V. Audurier

AIN plastic deformation between room temperature and 800°C. I. Dislocation substructure observations

Abstract Dislocations substructures in aluminium nitride (AIN) ceramics have been studied by transmission electron microscopy for samples plastically deformed under a high stress in the temperature range 20–800°C. The activation of basal slip ⅓ 〈1120〉(0001) as well as of the first-kind prismatic slip ⅓ 〈1120〉{1100} have been observed. In the basal plane, dislocations configurations attest to the role of the Peierls forces along the screw orientation. It is easier for a dislocation to cross-slip in the prismatic plane than to lie in the basal plane along an orientation other than the screw direction. In the basal plane as well as in the prismatic plane, no dissociation of dislocations has been resolved under weak-beam imaging conditions. Cross-slip is favoured by numerous pinning points on screw dislocations, giving rise to dipole dragging. From 20 to 500°C, AIN deformation proceeds by dislocation glide in the basal and the prismatic planes and frequent corss-slip between these two planes. At higher temper...

AIN plastic deformation between room temperature and 800°C. II. Dislocation core structures in basal and prismatic planes

Observations of dislocation substructures have shown that the basal and first-kind prismatic planes are easily activated in plastically deformed aluminium nitride between room temperature and 800°C as shown in the companion paper, part I. In this paper, several analyses are performed to determine the ability of dislocations to glide in one plane relative to others. According to the closest-packed criterion, the first-kind prismatic plane appears to be favoured. Line energy calculations under anisotropic elasticity do not allow one to conclude the preferential activation of one of the glide planes. The crystallographic structures of basal and first-kind prismatic planes, core structures of dislocations and dissociation reactions are investigated and stacking-fault configurations in prismatic planes are proposed.

Elastic stress field beneath an arbitrary axisymmetric punch

The nanoindentation test is commonly used for the local determination of mechanical properties (hardness, elastic modulus, etc.) and also to study the initial stages of plasticity (dislocation nucleation, dislocation interaction mechanisms) at the nanometre scale. In the latter case, the determination of the elastic stress field beneath the indenter is of primary interest. An analytical expression is derived for the elastic stress and strain fields beneath an axisymmetric punch. Most solutions, in the literature, are given for simple indenter shapes, such as flat, conical or spherical indenters. The complete solution proposed for an arbitrary indenter profile is described by a power law, in which the exponent can be an integer or not. The stress is given as the real part of complex analytical expressions.

Strain Build-Up, Swelling and Stacking Fault Formation in Implanted 4H-SiC

Ion implantation into 4H-SiC induces a local gradient of strain which increases with the nuclear energy losses. With the increase of temperature the strain tends to become uniform in the whole implanted area requiring the migration of particles. In case of helium implantation, defects are more stabilized and their evolutions observed post thermal annealing are concomitant with the surface swelling. The local modifications imputed to the ion process lead to the formation and the pile-up of stacking faults in the highly damaged region.

Effects of Helium Implantation on the Mechanical Properties of 4H-SiC

The evolution of mechanical properties of helium-implanted 4H-SiC at room temperature has been mainly studied by nanoindentation tests. The curves of hardness and elastic modulus present a maximum at low levels of damage while a degradation of the mechanical properties is observed for high levels of damage. However, when the concentration of implanted ions exceeds 0.5 %, complex defects (helium-vacancy defects) become predominant which results in the increase of both the hardness and the modulus. Under high fluence of helium implantation tiny bubbles form and the amorphous transition is observed above a critical level of damage.

Helium Implantation Damage in SiC

Single crystals SiC were implanted with 50 keV helium ions at room temperature and fluences in the range 1x1016 -1x1017 cm-2. The helium implantation induced swelling was studied through the measurement of the step height. The damage was studied by using X-ray diffraction measurements and the transmission electron microscopy observations. Degradation of mechanical properties is found after helium implantation.