Formation of a blocky ferrite in Fe–Cr–Ni alloy during directional solidification

Formation and evolution details of a blocky microstructure in AISI 304 stainless steel are studied by quenching method during directional solidification. Results show that a coupled growth microstructure, consisting of lathy ferrite and austenite, forms first from the melt. At solid-state transformation stage, most lathy ferrite disappears due to the phase transformation from ferrite to austenite. With further decreasing of the temperature, plenty of fine ferrite colonies occur in the original austenite region. The formation of the blocky ferrite indicates that reverse solid-state transformation from austenite to ferrite takes place. This transformation is due to the segregation and the instability of austenite during the growth of austenite under low cooling rate. The fine ferrite colonies transform into blocky ferrite at room temperature. TEM and EDS analyses were carried out to identify the phases and determine the phase composition, respectively.     

Zum Temperaturverhalten von Anpassungsversetzungen im gerichtet erstarrten NiAl?Cr-Eutektikum

Regular arrays of misfit dislocations at the interphase boundaries of a directionally solidified NiAl–Cr eutectic accommodate a certain amount of misfit between the chromium rods and the NiAl matrix. In this study the high temperature behaviour of these dislocation networks has been examined. Thin foils of the eutectic were heated slowly up to 700°C by a hot stage of the high voltage electron microscope and then cooled to room temperature. Investigations of bulk material quenched form 800° and 900°C complete the results. Although the misfit changes with temperature in consequence of different thermal expansions of the two phases the dislocation networks remain unchanged.     

Mode of solidification and strength properties for lead-tin binary system

Tensile strength and ductility of unidirectionally and randomly solidified Sn, Pb and Sn–Pb eutectic alloy are determined. Appreciable increase in the ductility from random to directional growth has been observed in case of Sn, Pb and Pb–Sn eutectic. Improvement in tensile strengths of pure metals and their eutectic composite by 15–20% and 15%, respectively, was evident in case of directionally solidified to randomly grown samples. The increase in tensile strength of the eutectic was 2–2.5 times to the strength of the pure metals. Tensile strength of directionally solidified Pb–Sn eutectic studied at different rates of solidification for constant temperature gradient was lower at higher and at lower rates of solidification than the value at medium rates of solidification.     

The structure of rapidly solidified ribbons of Al-12 at. % Zn alloy

The structure of melt spun rapidly solidified ribbons of the Al-12 at.% Zn alloy has been investigated by scanning electron microscopy and by X-ray diffraction. Four solidification processes differing in the extent of associated segregation of zinc have been found to be operative during the solidification of the ribbon. These successive processes beginning from above the sites of good contact of the melt with the rotating plane of the copper substrate are as follows: massive planar growth of grains, arrayed dendritic growth, unaligned growth of dendrites, and star-like dendritic growth out from fragments of dendrites. Smaller grains resulting from the last two mentioned processes have been also found within the regions located above the sites of poor contact of the melt with the substrate (lift-off regions). All indicated successive growth modes starting with the epitaxial regrowth of grains have been also found to proceed during the solidification of the melt flowed over the already solidified alloy ribbon. Grain boundary precipitation of equilibrium β (≈︁ Zn) phase has been detected in all stages of solidification. High density of lattice defects built-up during solidification is leading to the immediate intragranular heterogeneous precipitation of β phase at elevated temperatures within the two-phase region without the formation of intermediate metastable precipitates.     

Hair growth at a solid-liquid interface as a protein crystal without cell division

Concentrations of elements in single hair samples were evaluated by X-ray fluorescence by scanning with a narrow beam in the growth direction. Zn binds to the hair protein molecules, and is distributed uniformly from hair tip to root bulb by steady-state growth. To avoid the effect of thickness variation for the bulb, the hair elements were evaluated as the amount per protein molecule using the hair [Zn], resulting in the fault-bounded [S] change typical for a solid–liquid interface; the papilla is in a liquid state and the segregation of elements occurs so as to maintain the amount of shaft element equal to the element inflow into the papilla from the blood, leading to the relationship between hair and blood concentrations. The diffusion boundary layer of S segregation in the bulb gives the diffusion coefficient of D～1 × 10^?8 cm²/s. The liquid papilla during hair growth solidifies with temperature decrease with the formation of the hair specimen, and the results for solidified papilla are different from the state during growth. It is proposed that the serum protein supplied into dermal papilla changes into precursor keratin molecules, and then into insolvable keratin in the hair matrix cells, i.e., hair makes “protein-melt growth.” The pulsed or stepwise variations of [Ca] and [Sr] occur due to the ion channel gating of matrix cells; such variations can never be expected for the cell division growth as deduced from the solidified papilla. The hair growth reflects the status of ion channels and pumping only possible because of the solid–liquid growth interface driven by the gradient in chemical potential nearly perpendicular to the skin surface. Thus, a hair root is a solid–liquid system for hair formation from serum protein.     

