Deformation Microstructure in (001) Single Crystal Strontium Titanate by Vickers Indentation

Recent interests on the plastic deformation of strontium titanate (SrTiO₃) are derived from its unusual ductile-to-brittle-to-ductile transition (DBDT). The transition is divided into three regimes (A, B, and C) corresponding to the temperature range of 113–1053 K (−160° to 780°C), 1053 to ∼1503 K (780° to ∼1230°C), and ∼1503–1873 K (∼1230° to 1600°C), discovered by Sigle and colleagues in the MPI-Stuttgart. We report the dislocation substructures in (001) single crystal SrTiO₃ deformed by Vickers indentation at room temperature, studied by scanning and transmission electron microscopy. Dislocation dipoles of screw and edge character are observed and confirmed by inside–outside contrast using ± g -vector by weak-beam dark field imaging. They are formed by edge trapping, jog dragging, and cross slip pinching-off. Similar to dipole breaking off in deformed sapphire (α-Al₂O₃) at 1200°C and γ-TiAl intermetallic at room temperature, the dipoles pinch off at one end, and emit a string of loops at trail. Two sets of slip systems {110}〈     〉 and {100}〈011〉 are activated under both 100 g and 1 kg load. The suggestion is that plastic deformation has reached the stage II work hardening, which is characterized by multiplication of dislocations through cross slip, interactions between dislocations, and operating of multiple slip systems.     

Imaging and Diffraction Study of Continuous α-Fe2O3 Films on (0001)Al2O3

Continuous α-Fe₂O₃ films grown on bulk (0001)Al₂O₂ substrates by low-pressure chemical vapor deposition have been studied by transmission electron microscopy and the observations compared to those obtained from discontinuous films at an earlier stage of the growth process. Plan-view specimens revealed significant thermal stress in the continuous films, while cross-sectional specimens showed that cracking occurs in thicker films. The free surface of the film and the film/substrate interface appeared sharp and flat, apart from growth ledges and steps. Weak-beam imaging revealed a hexagonal misfit dislocation network consisting of perfect edge dislocations. Fine structure in the selected-area diffraction patterns which corroborates these observations is also discussed. The misfit network of partial dislocations previously observed in the discontinuous films was not observed for the continuous films, indicating an effect of film thickness, growth rate, or surface preparation on the Fe₂O₃/(0001)Al₂O₃ interface structure.     

Surface Structure in Corundum: II, Dislocation Structure and Fracture of Deformed Single Crystals

The dislocation structure in plastically deformed single crystals of corundum (α-A1₂O₃) was studied by etching and optical techniques. A special method of deformation was used which encouraged the participation of both the basal (0001), (1120) and the prismatic (1210), (1010) slip systems concurrently. This deformation resulted in highly stressed crystals which fractured spontaneously once the outside surface was scratched. Incoherent surface like imperfections, accompanied by distinctive dislocation arrays, were found embedded in the deformed crystals. These "surfaces" appeared as discontinuities, on each side of which dislocations etched differently. A possible origin of observed failure making use of the dislocation reaction ± [1010] = 3=1/3[2110] ± 1/3[1120] is suggested. A mechanism for the observed strain hardening in prismatic bending is presented.     

Influence of Aluminum–Oxygen–Yttrium Solid Solution on the Aqueous Tribological Behavior of Silicon Nitride

To investigate the influence of crystal structure and solid solutions of certain elements in Si₃N₄ on aqueous tribological properties, seven specimens were fabricated. Solid solutions of aluminum and oxygen made the friction coefficient and wear increase in both α-type and β-type crystal structures. In particular, α-SiAlON, in which yttrium is also incorporated into the α-Si₃N₄ lattice, showed a higher friction coefficient and wear than other specimens containing the same concentration of Al or O.     

Partially Reduced Cuprous Oxide Nanoparticles Formed in Porous Glass Reaction Fields

This paper describes the formation of cuprous oxide (Cu₂O) particles ∼10 nm in diameter and their reduction by hydrogen gas in porous glasses. Nanoparticles of Cu₂O coated with metallic copper are expected to show high third-order optical nonlinear susceptibilities (χ⁽³⁾). The porous glass is a medium in which nanoparticles can be partially reduced without forming agglomerations. Images obtained using transmission electron microscopy showed that particles with the desired core-shell structure were actually formed, even though some particles were not reduced uniformly from the surface. The χ⁽³⁾/α (α: absorbance) values in relevant conditions (10⁻¹³ esu·cm) were similar to that of copper because of the formation of a byproduct of copper nanoparticles on reduction.     

