Robert Pond - Academia.edu (original) (raw)
Papers by Robert Pond
Metallurgical and Materials Transactions A, 2006
Precipitate growth in a duplex stainless steel and a titanium aluminide alloy has been studied us... more Precipitate growth in a duplex stainless steel and a titanium aluminide alloy has been studied using transmission electron microscopy. Particles with similar crystallography and acicular form arise in both cases, and are bounded by two principal facets. One facet, designated C, is based on a commensurate singular interface structure, and the coherency strains are accommodated by interfacial defects. The other facet, designated I, is based on a singular configuration that is incommensurate in one dimension. The orientation relationship (OR) between the particle and the matrix for the singular C structure is Kurdjumov-Sachs (K-S), whereas that for the I facet is Pitsch. The angular incompatibility between these two types of facets must be accommodated to minimize the displacement field as particles grow. The present observations suggest that this is accomplished through the generation of crystal dislocations at facet junctions and their subsequent climb along the facets. The total density of defects needed to accommodate the angular discrepancy is fixed, but the partitioning of these dislocations between C and I facets is not. The actual partitioning determines the observed OR for a particle, and is determined by the kinetics of climb, which is likely to be different in the two facets. In the stainless steel, the observations are consistent with climb occurring in both the C and I facets, but faster in the I facet, leading to a distribution of observed ORs that is skewed away from Pitsch toward K-S. In the titanium aluminide alloy, no climb into the C facets was found, so a unique OR close to K-S arises.
MRS Proceedings, 1988
ABSTRACT
MRS Proceedings, 1999
ABSTRACT
MRS Proceedings, 1988
ABSTRACT
ABSTRACT The structure of disconnections in symmetrical low- and high-angle [0001] tilt boundarie... more ABSTRACT The structure of disconnections in symmetrical low- and high-angle [0001] tilt boundaries in an hcp metal are studied using atomic-scale simulation. Applied engineering strains cause such defects to move conservatively along the boundaries, producing coupled shear and migration. The Peierls stresses causing such motion are found to decrease precipitously through the transition from low- to high-angle boundaries. The reason underlying this behaviour is discussed.
Materials Science and Engineering: A, 2006
ABSTRACT A new model of martensitic transformations has been presented recently where the habit p... more ABSTRACT A new model of martensitic transformations has been presented recently where the habit plane consists of segments of coherent terraces reticulated by an array of line defects. These defects accommodate any coherency strains present and the transformation is effected by lateral motion of disconnections across the terraces. Provided the terraces and line defects satisfy certain criteria, the mechanism of transformation is conservative. Moreover, the topological parameters of the defects (Burgers vectors and step heights) can be determined rigorously enabling the overall habit plane inclination and orientation relationship to be determined.The objective of the present paper is to compare predictions for the habit planes according to this topological method with those of the classical theories in the case of transformations in ZrO2 and Ti. The two approaches lead to disparate solutions, and the origin and magnitude of the disparities are elucidated. Unlike habit planes in the classical model, which are invariant planes, topological ones are misfit-relieved semi-coherent configurations.
Interface Science, 2000
ABSTRACT
Journal of Materials Science, 2008
The classical theory of the crystallography of martensitic transformations developed in the 1950s... more The classical theory of the crystallography of martensitic transformations developed in the 1950s is based on the notion that the interface between the parent and product phases is an invariant plane of the shape deformation. Underlying this hypothesis is the expectation that such interfaces do not exhibit long-range strain, and the geometric theory is an algorithm for finding invariant planes, the orientation relationship and transformation displacement. In the context of ferrous alloys, the classical theory has been applied successfully to transformations with {295} habit planes, but is less satisfactory for {575} for example. A new model of martensitic transformations has been presented recently based on dislocation theory, incorporating developments in the understanding of the topological properties of interfacial defects. Topological arguments show that glissile motion of transformation dislocations, or disconnections, can only occur in coherent interphase interfaces. Hence, the interface in the model comprises coherent terraces with a superimposed network of disconnections and crystal dislocations. It is demonstrated explicitly that this defect network accommodates the coherency strains, and that lateral motion of the disconnections across the interface effects transformation in a diffusionless manner. Moreover, it is shown that a broader range of habit planes is predicted on the basis of the semi-coherent interface model than the invariant plane notion. In the case of ferrous alloys, it will be shown that a range of viable solutions arise which include {575}.
