Misfit dislocations in epitaxy

This article on epitaxy highlights the following: the definition and some historical milestones; the introduction by Frenkel and Kontorowa (FK) of a truncated Fourier series to model the periodic interaction at crystalline interfaces; the invention by Frank and van der Merwe (FvdM)—using the FK model—of (interfacial) misfit dislocations as an important mechanism in accommodating misfit at epilayer-substrate interfaces; the generalization of the FvdM theory to multilayers; the application of the parabolic model by Jesser and van der Merwe to describe, for growing multilayers and superlattices, the impact of Fourier coefficients in the realization of epitaxial orientations and the stability of modes of misfit accommodation; the involvement of intralayer interaction in the latter—all features that impact on the attainment of perfection in crystallinity of thin films, a property that is so vital in the fabrication of useful uniformly thick epilayers (uniformity being another technological requirement), which also depends on misfit accommodation through the interfacial energy that function strongly in the criterion for growth modes, proposed by Bauer; and the ingenious application of the Volterra model by Matthews and others to describe misfit accommodation by dislocations in growing epilayers. This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology Sector, Structures.     

New frontiers in thin film growth and nanomaterials

This article reviews recent developments in thin film growth and formation of three-dimensional epitaxial nanostructures. First, we present a unified standard model for thin film epitaxy, where single-crystal films with small and large lattice misfits are grown by a new paradigm of domain matching epitaxy (DME). We define epitaxy as having a fixed orientation rather than the same orientation with respect to the substrate. The DME involves matching of integral multiples of lattice planes (diffracting as well as nondiffracting) between the film and the substrate, and this matching could be different in different directions. The idea of matching planes is derived from the basic fact that during thin film growth, lattice relaxation involves generation of dislocations whose Burgers vectors correspond to missing or extra planes, rather than lattice constants. In the DME framework, the conventional lattice matching epitaxy (LME) becomes a special case where matching of lattice constants results from matching of lattice planes with a relatively small misfit of less than 7 to 8 pct. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100) with 4/5 matching, and the ZnO/α-Al₂O₃(0001) with 6/7 matching of lattice planes across the film/substrate interface. By varying the domain size, which is equal to the integral multiple of lattice planes, in a periodic fashion, it is possible to accommodate additional misfit beyond the perfect domain matching. Thus, we can potentially design epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces as long as there is wetting or finite interatomic interaction across the interface and cores of dislocations do not overlap. In-situ X-ray diffraction studies on initial stages of growth of ZnO films on sapphire correctly identify a compressive stress and a rapid relaxation within one to two monolayers, consistent with the DME framework and the fact that the critical thickness is less than a monolayer. The DME examples ranging from the Ge-Si/Si(100) system with 49/50 matching (2 pct strain) to metal/Si systems with 1/2 matching (50 pct strain) are tabulated, strategies for growing strain-free films by engineering the misfit to be confined near the interface are presented, and the potential for epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces is discussed. In the second part, we discuss the formation of epitaxial nanodots/nanocrystals in crystalline matrices such as MgO and TiN. The formation Ni nanocrystals inside MgO involves lattice misfit ranging from 3.0 to 31.3 pct, and the formation of Ni nanocrystals in TiN has a misfit of about 17 pct. To form epitaxial nanodots, we use DME framework to grow nanodots inside crystalline matrices. Growth characteristics and crystallography of nanodots are controlled to enhance the overall properties of nanostructured materials. We illustrate this for nanostructured magnetic materials where coercivity and blocking temperature can be considerably enhanced by controlling the orientation for magnetic memory and storage applications, among others. In the last part, the DME principles are applied to grow self-assembled epitaxial nanodots using pulsed laser deposition. By controlling the clustering kinetics, it is possible to obtain a uniform distribution of epitaxial nanodots and overcome thermodynamically driven Ostwald ripening. This process allows the formation of epitaxial nanostructures via three-dimensional self-assembly, the “holy grail” of nanostructured materials processing, leading to unique and novel properties. Jagdish (Jay) Narayan holds The John C.C. Fan Family Distinguished Chair Professorship and is Director of the NSF Center for Advanced Materials and Smart Structures at North Carolina State University. The Edward DeMille Campbell memorial lecture entitled, “New Frontiers in Thin Film Growth and Nanomaterials,” was delivered at the Annual ASM International Meeting (October 17–20, 2004) in Columbus, Ohio. Professor Narayan received his M.S. (1970) and Ph.D. (1971) degrees in a record time of two years from the University of California, Berkeley, after his Bachelor’s of Technology with Distinction and Highest Honors from IIT, Kanpur in 1969. Professor Narayan has had a profound impact on our understanding of defects and interfaces in thin film heterostructures, laser-solid interactions and processing of novel materials, controlled thin film growth and self-assembled nanostructures, and nanoscale characterization and modeling of new materials and properties. He invented novel supersaturated semiconductor alloys formed by solid phase epitaxy, and by liquid-phase crystallization where melt-quenching rates are billions of degrees per second. This research, featured twice in Science Magazine, has resulted in numerous U.S. patents and three IR-100 Awards. More recently, Narayan has pioneered and patented a new concept of domain epitaxy, where an integral number of lattice planes of the film match that of the substrate in large lattice mismatched systems. The domain epitaxy is key to the formation of thin film heterostructures, such as TiN films on silicon with 4/3 matching, and III-nitrides and ZnO films on sapphire with 6/7 matching. Narayan invented new cubic ZnMgO alloys, which can be grown epitaxially on silicon (100) substrates for integrating optoelectronic and spintronic devices. Narayan discovered and patented a new method of self-assembly for processing nanostructured magnetic, photonic, electronic, and structural materials. His discoveries with Dr. John Fan. Kopin Corporation, related to domain epitaxy and quantum confinement by thickness variation (creating nanopockets) are being used by Kopin Corp. to manufacture high-efficiency light emitting diodes for solid-state lighting. He has published over 800 scientific articles, edited 9 books, and received 22 patents pertaining to novel materials and processing methods, domain epitaxy, and a new class of next-generation semiconductor alloys. He is a Fellow member of APS, AAAS, ASM International, TMS, and MRS-I. He has won numerous other honors, including three IR-100 Awards and a 1999 ASM Gold Medal. He also received Distinguished Alumnus Honors from IIT/K in 1997.     

