H. Garem

Influence of temperature on dissociation of dislocations and plastic deformation in spinel oxides

Abstract Depending upon the temperature at which spinel oxides are deformed, different dissociation configurations of dislocations have been observed. At elevated temperatures, edge dislocations are dissociated over several hundreds angstroms by a process involving climb of partials. At low temperatures, the long screw dipoles resulting from indentation tests suggest a classical dissociation of dislocations out of the preferred glide plane. These effects are compared to calculations of stacking-fault energy in the spinel structure. A good qualitative agreement is found whatever the dissociation mechanism. Quantitative agreement is satisfactorily obtained only in so far as glide dissociation is concerned. Discrepancies between calculated and observed climb dissociation suggest a strong interaction effect with point defects. The possible effects that such a dissociation dependence upon temperature has on deformation mechanisms in spinels are discussed.

Plastic deformation of GaAs single crystals as a function of electronic doping. I, Medium temperatures (150-650°C)

Abstract Plastic deformation of GaAs single crystals has been studied in the temperature range 20-650°C as a function of electronic doping, using different deformation techniques. In this paper, experiments conducted between 150°C and 650°C, using suitable pre-deformation conditions, are reported. The effects of electronic doping on macroscopic plasticity have been found to be consistent with dislocation velocity measurements for temperatures larger than 200°C. Activation Gibbs free energies were derived from activation volume measurements and found to be in reasonable agreement with activation energies of dislocation velocities. Observations of deformation substructures by transmission electron microscopy show that screw dislocations control the deformation at the lower temperatures.

Slip systems in gadolinium gallium garnet single crystals

Gd3Ga5O12 single crystals with compression axes of different orientations have been deformed under creep conditions in air at 1450° C and 1550° C (0.86 TM and 0.92 TM, respectively, where TM is the melting temperature). After a few per cent of permanent strain the deformation substructure has been studied by optical and transmission electron microscopy. Etching and bi-refringence patterns indicate that slip on {110}, {112} and {123} can be activated depending on the orientation of the compression load. Dislocations with a/2 〈111〉 Burgers vectors have been observed to glide in {110 planes. They exhibit a segmented aspect suggesting their dissociation out of the glide plane.

Sub-grain boundaries and dissociations of dislocations in yttrium iron garnet deformed at high temperatures

Abstract Sub-grain boundaries in YIG single crystals deformed at high temperatures (0·8-0·9 TM) have been studied by transmission electron microscopy. Dislocation networks form mainly by climb; they are formed from dislocations with α/2 〈111〉 Burgers vectors. Dislocation junctions with α 〈100〉 Burgers vectors are usually found in networks; however, junctions with α 〈110〉 Burgers vectors are occasionally observed. They are dissociated into two partial dislocations with collinear Burgers vectors and the resulting fault configuration has been shown to be one corresponding to climb.

Dissociation of dislocation with a/2〈111〉 Burgers vectors in YIG single crystals deformed at high temperature

Abstract It is shown that dislocations with a/2〈111〉 Burgers vector in yttrium iron garnet (YTG) deformed in compression above 0.8T M are dissociated into two partials following: a/2〈111〉→a/4〈111〉 + a/4〈111〉. The dissociation mode involves a climb component which renders the glide motion of the dislocation difficult. The dissociation width is not constant and cannot be used to derive a stacking-fault energy intrinsic to the crystal. The variations in width are probably associated with local interactions with point defects. Pure climb motion of the dislocations, consistent with the dissociation, is experimentally observed.

Plastic deformation of GaAs single crystals at room temperature and the influence of doping

Abstract The plasticity at room temperature of intrinsic, n- and p-doped GaAs single crystals has been studied in uniaxial compression under a confining pressure of 1 4 GPa. The n-type crystals were found to be softer than intrinsic or p-type crystals, in contrast to previously reported experiments carried out at intermediate temperatures. This feature can be related to the deformation substructure which exhibits long screw dislocation segments and evidence of cross slip. Microtwinning was also observed as well as large dissociation widths.

Transmission electron microscopy determinations of dislocation Burgers vectors in plastically deformed yttrium iron garnet single crystals

Dislocations have been studied in YIG by transmission electron microscopy. Loops are found to have (a/2) 〈111〉 Burgers vectors whereas dislocation segments may have (a/2) 〈111〉 or a 〈001〉 ones, the latter resulting from the attractive reaction of dislocations with (a/2) 〈111〉 Burgers vectors. Owing to the large unit cell of YIG, experimental imaging conditions prevented from concluding on the possibility of dislocation dissociation.

Microstructural defects induced by implantation of hydrogen in (111) silicon

Abstract Silicon crystals have been implanted with hydrogen at high energy. In order to obtain a constant distribution of hydrogen through a wide zone located from 20 to 50 pm from the surface, 23 different energies have been used, ranging from 1 to 2 MeV. The samples have then been annealed 30 min at temperatures ranging from 100 to 1100P°C and examined by transmission electron microscopy (TEM). Only two types of planar defects lying in {111} planes have been observed: fault-like defects (F) and loop-like defects (L). The present paper reports a detailed TEM study of the structure of the F and L defects observed in a (111) crystal. We propose a model which gives a coherent interpretation of the various characteristics of these defects.

Dislocation dissociation widths in silicon at low temperature under controlled high-stress orientations

Abstract Dissociation widths of dislocations in silicon submitted to high stress and low temperature (HS/LT) are reported. In the first part of the paper, force systems (glide and climb components) acting on a pre-existing dissociated dislocation loop are analysed in terms of the deformation compression axis K. Then two main force systems are experimentally examined: (i) static cases for which the whole dislocation is at rest while partials are submitted to forces with directions which should result in a narrowing or a widening of the stacking fault ribbon; and (ii) dynamic cases (K = 〈120〉) with very different force systems. In all cases stacking faults have been found to be intrinsic. Climb forces seem to act on friction forces of 30[ddot] partials according as to whether they are positive or negative. Models developed by Alexander to explain the mobility of dislocations in the HS/LT conditions cannot account for our results. It is suggested that point defects could have an important effect on the mobil...

Transmission electron microscope observation of the {001} 〈110〉 glide system in spinel nickel ferrite at room temperature

Abstract It is confirmed that the fine structure of screw dislocations in the spinel structure consists of a core appreciably dissociated in a {001} plane. An example of a {001} 〈110〉 glide system is given by electron microscope analysis of a deformed area close to a room-temperature Vickers indentation. It is concluded that isolated dislocations behave as if their glide were easier on {001} planes than on {111} planes at room temperature.