Mechanical Properties of Polycrystalline TiC

The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests. The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size. Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature. The yield stress as a function of grain size can be described by a Hall-Petch type of relation, i.e. yield stress α (grain size)-1/2. The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present. As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density. A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures. Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip.     

Dislocations in Rutile as Revealed by the Etch-Pit Technique

Potassium hydroxide and sodium hydroxide were found to be satisfactory for polishing various surfaces of rutile. Dislocation etch pits could be produced by alkali fusions, 85% orthophosphoric acid, and concentrated sulfuric acid. The {l 10}-, near {111}-, (001)-, and {100}-type surfaces were etched and the etch pits were analyzed in terms of their relation to the deformation systems and crystal symmetry. The correlation between etch pits and dislocations was substantiated by means of matched cleavage surfaces, existence of substructures, generation of dislocations, dislocation densities, and polygonization phenomena. Dislocations were introduced by impacting and macroscopic plastic deformation. Generation of dislocations was observed from 1050°C to as low as room temperature. The slip planes confirmed by means of dislocation traces and slip lines were {110}-and {101}-type planes. The slip direction corresponding to the {110)-type plane was [OO11, the close-packed direction. The Burgers vector, structure, and motion of dislocations in the edge orientation on the {110) [001] system were determined by analyzing the crystal structure. The Burgers vector was c [001], the lattice translation vector, and the dislocations did not dissociate into partial dislocations. Another possible slip system was the (100) [010] system.     

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.     

Kinking and Cracking Caused by Slip in Single Crystals of Silicon Carbide

α-SiC single crystals were compressed parallel to the basal plane, (0001), at temperatures between 900° and 1500°C. Plastic deformation by slip on the basal planes which accompanied kinking occurred above 1000°C. At kink boundaries, two kinds of cracks were observed. One was the cracks elongated parallel to the basal plane. This kind of crack was initiated by the tensile stress produced by piled-up dislocations on the basal planes against a kink boundary. The other was on a kink boundary, and was induced by the stress of dislocations, heterogeneously distributed on the kink boundary. The initiation of cracks produced by dislocations was considered to be a possible cause of fracture in polycrystalline SiC at high temperatures.     

Dissociation of the 〈001〉 Dislocations and Their Interactions with Dislocation Loops in Tetragonal BaTiO3

Dislocations in pressureless-sintered BaTiO₃ ceramics have been analyzed using transmission electron microscopy. Subjected to effective sintering stresses, dislocations were generated and multiplied in plastically deformed BaTiO₃ crystals by the Frank–Read mechanism from both single- and double-ended sources. This is represented by dislocations encompassing a series of square-like borders that shared a common center. All border dislocations exhibited the characteristic scallop shape. True dislocation line directions ( u ) were determined by trace analysis and Burgers vectors ( b ) by contrast analysis for the dislocations dissociated from b =〈001〉 into two half-partials following the type (I) reaction of     by climb on {001}. Dislocation interactions between the main dislocations created from plastic deformation and dislocation loops of b =〈100〉 or 〈110〉 forming condensation of intrinsic Schottky vacancies were also found to obey the type (IV) reaction of     , the type (V) reactions of     . Migrating dislocations and loops interacting mutually in several stages, illustrated schematically, before arriving at the configuration described by types (IV) and (V) were observed and discussed.     

Deformation Behavior of Sapphire Via the Prismatic Slip System

The deformation behavior of α -axis sapphire in tension from 1550° to 1850°C was studied in constant strain-rate, differential strain-rate, and differential-temperature tests. The crystals deformed via the prismatic system,     , resulting in stress-strain curves with a significant yield drop, a region of work-hardening which decreased in magnitude with increasing temperature, and a region of constant flow stress. The deformation was inhomogeneous with irregularly spaced slip bands containing a high density of glide dislocations and also basal dislocation debris. The flow behavior did not conform to simple diffusional or Peierls-process models but could be rationalized on the basis of the creation of a super saturation of vacancies generated by the motion of jogs on gliding screw dislocations.     