New frontiers in thin film growth and nanomaterials

This article reviews recent developments in thin film growth and formation of three-dimensional epitaxial nanostructures. First, we present a unified standard model for thin film epitaxy, where single-crystal films with small and large lattice misfits are grown by a new paradigm of domain matching epitaxy (DME). We define epitaxy as having a fixed orientation rather than the same orientation with respect to the substrate. The DME involves matching of integral multiples of lattice planes (diffracting as well as nondiffracting) between the film and the substrate, and this matching could be different in different directions. The idea of matching planes is derived from the basic fact that during thin film growth, lattice relaxation involves generation of dislocations whose Burgers vectors correspond to missing or extra planes, rather than lattice constants. In the DME framework, the conventional lattice matching epitaxy (LME) becomes a special case where matching of lattice constants results from matching of lattice planes with a relatively small misfit of less than 7 to 8 pct. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100) with 4/5 matching, and the ZnO/α-Al₂O₃(0001) with 6/7 matching of lattice planes across the film/substrate interface. By varying the domain size, which is equal to the integral multiple of lattice planes, in a periodic fashion, it is possible to accommodate additional misfit beyond the perfect domain matching. Thus, we can potentially design epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces as long as there is wetting or finite interatomic interaction across the interface and cores of dislocations do not overlap. In-situ X-ray diffraction studies on initial stages of growth of ZnO films on sapphire correctly identify a compressive stress and a rapid relaxation within one to two monolayers, consistent with the DME framework and the fact that the critical thickness is less than a monolayer. The DME examples ranging from the Ge-Si/Si(100) system with 49/50 matching (2 pct strain) to metal/Si systems with 1/2 matching (50 pct strain) are tabulated, strategies for growing strain-free films by engineering the misfit to be confined near the interface are presented, and the potential for epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces is discussed. In the second part, we discuss the formation of epitaxial nanodots/nanocrystals in crystalline matrices such as MgO and TiN. The formation Ni nanocrystals inside MgO involves lattice misfit ranging from 3.0 to 31.3 pct, and the formation of Ni nanocrystals in TiN has a misfit of about 17 pct. To form epitaxial nanodots, we use DME framework to grow nanodots inside crystalline matrices. Growth characteristics and crystallography of nanodots are controlled to enhance the overall properties of nanostructured materials. We illustrate this for nanostructured magnetic materials where coercivity and blocking temperature can be considerably enhanced by controlling the orientation for magnetic memory and storage applications, among others. In the last part, the DME principles are applied to grow self-assembled epitaxial nanodots using pulsed laser deposition. By controlling the clustering kinetics, it is possible to obtain a uniform distribution of epitaxial nanodots and overcome thermodynamically driven Ostwald ripening. This process allows the formation of epitaxial nanostructures via three-dimensional self-assembly, the “holy grail” of nanostructured materials processing, leading to unique and novel properties. Jagdish (Jay) Narayan holds The John C.C. Fan Family Distinguished Chair Professorship and is Director of the NSF Center for Advanced Materials and Smart Structures at North Carolina State University. The Edward DeMille Campbell memorial lecture entitled, “New Frontiers