Ledge Growth Mechanism Research Articles

The phase transformation behavior between γ lamellae and massive γ in a Ta containing TiAl-based alloy

The phase transformation behavior between massive γ (γm) phase and γ lamellae was studied through interrupted quenching from single α-phase region and α+γ two-phase region in a Ti–48Al–3Nb-0.5Ta alloy. Massive α→γm phase transformation temperature T0 is confirmed to be 1400 °C (in α+γ two-phase region). Combining with the thermodynamic analysis, it indicates that the formation of γm phase is gradually inhibited with the decrease of quenching temperature, instead fine γ lamellae are preferred. In this case EBSD (Electron back scattered diffraction) and HRTEM (high resolution transmission electron microscope) have shown that besides nucleating on α grain boundary, γm phase can nucleate on defects in the interior of an α grain with Blackburn orientation relationship or nucleate on fine γ lamellae along close-packed {111}γ with orientation relationship of [121‾] γlamellae//[1‾11]γm1// [101‾1‾]α2 at their interface. Furthermore, the ledge growth mechanism of γm phase is identified.

Investigation of the Ledge Structure in Aluminum Smelting Cells

In aluminum smelting cells, ledges freeze on to cell walls from the cryolitic bath when the temperature drops below the bath liquidus point. Modern cell design and control cause a suitable ledge profile to form and be maintained, in order to protect the cell walls from corrosive liquids (molten salts and Al metal) and ensure efficient current distribution and cell heat balance. During cell operation, a significant ledge, freezing and melting does occur following heat balance changes due to batch operations. The ledge formation mechanism has been studied at the laboratory scale in our previous work. It shows a linkage between the rate and directional nature of ledge growth and its structure as affected through a superheat change. An open ledge structure can dominate the laboratory ledge material growth or melt it out quickly when the superheat either decreases or increases, respectively. This paper begins the investigation of industrial ledge samples, in terms of structure and composition, primarily to identify whether the same ledge formation mechanism exists in industrial cells. In this study, as expected, the industrial ledge shows more complexity than the laboratory ledge; the open structure is different compared to the laboratory ledge due to the inclusion of carbon dust, a large thermal gradient across the ledge, and sufficient aging of the ledge in the cell. The comparison between the laboratory ledge and the industrial ledge has provided insight into the ledge growth mechanism in aluminum smelting cells.

Synthesis and Reaction Mechanism of Ti<sub>3</sub>SiC<sub>2</sub> by Molten Salt Method from Ti-Si-Fe Alloy

Titanium silicon carbide (Ti3SiC2) were obtained by molten salt synthesis method using the Ti-Si-Fe alloy extracted from high titania blast furnace slag and natural graphite as the raw materials. The phase composition, microscopic structure of the products were characterized by powder X-ray diffraction, scanning electron microscope and transmission electron microscope. The influence of firing temperature and chloride salts species on the phase and morphology of the products were investigated. The results indicated that the synthetic temperature of Ti3SiC2 by molten salt synthesis method was about 100 °C, which was lower than that without molten salts. The “dissolution-precipitation” mechanism governed the overall molten salt synthesis process. The lamellar Ti3(Si,Al)C2 crystal growth obeyed by a two-dimensional ledge growth mechanism.

Nucleation and growth mechanism of Ag precipitates in a CuAgZr alloy

CuAgZr alloy is a variant of the CuAg alloy that is developed for high strength and high conductivity applications. Its strengthening is accomplished mainly by the precipitation of Ag precipitates, which tend to align on {111} planes in the Cu matrix. This alignment has been reported to be caused by stacking faults on {111} planes. Contrary to these reports, this research presents evidence for Ag precipitates formation on {111} planes due to the minimization of elastic energy. The Ag precipitates were formed by clustering of Ag atoms while maintaining the fcc crystal structure of the matrix. They have faceted {111} interfaces with the matrix. The thickening of precipitates appears to be by the ledge growth mechanism, which is resulted in by misfit dislocation networks on the interface.

Orientation-tuning in self-assembled heterostructures induced by a buffer layer.

Anisotropic nano-plate structures in self-assembled perovskite-spinel thin films, BiFeO3-NiFe2O4 and BiFeO3-CoFe2O4, which were deposited on (001)c SrRuO3/SrTiO3 and DyScO3 substrates, respectively, have been demonstrated using transmission electron microscopy combined with strain analysis. Unlike the unitary cube-on-cube orientation relationship reported widely, the growth direction of the CoFe2O4 and NiFe2O4 plates was tuned to [011]c while the BiFeO3 matrix kept [001]c in both systems. In particular, a thin stress-sensitive BiFeO3 buffer layer between the spinel nanostructure and the substrate was introduced for providing a complex strain state in both film systems. The novel orientation tuning and the pattern configuration of the heterostructures are mainly attributed to the strain imposed on the films and the anisotropic ledge growth mechanism of spinels.

