evolution. Results show that less plastic strain can be achieved in the gauge of the two ISO standard geometries. For the remaining cruciform geometries, a strong non-linear coupling between applied forces in arms and gauge stresses is generated. The evolution of this non-linear coupling depends on the geometry type, applied biaxial load ratio and the elastic-plastic properties of the material. Geometry selection criteria are proposed to reduce this non-linear coupling.

Precursors of failure are dislocation mechanisms at the nanoscale and dislocation organization at the mesoscale responsible for long-range internal stresses and lattice rotation. Detailed information on the link between both scales is missing, computationally and experimentally. Here we present a method based on x-ray Laue diffraction scanning providing time and sub -micron spatially resolved evolution of geometrical necessary dislocations in volumes that are similar to what advanced computational models can achieve. The approach is used to follow dislocation patterning during accumulation of fatigue cycles using a newly developed miniaturized shear device. Performed on Cu during cyclic shear, it reveals early dislocation patterning influenced by pre-existing dislocation structures. The quantitative information on non -homogeneous structure formation and its evolution corresponds to the need for synergies with continuum dislocation plasticity simulations of fatigue or any other type of plastic deformation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY -NC ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Dynamic recovery in nanocrystalline Ni and Ni50Fe50 with respective grain sizes of 65 rim and 15 nm is studied using stress reduction tests during in situ X-ray diffraction. The results are compared with a previous study on NC Ni with 35 nm grain size. Defect recovery by means of grain boundary processes plays an important role in controlling the strength evolution resulting in a constant flow stress during uniaxial deformation. The relative contributions of dislocation and grain boundary accommodation mechanisms are discussed in terms of grain size and alloying. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In this paper we report about the formation of intergranular strains in superalloy Haynes 282 studied by neutron diffraction during uniaxial tensile testing. The results gained from the initial bar material are compared to results of a peak-aged sample state at ambient temperature. A comparison of the results with data from a fully aged IN 718 alloy sample shows that intergranular strains are much lower in Haynes 282 than in the conventional nickel based superalloy. In contrast to IN 718 the formation of intergranular residual strains between bar material and heat treated sample state shows no significant differences. The relevance of the results to the macroscopic residual stress analysis by neutron diffraction on both continuous wavelength and spallation neutron sources is discussed in detail.

An in situ method to study the behavior of alloys during rapid heating and cooling combining laser heating with synchrotron micro X-ray diffraction and high-speed imaging is presented. Near-spherical Ti-48Al specimens were melted using a laser-based heating system consisting of two diode lasers and subsequently rapidly solidified using a Cu support structure. The Al-rich Ti-48Al forms a fine grained dual phase alpha(2) + gamma microstructure. Upon solidification, the formation of the beta-phase is fully suppressed. Time-resolved peak evolution and temperature-corrected micro X-ray diffraction data provides clear observations of solidification and solid state phase transformations under non-equilibrium conditions similar to beam-based additive manufacturing. (C) 2015 Elsevier Ltd. All rights reserved.

In situ multiaxial loading during neutron diffraction tests were undertaken on a low-alloyed Quenched and Partitioning (QP the amount of deformation-induced martensite is similar under uniaxial and equibiaxial tension but it is significantly smaller under tension/compression. The transformation kinetics are modeled using a recently developed kinetic model that accounts for the stress state and the stability and size of the austenite particles. The larger austenite particles transform first and the mean volume of the austenite particles decreases with increasing strain; the decreasing austenite particle size impedes the phase transformation as the deformation proceeds. It is concluded that stress triaxiality alone cannot account for the differences in the transformation kinetics between different loading states and that the number of potential nucleation sites depends on the stress state.

Indium Antimonide (InSb) single-crystalline micro-pillars were mechanically deformed by uniaxial compression loading-unloading cycles up to the beginning of the plastic regime. After deformation, 2D spatially resolved maps were collected via two X-Ray Diffraction (XRD) techniques: polychromatic micro-Laue and monochromatic micro-diffraction. In both techniques, the integrated diffracted intensity shows strong variations inside the pillar. Moreover, the shift and streaking of one spot in polychromatic XRD as well as the lattice strain and tilt components derived from monochromatic XRD reveal that the plastically deformed area is localized on the top of the pillar, in agreement with scanning electron microscopy images. The two XRD techniques are thus providing correlated but yet complementary information.

