Dendritic Single Crystals Research Articles

Mapping single-crystal dendritic microstructure and defects in nickel-base superalloys with synchrotron radiation

Solidification of single-crystal nickel-base superalloys introduces large-scale segregation of constituent elements and defects such as dislocations and mosaicity. By exploiting the energy tunability and interference capabilities of high-brilliance X-ray radiation, key structural features of the dendritic single crystals were mapped over large areas. Interference and diffraction of synchrotron X-rays revealed significant misorientations between individual dendrites in the as-solidified state. For the first time this mosaic structure was quantified for an array of dendrites and correlated with the density of “grown-in” dislocations whose density ranged from 10 7 to 10 8 cm −2. Absorption contrast permitted simultaneous mapping of the distribution of refractory metal additives (e.g. rhenium and tungsten), which segregated preferentially to the dendrite cores with a linear composition gradient toward the interdendritic regions. The results demonstrate that synchrotron X-ray imaging is promising for in situ studies of single-crystal structure and defects in nickel-base superalloys.

Micromorphology of pure and PbI2‐doped CdI2 dendritic single crystals

AbstractUndoped and PbI2‐doped dendritic single crystals were grown by vapour growth technique. The basal surfaces of the as grown crystals were examined by optical and electron microscopy to observe wide variety of growth and defect features. Apart from typical features of dendritic growth, features of overgrowth, slip bands, growth steps and their bunching, etc. were observed. The basal surfaces of the crystals were then etched by controlled condensation of water vapour, after optimizing the etching condition, and the microscopic studies were repeated. Etch pits of hexagonal and triangular shape, both symmetric and asymmetric, and of different density, were observed in the case of undoped and doped crystals, respectively. In some cases, crystallographic hillocks were also observed. The crystals were also examined by X‐ray diffraction for their polytypism and related behaviour. The results are analyzed to elicit information on the correlation of structure, defects and surface features of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Devitrification phase transformations in amorphous Al85Ni7Gd8 alloy

Non-isothermal devitrification phase transformations in amorphous Al85Ni7Gd8 over the temperature range from 100 to 1300 °C were systematically investigated using differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) techniques. Continuous heating DSC scans revealed that the crystallization proceeds through multiple stages. The only crystalline phase formed in the first two stages is fcc-Al, appearing exclusively as dendritic single crystals. A metastable phase (τn) is formed in the 3rd stage, and another metastable phase (τu) is formed in the 4th stage, together with the equilibrium ternary compound τ1. The equilibrium “binary” compound M3Gd (M=Al, Ni) with 0.4 at.% Ni solubility is formed only in the 5th stage. Further heating initiates eutectic melting at 635 °C, followed by other melting events at higher temperatures, until fully liquid when T>919 °C. Isothermal annealing at 260 °C readily induces formation of another metastable phase (τm) and fcc-Al. Fcc-Al nanocrystal development and interpretation of isothermal DSC technique is discussed.

Deformed microstructure of the single-crystal superalloy NASAIR 100 at 1050 °C

The microstructure of the [001] oriented single-crystal superalloy NASAIR 100, solidified respectively with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces, deformed at 1050 °C and 160 MPa, was investigated with the attempt to reveal the variation of γ′ structures in the necked zones of creep specimens and, furthermore, to explain that the dendritic single crystals constantly possessed superior stress-rupture lives. During the stress-rupture tests, it was found that directional coarsening of γ′ precipitates occurred and that γ′ rafts perpendicular to the tensile load axis, [001] orientation, formed. The γ′ rafts in the necked zones of tensile creep specimens were thickened significantly and were no longer perpendicular to, but gradually inclined to, the tensile load axis. Additionally, the γ′ rafts were much thicker and showed less perfect morphology in both the planar and cellular single crystals than in the dendritic single crystals. From the observation of dislocation configurations, it was evident that the shearing of γ′ precipitates was the main movement of dislocations when passing the γ′ particles. At the primary creep stages of both the planar and cellular single crystals, the slip of dislocations was inhomogeneous. However, the shearing of γ′ precipitates in the dendritic single crystals was homogeneous, resulting in higher creep resistance.

