Two distinctly different morphologies of Mo5Si3 precipitates have been observed in a single cryst... more Two distinctly different morphologies of Mo5Si3 precipitates have been observed in a single crystal MoSi2. Both precipitates are laths elongated along the (110) direction common to the matrix and the precipitate but differ in their cross-sectional shape and lattice orientation. Type I precipitates exhibit a rectangular cross-section, with interfaces parallel to low-index planes (110)m∥(002)p and (002)m∥(220)p, while type II precipitates are parallelograms, with their major interface at 13° to the low-index planes (002)m∥(220)p (the subscripts m and p denote the MoSi2 matrix and Mo5Si3 precipitate, respectively). The orientation relation-ships corresponding to the two characteristic morphologies differ by a 1 8° rotation around the lath axis. A periodic array of dislocation loops and associated ledges enveloping the precipitates were revealed by high-resolution electron microscopy and selected area electron diffraction. The Burgers vector of these dislocations was determined unambiguously from high-resolution images in orthogonal viewing directions. The differences between the two characteristic morphologies and their orientation relationships are due to a difference in the stacking sequence of ledges.
Hot-stage high voltage electron microscopy (HVEM) combined with high resolution electron microsco... more Hot-stage high voltage electron microscopy (HVEM) combined with high resolution electron microscopy (HREM) has been employed to study the microstructural evolution during in-situ decomposition of thin Mg(OH)â flakes and during early stage sintering of the resultant nanocomposite. The decomposition reaction is known to be pseudomorphic and topotactic, resulting in a regular composite of highly nearly cubic MgO nanocrystals interspersed with
ABSTRACTA “map” of all possible high-resolution images may be simulated for a crystalline specime... more ABSTRACTA “map” of all possible high-resolution images may be simulated for a crystalline specimen in a chosen orientation for any particular transmission electron microscope (HRTEM). These maps are useful during experimental high-resolution electron microscopy and make it possible to locate optimum imaging conditions even for foil thicknesses beyond the weak-phase object limit. Although defects such as grain boundaries are not generally periodic, image maps of perfect crystal can be used to optimize defect contrast during operation of the microscope by reference to the image of the perfect crystal neighboring the defect.
Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates us... more Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates using physical vapor deposition (PVD). The resulting microstructure was that of a mazed bicrystal in which two equivalent grain orientations, related to each other by a 90° rotation, are arranged in a morphology of irregularly shaped, convoluted grains. Quantitative morphological analysis showed a strong dependence of grain shape on size, with larger grains being more convoluted and smaller grains more compact. The evolution of grain size, anisotropy and shape during heating in the temperature range from 300-340 °C was studied by in-situ transmission electron microscopy (TEM).
The study of enclosed crystal shapes in relation to their interface structure is an area of phase... more The study of enclosed crystal shapes in relation to their interface structure is an area of phase transformations rich in new and interesting phenomena. It is shown that transmission electron microscopy (TEM) can make an important contribution to the understanding and control of materials properties through the experimental study of these phenomena by conventional, analytical, high resolution, diffraction and dynamic in-situ techniques. There is a close connection between the morphology, interface structure, defect substructure, and formation mechanism of precipitates in a solid matrix. Complementary to static observations, dynamic in-situ experiments by high voltage electron microscopy show that germanium precipitates in an aluminum matrix can be transformed between a strongly faceted, anisotropic shape and a rounded, isotropic shape.
We present a theoretical analysis, together with a simulation, of a grain boundary in gold that h... more We present a theoretical analysis, together with a simulation, of a grain boundary in gold that has been studied by high resolution electron microscopy. This interface is non symmetric and non periodic. Using a suitable interatomic potential, we build a model whose atomic positions are found to be in agreement with the electron microscopy images. We describe this structure, related to the irrational number &surd;2, as a periodic structure in a 4D hyper-space using the quasicrystal theory. The hypo-friction property of the interface can be derived from this analysis. It turns out that this system is analogous to the simple linear chain model of Frenkel-Kontorova. Using a minimum energy path calculation, we test numerically that the static friction force is zero and we find the corresponding atomic mechanism that allows a grain to slide on the other one.
Twinning plays an important role in phase transformations and can have significant effects on mic... more Twinning plays an important role in phase transformations and can have significant effects on microstructural evolution. Different roles of twinning in the development of microstructures during precipitation and phase transformations are reviewed and illustrated with examples from investigations by high-resolution electron microscopy, including the effect of multiple twinning on the development of Ge precipitates in Al-Ge and Ag-Ge alloys, the twin dissociation of grain boundaries in Au, the formation of hexagonal Si at twin intersections and the effect of twin boundaries on the equilibrium shape of Pb inclusions in Al.
Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tenness... more Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.
