W.J. Clegg

Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO

Microcompression has attracted considerable interest in the study of size effects, mainly in soft metals. Little data is available in the literature on experiments on materials with a higher bulk flow stress, although it has been shown that the technique can be successfully employed to suppress cracking due to the small specimen dimensions. Here, microcompressions on MgO were carried out to demonstrate the possibility of individually activating different slip systems. The yield stresses obtained in conjunction with transmission electron microscopy show that both the hard and soft slip system in MgO can be characterised individually. Microcompression is, therefore, a potential alternative to macroscopic testing of brittle materials under confining pressure or at high temperatures. To determine the influence of size on such measurements, results on the two slip systems in MgO and from the literature are compared. It is found that the bulk yield stress of a material might be used to estimate the effect of size on its yield stress at the microscale.

Growth of interface defects and its effect on crack deflection and toughening criteria

The role of interface defects in crack deflection at planar interfaces has been studied by the direct observation of interface and crack growth in laminates of poly(methyl methacrylate) and a finite element analysis. It was observed that crack deflection occurred by the growth of interface defects ahead of the growing primary crack. It was also observed that the growth of these defects is not sufficient to ensure toughening through crack deflection, but that a further condition, that any growing interface defects must not kink out of the interface, must be also satisfied. A finite element analysis based on the observed deflection mechanism suggests that initial crack deflection is possible when the interfacial fracture energy is less than 64% of that of the bulk material, which is consistent with experimental values. However, the analysis also predicts that this is somewhat dependent on layer thicknesses and global loading states for relatively longer interface defects.

Crack deflection in ceramic laminates using porous interlayers

Abstract Ceramic laminates have been made from alternating layers of silicon carbide and silicon carbide containing a fugitive polymer, which can be pyrolysed to produce porous interlayers. It is shown that such interlayers can be used to deflect cracks and that the volume fraction of porosity, due to the added polymer, that is required to cause crack deflection is approximately 0·4. A simple model has been developed which describes the fracture behaviour of porous solids and also predicts the volume fraction of porosity required to give crack deflection in the laminate and which is in good agreement with experiment.

Ductile-brittle transition in micropillar compression of GaAs at room temperature

Experiments have been carried out on how compressive failure of <100> axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 µm did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile–brittle transition at a micropillar diameter of approximately 1 µm, consistent with experimental observations.

Deformation under nanoindents in Si, Ge, and GaAs examined through transmission electron microscopy

Cross sections through nanoindents on Si, Ge, and GaAs {001} were examined through transmission electron microscopy. A focused ion beam workstation was used to machine electron transparent windows through the indents. In both Si and Ge there was a transformed zone immediately under the indent composed of amorphous material and a mixture of face-centered-cubic and body-centered cubic crystals. Cracking and dislocation generation were also observed around the transformed zone. In GaAs the dominant deformation mechanism was twinning on the {11} planes. The hardness of these materials is discussed in light of these observations and their macroscopic material properties such as phase transformation pressure.

Solidification and Ordering during Directional Drying of a Colloidal Dispersion

During drying, colloidal dispersions undergo processes such as solidification, cracking, and the draining of interstitial pores. Here we show that the solidification of polystyrene and silica dispersions, during directional drying, occurs in two separate stages. These correspond to the initial ordering and subsequent aggregation of the colloidal particles. Transitions between these stages are observed as changes in transparency and color that propagate as distinct fronts along the drying layer. The dynamics of these fronts are shown to arise from a balance between compressive capillary forces and the electrostatic and van der Waals forces described by DLVO theory. This suggests a simple method by which the maximum interparticle repulsion between particles can be measured through the optical inspection of the dynamics of a drying dispersion, under a microscope.

Structural defects in electrically degraded 4H-SiC p+/n−/n+ diodes

Triangular structural defects are occasionally generated during the long-term operation of 4H-SiC pin diodes and degrade the forward characteristics of the diode. We have used synchrotron white beam x-ray topography, scanning electron microscopy, in situ cathodo luminescence, and transmission electron microscopy to characterize the structure and formation of these defects. It is shown that the defects are stacking faults on the (0001) basal planes, bound by partial dislocations with Burgers vectors 1/3〈1010〉 and 1/3〈0110〉. These partials are suggested to form by the dissociation of existing dislocations.

Evolution of mud-crack patterns during repeated drying cycles

In mud, crack patterns are frequently seen with either an approximately rectilinear or hexagonal tiling. Here we show, experimentally, how a desiccation crack pattern changes from being dominated by 90° joint angles, to 120° joint angles. Layers of bentonite clay, a few mm thick, were repeatedly wetted and dried. When dried, the layers crack. These cracks visibly close when rewetted, but a similar crack pattern forms when the layer is redried, with cracks forming along the lines of previously open cracks. Time-lapse photography was used to show how the sequence in which individual cracks open is different in each generation of drying. The geometry of the crack pattern was observed after each of 25 generations of wetting and drying. The angles between cracks were found to approach 120°, with a relaxation time of approximately 4 generations. This was accompanied by a gradual change in the position of the crack vertices, as the crack pattern evolved. A simple model of crack behavior in a layer where the positions of previously open cracks define lines of weakness is developed to explain these observations.

Deformation under nanoindents in sapphire, spinel and magnesia examined using transmission electron microscopy

Abstract Cross-sections through nanoindentions in the (001) surface of sapphire, spinel and magnesia have been examined in the transmission electron microscope. Electron-transparent sections were prepared using a focused ion beam microscope. All three recognized high-temperature slip systems were observed in sapphire, while spinel deformed in slip bands on the {111} planes. Evidence was obtained for slip on {110} planes in magnesia and the possibility that slip also occurs on {100} planes is discussed as an explanation of its high ratio of hardness to yield stress.

New phases in the superconducting Y:Ba:Cu:O system

An electron diffraction and microscopy study is presented of a variety of phases in the Y:Ba:Cu:O system in which superconductivity occurs. The superconducting phase is demonstrated by convergent beam electron diffraction to be centrosymmetric with space group Pmmm, in contrast to a previous determination of Pmm2. This discrepancy arises from local symmetry‐breaking defects. In addition to this phase and a cubic BaCuO2 phase, we characterize two other phases. One is the Y‐rich orthorhombic phase: Pnma with a=13.5 A, b=6.3 A, and c=7.6 A. The second occurs by a phase transition of the superconducting Pmmm phase to P4/mmm with a=3.85 A, c=11.7 A. The superconducting phase may now be described as either an ordered array of oxygen vacancies in the perovskite structure, or an ordered array of oxygen interstitials in the new tetragonal phase, which may explain how the material can lose oxygen reversibly.