High Temperature Tensile Creep of Magnesium Oxide Single Crystals

The creep behavior and the dislocation substructure developed during creep were investigated for 〈011〉 oriented MgO single crystals creep tested in tension. Creep deformation was studied over stress and temperature ranges of 29.0 to 86.2 MN/m² and 1200 to 1500°C, and the minimum creep rate, ε, was found to obey the relation:

where σ = applied tensile stress, k = the Boltzmann constant, T = absolute temperature, n = 3.8 to 4.5, and A = ll × 10⁻² (MN/m²)^-4 s^-1. Dislocation substructures developed during creep were studied by transmission electron microscopy and etch pitting techniques. At 1400°C, the dislocation density, ρ , at 0.10 tensile creep strain depended on applied stress as ρασ ^2.1. Numerous dislocation loops and long straight dislocations were present, but subboundaries were seldom observed. The results are discussed in terms of two possible operative creep mechanisms: (1) a recovery process based on annealing out of dislocation dipoles and loops, and (2) dislocation glide limited by atmospheres of charged defects surrounding dislocations.     

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al₂O₃. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.     

Deformation of Hyperstoichiometric UO2 Single Crystals

Hyperstoichiometric UO₂ single crystals having O/M ratios from 2.06 to 2.10 were deformed in compression. The active slip plane for deformation, as evaluated from slip traces, has an orientation between (112) and (111). The critical resolved shear stresses for hyperstoichiometric specimens are approximately the same as those for stoichiometric specimens; the dislocation structures, as revealed by transmission electron microscopy, however, are very different and suggest more rapid motion and/or climb of dislocations in the hyperstoichiometric samples. The response to strain rate changes during deformation also reflects the difference in deformation behavior of hyperstoichiometric and stoichiometric crystals.     

Surface Domain Reorientation Produced by Abrasion and Annealing of Polycrystalline BaTiO3

The abraded surface of polycrystalline BaTiO₃ was examined by X-ray diffraction methods. The distorted layer produced by abrasion on 320-grade SiC consists of (1) a region extending to the bottom of the deepest scratches in which the domains tend to be oriented with their c axes perpendicular to the surface and (2) a transition layer below this in which the orientation varies with depth. The orientation changes little on annealing until recrystallization begins at #900°C; however, annealing at 1100° to 1400°C produces an α-domain reorientation. Possible explanations for these effects are discussed in terms of dislocations introduced by deformation during abrasion and grain size in the recrystallized surface layer.     

Correlation Between Microstructure and Creep Behavior in Liquid-Phase-Sintered α-Silicon Carbide

The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in α-silicon carbide (α-SiC) sintered in argon atmosphere with Y₂O₃–Al₂O₃ (10% weight) as liquid phase (LPS-α-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 μm, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450°C and 1625°C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 × 10⁻⁸ and 1.5 × 10⁻⁶ s⁻¹. The stress exponent n and the activation energy Q were determined, finding values of n between 2.4±0.1 and 4.5±0.2 and Q =680±35 kJ/mol for samples sintered for 1 h, and n between 1.2±0.1 and 2.4±0.1 and Q =710±90 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep.     

Microstructural Model on the Evolution of Domain Structure during Processing of PbTiO3 Thin Films on MgO(100) Substrates

A two-dimensional microstructural model is proposed to explain the origin of the 90° domain structure of epitaxial PbTiO₃ thin films grown on MgO(100) substrates by radio frequency sputter deposition. During film deposition at or above 600°C, the film grows epitaxially in a cubic phase without any domain structure. At or just below the Curie temperature, the film mainly consists of α-domains with a small portion of c -domains nucleated in a triangular shape. The c -domains gradually expand with decresasing temperature to relax the strain energy caused by the difference in thermal contraction rate between the c -axis of α-domains and the substrate, forming consequently thin strips of α-domains distributed in the matrix of c -domains at room temperature.     

Decoration of Dislocations by the Precipitation of Alumina in MgO · 2.9Al2O3 Spinel

A thermal etching technique is developed to reveal dislocations in MgO · nAl₂O₃ (n=2.9) single crystals decorated by Al₂O₃ precipitation. Large plastic deformation leads to a larger density of thermal etch pits. Also, dense arrays of pits, parallel to the traces of slip planes with a surface, are observed around a Vickers identation. The cracks developed by an indentation run selectively along the 〈100〉 directions and accompany dislocations arrayed in the possible slip planes.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

Effect of Nitridation Rate on the Composition and Conductivity of Titanium Nitride Films Prepared from Sol–Gel Titania

Nitridation of TiO₂ films in NH₃ was studied as a function of heating rate by ¹⁴N(α, α)¹⁴N and ¹⁶O(α, α)¹⁶O nuclear resonance, Auger depth profiling, X-ray diffraction, and electrical conductivity measurements. TiN films containing minimal amount of oxygen and exhibiting the lowest resistivity were obtained when the fastest nitridation rate (70°C/min) was used. Nitridation at slower rates not only increased the film-oxygen content and electrical resistivity but also produced an oxygen concentration gradient across the film.