In Situ Study on the Generation of Radiation Damage in Cubic-Zirconia in the High-Voltage Electron Microscope

Deformed cubic-zirconia single crystals have been irradiated in the high-voltage electron microscope at room temperature. Prismatic dislocation loops of extrinsic character grow on {111} planes as secondary radiation damage, preferentially along dislocations. At about 200°C, the loops grow much faster and instantaneously transform into dislocations with a a /2〈110〉-type Burgers vector.     

Surface Structure in Corundum: I, Etching of Dislocations

Triangular etch pits, corresponding to the intersection of individual edge dislocations with the surface, have been observed in corundum single crystals after about 5 minutes of immersion in boiling phosphoric acid. Dislocations of both the (0001), (1120) and (1210), (1010) slip systems (where reference is made to the slip plane and slip direction, respectively) have been detected by this method. Edge dislocations of the (0001), (1120) system etched on surfaces close to and including the (1011) and {2021} planes. Dislocations of the {1210}, (1010) systems etched on the basal (0001) plane. The orientation dependence and technique of etching is described. The agreement with the Nye formula and with some of the expected properties of dislocations is cited as evidence that individual edge dislocations are actually detected. Typical photomicrographs of the etched surface in as-received, deformed, flame-polished, and polygonized crystals are shown. The dislocation structure produced in basally deformed crystals was found to depend critically on the constancy of the temperature during deformation. Crystals held at 2000°C. during bending exhibited dislocations arrayed in rows in the slip planes in good agreement with the Nye formula. Crystals bent while cooling (quench-bent) had about twice as many dislocations as predicted by the Nye formula and these were distributed randomly. The possibility of vacancy condensation as a method for dislocation generation in quench-bent crystals is discussed.     

Plasticity and Creep in Single Crystals of Zirconium Carbide

High-temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf-coupling floating-zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady-state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.     

Creep Deformation of 0° Sapphire

Creep deformation of 0° sapphire was studied between 1600°and 1800°C at stresses up to 114 MN/m². Microscopical evidence (dislocation structures observed by transmission electron microscopy (TEM) and by etch pits) suggested that Nabarro climb was the predominant deformation mechanism. Both the experimental creep rates and stress exponents were in good agreement with those predicted by this model. Although non-basal dislocations with a ½〈101〉 Burgers vector were present, the good creep resistance of 0° sapphire was attributed to the difficulty of activating pyramidal slip.     

3D Dislocation structure evolution in strontium titanate: Spherical indentation experiments and MD simulations

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch‐pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load‐displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile‐ups are all aligned in 〈100〉 directions, lying on {110}₄₅ planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile‐ups along 〈110〉 directions appear, lying on {110}₉₀ planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth.     

{0001}<1010> Slip and Basal Twinning in Sapphire Single Crystals Shock-Loaded at Room Temperature

The microstructures of sapphire single crystals explosively shocked at pressures of 5, 12, and 23 GPa, with the basal plane perpendicular to the shock wave direction, have been studied using transmission electron microscopy. There was no evidence of microplastic deformation, dislocations or twins, for crystals shocked at 5 GPa. Perfect dislocations with 10 1 0 and ⅓11 2 0 Burgers vectors on the basal planes were observed at the higher pressures. The gliding of 10 1 0 dislocations on the basal plane established the activation of {0001}10 1 0 slip, which has not been experimentally verified previously. In addition to dislocation generation and motion, a significant number of basal twins were observed.     

Mechanical Behavior of Single-Crystal and Polycrystalline Cesium Bromide

Plastic-deformation studies were made of cesium bromide which exhibits no cleavage planes. Single crystals were found to be "soft" and ductile if they were favorably oriented relative to the loading axis to activate the {110}∝001〈 slip systems. Kink bands were formed by applying a compressive load normal to a {100} plane. Cross slip was found to occur during deformation at all test temperatures. Tensile loads applied normal to {100} and {110} planes caused fracture along {110} planes, whereas compressive loads normal to {100} planes forced fracture along {112} planes. The ductility of polycrystalline cesium bromide was found to be limited because of the lack of five independent slip systems. The specimens with smaller grains showed less strain at fracture and greater strain hardening than the larger-grained specimens. The mechanism fracture was intergranular up to 300° C.     

