Deformation and fracture of yttria-stabilized zirconia single crystals

Deformation and failure mechanisms are characterized for fully and partially yttria-stabilized single crystals tested in compression from 23 to 1150° C. It is found that both types of material exhibit extensive plastic flow over most of this temperature range, producing rapid decreases in strength with increasing temperature. Tetragonal to monoclinic transformation-induced plasticity is not observed; rather, plastic flow is related solely to dislocation activity. Evidence is found for apparent cubic to rhombohedral transformation during polishing, and reverse rhombohedral to cubic transformation during imposed compressive stress.     

Creep and stress relaxation of germanium single crystals

Isothermal creep and stress relaxation were studied in germanium single crystals oriented for double slip at 400 to 700? C under compressive stresses of 180 to 490 kgf cm^?2. The curves, S-shaped in both types of experiment, were compared with correspondingones derived from a stochastic model involving a spectrum of energy barriers to dislocation movement. Agreement was satisfactory for stress relaxation; discrepancies in the case of creep point to the difficulty of correlating creep and stress relaxation when an equation of state is inadequate as an approximation for representing the plastic response. Differences between predicted and observed behaviour are, however, of diagnostic value concerning the extent of dislocation multiplication and other structural changes.     

Internal stresses and dislocation structure in germanium single crystals

The thermoelastic stress developing during the growth of disk-shaped germanium single crystals for IR applications has been evaluated theoretically. The results have been shown to correlate with the dislocation structure of large germanium single crystals grown by the Stepanov method and directional solidification.     

Deformation of single crystals of gallium arsenide

Deformation of single crystals of gallium arsenide is reported in bend and tension up to 1000° C whilst maintaining stoichiometry in an arsenic atmosphere. Surface defects and impurity segregation are shown to be dislocation sources. The dislocation density is low enough, however, to show large yield drops which are analysed in detail. Strains of 39% are possible. The activation energy for dislocation movement is increased by heat-treatment owing to an increase in point defect population.Electron microscopy shows that the predominant slip systems are {111} 110 and the majority of dislocations have b=a/2 110, the axes lying along 110 and 112 directions. Sub-cell formation occurs with sub-boundaries lying along 110 directions.     

Deformation mechanisms in titanium dioxide single crystals

Single crystals of TiO₂ grown by a floating-zone method have been compressed in two directions, [1 0 0] and [0 0 1], at temperatures between 300 and 1300°C in an argon atmosphere. They deformed by {10¯1 1 01 slips, except [1 0 0] specimens tested below 600° C, where twinning preceded the slip deformation. Above 800° C, an oxygen reduction occurred during compression tests and the flow stress data were not reproducible. Below 700° C the data were reproducible and the resolved yield stress increased steeply with decreasing temperature. From the temperature and strain-rate dependences, activation analyses have successfully been performed. Activation volume becomes as small as 2b ³ at high stress, and the deformation below 700° C is concluded to be governed by the Peierls mechanism. The total activation enthalpy of deformation by the Peierls mechanism is 0.32eV. Applying the string model for the kink-pair formation, the results are consistent if the 101 dislocation is dissociated into two 1/2101 partial dislocations. No twinning-anti-twinning asymmetry of slip was observed.     

Microhardness and brittle fracture of garnet single crystals

Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd₃Ga₅O₁₂ (GGG) and Ca₃Ga₂Ge₃O₁₂ (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m^–2, for [111] oriented CaGeGG it is 12 GN m^–2, the average fracture toughness beingK _c=1.2 and 0.8 MN m^–3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.     

Deformation of single crystals of Cd3Mg

The deformation behaviour of single crystals of Cd₃Mg and Cd-28.5 at.% Mg has been studied over a range of temperature from –196 to 200° C, and the critical resolved shear stresses associated with both basal and {1¯100} 11¯20 prismatic slip have been measured. The CRSS for prismatic slip has been found to show a marked increase with increasing temperature prior to the onset of appreciable disordering while that for basal slip remains athermal over the same range. Both slip systems, however, showed a CRSS peak within the two-phase, ordered plus disordered, region. The athermal CRSS for basal slip in the stoichiometric alloy is shown to be too low to be consistent with a previous explanation advanced to account for the CRSS for basal slip in Mg₃Cd.     