A water-quenched martensitic CuAlNi shape-memory alloy was investigated by a combination of coincidence Doppler broadening and positron-lifetime spectroscopy, supported by positron-lifetime calculations. We find a high defect concentration in the as-quenched samples. The positron-lifetime calculations suggest that the defects are not only single vacancies but also vacancies associated with dislocations and stacking faults. Annealing in the martensitic phase has no significant influence on the vacancy concentration but results in a different chemical environment around the vacancies. After aging in the austenitic phase the vacancy concentration decreases significantly.

Beam-based additive manufacturing (AM) typically involves high cooling rates in a range of 103–104 K/s. Therefore, new techniques are required to understand the non-equilibrium evolution of materials at appropriate time scales. Most technical alloys have not been optimized for such rapid solidification, and microstructural, phase, and elemental solubility behavior can be very different. In this work, the combination of complementary in situ synchrotron micro-x-ray diffraction (microXRD) and small angle x-ray scattering (SAXS) studies with laser-based heating and rapid cooling is presented as an approach to study alloy behavior under processing conditions similar to AM techniques. In rapidly solidified Ti-48Al, the full solidification and phase transformation sequences are observed using microXRD with high temporal resolution. The high cooling rates are achieved by fast heat extraction. Further, the temperature- and cooling rate-dependent precipitation of sub-nanometer clusters in an Al-Cu-Mg alloy can be studied by SAXS. The sensitivity of SAXS on the length scales of the newly formed phases allows their size and fraction to be determined. These techniques are unique tools to help provide a deeper understanding of underlying alloy behavior and its influence on resulting microstructures and properties after AM. Their availability to materials scientists is crucial for both in-depth investigations of novel alloys and also future production of high-quality parts using AM.

The microstructure of porous silver layers produced under different low temperature pressure-assisted sintering conditions is characterized and linked with the mechanical behavior studied insitu during X-ray diffraction. Peak profile analysis reveals important strain recovery and hardening mechanism during cyclic deformation. The competition between both mechanisms is discussed in terms of porosity and grain size. (C) 2015 Acts Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report on the diffraction peak shape evolution from cementite particles in three steel microstructures, recorded during in-situ tensile deformation at room temperature. We demonstrate that in a bainitic tempered steel the observed peak broadening and asymmetry of cementite is a purely elastic effect, related to the heterogeneity of the microstructure. This result warns against the indiscriminate use of peak profile analysis to derive information on plastic deformation in cementite. (C) 2014 Elsevier Ltd. All rights reserved.

The role of misfit stress in kinematic hardening under reversed straining of a Type 316H austenitic stainless steel has been investigated by using neutron diffraction combined with in situ deformation. Initial misfit stresses, often referred to an intergranular internal stresses, were created by the tensile pre-straining at high temperature. The misfit stresses at the length-scale of grain families, measured by neutron diffraction, were shown to be a function of the magnitude of the tensile pre-strain. The pre-strained specimens were further subjected to either continued (tensile) straining or reversed (compressive) straining at room temperature. In situ neutron diffraction measurements were undertaken to monitor the change of the misfit stresses during loading. The macroscopic stress–strain behaviour was used to derive isotropic and kinematic hardening stresses developed in the pre-strained specimens. Results show that the change of the transient softening stress towards a zero value is accompanied by a decrease in the change of the misfit stresses. A multi-scale self-consistent model has been developed to assist in understanding the measured change of the misfit stresses when subjecting the material to strain reversal. An important conclusion is that the origin of the kinematic hardening of Type 316H austenitic stainless steel arises from the misfit stress between grains.