Nanobelt Structure in Perovskite–Spinel Composite Thin Films

We report a nanobelt structure along the 〈110〉 direction in perovskite–spinel (BiFeO3–CoFe2O4 or BFO–CFO) composite epitaxial thin films deposited on SrTiO3 (STO) substrates by pulsed laser deposition. The results reveal the nucleation and growth mechanism of a nanostructure: first nucleation of perovskite phase regions on STO, followed by segregation of spinel out of the perovskite phase, and finally by evolution of spinel nanobelts that are elongated along the 〈110〉. The reason for control of the nanostructure evolution is a ledge growth mechanism of spinel along the 〈110〉 direction.

The massive transformation in titanium aluminides: Initial stages of nucleation and growth

Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γm massive transformation in titanium aluminides. Splat quenching Ti52Al48 and Ti50Al48Cr2 from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 106 K/s arrest the α → γm massive transformation with 1- to 5-µm-sized γm nuclei, especially in the Ti50Al48Cr2 alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111]γ//[0001]α and \(\{ 110\} _\gamma //\{ 11\bar 20\} _\alpha \). The γm nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111}γ facets. Orthogonal variants of the tetragonal c-axis in the γm product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L10 phase. Nucleation of γm is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γm nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular nuclei. However, the intragrain nuclei do not form {111}γ facets and retain a curved α:γm interface. Although analysis of the {111}γ faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations, lattice imaging of the {111}γ facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing (λ) to ledge height (h) for the {111}γ facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.

Discrete β-Ta2O5 crystallite formation in reactively sputtered amorphous thin films

The crystallization of thin amorphous TaOx films formed by d.c. reactive sputtering was investigated at temperatures from 500–700 °C. The films remained amorphous for times up to 100 h at 500 °C. The formation of discrete, single crystallites of the orthorhombic β-Ta2O5 phase was observed after annealing at 600 °C for times from 8–108 h. The crystallites were 0.35 μm×0.35 μm after 8 h and grew to approximately 2.5 μm×2.0 μm after 108 h. A (2 0 0) fibre texture with a 6° spread was observed. More rapid in-plane growth in the [0 1 0] direction resulted in a near-rectangular shape and is attributed to a ledge growth mechanism. Higher temperature anneals at 650 and 700 °C produced less-textured polycrystalline films with remnant amorphous regions. © 1998 Kluwer Academic Publishers

The Effect of Heat Treatment on the Microstructure and Properties of FeAl+Cr

ABSTRACTMicrostructural studies were performed on an alloy of composition Fe45Cr5Al50 heat treated at 950°C and oil-quenched and then given isothermal annealing treatments for times up to 200 hours at either 400°C or 500°C. The observed microstructures were correlated with variations in hardness during isothermal annealing. It was deduced that the thermal vacancies retained following the initial heat treatment are removed relatively rapidly from the lattice, which leads to an initial drop in hardness. However, during prolonged annealing, the coarsening of bothFeAl2 particles and a disordered a(Fe,Cr) phase leads to further softening. It was also deduced that the chromium atoms, which remain in solution, are effective solute strengtheners. The a(Fe,Cr) phase, which is coherent with the B2 matrix, appears to coarsen by a ledge growth mechanism.

Appraisal of recent work on role of crystallography in ferrous and non-ferrous pearlite

Work on pearlite up to 1962 is briefly outlined and work on the crystallography of pearlite before 1984 is discussed. Studies of the heterophase boundaries in ferrous and non1errous pearlite by transmission electron microscopy over the past decade are introduced. The ‘ledge growth mechanism’ based on such studies is evaluated against the ‘classic’ view of pearlite growth. It is concluded that the hypothesis of the ledge growth mechanism offers a comprehensive explanation of pearlite phenomena and that the classic view fails to account for some features of pearlite. The implications of the new research on pearlite are addressed. The details of ledge motion and the velocity–spacing problem are discussed. The lack of work on the importance of crystallography on morphological evolution of pear lites is identified. Further work necessary to evaluate and advance the new understanding of pearlite is outlined.MST/3253

Appraisal of recent work on role of crystallography in ferrous and non-ferrous pearlite

Work on pearlite up to 1962 is briefly outlined and work on the crystallography of pearlite before 1984 is discussed. Studies of the heterophase boundaries in ferrous and non1errous pearlite by transmission electron microscopy over the past decade are introduced. The ‘ledge growth mechanism’ based on such studies is evaluated against the ‘classic’ view of pearlite growth. It is concluded that the hypothesis of the ledge growth mechanism offers a comprehensive explanation of pearlite phenomena and that the classic view fails to account for some features of pearlite. The implications of the new research on pearlite are addressed. The details of ledge motion and the velocity–spacing problem are discussed. The lack of work on the importance of crystallography on morphological evolution of pear lites is identified. Further work necessary to evaluate and advance the new understanding of pearlite is outlined.MST/3253

In situ HREM of interface interactions

We have applied in situ high-resolution electron microscopy (HREM) to the study of interface reactions, particularly in metal-semiconductor systems. There is contrasting behavior whether or not the manufactured interface undergoes a chemical reaction. The in situ technique allows determination of the reaction mechanisms on an atomic scale.Reactive interfaces are characterized by systems in which new chemical compounds are formed (e.g., silicides for metal-silicon interfaces, metal gallides and arsenides for GaAs, etc.). We found that the equilibrium phase formation is often preceded by a solid-state amorphization reaction. In situ observations allow very precise measurement of the reaction rate in a sufficient temperature range to confirm that this process is diffusion controlled. Crystallization of the amorphous material can be followed as well as the development of any crystallographic orientation relationships. A ledge growth mechanism can easily be distinguished from a random process.It might be expected that non-reactive interfaces are stable upon heating.