Dislocation Etchant for Beryllium Oxide

A hydrochloric acid chemical etchant for BeO was refined and evaluated. The etchant, when used for 20 min at 120°C on chemically polished surfaces, produces distinctive pits at dislocations intersecting the (0001) and (101) surfaces. Correspondence of etch pits and dislocations was inferred by comparing the enlargement of pits with etching time and the constancy of pit density with continued etching. On {1010} surfaces, the etchant is not sensitive to sites of defect emergence and hence leaves a relatively flat, featureless surface. The etch rates and activation energies are given for {0001}, {1010}, and {101} surfaces. The reaction mechanism is discussed.     

Dislocations and Mechanical Properties of MgO-Al2O3 Spinel Single Crystals

MgO· n Al₂O₃ spinel single crystals can be deformed plastically at high temperatures, displaying a range of interesting features. Stress-strain curves often exhibit strong work hardening followed by prominent work softening due to glide and climb processes. The critical resolved shear stress (CRSS) at a given temperature decreases dramatically, by almost 2 orders of magnitude, with increasing deviation from stoichiometry, i.e., as n increases from 1 to 3.5. The CRSS is proportional to exp(- T / T ₀) and to [ V _c]^-2, where T is the temperature in kelvin, T ₀ a characteristic temperature, and [ V _c] the concentration of charge-compensating cation vacancies. The Burgers vector is 1/2<110>, and slip can occur on {111} and {110} planes. Slip on {111} planes is believed to occur between the Kagomé cation layer and the adjacent anion layer. Slip on {110} planes is slightly easier (and has a higher T ₀), because the planes are more widely separated. The temperature dependence of the CRSS can be explained in terms of the Peierls stress for partial dislocations, either in terms of a steep and high Peierls potential or in terms of temperature and stress-dependent kink diffusion. The dependence of CRSS on [ V _c]^-2 can be explained in terms of kink nucleation at cation vacancies.     

Comparison of Whisker Growth Sites and Dislocation Etch Pits on Single-Crystal Sapphire

It has been shown previously that alumina (sapphire) whiskers grown on single-crystal alumina grow coherently with and in the directions of screw dislocations in the substrate. A possible explanation is that the whiskers grow by the screw-dislocation mechanism at the site of emergent screw dislocations on the substrate surface. The present study was undertaken to determine if the whiskers grow at the sites of substrate dislocations as revealed by etching techniques. To do this, whisker growth sites were compared with the positions of dislocation etch pits. This was done for the prismatic [{112¯0}, <11¯00>] and basal [(0001), (112¯0)] slip systems on (112¯0), (11¯01), and (0001) crystal surfaces. The whiskers and etch pits did not form at the same places on the crystal surfaces. The reason may be that (1) the etch pits form at pure edge dislocations only, (2) surface nucleation generates screw dislocations at which the whiskers grow, or (3) screw dislocations play no part in the growth of these whiskers.     

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.     

Deformation and Fracture of MnTe and MnSe-MnTe Solid Solutions

Single crystals of MnTe and solid solutions in the system MnSe-MnTe were prepared; selected surfaces were indented with either a Vickers or a Knoop microindenter. Information on mechanical behavior was obtained by observation of slip traces and by study of Knoop hardness anisotropy. Hexagonal NiAs-type MnTe and MnTe-rich Mn(Te,Se) solid solutions show plastic deformation attributable to {1012} twinning, basal slip, and pencil glide in 〈1120〉. Fracture occurs as primary {0001} and secondary {1010} cleavage. Vickers and Knoop hardnesses were greater on the basal plane than on prism planes. Cubic MnSe-rich Mn(Te,Se) solid solutions show both {111}〈110〉 and {110}〈110〉 slip with {100} and {110} fracture in crushed fragments and around surface indentations. Knoop hardness anisotropy is like that in MnSe. The rate of solid solution hardening is greater than for comparable substitutions of sulfide ions in the MnSe matrix.     