The constant flow stress reached during uniaxial deformation of electrodeposited nanocrystalline Ni reflects a quasi-stationary balance between dislocation slip and grain boundary (GB) accommodation mechanisms. Stress reduction tests allow to suppress dislocation slip and bring recovery mechanisms into the foreground. When combined with in situ X-ray diffraction it can be shown that grain boundary recovery mechanisms play an important role in producing plastic strain while hardening the microstructure. This result has a significant consequence for the parameters of thermally activated glide of dislocations, such as athermal stress and activation volume, which are traditionally derived from stress/strain rate change tests. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Deformation mechanisms in nanocrystalline electrodeposited Ni (26 nm) and Ni-Fe (12 nm) are studied by strain rate sensitivity experiments and in situ mechanical testing during X-ray diffraction. We find a relatively low strength for Ni-Fe, which may be ascribed to a weakening of dislocation pinning points. Furthermore, the strain rate sensitivity of Ni-Fe is relatively low. The in situ studies allow the macroscopic mechanical response of Ni to be divided into three different deformation regimes: elastic, microplastic and macroplastic. For Ni-Fe, recovery mechanisms seem particularly important, resulting in an extended microplastic regime and necking before a state of homogeneous deformation can be reached. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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.

In-situ mechanical testing during X-ray diffraction is performed on electro-deposited nanocrystalline Ni. It is found that the evolution of inter-granular stresses with plastic deformation is altered compared to what is usually found for coarse-grained metals. The results are rationalized by insights gained from molecular dynamics and quantized crystal plasticity modelling.

International audience; A novel miniaturized biaxial deformation rig is presented. It allows one to apply in-plane biaxial stress states with arbitrary stress ratios and to perform strain path changes on thin-sheet metals. The device is optimized for in situ usage inside a scanning electron microscope and at synchrotron beam lines. The sample has a cruciform shape and the geometry is optimized with the aid of finite element simulations. A proof-of-principle experiment confirms the successful operation of this rig.

In situ Laue analysis during microcompression is carried out on Mo-alloy pillars containing defects due to focused ion beam milling or pre-strain. Independently of how the defects are introduced, if the diffraction peaks are streaked prior to deformation, slip starts in the direction matching the observed streaking, even if this corresponds to a low Schmid factor. The amount of pre-strain influences the deformation behaviour in terms of hardening and strain bursts. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In situ Laue analysis during microcompression reveals plasticity in [0 0 1]-oriented, directionally solidified Mo alloy pillars to start with slip on the {1 1 2} system having the highest Schmid factor followed by slip on the (1 1 0) plane containing the same Burgers vector. The results are interpreted in terms of the microstructure analyzed by scanning transmission electron microscopy and 3-D atom probe. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The present investigation aims to quantify the size and distribution of residual stresses remaining in rectilinear aluminium forged alloy blocks after they had been partially and fully heat treated. Various quench conditions were used, including water at different temperatures and polyoxyethylene glycol in two concentrations. The influence of standard and novel aging procedures, including retrogression and reaging, has been determined. Residual stresses were characterised using X-ray and neutron diffraction. Residual stress magnitudes were found to vary significantly with rapidity of cooling from the solution treatment temperature, with the subsequent aging treatments having a much smaller effect. The influence of heat treatments on the mechanical properties of the alloy was predicted by means of quench factor analysis and measured using indentation hardness and tensile testing. Quench factor analysis is also used to predict the change in lattice parameter arising from different quench paths in the strain...

The mechanical properties of electrodeposited nanocrystalline Ni–50 wt.% Fe are compared with those of electrodeposited Ni. Load–unload experiments and strain rate jump tests demonstrate the presence of a yield point after a change in strain rate in the plastic regime. The strain rate sensitivity is considerably reduced in Ni–Fe, in spite of the smaller grain size.

Neutron and synchrotron X-ray diffraction spectra have been acquired during room temperature tensile deformation of a creep-resistant bainitic 1% CrMoV steel, in order to study the evolution of internal microstresses and load-sharing mechanisms between the ferrite matrix and the various carbides. Cementite takes load from the plastifying matrix at the onset of macroscopic plasticity resulting in residual interphase stresses. Single peak fitting indicates an elastic anisotropic behaviour of cementite.

