Jean-Pierre Michel

Dislocations at grain boundaries in deformed silicon

Abstract Regions adjacent to Σ = 9 and Σ = 25 boundaries in silicon bicrystals are found to be in an advanced stage of hardening. Electron microscopy shows multiple slip, cross-slip, network formation and some transmission of slip across the boundary.

Plastic deformation of gallium antimonide single crystals : yield stresses, activation parameters and dislocation structures

Abstract Single crystals of undoped GaSb were deformed by compression at low strain rates from 0.48 Tm to 0.3 T m (where T m is the melting temperature). The activation enthalpy derived by standard techniques at the yield stress in pre-strained GaSb is about 1.6eV in fair agreement with activation energies for dislocation velocities. The dislocation structures were estimated by transmission electron microscopy. A progressive transition is observed from curved dislocations and edge-type lenticular loops and dipoles at high temperatures and low stresses to predominant long screw dislocations at low temperatures and high stresses. This transition is explained in terms of dislocation-dislocation interactions as against Peierls lattice friction, the onset of thermally activated cross-slip and the large anisotropy of dislocation velocities. Quantitative estimates show that screw dislocations are rate controlling and give the major contribution to the plastic strain at T 10MPa.

Plastic deformation, extended stacking faults and deformation twinning in single crystalline indium phosphide. 2. S doped InP

Abstract Sulphur doped InP single crystals were deformed by compression in dual and multislip conditions at a low shear strain rate (∼2 × 10−5 s−1) and moderate temperature (∼ 0.4T m). Hardening curves show a very different behaviour from pure InP, although the S does not have a net hardening effect at the yield point for the investigated testing conditions, contrarily to what is observed at higher temperatures. Transmission electron microscope observations of deformed samples have revealed extended stacking faults similar to those observed in pure InP, but also numerous twins or microtwins which were not observed in the latter case. Partial dislocation configurations suggest that pole mechanisms are operative. No evidence could be found that extrinsic stacking faults act as twin nuclei. The role of S in twinning is discussed.

On the yield point of floating-zone silicon single crystals II. A quantitative analysis of the dislocation structure at the lower yield point

Abstract The dislocation structures in silicon single crystals deformed up to the lower yield point at moderate temperatures (823 K ≲ T ≲ 1073 K) and low shear strain rates (γ<2×10−4s−1) were estimated by transmission electron microscopy. Both the total dislocation density ρt and the mobile-dislocation density ρm could be measured rather accurately, at least when the dislocations lie preferentially along ⟨110⟩ orientations. Local shear stresses were estimated from the radius of curvature at dislocation bends. With the distribution of local stresses and the known dislocation velocities, an average velocity v and the corresponding effective stress τeff could be derived. The internal consistency of the measured ρm and τeff was checked using Orowans relation where b is the Burgers vector, and was found to be good at sufficiently high stresses (τ1y ≳ 10 MPa), giving support to the view that, in those conditions, τeff is probably much closer to τly than predicted by current models.

TEM observations of dislocations in aluminium nitride after high temperature deformation

Abstract AlN ceramics were deformed up to 10% strain in compression at elevated temperatures (1820–1920 K) under constant stress in the range 150–250 MPa or at a constant fixed strain rate of 5×10−6 s−1. Several kinds of dislocations were identified by TEM in deformed crystals. Most of them have a → type Burgers vectors and were seen to glide in the basal plane or in prismatic planes. They looked usually undissociated but large dissociations were observed after deformation tests of long durations. In such cases, a → dislocations could be dissociated either by pure glide in two Shockley partials or by combined glide plus climb in two Frank–Shockley partials. In any case, the enclosed stacking fault was in the basal plane. c → + a → dislocations were also observed but seldom. Different kinds of faulted loops are reported, delineated by Shockley, Frank or Frank–Shockley partials.Indications are given that oxygen segregation should play a role in the dissociation of dislocations.

On the yield point of floating-zone silicon single crystals. III: The evolution of the dislocation structure through the yield point at low temperature

Abstract The dislocation structures in silicon single crystals deformed up to different points in the yield region, at 823 K and at the shear strain rate γ of 2 × 10−5 s−1, have been estimated by transmission electron microscopy. In addition to the total dislocation density ρt, the mobile dislocation density ρm can be measured with reasonable accuracy, thanks to the characteristic shape of dislocation loops. Local shear stresses are estimated from the radius of curvature at dislocation bends. Estimation of the effective stress τeff is difficult before the lower yield point because of micro-scale inhomogeneities. The internal consistency of the measured τeff and ρ has been checked using Orowans relation, leading to the conclusion that, in the experimental conditions realized, τeff is very close to the applied stress and nearly all dislocations are mobile from the onset of plastic deformation up to the lower yield point.

Dislocations and extended defects in AlN deformed at high temperatures

Abstract Polycrystalline AlN could be deformed up to 10% strain in compression at elevated temperatures (1820–1920 K) under constant stress in the range 150–250 MPa or at a fixed strain rate of 5.6 × 10 −6 s −1 . Stress-strain curves show a marked temperature dependence. TEM investigations point to cavitation at grain boundaries as the main source of damage. Intragranular plasticity is achieved mainly by a-type dislocations. A few dislocations with c + a Burgers vectors could also be observed.

The Relation Between Flow Stress and Dislocation Structure in Gallium Antimonide

ABSTRACT Single crystals of undoped GaSb were deformed by compression at low strain rates from 0.48 Tm to 0.73 Tm (Tm melting temperature). The activation enthalpy derived from activation volume measurements at the yield stress is about 1.6 eV in fair agreement with dislocation velocity measurements. Systematic TEM observations in the “stress-applied state” revealed a progressive transition from curved dislocations and edge dipoles at high temperature and low stress to predominant long screw dislocations at low temperature and high stress. Screw dislocations are rate-controlling at T 10 MPa.

Cross Slip and Latent Hardening in Silicon Single Crystals

ABSTRACT A two-step deformation experiment, designed to characterize the cross slipping process and first successfully performed in copper single crystals by Bonneville and Escaig (1) was repeated in silicon. In contrary to the case of Cu, no evidence of cross-slip could be observed in Si. Critical shear stresses are consistently explained in terms of latent hardening.