Volume 98, Issue 8

This paper presents the results of an in-situ Scanning Electron Microscopy study of the local strain partitioning between ferrite- and martensite-rich regions in a commercial dual-phase steel. A Scanning Electron Microscopy tensile micro-stage, coupled with strain measurement methodologies based on gold micro-grids and digital image correlation, has been used to measure inhomogeneous strain fields at the micron scale. It has been found that when martensite is distributed non-uniformly, local strain partitioning depends significantly on the local spatial phase distribution and morphology. Strain distribution maps can be developed which provide valuable information about local strain paths for both phases. The results suggest that a rather detailed description of the two-phase microstructure of such materials is needed in order to fully understand their mechanical behaviour.

We dedicate this work to Wolfgang Pompe, a very good friend and a good friend of the Materials Faculty at UCSB where he and his wife, Gisela, shared their lives and fellowship. Three different zirconia – alumina laminates, AZ50, AZ80, and AZ95, were fabricated via tape-casting thick Zr(3Y)O 2 layers, dip-coating the zirconia tapes in a slurry containing a mixture of zirconia and alumina, stacking and bonding the coated zirconia tapes, followed by densification. Each composite had a different compressive stress by using different mixtures of alumina and zirconia (the number associated with each composite system refers to the volume fraction of alumina in the thin, compressive layers). After densification, the Zr(3Y)O 2 layers were ∼ 425 μm thick, and the thin alumina/zirconia layers were 60 ∼ 65 μm thick. The threshold strengths, below which the probability of failure is zero, were determined to be 255 ± 8 MPa, 311 ± 7 MPa, and 421 ± 12 MPa for AZ50, AZ80, and AZ95 laminates, respectively. These values are about 60 ∼ 70 % of those calculated from a previously reported function. The surface and interior stresses were determined using a finite element analysis. The compressive stresses of the outer alumina/zirconia layers were determined using a piezospectroscopy method. The surface stresses for the thick zirconia layers were estimated using an indentation – crack length relation. The piezospectroscopy, analytical and finite element analysis results for the outer compressive layer were in good agreement. But, the finite element analysis showed that the compressive stress component on the surface of the compressive layers was approximately half the value of the biaxial compressive stresses deep within the thin, compressive layers. It appears that the much smaller compressive stresses on the surface of the compressive layers is one important factor that resulted in the lower threshold strength than predicted by the previously developed function used to estimate the threshold strength.

Thin compressive layers sandwiched between thicker tensile layers in laminar ceramics can stop unstable cracks. As shown by Lange and co-workers, the arrested crack propagates stably through the compressive layer under increasing load until a critical load or threshold strength is reached. In 4-point flexural loading tests, cracks can bifurcate after entering into the compressive layer. This paper presents finite element simulations of bifurcated crack paths and threshold strength prediction in two-dimensional approximation. In rather good agreement with experiments the calculated threshold strength increases with compression and is higher with bifurcated cracks than with straight cracks, and higher with loading axis parallel to the laminar plane than normal to it.

In this paper we describe the structure and formation of a new intermetallic phase in the Cu – Al system, which has not been reported before. The phase was found in Cu/Al bilayer films, which were deposited at room temperature on (0001) sapphire substrates using molecular beam epitaxy. The interfacial intermetallic phase is 8 nm thick, and possesses a hexagonal close-packed structure. The lattice parameters of the phase gradually increase from the near-Cu-side to the near-Al-side. In parallel with the described lattice expansion, the chemical composition of the interlayer also varies from 27 to 58 Al at.% from the near-Cu-side to the near-Al-side. The formation and microstructural characteristics of this new phase are explained by Hume-Rothery laws and Shockley partial dislocations. In addition, in-situ heating experiments were performed in a transmission electron microscope at ∼600 °C to investigate the thermodynamic stability of this new Cu – Al intermetallic phase. During annealing the intermetallic layer disappears and other known equilibrium intermetallic phases develop. This indicates that this new hexagonal close-packed Cu – Al intermetallic phase is metastable.

An (001) single-crystal plate of a solid solution of PbMg 1 /3Nb 2 /3O 3 -28 mol.% PbTiO 3 was investigated by Wide-Angle X-ray Scattering techniques at room temperature in situ under the influence of an external static electric field (field strength E up to 13 kV cm −1 ) applied along the [001] direction of the wafer. Structural changes attributed to a reversible phase transition between ferroelectric modifications of the solid solution in a near electrode region of about 1 mm depth are found. Part of the near-electrode layer (of about 5 vol.% at E = 4.5 kV cm −1 and about 15 vol.% at 13 kV cm −1 ) transforms from the initial rhombohedral structure to another one, most probably, the monoclinic M b phase. Changes of the profile shape of X-ray rocking curves suggest possible technical use of the electric-field induced phase transition for adaptive X-ray optics.

The formation of ultrathin oxide layers on metal surfaces is a non-thermally-activated process which takes place spontaneously at very low temperatures within nanoseconds. This paper reports mechanistic details of the initial oxidation of bare metal surfaces, in particular Al(111) and TiN(001), as obtained by means of first-principles molecular dynamics modelling within the Density-Functional Theory. It is shown that the reactions of bare metal surfaces with O molecules take place according to a “hot-atom” dissociative mechanism which is triggered by the filling of the σ* antibonding molecular orbital and is characterised by a sudden release of a large amount of kinetic energy. This released energy provides a driving force for metal/oxygen place-exchange processes which are responsible for the onset of oxide formation at virtually 0 K and at oxygen coverages well below 1 monolayer (ML). Further simulations of the oxidation reactions reveal that a disordered ultrathin oxide forms on Al(111), whereas a rather ordered structure develops on TiN(001) following a selective oxidation process which leaves clusters of Ti vacancies in the TiN lattice underneath the oxide layer.