Deformation behaviour of single crystals of titanium carbide

Single crystals of titanium carbide were deformed in compression over a wide range of temperature, and the operative slip systems were determined by etch-pitting and electron microscopy. Around the brittle-ductile transition temperature, the slip system undergoes a gradual change from {1 1 0} 1 ¯1 O to {1 1 1} 1 ¯1 0; this is interpreted to be the mechanism governing the brittle-ductile transition in titanium carbide.On leave from Defense Metallurgical Research Laboratories, Hyderabad 500258, India.     

Deformation and recrystallization textures of copper single crystals and bicrystals

Recrystallization textures in rolled copper specimens have been investigated so as to elucidate the mechanisms of cube texture formation. The specimens are single crystals with the orientations corresponding to the main components of the rolled textures, such as {112}111,{110}112, etc. and bicrystals consisting of such oriented crystals. The cube texture was not observed in any single crystal specimens, but observed in only two bicrystal specimens with {112}/111{100}001 and {110}112/{100}001 orientations. The formation of cube texture seems to require the existence of a cube oriented region in the deformed state, and the favourable oriented matrix to allow the growth of such nuclei.     

Deformation tracks distribution in iridium single crystals under tension

The deformation tracks distribution in a single crystal of the high melting FCC-metal iridium, which exhibits the cleavage after considerable elongation, is considered. The octahedral slip is the sole deformation mechanism in iridium single crystal at room temperature, and, therefore, its mechanical behavior is similar to the behavior of a FCC-metal. However, in contrast to other FCC-metals, the resource of plasticity of the iridium single crystalline samples is exhausted at the initial/early stages of plastic deformation, when the octahedral slip bands are homogeneously distributed on the working surface and the necking is absent in vicinity of the dangerous crack.

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Deformation behaviour of single crystals of InP in uniaxial compression

The deformation characteristics of indium phosphide (InP) single crystals under uniaxial compression have been examined as a function of strain rate, temperature and orientation. It has been shown that at temperatures below 0.55T _m (T _m=melting point; 1335 K) the material fractures in a brittle manner whereas at higher temperatures, within the range 0.55 to 0.71T _m, plastic deformation occurs by both slip and deformation twinning; above 0.71T _m, slip alone is the operative deformation mechanism. The observed operative slip systems are of the type {1 1 1} 0 1 1 which are characteristic of most Group IIIb-Group Vb compounds. Deformation twinning occurs predominantly on {1 1 1} planes but some activity is also observed on planes of the type {3 4 5}.     

Creating standard resistors based on germanium and silicon single crystals grown under microgravity conditions

Requirements on the creation of standard resistors (SRs), which are necessary for the calibration of microprobe complexes used for the diagnostics of electrical homogeneity of single crystal semiconductors, are considered. SR prototypes have been created based on Sb-doped Ge single crystals grown by float zone melting under microgravity conditions aboard the Photon series satellites, in which the inhomogeneity of the resistivity distribution does not exceed 1%. The main factors influencing the homogeneity of doping for Ge and Si crystals grown from melt under orbital flight conditions are formulated; methods for the optimization of this technological process are described.     

Deformation of NiO single crystals below room temperature: dislocation configurations

NiO single crystals prepared by two crystal growth techniques (zone melting in an arc-image furnace and Verneuil crystallization have been deformed by compression along 001 at temperatures as low as 4.2 K, and the dislocation substructure observed by transmission electron microscopy. The Peierls mechanism has been suggested as the mechanism controlling the mechanical behaviour at the lower temperatures. The dislocations generated at cavities found in the zone-melted crystals may be responsible for the increase of the flow stress of these crystals compared with the Verneuil ones.