in Thin Film Growth and Nanomaterials,” was delivered at the Annual ASM International Meeting (October 17–20, 2004) in Columbus, Ohio. Professor Narayan received his M.S. (1970) and Ph.D. (1971) degrees in a record time of two years from the University of California, Berkeley, after his Bachelor’s of Technology with Distinction and Highest Honors from IIT, Kanpur in 1969. Professor Narayan has had a profound impact on our understanding of defects and interfaces in thin film heterostructures, laser-solid interactions and processing of novel materials, controlled thin film growth and self-assembled nanostructures, and nanoscale characterization and modeling of new materials and properties. He invented novel supersaturated semiconductor alloys formed by solid phase epitaxy, and by liquid-phase crystallization where melt-quenching rates are billions of degrees per second. This research, featured twice in Science Magazine, has resulted in numerous U.S. patents and three IR-100 Awards. More recently, Narayan has pioneered and patented a new concept of domain epitaxy, where an integral number of lattice planes of the film match that of the substrate in large lattice mismatched systems. The domain epitaxy is key to the formation of thin film heterostructures, such as TiN films on silicon with 4/3 matching, and III-nitrides and ZnO films on sapphire with 6/7 matching. Narayan invented new cubic ZnMgO alloys, which can be grown epitaxially on silicon (100) substrates for integrating optoelectronic and spintronic devices. Narayan discovered and patented a new method of self-assembly for processing nanostructured magnetic, photonic, electronic, and structural materials. His discoveries with Dr. John Fan, Kopin Corporation, related to domain epitaxy and quantum confinement by thickness variation (creating nanopockets) are being used by Kopin Corp. to manufacture high-efficiency light emitting diodes for solid-state lighting. He has published over 800 scientific articles, edited 9 books, and received 22 patents pertaining to novel materials and processing methods, domain epitaxy, and a new class of next-generation semiconductor alloys. He is a Fellow member of APS, AAAS, ASM International, TMS, and MRS-I. He has won numerous other honors, including three IR-100 Awards and a 1999 ASM Gold Medal. He also received Distinguished Alumnus Honors from IIT/K in 1997.     

Why do dislocations assemble into interfaces in epitaxy as well as in crystal plasticity? To minimize free energy

Dislocations commonly form planar arrays that minimize the free interfacial energy between relatively mismatched crystal volumes. In epitaxy and phase transformations, the causative misfit is that between differences in lattice structure and/or orientations of different phases. In deformed homogeneous crystalline materials, the planar dislocation arrays are grain and mosaic block boundaries that accommodate relative misorientations within the same crystal structure. Thus, overwhelmingly, planar dislocation arrays have a basically common origin, namely minimization of interfacial energies. Consequently, they are all subject to the low-energy dislocation structures (LEDS) hypothesis. While the specific applications of the underlying general theory are well advanced in terms of epitaxy, phase, and grain boundaries, in connection with plastic deformation that very basis is widely overlooked, if not denied. The present article aims to (a) document the fact that, while being formed, dislocation structures due to plastic deformation are in thermodynamical equilibrium, (b) firmly establish the outlined connection between planar dislocation arrays of all types, and, thereby, (c) establish the kinship between epitaxy and plastic deformation of crystalline materials. This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology Sector, Structures.     

