Thermal Grooving Research Articles

The effect of bismuth segregation on the faceting of Σ3 and Σ9 coincidence boundaries in copper bicrystals

Abstract The faceting of a cylindric tilt grain boundary (GB) in Cu and Cu-160 ppm Bi bicrystals was studied. The crystal lattices of both grains form a superlattice called a coincidence site lattice (CSL) with an inverse density of coincidence sites relating to the density of lattice sites Σ = 3 and Σ = 9. The Σ3 and Σ9 <110> tilt GBs became faceted upon annealing at 0.95 Tm. The ratio of GB energy σGB and surface energy σsur of the specimen was measured, applying atomic force microscopy, from the profile of the GB thermal groove formed upon additional annealing. The 160 ppm Bi concentration in the Bi-containing bicrystal is between GB and bulk solidus lines in the Cu – Bi phase diagram. In this concentration region GBs contain a multilayer Bi-rich film which is supposed to be liquid-like. Wulff – Herring diagrams were constructed using the measured σGB/σsur values, revealing stable and metastable GB facets.

Grain-Boundary Grooving and Agglomeration of Alloy Thin Films with a Slow-Diffusing Species

We present a general phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on the grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant diffusion path is along the boundary, while at late times it is parallel to the substrate. This change in path strongly affects the time dependence of grain-boundary grooving and increases the time to agglomeration. The present model provides a tool for agglomeration-resistant thin film alloy design.

Using confocal scanning laser microscopy for the in situ study of high-temperature behaviour of complex ceramic materials

The confocal scanning laser microscopy (CSLM) technique has been successful in many metallurgical fields. This paper assesses its applicability to the in situ investigation of the high-temperature behaviour of complex ceramic materials. Magnesia–chromite refractory is selected as a ceramic test material. At room temperature, CSLM images correspond well to typical light optical microscopy (LOM) and backscattered electron (BSE) micrographs. In fact, because of the high axial resolution (short focal depth) and the confocal optics of the CSLM technique, the porosity level of the mirror polished ceramic specimens is more truthfully assessed by the CSLM (2-D) images than by the BSE micrographs (long focal depth). However, at high-temperatures (1550–1650 °C) the observed CSLM (2-D) image quality is slightly poorer. The principal explanation is the in situ roughening of the specimen surface during heating. The roughening has two causes: differential thermal expansion of the two primary phases in the ceramic test material and, to a lesser extent, thermal grooving. Nevertheless, it is shown that the CSLM image quality suffices for an in situ study of the high-temperature behaviour of ceramic materials. This is illustrated for the magnesia–chromite system by examining the dissolution mechanism of secondary (magnesiochromite) spinel into the periclase phase.

Micro processing on Cr films by Nd:YAG pulsed laser oxidation method

The surface oxidation of Cr films by Nd:YAG pulsed laser was studied using a combination of SEM, FESEM, AFM and XRD. The oxide growth is near homogeneous by laser irradiation at the lower power density but it presents hill-like growth mode by the laser irradiation at the higher power density. The formation of the thermal groove that is harmful for the next processing step of etching was considered as the relaxation of the initial tensile stress in Cr film and the lower diffusion rate during laser oxidation at the lower temperature. It is harmful for the next processing step of etching. The hill-like oxide growth is due to the rapid outward diffusion of Cr ions by laser oxidation at high-power density induced high temperature.

Grain‐Boundary Grooving of Plasma‐Sprayed Yttria‐Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma‐sprayed 7 wt% Y2O3–ZrO2 thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma‐sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain‐boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat‐treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.

Thermally stable nanomultilayer films of Cu/Mo

Thermal decomposition of nanoscaled (5, 50, and 100 nm) multilayer films has been studied in an immiscible Cu and Mo system. While the onset of nanolayer instability is by thermal grooving at elevated temperatures, the entire nanomultilayer film decomposition can be differentiated into three distinct stages, over a range of temperatures (848 to 1073 K). Stage I is characterized by the onset of grooves, which appear as minor perturbations in otherwise flat interfaces. This is followed by the occurrence of prominent grooves in stage II. Stage III consists of a complete breakdown of the layered structure, with the microstructure composed of grains of Cu and Mo. However, a good layer stability was observed in some of these nanomultilayers (50 Mo:5 Cu and 5 Mo:5 Cu), and stage II is retained up to long times at elevated temperatures. This is attributed to the large difference in the individual layer melting temperatures, combined with unequal film thickness (and hence volume fractions), which inhibits the attainment of an equilibrium groove configuration, up to extended periods of time. Analytical models for thermal grooving in bulk polycrystalline materials were applied to the case of thin film nanomultilayers. The predicted instability kinetics were found to corroborate with the experimentally observed stability of the nanomultilayers, except at very small size ranges (5 nm). A methodology for generating stable nanomultilayer films is suggested as an outcome of this study.