Direct observation of crystallization in silicon by in situ high-resolution electron microscopy

We have studied the nucleation and growth of crystalline silicon by in situhigh-resolution electron microscopy. Amorphous silicon thin films, deposited onto oxidized silicon wafers by low-pressure chemical vapor deposition, were heated in the microscope in the nominal temperature range of 700–775°C. Many sub-critical crystal embryos exist at the a-Si/SiO 2 interface, very few of which develop into viable nuclei. One nucleation event was recorded successfully at lattice resolution, allowing an estimate of the critical radius as 2.5±1.0 nm and the a-Si/c-Si interfacial energy as 600±200 mJ m −2. The crystal growth was followed for extended periods of time. It appeared to be characterized by sporadic growth bursts rather than a continuos advance of the crystal-amorphous phase interface. We specifically sought evidence for a ledge growth mechanism but did not find such behavior.

Interface structure between decagonal and icosahedral quasicrystals in Al-Pd-Mn alloy

Abstract The decagonal (D) phase was formed in a melt-spun Al70Pd20Mn10 ribbon after annealing at 800°C, and its interface to the icosahedral (I) phase was examined by conventional and high-resolution electron microscopy. The results show that growth inter race of the D-phase is characterized by a stepped structure constructed of interfacial facets and ledges. The interfacial facets are parallel to the (tenfold plane)D ∥ (fivefold plane)I, along which a good match between the D- and I-phases has been observed. These observations suggest a possible ledge growth mechanism of the D-phase.

High Resolution In Situ TEM Studies of Silicide-Mediated Crystallization of Amorphous Silicon

ABSTRACTWe report in situ high resolution transmission electron microscopy studies of NiSi2-Medi-ated crystallization of Amorphous Si. Compared to conventional solid phase epitaxy of (111) Si, an enhancement of the growth rate by three orders of magnitude was observed and high quality twin-free needles of <111> Si were formed. Crystallization occurred via a ledge growth mechanism at the epitaxial Type A NiSi2/crystalline Si (111) interface. A Model for NiSi2-Mediated crystallization of Amorphous Si involving the passage of kinks along <110> ledges at the NiSi2/crystalline Si (111) interface is proposed.

Surface outgrowth problem in c-axis oriented Y-Ba-Cu-O superconducting thin films

The origin of surface outgrowths in c-axis oriented superconducting Y-Ba-Cu-O thin films has been identified. We find that a,b-axis regions nucleate heterogeneously at second phase regions and rapidly grow outward, leading to the formation of lenticular-shaped outgrowths. The second phase has been identified by electron diffraction and high-resolution electron microscopy to be the Y1Ba3Cu2O7−x phase that forms as microscopic platelet-shaped precipitates on the a-b plane. The heterogeneous nucleation of such outgrowths is general and has been observed in both in situ sputtered and laser-deposited films. It is probably related to the ledge growth mechanism of the c-axis oriented Y-Ba-Cu-O thin film and to the precise overall composition of the film and needs to be studied in detail in order to produce films with smooth surfaces.

Equilibrium And Non-Equilibrium Metal-Ceramic Interfaces

ABSTRACTMetal-ceramic interfaces in theimodynamic equilibrium (Au/ZrO2) and non-equilibrium (Au/MgO) have been studied by TEM and HREM. In the Au/ZrO2 system, ZrO2 precipitates formed by internal oxidation of a 7%Zr-Au alloy show a cubic ZrO2 phase. It appears that formation of the cubic ZrO2 is facilitated by alignment with the Au matrix. Most of the ZrO2 precipitates have a perfect cube-on-cube orientation relationship with the Au matrix. The large number of interfacial steps observed in a short-time annealing experiment indicate that the precipitates are formed by the ledge growth mechanism. The lowest interfacial energy is indicated by the dominance of closed-packed {111} Au/ZrO2 interfaces. In the Au/MgO system, composite films with small MgO smoke particles embedded in a Au matrix were prepared by a thin film technique. HREM observations show that most of the Au/MgO interfaces have a strong tendency to maintain a dense lattice structure across the interfaces irrespective of whether the interfaces are incoherent or semi-coherent. This indicates that there may be a relatively strong bond between MgO and Au.

Growth of α‐Al2O3 Within a Transition Alumina Matrix

The growth of α‐Al2O3 from a planar specimen of thermally grown γ‐alumina on a molybdenum transmission electron microscope grid was studied. The α‐Al2O3 grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of α‐Al2O3 ledges and are larger on α‐Al2O3 than on the transition alumina matrix.