C. Rentenberger

The influence of the fault energies on the anomalous mechanical behaviour of Ni3Al alloys

Single crystals of the intermetallic, L12 ordered alloy Ni78Al22 were deformed in compression at RT and at 400°C, a temperature below and within the anomalous regime. Transmission electron microscopic (TEM) images were used to analyse the dislocation structures. At RT edge dipoles are prevailing (as in f.c.c. metals) whereas at 400°C locked screws, screw dipoles and near screw dislocations bowed out on the cube cross-slip plane (010) are predominant. Their formation is caused by a gradual transition from “normal” octahedral cross-slip to the thermal activated cube cross-slip. By comparing the experimental weak-beam TEM images with computer simulations and using anisotropic elasticity theory the complete set of fault energies was determined: γCSF = 235 ± 40 mJ/m2, γAPB (111) = 175 ± 15 mJ/m2, γAPB (010) = 104 ± 15 mJ/m2 and γSISF = 6 ± 0.5 mJ/m2. These values can be used to explain the shift of the anomalous increase of the yield stress to higher temperatures observed in Ni3Al as compared with Ni3(Al, 1 at.%Ta). The value of R = γAPB(111)γAPB(010) determines the driving force for cube cross-slip (by comparing the R values of the two alloys the reverse behaviour of the shift might be expected). The value of γCSF is the decisive parameter, it determines the dissociation width and therefore the constriction energy of the Shockley partials of the screws. A low value of γCSF reduces the thermal activation necessary for the formation of KW locks and screws bowing out on (010).

The influence of pH on iron speciation in podzol extracts: Iron complexes with natural organic matter, and iron mineral nanoparticles

The quantities of natural organic matter (NOM) and associated iron (Fe) in soil extracts are known to increase with increasing extractant pH. However, it was unclear how the extraction pH affects Fe speciation for particles below 30 nm. We used flow field-flow fractionation (FlowFFF) and transmission electron microscopy (TEM) to investigate the association of Fe and trace elements with NOM and nanoparticulate iron (oxy)hydroxides in podzol extracts. For extracts prepared at the native soil pH (~4), and within a 1-30 nm size range, Fe was associated with NOM. In extracts with a pH≥7 from the E and B soil horizons, Fe was associated with NOM as well as with iron (oxy)hydroxide nanoparticles with a size of approximately 10 nm. The iron (oxy)hydroxide nanoparticles may have either formed within the soil extracts in response to the increase in pH, or they were mobilized from the soil. Additionally, pH shift experiments showed that iron (oxy)hydroxides formed when the native soil pH (~4) was increased to 9 following the extraction. The iron (oxy)hydroxide nanoparticles aggregated if the pH was decreased from 9 to 4. The speciation of Fe also influenced trace element speciation: lead was partly associated with the iron (oxy)hydroxides (when present), while copper binding to NOM remained unaffected by the presence of iron (oxy)hydroxide nanoparticles. The results of this study are important for interpreting the representativeness of soil extracts prepared at a pH other than the native soil pH, and for understanding the changes in Fe speciation that occur along a pH gradient.

Interplay between structural and magnetic properties of L10-FePt(001) thin films directly grown on MgO(001)

We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10 FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.

Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals.

A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80–400 nanometers) and grain sizes (50–220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts.

Electron microscopy of severely deformed L12 intermetallics

Severe plastic deformation (SPD) can be used to make bulk, nanostructured materials. Three L12 long-range ordered (LRO) intermetallic compounds were studied by TEM methods. The superlattice glide dislocations can dissociate according to two schemes: antiphase boundary (APB) coupled unit dislocations or superlattice intrinsic stacking fault (SISF) coupled super Shockley partials; both of them are analysed by weak-beam TEM methods. The nanostructures resulting from SPD carried out by high pressure torsion (HPT) are strongly affected by the different dissociation schemes of the dislocations. APB-dissociated superlattice dislocations and especially the APB tubes they form lead to the destruction of the LRO by HPT deformation as observed in Cu3Au and Ni3Al, whereas in Zr3Al heavily deformed (∼100,000% shear strain) at low temperatures the order is not destroyed since the deformation occurs by SISF-dissociated dislocations. In addition to the effects on the LRO the different dissociation schemes of the dislocations have a strong impact on the refinement and destruction of the crystalline structure by SPD. They seem to be decisive for the dynamic recovery considered as the limiting factor for the final grain sizes and the possibility of reaching amorphisation. Finally, the correlation between the reduction of the LRO and the structural refinement occurring during SPD is different in the three different alloys: In Cu3Au, the LRO is already strongly reduced before the structural refinement reaches saturation, in Ni3Al both are occurring simultaneously, whereas in Zr3Al, the formation of the nanograins does not seem to be connected with disordering.