Evolution of cellular spacing during directional solid-state ferrite–austenite transformation of Fe–Mn–Al alloy

A series of isovelocity experiments was performed to investigate the interface morphology evolution during the solid-state ferrite–austenite transformation of the Fe–Mn–Al alloy, and a quantitative relationship between cellular spacing and growth velocity had been measured. The corresponding cellular spacing of the product phase decreases with the growth velocity increasing. Theoretical model modified from directional solidification process was also applied to study the relationship between cellular spacing and growth velocity, and the theoretical prediction corresponds well with the measured results.     

Effect of minor alloying additions on the solidification of single-crystal Ni-base superalloys

Grain boundary elements, such as carbon and boron, were initially removed from single-crystal alloys, but more recently they have been added back into single-crystal alloys for increased castability and defect tolerance. The mechanism(s) for the increased castability is not completely understood. In this study, carbon was added to the second generation, single crystal, Ni-base superalloy and CMSX-4, to form MC-type carbides. Then either nitrogen or boron was added to the carbon-containing alloy to alter the carbide morphology and the castability of the alloy. The segregation coefficients of the alloying elements in CMSX-4 were measured with varying techniques, but no changes in partitioning were observed. The addition of carbon to CMSX-4 did reduce the number of solidification defects observed in the samples. The addition of carbon and boron to CMSX-4 did not significantly change the number of solidification defects compared to the carbon-only alloy. However, the addition of nitrogen and carbon resulted in an increase in the number of solidification defects in the CMSX-4 samples compared to the baseline CMSX-4 and carbon-containing CMSX-4 samples. The effects of these alloy additions on the carbide morphology and solidification defects are discussed.     

An analysis of the applicability of the Scheil equation in the determination of segregation in monocrystals of the manganese-zinc ferrites Mn1?xZnxFe2O4

Theoretical assumptions concerning the segregation phenomenon of alloying components during formation of monocrystals of the manganese-zinc ferrite Mn_{1 x}Zn_xFe₂O₄ have been presented. Taking into account the case of the unidirectional solidification of the above ferrite by means of the Bridgman furnace, which is working in:

• Closed system.
• Open system, with adding of the pure component (melting at an elevated temperature).
• Open system, with adding of ferrite of a nominal composition.
• The method for calculation of alloying components segregation has been analysed. A possibility of modelling the segregation by means of the choice of the size of the melting zone has been considered for a given monocrystal.

     

Influence of the starting powders on the synthesis of nickel ferrite

The thermal decomposition of freeze‐dried nickel(II)‐iron(III) formate was investigated by means of DTA, TG, mass spectrometry and X‐ray powder diffractometry. For the preparation of homogeneous freeze‐dried nickel(II)‐iron(III) formate precursors, a rigorous control of nickel ion concentration in the precursor solution was required. The decomposition of the reactive nickel(II)‐iron(III) formate does not only reflect aspects of single formates, but also an interaction between components which lowers the decomposition temperature. Crystalline nickel ferrite powders were obtained at 600‐800°C. This temperature is quite lower than 1100°C employed for the ceramic method. In the presence of air, the regeneration of nickel ferrite from the taenite phase (γNi,Fe) is accomplished at 800°C. This temperature is also 300°C below the temperature employed when the mixtures NiO:α‐Fe₂O₃ or Ni:2Fe are the starting powders. The main reason for the high reactivity of the freeze‐dried formates and the taenite alloy is the large homogeneity of these precursors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nitrogen segregation on the (100) surface of an austenitic chromium-nickel steel studied by LEED and AES

At temperatures above 700 K, the annealing of austenitic stainless steel under ultra-high vacuum conditions results in the segregation of nitrogen, accompanied by the increase of chromium concentration at the surface. Both the elements enrich at the surface, so that a constant ratio of the detected amounts is reached. Thus we conclude the formation of a chromium-nitrogen compound. — Due to the segregation of nitrogen, we observed the formation of a p (2 × 2) superstructure in the LEED pattern. From a kinematic interpretation of the (00) beam intensity measured we suppose the nitrogen to be arranged in fourfold sites and to have an atomic radius of 0.054 nm.     