Micromechanics is a booming research area experiencing the development of new advanced testing methods at small dimensions. A relatively young but very popular technique involves uniaxial compressing micrometer and sub-micrometer sized objects, usually in the shape of pillars. Research in this field has focused mainly on exploring size effects in single crystal metals. This article demonstrates that Laue microdiffraction allows exploring in-situ the evolving microstructure in the transition regime from elasticity to plasticity, a feature that is not accessible with other techniques but which is essential for the understanding of small-scale plasticity.

White beam Laue micro-diffraction was performed on directionally solidified, single-crystal Mo pillars in the as-grown state, after focused ion beam (FIB) milling and after pre-straining. The Laue diffraction peaks from the as-grown pillars are very sharp and show no broadening, similar to those from single-crystal Si wafers. Significant broadening and streaking of the peaks occurred after FIB milling and pre-straining, indicative of the damage these treatments induce in the nearly perfect crystal structure of the directionally solidified Mo pillars.

The high frequency phonon properties of a computer-generated nanocrystalline (nc) Ni are investigated by directly calculating the onsite phonon Green function using a recursion technique based on a continued fraction representation. Past work found that a high frequency enhancement of the vibrational density of states exists and arises from localised vibrational modes forming within the nc grain boundary regions at atoms of reduced coordination [Phys. Rev. Lett. 92 (2004) p.035505]. In the present work, the positron wave function for such nanocrystalline grain boundaries was calculated and used to identify regions of localised free volume that, in turn, were found to correlate well with the localised vibrational modes. The work also demonstrates that, irrespective of the form of the central-force inter-atomic potential used, the observed effect can be more generally understood with respect to local variations in hydrostatic pressure within the grain boundary.

Single-crystal face-centered cubic metal pillars synthesized using a focused ion beam are reported to be stronger when compressed in smaller volumes. Using in situ Laue diffraction and crystal plasticity simulations it is shown that plastic deformation is initially controlled by the boundary constraints of the microcompression tests, followed by classical crystal plasticity for uniaxial compression. Taking the stress at which the change between the two modes occurs as strength of the pillar instead of the flow stress at a fixed amount of strain, the “smaller is stronger” trend is considerably reduced, if not eliminated, and what remains is a size dependence in strain hardening. The sizedependent increase in flow stress is a result of the early activation of multiple slip systems and thus the evolution of the microstructure during compression.

Laue micro-diffraction performed on metallic micropillars prior to deformation revealed the presence of strain gradients and planar defects in samples made by focused ion beam (FIB) milling. In situ Laue micro-diffraction shows that such pre-existing gradients can play a role in the determination of the first activated slip system, and thus leading to un-expected geometrical strengthening. Lattice rotations resulting in the formation of substructures are observed at stresses well below the strength of the pillars usually defined as the stress at 5% strain.

The Young's modulus of electrodeposited amorphous and nanocrystalline Ni–P with grain sizes ranging from 4 to 29 nm was examined by the nanoindentation technique. A continuous decrease in the Young's modulus with decreasing grain size followed by a smooth transition towards the modulus of the amorphous structure was observed. Positron annihilation lifetime spectroscopy measurements show that excess free volume exists in the interface regions of the nanocrystalline materials, predominantly with a size somewhat smaller than mono-vacancies. The associated effective increase in interatomic distances explains the contribution from the interfaces in leading to the observed reduction in Young's modulus at very small grain sizes. In addition, vacancy clusters was found to have negligible effect on the Young's modulus in nanocrystalline Ni–P.

Stress-dip experiments followed by creep have been performed on nanocrystalline Ni during in situ X-ray diffraction. The signatures of the peak profile at different creep stress levels demonstrate the presence of a creep mechanism that broadens the diffraction peak width and of a creep mechanism that reduces the peak width. The balance between the two mechanisms is discussed in terms of the applied creep stress.

The behavior of vacancies in selected coherent grain boundaries (GBs) in Fe and Ni is studied by means of molecular dynamics simulations. Corresponding positron lifetimes are calculated using the atomic superposition method. There is a difference between the vacancy behavior in Fe and Ni in dependence on temperature. In Ni, vacancies at GBs appear to diminish substantially their free volume (and lifetime) with the increasing temperature, which can be attributed to ‘vacancy delocalization’. Contrary, GB vacancies remain stable up to apparently higher temperatures in Fe.