Studying alumina boundary migration using combined microscopy techniques

Thermal grooving and migration of grain boundaries in alumina have been investigated using a variety of microscopy techniques. Using two different methods, polycrystalline alumina was used to investigate wet, (implying the presence of a glassy phase), and dry grain boundaries. In the first, single-crystal Al2O3 was hot-pressed via liquid phase sintering (LPS) to polycrystalline alumina with an anorthite glass film at the interface. Pulsed laser deposition was used to deposit approximately 100-nm thick glass films. Specimens were annealed in air at 1650°C for 20 h to induce boundary migration. Boundary characterization was carried out using visible light (VLM) and scanning electron (SEM) microscopies. Effects on migration due to surface orientation of grains were investigated using electron backscatter diffraction (EBSD). The second method dealt with heat treating dry boundaries in polycrystalline alumina to monitor boundary migration behavior via remnant thermal grooves. Heat treatments were conducted at 1650°C for 30 min. The same region of the sample was mapped using VLM and atomic force microscopy (AFM) and followed over a series of 30 min heat treatments. Boundary migration through a pore trapped inside the grain matrix was of particular interest.

Temperature influence on the faceting of Σ3 and Σ9 grain boundaries in Cu

The faceting of a tube-like tilt grain boundary (GB) in Cu bicrystals has been studied in the temperature interval from 0.5 to 0.95 of the melting temperature T m ( T m = 1356 K). The grains of the bicrystals form the coincidence site lattice (CSL) with inverse density of coincidence sites Σ = 3 and Σ = 9. No rounded edges between facets were observed up to 0.95 T m. With decreasing temperature new facets of increasingly higher CSL indices appear. At 0.5 T m six crystallographically different Σ = 3 facets exist simultaneously. The appearance of high-index CSL facets can be explained by the roughening phase transition of a Kosterlitz–Thouless type. The ratio of GB energy σ GB and surface energy σ sur of the specimen was measured by applying atomic force microscopy to the profile of the GB thermal groove formed upon additional annealing. The Wulff–Herring diagrams were constructed using measured σ GB/ σ sur values.

Faceting of Ʃ3 Grain Boundaries in Cu: Three-Dimensional Wulff Diagrams

Diffusional growth of the grain boundary (GB) groove permits one to measure the ratio between GB energy sGB and surface energy ssur. The faceting of twin tilt grain boundaries in Cu has been studied using the GB thermal groove method. No rough facet edges were observed. It means that melting temperature is lower than the roughening temperature for the observed facets in Cu. The influence of orientation and misorientation deviation Dq = ½q – qS½ from coincidence misorientation qS has been studied. By increase of Dq the energy of (100)CSL facet increases. The convenient method for construction 3D three-dimensional Wulff diagrams was found. The 3- dimensional Wulff diagrams were constructed using this method and measured sGB / ssur values.

On the evolution of faceted grain-boundary grooves by surface diffusion

Thermal grooving at a grain-boundary is well-understood when the surface of the groove is entirely rough (finite surface stiffness) at its root. In this situation, the grain-boundary energy is uniquely related to the surface energy of the groove at its root via the dihedral angle that is obtained by enforcing equilibrium (balance of capillary forces) at the triple-junction. However, when a groove-root is faceted, the non-analyticity of the surface energy at the facet orientation leads to ambiguities in both the enforcement of equilibrium at the groove-root and the relation between the facet energy and the grain-boundary energy. In this work, we show that equilibrium at the groove-root requires careful consideration of the torque, which can adjust itself to maintain the fixed facet orientation for a range of grain-boundary energies thereby achieving a net balance of capillary forces. This is in stark contrast to the case of a rough root where the grain-boundary energy is in one-to-one correspondence with the observed dihedral angle. Using this insight, we show via a simple graphical method that the grain-boundary energy and surface energy function are entirely adequate to determine if a groove-root is faceted or rough thereby obviating the need for any ad hoc specification of the dihedral angle as a boundary condition. The evolution of thermal grooves is studied using a variational approach that can handle infinite surface stiffnesses and naturally allow for facet formation. The mobility of the triple-junction, which is know to lead to deviations from Mullins’ t 1/4 scaling law, is also explicitly included in the model. A key observation is that low junction-mobilities can lead to the formation of kinetic groove-shapes – in particular, groove-roots that are anticipated to be rough from energetic considerations alone are seen instead to attain nearby facet orientations at low junction mobilities. Such kinetically limited facets at groove-roots can never be obtained if the equilibrium angle is prescribed as a boundary condition for the evolution equation as is often done. We also extend our analysis to the more realistic case of asymmetric grooves and present a graphical procedure that elucidates their structure.