Analysis of the Cu-3 Wt pct Ti cellular interphase boundary by various models

The cellular-interlamellar interface in Cu-3 wt pct Ti is modeled employing three different theoretical approaches, which are then compared to experimental observations obtained by transmission electron strain contrast and high-resolution microscopy. The simplest modeling technique used is interfacial superposition, which is shown to give useful information on the general nature of the interface. Van der Merwe’s ideal interfacial-configuration model, which calculates the relative energies of different misorientations and dilatations, establishes three nearby energy minima over azimuthal rotations of less than 1 deg for a given interlamellar habit plane. This is too small a rotation to differentiate by experiment alone. From this ideal interfacial-configuration model, a different orientation relationship than previously reported in the literature is proposed that more closely represents the experimental observations when combined with O-lattice calculations, viz., (111) _α ‖ (010) _β with [-101] _α ‖ [501] _β , where α is the fcc matrix, and β is the Cu₄Ti orthorhombic phase. Probable defect networks were determined by the O-lattice method for the new and previously proposed orientation relationships (ORs). This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology, Sector, Structures.     

Comparison of interfacial structure-related mechanisms in diffusional and martensitic transformations

Some central problems in understanding the similarities of and the differences between ledgewise martensitic and ledgewise diffusional growth are examined. Martensitic growth can be described in terms of a lattic correspondence and a plane undistorted by the shear transformation. Diffusional growth can be similarly described in some cases but not in others. On the basis of the Sutton-Balluffi definitions of glissile and sessile boundaries, only misfit dislocations (on terraces or risers) or orthogonal sets of disconnections provide a truly sessile interface. When closely spaced structural ledges (now termed “structural disconnections”) are present during diffusional growth, they must have been glissile in the formation of a local equilibrium structure during the initial stages of growth. Once they are in local equilibrium and evenly spaced, however, they can only move synchronously because of their local strain interaction. Under these circumstances, extrinsic sources of growth ledges are required to move such interfaces in a diffusional manner. During martensitic growth, however, disconnections in the form of transformation dislocations can move freely in a synchronous manner. Also, on this basis, the apparent (undistorted) habit plane is generally useful in deducing the transformation mechanism during martensite formation, but is only occasionally so during diffusional growth, where only the terrace plane is generally useful. This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology Sector, Structures.     

The cellular interlamellar and growth-front interphase boundaries in Cu-3 Wt pct Ti

Examination of the cellular colony interlamellar and growth-front interphase boundaries in Cu-3 wt pct Ti reveals an influence of crystallography at both of these interface types. Analysis of the interlamellar boundaries demonstrates that different arrangements of interphase misfit-compensating defects exist and combinations of misfit dislocations (MDs), structural ledges (SLs), or direction steps (DSs) were observed to dominate strain reduction between lamellae, even within the same colony. Detailed analysis also demonstrated that the actual interlamellar orientation relationship (OR) is (111) _α ‖ (010) _β with [-101] _α ‖ [501] _β , which is 0.28 deg in misorientation from the reported OR. The effect of crystallography was also apparent at the cellular growth front, as evidenced by the misfit-compensating structure observed with transmission electron microscopy (TEM) at the grain-boundary segments and the sharp faceting of all β precipitate-growth interfaces. This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology, Sector, Structures.     

Theory of shape bifurcation during nucleation in solids

The shape bifurcation theory of Johnson and Cahn [su1] is extended to a solid-state nucleation in order to predict the critical nucleus shape as a function of the nucleation driving force. For the case of homogeneous nucleation, the relationship between the nucleation driving force and the nucleus shape reveals characteristics similar to the well-known temperature effects on the isothermal pressure-volume variations of a van der Waals fluid. When the elastic strain energy is a strong function of the nucleus aspect ratio in the neighborhood of a high-symmetry morphology, such as a sphere or a cube, the shape transition is a continuous type, analogous to a smooth pressure-volume change in a van der Waals fluid above the critical temperature. On the other hand, if the elastic energy changes significantly only with small aspect ratios, the shape transition becomes a discontinuous type, analogous to the first-order liquid-to-vapor phase transition below the critical temperature. Results of an elementary model for heterogeneous nucleation also exhibit a similar bifurcation nature, here the potency of a heterogeneous nucleation site replacing the role of the temperature in the phase diagram of a van der Waals fluid. This paper is based on a presentation made in the “G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations” presented as part of the 1990 fall meeting of TMS, October 8–12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee.     