In one design of flexible electronics, thin-film islands of a stiff material are fabricated on a polymeric substrate, and functional materials are grown on these islands. When the substrate is stretched, the deformation is mainly accommodated by the substrate, and the islands and functional materials experience relatively small strains. Experiments have shown that, however, for a given amount of stretch, the islands exceeding a certain size may delaminate from the substrate. We calculate the energy release rate using a combination of finite element and complex variable methods. Our results show that the energy release rate diminishes as the island size or substrate stiffness decreases. Consequently, the critical island size is large when the substrate is compliant. We also obtain an analytical expression for the energy release rate of debonding islands from a very compliant substrate.

The problem of crack formation in surface layers where a linear profile of elastic strain prevails through the thickness of a layer is investigated. Such strain gradients can be generated, for example, in graded alloy semiconductor layers or due to specific stress relaxation mechanisms in lattice mismatched layers or previously unanticipated dislocation-related strain gradients. The operation of the relaxation mechanisms related to threading dislocation inclination and leading to the strain gradients in nominally compressed Al x Ga 1 – x N layers grown on buffer layers with smaller lattice constants is discussed. A fracture mechanics model is developed for the calculation of the stress intensity factor of mode I cracks initiated at the surface of the layer exhibiting a linear strain dependence. The critical layer thickness for crack formation in such a gradient elastic field has been found in the framework of this fracture mechanics model. Results of the modeling are compared with experimental observations of crack onset in nominally compressed layers of Al x Ga 1− x N semiconductors. Good agreement between the model predictions and the experimental data is found.

As material structures, and devices fabricated from them, shrink to the nano-scale, it is ever more difficult to measure accurately the mechanical properties needed for designing small-scale devices and for assessing reliability. A promising test configuration for localized, small-scale uniaxial tensile measurements is the theta specimen. Stress distributions in this test configuration are examined via finite element analysis. When loaded uniformly with a double-articulating loading platen, a uniform uniaxial stress state is realized in the test gauge. When alternative, more easily realized experimentally, loading configurations are used (i. e., line or point contact), large secondary stresses develop. For each loading configuration, influences of non-ideal load application are elucidated. Two design modifications are considered to mitigate undesirable stresses: a compliant layer and an extended upper loading tab. The latter yields the most favorable stress state and is the most tolerant to non-ideal loading.

Fabrication, handling and processing of wafers of intrinsically brittle and anisotropic single-crystalline GaAs require a high level of control of the material's response to different loading conditions. The present work is focused on the response to hardness indentation. A short overview on the behaviour of (100) GaAs wafers in several doping conditions over a wide range of indentation loads from nano-indentation up to macro-indentation including sharp and blunt indenters is given. Special attention is paid to the pop-in effect in depth-sensing nano-indentation, to the anisotropy of indentation-induced radial crack formation and to the material's crack resistance obtained from the indentation fracture mechanics approach. We have observed that, under certain conditions, the frequency of formation of radial cracks is essentially different for the two in-plane <110> directions. This observation is attributed to the occurrence of two different kinds of dislocations and to the lack of inversion symmetry. The effect turns out to be closely related to a left – right asymmetry in the material removal caused by wire sawing. This insight has paved the way to the optimisation of the process of wire sawing of GaAs single crystals.

A physical self-assembly process based on ac-dielectrophoretic deposition of single-walled carbon nanotubes between metallic electrodes and subsequent gate-voltage controlled elimination of the metallic nanotubes is reported that allows fabrication of multi-tube carbon nanotube field-effect transistors with electrical ON/OFF ratios up to more than seven orders of magnitude. Before deposition, the carbon nanotubes were suspended by means of single-stranded DNA or sodium-taurodeoxycholate. Exposure of the devices to ethanol vapor revealed a higher sensitivity for devices made from DNA-suspended carbon nanotubes.

For the regenerative therapy of osteochondral defects – deep lesions of the articular cartilage in which the underlying bone tissue is already affected too – special implant materials and scaffolds are needed. In this study, two new approaches will be presented, leading to biphasic, but monolithic scaffold materials. Both consist of a mineralised layer for filling of the bony part of the defect and a non-mineralised one for the chondral part. Due to the preparation methods, both layers are fused together to give a unified whole without need of any artificial joining. The resulting materials, either based on collagen, hyaluronic acid and hydroxyapatite or calcium alginate gels and hydroxyapatite, seem to be suitable scaffolds for cultivating chondrocytes and osteoblasts – and therefore can act as matrices for tissue engineering of osteochondral grafts.

Strain produces small but systematic changes in the Raman and luminescence spectra of solids. Piezospectroscopy is a technique that exploits these changes for determining stresses, usually from shifts in a characteristic spectral line. In this review in honor of Wolfgang Pompe, recent advances in piezospectroscopy applied to the determination of stresses in alumina materials are described.