Characterisation of fine-grained oxide ceramics

A range of high resolution techniques have been used to characterise the grain boundary segregation behaviour of rare earth (RE) doped (La, Gd, Eu and Yb) alumina and spinel. TEM based techniques (HR-TEM, HAADF STEM and EDS) have been used to study the structure and chemistry of grain boundaries. The use of a HAADF detector in STEM provides atomic number contrast and easy identification of heavy (RE) segregants. This has been used to produce high resolution RE elemental maps, showing the width of the segregated region to be less than the size of the electron probe (1 nm) for all boundaries studied. EDS showed that within a 1 nm thick boundary region (an upper limit) the RE cations would account for 10 (+/-2)% and 15 (+/-2)% of the cation total in alumina and spinel respectively. Preliminary results from ultra-high resolution STEM (probe size similar to0.1 nm) suggest that, in spinel, the segregated region is actually composed of a much thinner continuous monolayer of RE atoms at the grain boundary. This is consistent with HR-TEM, which showed spinel grain boundaries possessed minimal grain boundary structural disorder. AFM has been used to study the effect of RE grain boundary segregation on thermal grooving behaviour. The improvement in resolution that is achieved by operating in Tapping(TM) mode is shown to translate into an improved profile of the groove root. This has been used in conjunction with Electron Backscattered Diffraction (EBSD) to examine the relationship between grain boundary geometry and misorientation. The addition of RE dopants to alumina was found to significantly increase the size of grain boundary grooves. This can be attributed to the out-diffusion of RE segregants, an effect which compromises grain boundary energy calculations for materials with grain boundary segregation. AFM and EBSD are also used to relate anisotropic tribo-chemical polishing-wear with grain orientation. (C) 2004 Kluwer Academic Publishers.

Remnant grooves on alumina surfaces

Thermal grooving of grain boundaries in alumina has been studied using a combination of visible-light microscopy (VLM) and atomic-force microscopy (AFM). The partial angles of grooves that develop during heat treatments at 1650 °C were estimated by AFM. The observation of remnant grooves is key to determining the magnitude of the movement of a grain boundary. Imaging using VLM and AFM enabled the smoothing of remnant grooves to be observed through a progression of heat treatments, which enables the anisotropic nature of the surface diffusivity of alumina to be monitored.

Grain Growth in Films with Initial 3D Microstructure

Grain growth in films with h/〈D〉0=3.5−7.5, where h is the film thickness and 〈D〉0 the initial average grain diameter, was studied by means of computer simulation. The growth kinetics during 3D®2D microstructure transformation do not obey the parabolic law commonly used for the grain growth description. By the end of the transformation, 〈D〉/h is not grater than 2. The drag effect of thermal grooves also results in 〈D〉/h < 2 to the end of the microstructure transformation.

Grain boundary faceting close to the Σ3 coincidence misorientation in copper

The faceting of a cylindric tilt grain boundary (GB) in Cu bicrystal has been studied. The crystal lattices of both grains give rise to a superlattice called coincidence site lattice (CSL) with inverse density of coincidence sites Σ = 3. The (100) C S L , (210) C S L, (130) C S L facets and the non-CSL 82°9R facet develop upon annealing at 0.95 T m , where T m is the melting temperature. The ratio of GB energy σ G B and surface energy σ s u r of the specimen was measured applying atomic force microscopy from the profile of the GB thermal groove formed upon additional annealing. The influence of deviation from the exact coincidence misorientation θ Σ , Δθ = ‖θ - θ Σ ‖, has been studied on the basis of electron backscattering diffraction patterns. The Wulff-Herring diagrams were constructed using measured σ G B / σ s u r values, revealing stable and metastable GB facets. T m is lower than the roughening temperature for the (100) C S L and 9R facets facets in Cu.

Investigation of the nickel/emissive oxide interface in thermionic emitters

AbstractOxide cathodes act as electron sources in cathode‐ray tubes (CRTs). In an attempt to understand the processes leading to degradation in their performance over their lifetimes, SEM, EDX and AES have been used to examine the Ni base/emissive oxide interface. Oxide cathodes from dummy electron gun tubes were investigated after 0 h and 1000 h lifetime testing. The interface was studied (a) from above, by stripping away the emissive oxide using adhesive tape, and (b) in cross‐section. After fabrication and processing (0 h exposure), cracks exist at grain boundaries on the surface of the Ni base, probably due to thermal grooving. The activator elements, Al and Mg, are enriched on the metal surface: Al on the Ni surface and Mg at the crack mouths. After 1000 h of lifetime testing the cracks had widened and deepened, and the activator concentrations in the interface region had increased. Cross‐sections reveal that chemical processes involving Mg occur primarily within the cracks, whilst those involving Al predominate at the Ni/oxide interface and on the oxide layer itself. An interfacial layer develops during cathode operation containing the elements Al, Mg, Ba and Sr. Further work is required to fully understand the degradation and activator reaction processes and to clarify the principal mechanism leading to reduced cathode performance as a function of lifetime. Copyright © 2004 John Wiley & Sons, Ltd.