Lattice Distortion Formations by Low-Energy Ar<Superscript>+</Superscript> Bombardment of Thin Silicon Films Grown on Silicon (100)

The transmission electron microscopy (TEM) and X-ray characterization of lattice distortion forms caused by low-energy Ar⁺ bombardment of grown thin silicon films on a silicon (001) substrate were studied. The isotropic case (of spherical distortions) takes place in epitaxial silicon “as grown” processes. The intensity distribution consists of two maxima—one from the distorted layer and the other from the original unaffected silicon lattice. Significant changes in the 2θ location, peak broadening, and integrated intensity from the (004)* reflections were obtained as functions of aging temperatures. First, aging heat treatment, affects the distribution of distortions obtained from local regions at the “as grown” layer, which changes to a special topography of continued distortions at higher aging temperatures. At aging temperatures above 923 K (650 °C), this extra diffraction peak disappears. The TEM observations reveal the appearance of dislocation lines with dark and bright contrasts around the lines and interdislocation strain contrasts and disorder of Ar atoms in Si matrix regions with coherent interfaces.     

Texture enhancement by inoculation during casting of ferritic stainless steel strip

Melt/substrate contacting experiments, designed to approximate conditions encountered during strip casting, were carried out to produce as-cast ferritic stainless steel strip. The results show that inoculation of the melt to produce TiN particles, together with casting onto a smooth substrate, results in the optimum conditions for nucleation and subsequent growth of an exceedingly high volume fraction of ferrite grains with 〈001〉 oriented within a few degrees of the normal direction (ND) of the strip surface. It is argued that, during casting, TiN particles either nucleate or deposit onto the substrate with 〈001〉 parallel to the ND, and since these particles exhibit crystallographic features similar to δ-ferrite, subsequent epitaxial growth inherits the initial particle orientation. Such oriented nucleation of ferrite from a smooth substrate results in the optimum heat-transfer conditions for further growth of dendrites with 〈001〉 perpendicular to the substrate, thus producing the intense through-thickness 〈001〉//ND fiber texture in the as-cast strip. The potential for producing grain-oriented silicon iron by direct strip casting is outlined.     

The motion of multiple height ledges and disconnections in phase transformations

For phase transformations with well-defined terrace planes, interface motion can occur by the motion of ledges or disconnections (ledges with added dislocation character). Symmetry imposes restrictions on the nature of these defects and may lead to the need for multiple height ledges. The structure of the ledge riser can also be variable. These possibilities impose constraints that can influence the motion of the defects and hence affect the rate of phase transformation. Examples of these phenomena are presented for interfaces with differing degrees of lattice matching. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

Classification of processes of oriented and random intergrowth and methods of producing epitaxial layers of various crystalline substances

Conclusions The development of microelectronics is mainly due to epitaxial and thin-film technology; therefore, it is quite natural that perfection of this technology is very necessary. We can hope that an investigation of the processes of epitaxy and endotaxy will find support also among specialists working in the area of cermets and powder metallurgy.The presented classification of methods of producing epitaxial layers of various crystalline substances based on the physicochemical characteristics of the external environment and processes of auto-, hetero-, and chemoepitaxy can be used both for further perfection of these methods and for refining the mechanism of epitaxial processes and development of an acceptable theory of oriented overgrowth.The large-scale use of methods of producing EL from an external environment differing in its physicochemical nature can open great opportunities for perfecting technological processes. For example, the production of quality HEL from the gaseous phase can ensure subsequent production of a uniform AEL (during melting in of the HEL), etc.The presented classification of methods of producing EL can be successfully used also for systematizing a number of micrometallurgical processes allied with epitaxial technology, for example, for the production of various layers with rigorously prescribed properties by chemical-heat treatment methods, in the technology of welding and soldering of metals and alloys, and in a number of other areas related with processes of intergrowth of crystalline substances.Translated from Poroshkovaya Metallurgiya, No. 8 (68), pp. 35–47, August, 1968.     

Edge-to-edge matching of lattice planes and coupling of crystallography and migration mechanisms of planar interfaces

The structure and migration mechanisms of planar interfaces are examined geometrically using the concept of edge-to-edge matching of lattice planes and the Moiré plane approach derived from this concept. The selected examples of planar interfaces include those associated with rational, nearrational, or irrational orientation relationships. It is demonstrated that the orientation and structure of planar interfaces associated with these orientation relationships can be rationalized by the Moiré plane approach, and that the migration of these interfaces in their normal directions can occur via successive nucleation and lateral gliding of growth ledges that are in the form of transformation disconnections, for low-index interfaces, and of Moiré ledges, for high-index interfaces. It is further demonstrated that a shear, and thus a shape change, is associated with the motion of all planar interfaces defined by the edge-to-edge matching of lattice planes. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformation Committee.     