Acta Materialia, 2000
AbstractÐThe displacive±diusive transformation, which gives rise to lamellar g-TiAl+a 2 -Ti 3 Al ... more AbstractÐThe displacive±diusive transformation, which gives rise to lamellar g-TiAl+a 2 -Ti 3 Al microstructures, has been analysed theoretically. Using the formulation due to Hirth and Pond, the shear produced and extent of diusional¯ux associated with interfacial defect motion can be quanti®ed in terms of the Burgers vectors, step heights of the defects and the chemical compositions of the adjacent crystals. Moreover, this formulation also enables the eects of chemical composition, mis®t at the interface and ordering to be investigated systematically. It is shown that for the defects observed on such interfaces in recent high-resolution transmission electron microscopy studies, their motion would lead to the conservation of sites and, correspondingly, to equal and opposite¯uxes of Ti and Al atoms. Consequently, although this transformation is diusional, it exhibits crystallographic features consistent with martensitic processes. 7
Metallurgical and Materials Transactions A, 2006
Precipitate growth in a duplex stainless steel and a titanium aluminide alloy has been studied us... more Precipitate growth in a duplex stainless steel and a titanium aluminide alloy has been studied using transmission electron microscopy. Particles with similar crystallography and acicular form arise in both cases, and are bounded by two principal facets. One facet, designated C, is based on a commensurate singular interface structure, and the coherency strains are accommodated by interfacial defects. The other facet, designated I, is based on a singular configuration that is incommensurate in one dimension. The orientation relationship (OR) between the particle and the matrix for the singular C structure is Kurdjumov-Sachs (K-S), whereas that for the I facet is Pitsch. The angular incompatibility between these two types of facets must be accommodated to minimize the displacement field as particles grow. The present observations suggest that this is accomplished through the generation of crystal dislocations at facet junctions and their subsequent climb along the facets. The total density of defects needed to accommodate the angular discrepancy is fixed, but the partitioning of these dislocations between C and I facets is not. The actual partitioning determines the observed OR for a particle, and is determined by the kinetics of climb, which is likely to be different in the two facets. In the stainless steel, the observations are consistent with climb occurring in both the C and I facets, but faster in the I facet, leading to a distribution of observed ORs that is skewed away from Pitsch toward K-S. In the titanium aluminide alloy, no climb into the C facets was found, so a unique OR close to K-S arises.
MRS Proceedings, 1988
ABSTRACT
MRS Proceedings, 1999
ABSTRACT
MRS Proceedings, 1988
ABSTRACT
ABSTRACT The structure of disconnections in symmetrical low- and high-angle [0001] tilt boundarie... more ABSTRACT The structure of disconnections in symmetrical low- and high-angle [0001] tilt boundaries in an hcp metal are studied using atomic-scale simulation. Applied engineering strains cause such defects to move conservatively along the boundaries, producing coupled shear and migration. The Peierls stresses causing such motion are found to decrease precipitously through the transition from low- to high-angle boundaries. The reason underlying this behaviour is discussed.
Materials Science and Engineering: A, 2006
ABSTRACT A new model of martensitic transformations has been presented recently where the habit p... more ABSTRACT A new model of martensitic transformations has been presented recently where the habit plane consists of segments of coherent terraces reticulated by an array of line defects. These defects accommodate any coherency strains present and the transformation is effected by lateral motion of disconnections across the terraces. Provided the terraces and line defects satisfy certain criteria, the mechanism of transformation is conservative. Moreover, the topological parameters of the defects (Burgers vectors and step heights) can be determined rigorously enabling the overall habit plane inclination and orientation relationship to be determined.The objective of the present paper is to compare predictions for the habit planes according to this topological method with those of the classical theories in the case of transformations in ZrO2 and Ti. The two approaches lead to disparate solutions, and the origin and magnitude of the disparities are elucidated. Unlike habit planes in the classical model, which are invariant planes, topological ones are misfit-relieved semi-coherent configurations.
Interface Science, 2000
ABSTRACT
Journal of Materials Science, 2008
The classical theory of the crystallography of martensitic transformations developed in the 1950s... more The classical theory of the crystallography of martensitic transformations developed in the 1950s is based on the notion that the interface between the parent and product phases is an invariant plane of the shape deformation. Underlying this hypothesis is the expectation that such interfaces do not exhibit long-range strain, and the geometric theory is an algorithm for finding invariant planes, the orientation relationship and transformation displacement. In the context of ferrous alloys, the classical theory has been applied successfully to transformations with {295} habit planes, but is less satisfactory for {575} for example. A new model of martensitic transformations has been presented recently based on dislocation theory, incorporating developments in the understanding of the topological properties of interfacial defects. Topological arguments show that glissile motion of transformation dislocations, or disconnections, can only occur in coherent interphase interfaces. Hence, the interface in the model comprises coherent terraces with a superimposed network of disconnections and crystal dislocations. It is demonstrated explicitly that this defect network accommodates the coherency strains, and that lateral motion of the disconnections across the interface effects transformation in a diffusionless manner. Moreover, it is shown that a broader range of habit planes is predicted on the basis of the semi-coherent interface model than the invariant plane notion. In the case of ferrous alloys, it will be shown that a range of viable solutions arise which include {575}.
Acta Materialia, 2000
AbstractÐThe displacive±diusive transformation, which gives rise to lamellar g-TiAl+a 2 -Ti 3 Al ... more AbstractÐThe displacive±diusive transformation, which gives rise to lamellar g-TiAl+a 2 -Ti 3 Al microstructures, has been analysed theoretically. Using the formulation due to Hirth and Pond, the shear produced and extent of diusional¯ux associated with interfacial defect motion can be quanti®ed in terms of the Burgers vectors, step heights of the defects and the chemical compositions of the adjacent crystals. Moreover, this formulation also enables the eects of chemical composition, mis®t at the interface and ordering to be investigated systematically. It is shown that for the defects observed on such interfaces in recent high-resolution transmission electron microscopy studies, their motion would lead to the conservation of sites and, correspondingly, to equal and opposite¯uxes of Ti and Al atoms. Consequently, although this transformation is diusional, it exhibits crystallographic features consistent with martensitic processes. 7