Intragranular plasticity of β Si3N4 between 20 °C and 700 °C

Intragranular plasticity induced by indentation is studied in β Silicon Nitride between room temperature and 700°C by Transmission Electron Microscopy (TEM). It is shown that all the dislocations resulting unambiguously from plastic deformation have a Burgers vector b = [0001] and a dominant screw character. This can account for a large Peierls stress along the [0001] direction and a larger mobility of edge dislocation segments as compared to screw ones. Dislocations bowing out of screw direction are found to glide in {10-10} prismatic planes. Cross-slip is evidenced from the dislocation microstructure observations. It is found to occur between {10-10} prismatic planes at 20°C whereas {11-20} is found to be a new deviation plane at 700°C. Those observations are discussed in relation with the possible core structure of [0001] screw dislocations

Stacking faults and phase transformations in silicon nitride

From observations of extended dislocation nodes in β silicon nitride, possible stacking fault structures in the basal plane of this compound have been investigated. It has been found that stacking fault structure is locally analogous to α silicon nitride. A phase transformation α to β or β to α can also be achieved by cooperative shear of partial dislocations with $1/3\langle1\bar{1}00\rangle$ Burgers vectors

Consolidation of iron powders through the influence of phosphate thin films

In this work, the compaction and sintering behaviour of iron-phosphated powders have first been studied. Then, the iron powders consolidation mechanism has been investigated under the influence of the phosphating treatment, by means of mechanical tests. Iron powders with spongy grains have been used in order to be first phosphated. Then, green compacts with both untreated and phosphated grains have been prepared. The compaction behaviour of the powders has been analysed using the Heckel law. It indicates that the phosphating process does not influence the compaction behaviour. Besides, a desaggregation stage of the powders occurs at the end of the compaction process. The subsequent sintering of these green compacts does not induce any significant evolution of the densities values. Moreover, the sintering conditions have been optimised in order to be a compromise between the persistence of the phosphate film and the strengthening. Then, strength properties of compacts made of densified iron powders are studied and the influence of the initial phosphating treatment is explored. The mechanical strength increases under the influence of either the phosphating treatment or the sintering process. Moreover, when both are combined, it leads to the better characteristics. The phosphate film acts as a binding agent and the corresponding green body gets its strength only by the retentive force of the metallic phosphate. By sintering, the sample's strength is increased by metallic bonds that mainly occur by necks formation. In sintered compacts obtained from phosphated iron powders, the strength is the most important as revealed by the fracture morphology where transgranular brittle fracture is indicative of reinforced links between powder grains. © 2007 Elsevier B.V. All rights reserved

Ion beam nitriding of single and polycrystalline austenitic stainless steel

Article Number: 083531Polycrystalline and single crystalline [orientations (001) and (011)] AISI 316L austenitic stainless steel was implanted at 400 degrees C with 1.2 keV nitrogen ions using a high current density of 0.5 mA cm(-2). The nitrogen distribution profiles were determined using nuclear reaction analysis (NRA). The structure of nitrided polycrystalline stainless steel samples was analyzed using glancing incidence and symmetric x-ray diffraction (XRD) while the structure of the nitrided single crystalline stainless steel samples was analyzed using x-ray diffraction mapping of the reciprocal space. For identical treatment conditions, it is observed that the nitrogen penetration depth is larger for the polycrystalline samples than for the single crystalline ones. The nitrogen penetration depth depends on the orientation, the being more preferential for nitrogen diffusion than . In both type of samples, XRD analysis shows the presence of the phase usually called "expanded" austenite or gamma(N) phase. The lattice expansion depends on the crystallographic plane family, the (001) planes showing an anomalously large expansion. The reciprocal lattice maps of the nitrided single crystalline stainless steel demonstrate that during nitriding lattice rotation takes place simultaneously with lattice expansion. The analysis of the results based on the presence of stacking faults, residual compressive stress induced by the lattice expansion, and nitrogen concentration gradient indicates that the average lattice parameter increases with the nitrided layer depth. A possible explanation of the anomalous expansion of the (001) planes is presented, which is based on the combination of faster nitriding rate in the (001) oriented grains and the role of stacking faults and compressive stressVytauto Didžiojo universiteta

Evolution of the nanoporous microstructure of sintered Ag at high temperature using in-situ X-ray nanotomography

International audienceSilver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction, thus reducing the density of the material compared to bulk silver. The mechanical properties (Young's modulus, yield strength and ultimate tensile stress) are then drastically altered. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal aging and more specifically during temperature jumps endured by the assembly during operation. Here, high temperature evolutions of the 3D nanoporous structure have been observed in-situ using a heater fitted into the beamline 6-2c at SLAC-SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal a complex evolution of the porous nanostructure including growth, separation and coalescence at a constant density. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service

Evolution of the nanoporous microstructure of sintered Ag at high temperature using in-situ X-ray nanotomography

International audienceSilver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction, thus reducing the density of the material compared to bulk silver. The mechanical properties (Young's modulus, yield strength and ultimate tensile stress) are then drastically altered. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal aging and more specifically during temperature jumps endured by the assembly during operation. Here, high temperature evolutions of the 3D nanoporous structure have been observed in-situ using a heater fitted into the beamline 6-2c at SLAC-SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal a complex evolution of the porous nanostructure including growth, separation and coalescence at a constant density. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service

Evolution of the Thermal Conductivity of Sintered Silver Joints with their Porosity Predicted by the Finite Element Analysis of Real 3D Microstructures

International audienceSilver paste sintering is a very promising technology for chip bonding in future power electronics modules owing to its high melting temperature and the good electrical and thermal properties among other classic solder alloys. However, in its sintered form, these joints contain nanometric/submicrometric pores that affect their thermal performance. The present study gives insight into the relationship between the material thermal conductivity and the real three-dimensional porous structure using finite element modelling. It is shown that over a certain pore fraction threshold (∼ 13%), the pore morphology has a non-negligible influence on the thermal conductivity. Results are also compared to predictions obtained by analytical models available in the literature