Edge-to-edge matching of lattice planes and coupling of crystallography and migration mechanisms of planar interfaces

The structure and migration mechanisms of planar interfaces are examined geometrically using the concept of edge-to-edge matching of lattice planes and the Moiré plane approach derived from this concept. The selected examples of planar interfaces include those associated with rational, near-rational, or irrational orientation relationships. It is demonstrated that the orientation and structure of planar interfaces associated with these orientation on relationships can be rationalized by the Moiré plane approach, and that the migration of these interfaces in their normal directions can occur via successive nucleation and lateral gliding of growth ledges that are in the form of transformation disconnections, for low-index interfaces, and of Moiré ledges, for high-index interfaces. It is further demonstrated that a shear, and thus a shape change, is associated with the motion of all planar interfaces defined by the edge-to-edge matching of lattice planes. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occured during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformation Committee.     

Edge-to-edge matching of lattice planes and coupling of crystallography and migration mechanisms of planar interfaces

The structure and migration mechanisms of planar interfaces are examined geometrically using the concept of edge-to-edge matching of lattice planes and the Moiré plane approach derived from this concept. The selected examples of planar interfaces include those associated with rational, near-rational, or irrational orientation relationships. It is demonstrated that the orientation and structure of planar interfaces associated with these orientation on relationships can be rationalized by the Moiré plane approach, and that the migration of these interfaces in their normal directions can occurvia successive nucleation and lateral gliding of growth ledges that are in the form of transformation disconnections, for low-index interfaces, and of Moiré ledges, for high-index interfaces. It is further demonstrated that a shear, and thus a shape change, is associated with the motion of all planar interfaces defined by the edge-to-edge matching of lattice planes. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occured during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformation Committee.     

The Effect of β Stabilizers on the Structure and Energy of α/β Interfaces in Titanium Alloys

The structure and energy associated with interfaces between the BCC and HCP lattices (β and α phase, respectively) in titanium alloys with commonly used β stabilizers were analyzed. For this purpose, the crystallographic structure of the matching facets of broad, side and end faces was described using misfit dislocations and structural ledges which compensate the mismatch in atomic spacing of the α and β phases. The effect of the β/α transformation temperature due to various concentration of β stabilizers on periodicity of misfit dislocations and structural ledges was estimated. The van der Merwe approach was used to calculate energy of different matching facets. An increase in the percentage of β-stabilizing elements was found to result in a decrease in the lattice-parameter ratio (a_β/a_α) and an increase in the energy of all faces. The dependence of the interface energy on the a_β/a_α ratio was for the first time quantified, and insight into the preferred shape of α-phase precipitates was obtained.

     

Dynamic reactive wetting and its role in hot dip coating of steel sheet with an Al-Zn-Si alloy

The effect of substrate preheat temperature on the dynamic wetting of 55Al-43.4Zn-1.6Si hot dip coating melts on low-carbon steel substrates has been investigated. An experimental apparatus based on the sessile drop technique was developed, which allowed the substrate to be preheated to a different temperature than that of the droplet. The initial wetting and spreading of the molten metal droplet on the substrate was recorded at 1000 frames per second using a high-speed digital camera. Wetting was improved (ϑ decreased from 120 to 25 deg) as the substrate preheat temperature was increased from room temperature and approached the droplet temperature, beyond which the improvement in wetting was negligible. Immersion experiments using a thermocouple instrumented substrate dipped into a coating bath were performed for various substrate preheat temperatures. Interfacial heat fluxes and interfacial resistances were calculated from the temperature responses. The “minimum” interfacial resistance was decreased by an order of magnitude (1 × 10⁻⁴ to 2 × 10⁻⁵ m² K/W) as the substrate preheat temperature was increased from room to bath temperature. The reduction in interfacial resistance was related to the improvement of the initial wetting and the increase in mass transfer of iron atoms from the substrate across the interface. There was an apparent increase in the minimum interfacial resistance for substrate temperatures greater than the bath temperature. This was due to the increased rate of alloy layer formation and the exothermic nature of the Fe-Al interfacial reactions. The significance of these findings was discussed with respect to the mechanism of alloy layer formation at the interface during the initial stages of solid-liquid contact. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.