Al-Cu-Si-Ge alloys display a unique combination of ultrarapid aging response, high peak hardness,... more Al-Cu-Si-Ge alloys display a unique combination of ultrarapid aging response, high peak hardness, and extended-aging microstructural stability. The purpose of this work is to explain these properties in terms of the role that the Si-Ge additions have on modifying the conventional Al-Cu aging sequence. In both AlCu and AlCuSiGe, the room-temperature microstructure consists of both Guinier-Preston (GP) zones and ϑ″
Two distinctly different morphologies of Mo5Si3 precipitates have been observed in a single cryst... more Two distinctly different morphologies of Mo5Si3 precipitates have been observed in a single crystal MoSi2. Both precipitates are laths elongated along the (110) direction common to the matrix and the precipitate but differ in their cross-sectional shape and lattice orientation. Type I precipitates exhibit a rectangular cross-section, with interfaces parallel to low-index planes (110)m∥(002)p and (002)m∥(220)p, while type II precipitates are parallelograms, with their major interface at 13° to the low-index planes (002)m∥(220)p (the subscripts m and p denote the MoSi2 matrix and Mo5Si3 precipitate, respectively). The orientation relation-ships corresponding to the two characteristic morphologies differ by a 1 8° rotation around the lath axis. A periodic array of dislocation loops and associated ledges enveloping the precipitates were revealed by high-resolution electron microscopy and selected area electron diffraction. The Burgers vector of these dislocations was determined unambiguously from high-resolution images in orthogonal viewing directions. The differences between the two characteristic morphologies and their orientation relationships are due to a difference in the stacking sequence of ledges.
Hot-stage high voltage electron microscopy (HVEM) combined with high resolution electron microsco... more Hot-stage high voltage electron microscopy (HVEM) combined with high resolution electron microscopy (HREM) has been employed to study the microstructural evolution during in-situ decomposition of thin Mg(OH)â flakes and during early stage sintering of the resultant nanocomposite. The decomposition reaction is known to be pseudomorphic and topotactic, resulting in a regular composite of highly nearly cubic MgO nanocrystals interspersed with
ABSTRACTA “map” of all possible high-resolution images may be simulated for a crystalline specime... more ABSTRACTA “map” of all possible high-resolution images may be simulated for a crystalline specimen in a chosen orientation for any particular transmission electron microscope (HRTEM). These maps are useful during experimental high-resolution electron microscopy and make it possible to locate optimum imaging conditions even for foil thicknesses beyond the weak-phase object limit. Although defects such as grain boundaries are not generally periodic, image maps of perfect crystal can be used to optimize defect contrast during operation of the microscope by reference to the image of the perfect crystal neighboring the defect.
Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates us... more Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates using physical vapor deposition (PVD). The resulting microstructure was that of a mazed bicrystal in which two equivalent grain orientations, related to each other by a 90° rotation, are arranged in a morphology of irregularly shaped, convoluted grains. Quantitative morphological analysis showed a strong dependence of grain shape on size, with larger grains being more convoluted and smaller grains more compact. The evolution of grain size, anisotropy and shape during heating in the temperature range from 300-340 °C was studied by in-situ transmission electron microscopy (TEM).
The study of enclosed crystal shapes in relation to their interface structure is an area of phase... more The study of enclosed crystal shapes in relation to their interface structure is an area of phase transformations rich in new and interesting phenomena. It is shown that transmission electron microscopy (TEM) can make an important contribution to the understanding and control of materials properties through the experimental study of these phenomena by conventional, analytical, high resolution, diffraction and dynamic in-situ techniques. There is a close connection between the morphology, interface structure, defect substructure, and formation mechanism of precipitates in a solid matrix. Complementary to static observations, dynamic in-situ experiments by high voltage electron microscopy show that germanium precipitates in an aluminum matrix can be transformed between a strongly faceted, anisotropic shape and a rounded, isotropic shape.
We present a theoretical analysis, together with a simulation, of a grain boundary in gold that h... more We present a theoretical analysis, together with a simulation, of a grain boundary in gold that has been studied by high resolution electron microscopy. This interface is non symmetric and non periodic. Using a suitable interatomic potential, we build a model whose atomic positions are found to be in agreement with the electron microscopy images. We describe this structure, related to the irrational number &surd;2, as a periodic structure in a 4D hyper-space using the quasicrystal theory. The hypo-friction property of the interface can be derived from this analysis. It turns out that this system is analogous to the simple linear chain model of Frenkel-Kontorova. Using a minimum energy path calculation, we test numerically that the static friction force is zero and we find the corresponding atomic mechanism that allows a grain to slide on the other one.
Twinning plays an important role in phase transformations and can have significant effects on mic... more Twinning plays an important role in phase transformations and can have significant effects on microstructural evolution. Different roles of twinning in the development of microstructures during precipitation and phase transformations are reviewed and illustrated with examples from investigations by high-resolution electron microscopy, including the effect of multiple twinning on the development of Ge precipitates in Al-Ge and Ag-Ge alloys, the twin dissociation of grain boundaries in Au, the formation of hexagonal Si at twin intersections and the effect of twin boundaries on the equilibrium shape of Pb inclusions in Al.
Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tenness... more Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.
Al-Cu-Si-Ge alloys display a unique combination of ultrarapid aging response, high peak hardness,... more Al-Cu-Si-Ge alloys display a unique combination of ultrarapid aging response, high peak hardness, and extended-aging microstructural stability. The purpose of this work is to explain these properties in terms of the role that the Si-Ge additions have on modifying the conventional Al-Cu aging sequence. In both AlCu and AlCuSiGe, the room-temperature microstructure consists of both Guinier-Preston (GP) zones and ϑ″
Uploads