Polytypism and Related Phenomena in CdBr2-Doped Dendritic Single Crystals of Cadmium Iodide

Dendritic single crystals of CdI 2 , doped with 3% CdBr 2 , were grown from vapour phase in vacuum. The crystals exhibited well developed dendritic morphology. They were characterized by X-ray diffraction for their polytypism and related phenomena of streaking and arcing. The CdI 2 common polytype 4H was found to occur exclusively in 28% of cases and in combination with the poylytype 12R in the rest of the cases. The frequency of occurrence of streaking and arcing were found to be rather high, viz 47 and 25%, respectively. The results point to a large-scale formation of dislocations and the consequent stacking faults inside the crystals, presumably created due to stresses produced in the host structure by the introduction of doped ions. Optical and scanning electron-microscopic study of the surface features shows a well developed dendritic morphology conforming with the hexagonal symmetry of the crystals.

Prediction of a process window for the investment casting of dendritic single crystals

Large dendritic single crystal blades operating in jet engines or land-based turbines are commonly produced in industrial plants. However, the production cost of some of these parts remains high due to the several types of defect which may appear during the different stages of the investment casting process. The prevention of stray crystal formation during solidification is the main problem which has to be faced by engineers. Analytical developments are first presented in order to define conditions under which single crystal growth is favored. This process window is described in a space where the thermal gradient and the withdrawal speed of a Bridgman furnace apparatus are the controlling parameters. The limits of this process window are then refined in order to determine the influence of other parameters such as the crystallographic orientation of the single dendritic grain, the size of the platform in which the single grain has to extend, the growth kinetics of the dendrite tips, or the existence of a lateral thermal gradient.

Influence of solid/liquid interfaces on the microstructure and stress-rupture life of the single-crystal nickel-base superalloy NASAIR 100

The [001] oriented single crystals of nickel-base superalloy NASAIR 100 with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces were prepared, respectively, and their microstructure and stress-rupture behavior at 1050 °C were investigated in both as-cast and solution heat-treated conditions. It was found that in as-cast single crystals of NASAIR 100, microsegregation and γ/γ′ eutectic produced in the solidification process increased, γ′ size decreased, and γ′ shape tended progressively to be cuboidal, with the successive transition of the S/L interface from planar to cellular, then to coarse-dendritic, and finally to fine-dendritic morphology. Furthermore, the solution temperature required to dissolve all as-cast γ′ and most of the γ/γ′ eutectic increased with the aforementioned successive transition of S/L interfaces. The reprecipitated γ′, after solution heat treatment (SHT), was usually fine and cuboidal. However, some W-rich phase was present in the heat-treated dendritic single crystals. Both the planar and the cellular single crystals of NASAIR 100 exhibited no superiority in stress-rupture life, irrespective of the heat-treatment conditions. Instead, the single crystals with dendritic morphology possessed excellent stress-rupture lives, after heat treatment at 1320 °C for 4 hours, followed by air cooling (AC). Perfect γ′ rafts with high-average aspect ratios formed during the stress-rupture tests of dendritic single crystals; in contrast, irregularly coarsening structures appeared in both the planar and cellular single crystals. The microstructure and solution behavior were illustrated in detail. Furthermore, the microstructural factors to affect the high-temperature stress-rupture life of the single crystals of NASAIR 100 were also analyzed.

Microscopic, etching and X-ray studies on doped CdI2dendritic single crystals

Microscopic, etching and X-ray studies on doped CdI₂dendritic single crystals

Growth and characterization of kdp-doped dendritic single crystals of cadmium iodide

Abstract Dendritic single crystals of KDP-doped cadmium iodide have been grown from vapour phase in vacuum. Polytypic behaviour of these crystals has been studied by X-ray diffraction. X-ray oscillation photographs have shown the occurrence of the most common CdI2 polytype 4H in 97% of cases. Streaking and arcing of reflections on the X-ray photographs are infrequent. Surface growth features on the basal faces have been studied by optical and scanning electron microscopy. It has been observed that doping with KH2PO4 has been found to result in marked changes in the surface features, as compared with doping with PbI2 and CdBr2.

Growth, twinning and etch figures of ferroelectric/ferroelastic dendritic BiFeO3 single domain crystals

Ferroelectric/ferroelastic dendritic single domain crystals of BiFeO3 were grown in the ferroelectric phase in a Bi2O3/B2O3/ Fe2O3 flux using a platinum crucible, placed in an evacuated sealed quartz tube. Etching of the crystals shows the non-centrosymetric polar character of the crystals. The single domains can be rendered ferroelastically polydomain by heating above the phase transition temperature of 1110 K and subsequent cooling or by mechanical twinning using stress components along the spontaneous polarization direction, which represents the longest pseudocubic space diagonal. Mechanical detwinning has also been observed. Etching of the twinned crystals at room temperature showed no 180° ferroelectric domains inside the ferroelastic domains.