Kinetics of defect recovery and long-range ordering in Ni3Al+B-I. Simultaneous recovery and ordering in cold-rolled material

Abstract Defect recovery and long-range ordering (LRO) during isochronal anealing of cold-rolled Ni 76 Al 24 + 0.19 at.% B (400 wt ppm) were studied by residual resistometry, transmission electron microscopy (TEM) and microhardness tests. The cold-rolling causes a decrease of the degree of LRO and an effective increase of the mobility of vacancies. The long-range ordering process during isochronal annealing proceeds, however, qualitatively in a similar way both in as-rolled and in homogenised samples. TEM observations revealed two stages of the defect recovery. Firstly, superlattice intrinsic stacking faults (SISF) of large density recovered almost completely in the temperature regime between 443 and 700 K showing that they are bounded by dislocations of opposite sign. Secondly, the recovery of antiphase-boundary (APB) dissociated superlattice dislocations occurred by the annihilation of dipoles within the whole temperature regime leading finally to a loss of all dislocations at 1273 K.

Radiation effects in bulk nanocrystalline FeAl alloy

Bulk-ordered nanocrystalline FeAl intermetallic compound with a grain size of 35 nm was prepared using severe plastic deformation. Nanocrystalline and coarse-grained counterparts with a grain size of 160 nm were subjected to 1.5 MeV Ar+ ion irradiation at room temperature. Enhanced irradiation resistance of nanocrystalline FeAl has clearly been identified by means of grazing-incidence X-ray diffraction and Mössbauer spectroscopy. At the identical damage dose, the nanocrystalline FeAl retains long-range ordering in the B2-superlattice structure, while the coarse-grained state becomes already substantially disordered. The present experimental studies verify that fully dense ordered intermetallic alloys are promising candidate materials for radiation environments.

TEM Study of Local Disordering: A Structural Phase Change Induced by High-Pressure Torsion

Long-range ordered intermetallic alloys with L12 (Ni3Al, Cu3Au) and B2 (FeAl) structures were deformed by high-pressure torsion at room temperature up to high grades of deformation. Transmission electron microscopy shows that disordering caused by the deformation occurs on a very local scale within coarse grains along glide planes (Cu3Au, Ni3Al) and in the form of well defined local regions (Ni3Al, FeAl). The latter leads to a duplex structure consisting of an ordered coarse-grained structure and a disordered nanocrystalline structure. The different mechanisms that can lead to disordering during severe plastic deformation are discussed on the basis of the different ordering energies and on the basis of antiphase boundaries associated with gliding dislocations. The results indicate that in intermetallic alloys the formation of a nanocrystalline structure by severe plastic deformation is facilitated by the loss of order.

Laser-Assisted Synthesis of Colloidal Ni/NiOx Core/Shell Nanoparticles in Water and Alcoholic Solvents

The nanosecond-pulse laser-assisted generation of Ni/NiOx core/shell nanoparticles (NPs) in water and alcoholic fluids can yield colloidal solutions without surfactants. The size distribution can be controlled by the nature of the alcohol, the number of laser pulses and the laser fluence. The incubation of the nickel target ablation in liquid contact shows a dependence on the carbon number of the respective alcohol. The laser-generated NPs consist of crystalline nickel cores with face-centred cubic patterns and stacking fault defects surrounded by nickel oxide shells. The solvent butanol, in contrast to ethanol and isopropanol, yields a narrow, nearly unimodal, size distribution. The majority of NPs have low size distributions, with medians in the range of 10-20 nm. These can be related to a metal ablation plume interacting with a supercritical liquid that decelerates the ejected material in a low-density metal-water mixing region. NPs in the range above 30 nm result in a minority distribution tail that strongly depends on the fluid nature, the pulse number and the fluence. This coarse NP set may be correlated with the rupture of a superheated molten-metal layer into larger entities.

Local, atomic-level elastic strain measurements of metallic glass thin films by electron diffraction.

A novel technique is used to measure the atomic-level elastic strain tensor of amorphous materials by tracking geometric changes of the first diffuse ring of selected area electron diffraction patterns (SAD). An automatic procedure, which includes locating the centre and fitting an ellipse to the diffuse ring with sub-pixel precision is developed for extracting the 2-dimensional strain tensor from the SAD patterns. Using this technique, atomic-level principal strains from micrometre-sized regions of freestanding amorphous Ti0.45Al0.55 thin films were measured during in-situ TEM tensile deformation. The thin films were deformed using MEMS based testing stages that allow simultaneous measurement of the macroscopic stress and strain. The calculated atomic-level principal strains show a linear dependence on the applied stress, and good correspondence with the measured macroscopic strains. The calculated Poissons ratio of 0.23 is reasonable for brittle metallic glasses. The technique yields a strain accuracy of about 1×10(-4) and shows the potential to obtain localized strain profiles/maps of amorphous thin film samples.