Plastic Deformation of Cuprous Oxide

Slip line and transmission electron microscopy observations on the plastic deformation of cuprous oxide were made on large-grained polycrystalline specimens. The specimens were prepared by the complete oxidation of OFHC copper strips in air followed by a high-temperature anneal. Plastic deformation occurred by motion of α(100) dislocations on (100) glide planes. Some dislocation segments of a(110) Burgers vector were present but were probably formed by recombination reactions between α(100) dislocations. The unusual structure of Cu₂0, which can be described as two interpenetrating and identical frameworks of copper and oxygen which are not cross-connected by any primary copper–oxygen bonds, did not result in any unusual behavior of dislocations. The effect of temperature on plasticity was explained in the same way as for other materials of less complex structure, such as MgO.     

Mechanical properties of ceria nanorods and nanochains; the effect of dislocations, grain-boundaries and oriented attachment

We predict that the presence of extended defects can reduce the mechanical strength of a ceria nanorod by 70%. Conversely, the pristine material can deform near its theoretical strength limit. Specifically, atomistic models of ceria nanorods have been generated with full microstructure, including: growth direction, morphology, surface roughening (steps, edges, corners), point defects, dislocations and grain-boundaries. The models were then used to calculate the mechanical strength as a function of microstructure. Our simulations reveal that the compressive yield strengths of ceria nanorods, ca. 10 nm in diameter and without extended defects, are 46 and 36 GPa for rods oriented along [211] and [110] respectively, which represents almost 10% of the bulk elastic modulus and are associated with yield strains of about 0.09. Tensile yield strengths were calculated to be about 50% lower with associated yield strains of about 0.06. For both nanorods, plastic deformation was found to proceed via slip in the {001} plane with direction <110>--a primary slip system for crystals with the fluorite structure. Dislocation evolution for the nanorod oriented along [110] was nucleated via a cerium vacancy present at the surface. A nanorod oriented along [321] and comprising twin-grain boundaries with {111} interfacial planes was calculated to have a yield strength of about 10 GPa (compression and tension) with the grain boundary providing the vehicle for plastic deformation, which slipped in the plane of the grain boundary, with an associated <110> slip direction. We also predict, using a combination of atomistic simulation and DFT, that rutile-structured ceria is feasible when the crystal is placed under tension. The mechanical properties of nanochains, comprising individual ceria nanoparticles with oriented attachment and generated using simulated self-assembly, were found to be similar to those of the nanorod with grain-boundary. Images of the atom positions during tension and compression are shown, together with animations, revealing the mechanisms underpinning plastic deformation. For the nanochain, our simulations help further our understanding of how a crystallising ice front can be used to 'sculpt' ceria nanoparticles into nanorods via oriented attachment.     

Electrochemical etching

The formation of two-dimensional negative nuclei (“Hohlkeime”) during the etching of edge dislocation emergence points has been appreciated as an atomistic process. Experimentally, by means of double-pulse potentiostatic method, {0001}-faces of zinc single crystals have been etched electrochemically according to a new “droplet” technique (in 1 N ZnSO₄ water solution). In this way the emergence points of screw and edge dislocations have been distinguished, the former being developed by deep pyramidal etch pits whereas the latter created flat-bottomed pits. By the electrochemical etching we observed ivariably 〈11$\text{2}$0〉 pit orientation (and, perhaps, in connection with this the appearance of non-symmetrical etch pits on the emergence points of screw dislocations with the Burgers vector $\text{1}\text{3}$ 〈11$\text{2}$3〉). In contrast, the chemical etchant of Rosenbaum and Saffren (alcoholic HCl solution) created 〈10$\text{1}$0〉 oriented pits. Anyway, our experiments showed that the addition of SO^2?₄ to this etchant in concentrations exceeding 1:4 changes the azimuthal orientation of the chemically produced etch pits in 〈11$\text{2}$0〉 and it was concluded that the SO^2?₄ selective adsorption changes significantly the polar diagram of the lateral dissolution rate.     