Effect of variation in PbI2doping on the polytypism of dendritic CdI2single crystals

Highly pure dendritic single crystals of cadmium iodide, variously (2 to 8%) doped with lead iodide, have been grown from the vapour phase in a vacuum. The study of polytypism and related phenomena in the crystals has been carried out by X-ray diffraction. An uneven high rate of increase in the occurrence of the 12-layered rhombohedral polytype 12R has been found to exist when the amount of PbI2 in the crystals rises from 2 to 8%. Simultaneously, the relative intensity of the 12R reflections has been found to rise rapidly. Another curious feature has been an unusually high frequency of occurrence of a particular high polytype 48H in the heavily (8%) doped crystals. The percentage occurrences of streaking and arcing have been found to be low and to vary slowly with the PbI2 content of the crystals. The observed polytypism has been explained in terms of the nucleation of two different thermodynamically stable end-member polytypes, 4H and 12R, of CdI2 and PbI2, respectively.

Influence of Crystallographic and Heat Flow Orientations on Growth of Cellular Dendrites.

A numerical program has been developed in order to calculate the time-dependent thermal gradient and isotherm velocity within a solidifying pool obtained by stationary laser treatment. The heat diffusion equation is solved for an axisymmetric geometry with an enthalpy formulation using a standard FDM implicit scheme. The program has been used to investigate to which extent the cellular dendritic growth orientation is affected by the direction and amplitude of the thermal gradient.Experiments were carried out on Al-1 mass%Si dendritic single crystals oriented by Laue back reflection technique and cut along precise crystallographic orientations prior to laser irradiation and on Al-3 mass%Cu eutectic polycrystalline alloys. Deviation of cellular dendritic growth direction from crystallographic and heat flux directions are reported. In addition, the comparison of the microstructural spacings with theoretical predictions outlines the influence of crystallography on the microstructural development during stationary laser treatment.

Dendritic growth of PbI2single crystals and study of their polytypism and growth features

Dendritic single crystals of highly purified lead iodide were grown from the vapour phase in vacuum. XRD was employed to characterize them and to study their polytypism and related behaviour. Most crystals were found to consist of the common polytype 12RO and 12RR. Streaking of the reflections was not seen in the X-ray photographs, but nearly 12% of the photographs showed arcing of the reflections. The surface microtopographic study of the basal (0001) faces of the crystals revealed typical dendritic growth features. Layer-by-layer growth by two-dimensional nucleation was frequently observed, while spiral growth was not observed at all.

Polytypism in PbI2-doped dendritic crystals of cadmium iodide

Cadmium iodide and lead iodide have been highly purified by a horizontal zone refining technique. Dendritic single crystals of PbI2-doped cadmium iodide have been grown from vapour phase in vacuum. The effect of doping on polytype formation and other structural characteristics has been studied by X-ray diffraction. The crystals have shown the formation of rhombohedral polytype 12R in about 50% of cases, along with the common polytype 4H of cadmium iodide. This is in sharp contrast to earlier findings for undoped dendritic crystals which exclusively contained the polytype 4H. Further, unlike the undoped crystals, some of the doped crystals also show streaking and arcing on their X-ray photographs. The observed structural changes may be governed by both thermodynamical considerations and the kinetics of crystal growth and the observed streaking and arcing result from internal stresses built up around the relatively large Pb2+ ions in the host CdI2 structure.

Microsegregation in dendritic single crystals of nickel-rich alloys

Segregation effects caused by unidirectional solidification of nickel-base alloys have been studied. It was found that: the composition in the branches of dendrites and y′-particles is constant along the growth path; the segregation of titanium, aluminium, and molybdenum increases towards the interdendritic region and of nickel and chromium increases towards the base of the dendrites; the occurrence of y–y′ nodules increases in those parts of the castings which solidified in regions remote from the chill plate.MST/237

Microsegregation in dendritic single crystals of nickel-rich alloys

Segregation effects caused by unidirectional solidification of nickel-base alloys have been studied. It was found that: the composition in the branches of dendrites and y′-particles is constant along the growth path; the segregation of titanium, aluminium, and molybdenum increases towards the interdendritic region and of nickel and chromium increases towards the base of the dendrites; the occurrence of y–y′ nodules increases in those parts of the castings which solidified in regions remote from the chill plate.MST/237