Development of cube-textured Ni–W alloy substrates for YBCO-coated conductor

We fabricated pure Ni and Ni–W alloy substrates for YBCO-coated conductor applications and evaluated the effect of W in Ni on texture, grain size, grain boundary and surface morphology, and hardness of the substrates. Pure Ni, Ni–2at.%W, and Ni–5at.%W alloy substrates were prepared by plasma arc melting, cold rolling, and recrystallization heat treatment at various temperatures (700–1300 °C). Substrate texture was evaluated by pole-figure and microstructure and surface morphology were investigated by optical and atomic force microscopy (AFM). We observed that the Ni–W alloy substrates had stronger cube texture that was maintained at higher annealing temperatures than seen for the pure Ni substrate: Full-width at half-maximum of in-plane texture was 13.40° for the Ni substrate and 4.42–5.57° for the Ni–W substrate annealed at 1000 °C. In addition, the Ni–W substrate had smaller grain size, less thermal grooving, and higher hardness, compared to those of the pure Ni substrate, indicating that the presence of W in Ni effectively restricts grain growth and enhances thermal stability by strengthening the grain boundary in the Ni substrate.

Effect of Ge-rich Si1−zGez Segregation on the Morphological Stability of NiSi1−uGeu Film Formed on Strained (001) Si0.8Ge0.2 Epilayer

ABSTRACTThe germanosilicidation of Ni on strained (001) Si0.8Ge0.2, particularly Ge segregation, grain boundary grooving, and surface morphology, during rapid thermal annealing (RTA) was studied. High-resolution cross-sectional transmission electron microscopy (HRXTEM) suggested that Ge-rich Si1−zGez segregation takes place preferentially at the germanosilicide/Si1−xGex interface, more specifically at the triple junctions between two adjacent NiSi1−uGeu grains and the underlying epi Si1−xGex, and it is accompanied with thermal grooving process. The segregation process accelerates the thermal grooving of NiSi1−uGeu grain boundaries at the interface. The segregation-accelerated grain boundary grooving has significant effect on the surface morphology of NiSi1−uGeu films in Ni-SiGe system.

The monitoring of grain-boundary grooves in alumina

The geometry of grain-boundary grooves in polycrystalline alumina has been studied using a combination of visible-light microscopy (VLM) and atomic force microscopy (AFM). Samples were heat treated in air to induce thermal grooving. A VLM map was made from the central region of each sample. Profiles of grooves located within the VLM maps were measured using AFM. Using again VLM maps, the same regions were re-examined using AFM after an additional heat treatment. This approach allowed the geometries of grooves which formed at migrating grain boundaries to be directly compared with those at stationary boundaries. The groove profiles measured were asymmetric at migrating grain boundaries and more nearly symmetric at stationary grain boundaries.

Grain Boundary Energy and Tensile Ductility in Superplastic Cation-doped TZP

Fine-grained 3Y-TZP has been known to show high superplasticity. Addition of a small amount of metal oxide influences the superplastic behavior in 3Y-TZP. In this study, 3Y-TZP doped with 1 mol% GeO2, TiO2 or BaO were fabricated, and respective grain boundary energy has been systematically measured by a thermal grooving technique with atomic force microscopy. It has been found that addition of Ge4+ or Ti4+ ions decreases the grain boundary energy to stabilize the grain boundaries in TZP whereas doping of Ba2+ ion increases the grain boundary energy to destabilize the grain boundaries. A change in the grain boundary energy should be due to segregation of dopant at grain boundaries. It has been also found that the elongation to failure of cation-doped 3Y-TZP is directly proportional to the stability of grain boundary. Grain boundary energy is thus one of the principal factors to determine the tensile ductility of TZP. In order to reveal the effect of dopant on the grain boundary energy, lattice static calculations and first principles molecular orbital calculations have been performed for supercells and model clusters including the present dopant, respectively. A series of results shows that substitution of Ge4+ or Ti4+ ion for Zr4+ ion increases the covalency of TZP, but the covalency of TZP is reduced by addition of Ba2+ ions. The grain boundary energy is found to have a relationship with covalency nearby grain boundaries in TZP.

Grain boundary energy vs. misorientation in Inconel ® 600 alloy as measured by thermal groove and OIM analysis correlation

Samples of Inconel ® 600 foil were annealed to form a bamboo structure with grain boundary grooves. Grain boundary misorientation was measured using OIM and groove root angle was measured by AFM. Analysis of the data showed a misorientation vs. grain boundary plot that agrees with literature trends for FCC materials.