Investigation of Lamella-Twinned Crystallites and Dislocations in Paraelectric Phases of Bi2O3-Doped BaTiO3

Defects in the paraelectric phases of BaTiO₃ doped with Bi₂O₃ were analyzed by transmission electron microscopy under two-beam conditions. (111) twin structures were characterized by selected area diffraction and bright-field images. The orientation relationships of the (111) twins were determined using stereograms. Lamella-twinned crystallites included in the paraelectric phases were found in this system. Pure wedge fringes were analyzed in these grains using electron diffraction and imaging techniques. Double diffraction was observed in the overlapped regions of the matrix and the microtwin in the [113] direction, and high-density dislocation loops were seen in some grains. Weak-beam dark-field microscopy techniques were used to observe the dislocation loops, which predominately lay on {100} crystal planes with Burgers vectors a 〈100〉, and were found to be pure edge dislocations. Some dislocations were transformed into crystallographic shear planes.     

Mechanical Behavior of Sodium Chloride Single Crystals in Flexure

Evidence is presented to show that plastic deformation of NaCl single crystals by transverse bending arbitrarily takes place in one of two distinct ways which result in quite different stress-strain behavior. The crystals deform initially by glide either along the {110} slip planes normal to the bending plane or along the {110} planes normal to the neutral plane. A higher rate of work hardening is observed when bending activates slip on the {110} planes normal to the neutral plane. Reliable studies of work hardening and related effects in NaCl single crystals therefore cannot be made in flexure unless careful attention is given to this orientation effect.     

High-Temperature Indentation Studies on the {110} Plane of Single-crystal Magnesium Oxide

The indentation behavior (Vickers) of Single-crystal MgO was studied as a function of temperature (20° to 1000°C). Indentations were made on the {110} plane, with the indents oriented such that one indent diagonal was parallel to the 〈001〉 direction. Using etchant techniques, the dislocation etch pit structures were examined both in the plane of the indentation and in cross section. All the observed slip traces were found to be consistent with primary slip ({110}〈 1 10〉), with no evidence of secondary slip, even at 1000°C. Radial cracking was observed only at the pair of indent corners joined by the indent diagonal parallel to 〈001〉. The crack length increased with temperature ( T ) for indentations conducted at T < 800°C. For indents made at 800°C or higher, however, no cracking occurred. These results are discussed both with respect to an existing slip-induced crack nucleation model, and the change in crack driving force and toughness with indentation temperature.     

Deformation and Fracture of Polycrystalline Lithium Fluoride

Techniques for the fabrication of polycrystalline LiF test specimens were developed and evaluated using single-crystal LiF as a control. An etch was developed which revealed dislocations on all crystallographic faces of LiF. Large-grained polycrystalline specimens tested in four-point loading underwent 0.076 to 0.798% plastic strain before fracture. In most cases their yield stress was similar to that for single crystals favorably oriented for flow on {110}〈110〉 slip systems. Deformation was inhomogeneous among the grains because of differences in orientation with respect to the applied stress and within individual grains because of interactions at grain boundaries. Grain boundaries were barriers to slip, but stresses resulting from slip in one grain were transmitted to neighboring grains and often caused local deformation near the boundary. In one case, local boundary slip occurred on an (010) plane. Three-grain junctions were areas of high residual stresses, and fractures originated at boundaries at or near three-grain junctions. Fractures were mixed transgranular and intergranular.     

Fracture Initiation in the Directionally Solidified NiO-CaO Eutectic

Initiation of fracture in the directionally solidified, lamellar NiO-CaO eutectic was examined using the indentation fracture technique. Fracture could not be induced along the interphase boundary on transverse sections of the directionally solidified eutectic. Instead, radial cracks evolved at angles of approximately 35° and 55° with respect to the lamellar interface and were consistent with median/radial crack formation on (TlO) and (001) planes, respectively. Indentation of single-crystal NiO resulted in fracture only along {110} planes. Crack initiation on {110} planes in both materials was attributed to a dislocation coalescence model proposed by Keh et al. while crack formation on (001) planes in the eutectic was believed to be initiated by a Stroh-type mechanism involving dislocation pile-ups. Variations in the interlamellar spacing resulted in a Hall-Petch-type behavior for the hardness but had little effect on the fracture toughness of the eutectic.