By in situ neutron diffraction during tensile tests of a strongly textured Nb wire, we have shown the development of internal tensile microstresses of the order of 20 % of the flow stress, acting normal to the longitudinally aligned {110} planes. Such stresses are retained as residual stresses upon unloading. They must be taken into account for preventing splitting failures of high strength BCC wires (e.g., highly drawn tungsten filaments or pearlitic steel wires) that, because of the drawing fabrication process, contain a high density of longitudinally oriented brittle interfaces.

White beam Laue microdiffraction has been performed to investigate the initial microstructure of undeformed Au, Ni, Cu, and NiTi micropillars fabricated by focused ion beam milling. Various microstructural features have been shown, from which it is known that they contribute to classical hardening.

A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering instrument SANS-I at the SINQ neutron source. The polariser is needed for investigations of magnetic nanostructures as well as for spin contrast variation techniques relying on the spin-dependent neutron scattering length of polarised nuclei. The V-shaped polariser is installed in the first section of the collimator system of the SANS instrument and its performance is optimised for neutrons with a wavelength between 0.5 and 1.0 nm. For a precise polarisation analysis of a beam with selectable incident divergence, such as in SANS experiments, an opaque spin filter is ideal. We used a solid polarised proton target exploiting the strong spin-dependent neutron scattering cross-section of hydrogen and determined the neutron beam polarisation to a precision of δ p / p ∼ 0.5 % for different collimations in a broad wavelength band.

X-ray diffraction techniques allow the determination of mean grain size, root-mean-square strain and dislocation/twin content through peak profile analysis and the Williamson–Hall approach. The analysis is, however, based on assumptions that are questionable when applied to nanocrystalline (nc) structures. In the present work, two-theta X-ray diffraction profiles are calculated from multi-million atom computer-generated nc-Al samples containing a well-defined twin or dislocation content. The Williamson–Hall plots are compared with those obtained from in situ X-ray experiments performed on electrodeposited nc-Ni and on nc-Cu data available in the literature.

A change in strain path may have a significant effect on the mechanical response of metals. In order to understand or even predict the macroscopic behaviour under such conditions a detailed knowledge on the microstructural evolution is crucial. Yet relatively little work has been done to quantify and understand how the inter- and intragranular strains are affected during a change in strain path. In this work we present a new multiaxial deformation rig that allows performing in situ proportional and non proportional loading under neutron diffraction. We demonstrate the capabilities of this new setup for the case of a 316 L stainless steel. We show that the nature and magnitude of intergranular strain strongly depends on the applied stress state and demonstrate that micro yielding and internal strain recovery are responsible for the observed transient softening during a 90 strain path change. We anticipate that this new characterization method will provide previously inaccessible microstructural data that can serve as input for benchmarking current state-of-the-art crystal plasticity models. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd.

We investigate the positron response of simulated stacking fault tetrahedra in Cu. We find that in both the regular and truncated stacking fault tetrahedra positron lifetimes are about 10-15 ps larger than the bulk lifetime. This is an indication that the vacancy-like defects observed in neutron irradiated Cu by positron annihilation lifetime measurements do not arise during the formation of the stacking fault tetrahedra but rather during the interaction of the stacking fault tetrahedra with other defects. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation at selected tilt and twist grain boundaries in iron and nickel is examined theoretically. First the atomic structure of studied perfect and imperfect grain boundaries is obtained using molecular dynamics simulations. Characteristics of positrons trapped at such GBs are then calculated employing the atomic superposition method and are related to free volumes found at GBs. It is observed that in some cases vacancies introduced into ideal grain boundaries do not result in an increase of the positron lifetime.

Ultra short pulsed laser ablation is applied for preparing samples suitable for micromechanical testing. Laue microdiffraction shows that the micromachining hardly damages the initial microstructure. The technique has the potential to be a promising alternative to focused ion beam preparation for micromechanical objects since it is relatively fast, efficient and implants no ions. (C) 2015 Elsevier B.V. All rights reserved.