Characterization of dislocations in anisotropic materials by large angle convergent beam electron diffraction

It is shown that the characterization of the Burgers vector of dislocations, by large angle convergent beam electron diffraction, using the Cherns and Preston rules, is possible in elastically anisotropic materials, provided some care is taken in the choice of the Bragg lines used for the analysis

Study of Dislocations Generated by Thermal Cycling in Ni-Ti-Co Shape Memory Alloys

In the present work we report some results concerning the evolutions of transformation temperatures and microstructure during thermal cycling in Ti- 47%Ni - 3% Co (atomic percent). In the case of bulk material (obtained by a conventional solidification method) high amounts of dislocations form during cycling, which produce a considerable evolution of the transformation temperatures (up to ~40K), while in meltspun material containing a dispersion of fine precipitates, the formation of dislocations during thermal cycling is highly reduced, giving rise to stable transformation temperatures. The dislocation characteristics have been analysed by means of two-beam condition observations as well as the Large Angle Convergent Beam Electron Diffraction technique. Some energetical considerations concerning the dislocations are pointed out to interpret the evolution of transformation temperatures

Characterization of grain boundary misorientations in a alumina by large angle convergent beam electron diffraction

Large Angle Convergent Beam Electron Diffraction (LACBED) is used in order to characterize accurately the misorientation between grains situated on each side of grain boundaries in [MATH] alumina. It is shown that this method leads to an improved accuracy for the determination of misorientations between crystals at interfaces, when compared with the traditional diffraction and Kikuchi line techniques

ÉTUDE PAR MICROSCOPIE ÉLECTRONIQUE À BALAYAGE ET SPECTROMÉTRIE AUGER DE RUPTURES FRAGILES DU TUNGSTÈNE

Les effets de l'impureté phosphore et de la forme des grains sur la temperature de transition ductile-fragile et les faciès de rupture fragile du tungstène sont examinés par microscopie électronique à balayage et spectromètrie AUGER.The effect of phosphorous grain boundary segregation and of grain morphology on brittle to ductile transition temperatures and on metallographic aspects of brittle fracture are examined in tungsten samples by means of scanning electron microscopy and AUGER electron spectroscopy

DEFECTS IN Fe7C3 TYPE CARBIDE FORMED DURING THE CRYSTALLIZATION OF AMORPHOUS HIGH CARBON ALLOYS AND THEIR RELATION WITH THE AMORPHOUS STATE

Les carbures de type M7C3 ont une structure orthorhombique (R. FRUCHART et al.). Cette structure peut être masquée par des défauts qui sont propres au mode de préparation. Dans le cas du carbure Fe7C3 formé lors de la recristallisation d'un alliage amorphe Fe75C25 préparé par sputtering, la taille des défauts (inférieure à 100 Å) est du même ordre de grandeur que celle des domaines allongés suivant certaines directions. Pour les carbures primaires présents dans les fontes blanches et les aciers au carbone, les domaines sont environ cent fois plus grands. Ces résultats suggèrent que la croissance des domaines est bloquée pendant la cristallisation de l'amorphe. Un très grand nombre de germes peut déjà exister dans l'état amorphe ou se former très rapidement. L'apparition de Fe7C3 est discutée dans le cadre du modèle de GASKELL.M7C3 type carbides have an orthorhombic structure, as proposed by R. FRUCHART et al. This structure may be hidden by defects which are typical of the preparation method. In Fe7C3 formed by crystallization of sputter-deposited amorphous Fe75C25 alloys, the defects are only consistent with elongated domains whose size is less than [MATH] 100 Å, the order of the defect size. In primary M7C3 carbides, present in white cast-iron and chromium steels, domains are about 100 times larger than in the latter case. Both results suggest that domain growth is impeded during the crystallization of amorphous Fe75C25. Nuclei may thus already exist in or be easily formed with a large density from the amorphous state. The formation of Fe7C3 is discussed within the frame of the GASKELL model

Surface engineering by thermal spraying nanocrystalline coatings: X-ray and TEM characterisation of As-deposited iron aluminide structure

Iron Aluminide coatings were produced by thermal spraying atomized (microcrystalline) and milled (nanocrystalline) powders using the High Velocity Oxy-Fuel (HVOF) and the Atmospheric Plasma Spraying (APS) techniques. The as-sprayed coatings were investigated by X-ray diffraction. In particular, X-ray line shape broadening analysis was used to determine the size of the coherently diffracting domains and the internal microstrains. Significant peak broadening was observed whatever the thermal spraying technique and the starting feedstock powder. This surprising result is discussed, at the light of SEM and TEM observations, by considering chemical composition heterogeneities and the presence of structural defects in the coatings.Des revêtements d'aluminure de fer ont été projetés à l'aide des techniques HVOF et APS en utilisant comme précurseur de la poudre atomisée à structure microcristalline ou de la poudre à structure nanocristalline obtenue par broyage mécanique. La microstructure des revêtements a été analysée par diffraction des rayons X en utilisant, en particulier, des analyses de profils de raies. Des élargissements importants des raies de diffraction ont été mesurés quelque soit la technique de projection utilisée ou la nature des précurseurs. Ces résultats surprenants sont discutés, à l'aide d'observations effectuées en MEB et MET, en prenant en compte les gradients chimiques et la présence de défauts structuraux observés dans les revêtements