3D simulation of the effects of growth parameters on the growth of sapphire crystals using heat exchanger method

3D simulations using the commercial CFDRC and FIDAP code, which are based on finite element techniques, were performed to investigate the effects of anisotropic conductivity on the convexity of the melt–crystal interface and the hot spots of sapphire crystal in a heat‐exchanger‐method crystal growth system. The convection boundary conditions of both the energy input to the crucible by the radiation as well as convection inside the furnace and the energy output through the heat exchanger are modeled. The cross‐sectional flow pattern and the shape of the melt–crystal interface are confirmed by comparing the 3‐D modeling results with previous 2D simulation results. In the 3D model, the “hot spots” in the corners of the crucible are donut shaped, and the shape changes with the value of the conductivity of anisotropic crystal. The outline of the crystal becomes more convex as the conductivity in the z direction (k_sz) increases. The outline of melt–crystal interface is elliptical when the anisotropic conductivity is moving in the radial direction (k_sx and k_sy). The portion at the outline touching the bottom of the crucible is smaller than the maximum outline of the crystal, meaning that the shape at the “hot spot”, changes with the value of the conductivities of anisotropic crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation and validation of the Peltier pulse marking of solid/liquid interfaces

We describe a numerical approach of the solidification of binary alloys to study the motion of a crystal/melt interface submitted to current pulses involving a modification of the dopant concentration field. For the thermal aspect, the Thomson effect, the Peltier effect and Joule heating have been included in the heat flow. For the solutal segregation, our model is based on mass transports which occur in the liquid phase, namely diffusion and convection. Numerical computations are validated by comparison with experimental data and thus could find applications in the prediction of the effects of Peltier pulse marking in crystal growth.     

Dislocation Structure and Strain Localisation in Nickel Single Crystals Cyclically Deformed at 77 K

An experimentally based investigation is presented of the dislocation structure and of glide effects occurring in single slip oriented nickel crystals cyclically deformed at 77 K until saturation of the stress amplitude. Special attention is paid to a comparison of slip and structure phenomena observed in fatigue tests at 77 K and those found after cycling at room temperature (RT) and elevated temperatures. At strain amplitudes within the plateau region of the cyclic stress‐strain curve, where at higher temperatures in the crystal two structure types co‐exist, at 77 K nearly the entire specimen volume is occupied by one structure “phase”, a dislocation‐“condensed” wall configuration. On different scale levels the main characteristics of this extended wall structure were found to be independent of the imposed amplitude, and they turned out to fit in the temperature dependence of the structure features of the ladder‐like wall phase characterising the zones of intense slip (persistent slip bands) at RT and elevated temperatures. At 77 K the strain is localised in narrow slip bands (SBs) in the same way as at higher temperatures, although there is no indication of a “two‐phase” structure. From the experimental findings it is concluded that WINTERs “two‐phase” model remains valid, when averaging the plastic strain values over all SBs and over a sufficient number of cycles.     

The role of surfactant in synthesis of magnetic nanocrystalline powder of NiFe2O4 by sol–gel auto-combustion method

In this work, a new sol–gel auto-combustion method has been performed to synthesize single phase nickel ferrite nanocrystalline powders by using n-cetyltrimethylammonium bromide, as a cationic surfactant. The gels were prepared from ferric and nickel nitrates and citric acid. Ammonia was used as pH adjusting agent as well. The effects of the surfactant on the after combustion calcination process and the reduction of the resulting powder crystallite size which affects the magnetic properties of the material were investigated by XRD and DTA/TGA techniques. The results showed that the ignition of the gels in air have a self-propagating behavior. Addition of surfactant to the starting solution affected the crystallite size of the synthesized powders and their phase constitution. The crystallite size of nickel ferrite powder in sample with surfactant was obtained 31.2 nm. The other important result of this study was production of single phase nickel ferrite directly after combustion while without surfactant the nickel ferrite single phase was obtained only after a post-calcination process at 1000 °C.     

Effective convection coefficient for porous interface and solute segregation

Convective heat and mass transfer coefficients are used to calculate the rate at which convection sweeps heat and mass away from the interface. Convection in melt growth is driven by various forces, and the resulting convoluted flows are laminar or turbulent. Furthermore, cross-flow through the “porous” interface has a profound effect on convection. Thus, a general effective coefficient (h_eff ), which accounts for: (i) uniform flow “suction” through the porous interface, (ii) forced and/or natural convection, (iii) laminar or turbulent flow, and (iv) finite Schmidt numbers, is derived. Focusing next on solute segregation, mass conservation is used to derive a simple equation for k_eff (effective segregation coefficient) as a function of h_eff. Here, h_eff is an input, which provides the rate at which convection sweeps the rejected solute away from the interface.From the general expression, even simpler expressions are developed for restricted range of conditions, e.g., Czochralski growth under forced laminar convection (no natural convection or turbulence). h_eff utilizes numerous established correlations, all developed for impermeable solids.     

On the correlation between solidified microstructures and liquid structural states in Bi-40 wt%Te alloy

The compound-forming alloy Bi-40 wt%Te was selected to explore effects of the temperature-induced liquid–liquid structure transition (TI-LLST) on solidification. The resistivity–temperature pattern of the molten alloy suggested that an irreversible TI-LLST occurred from 694.6 to 723.2 °C as temperature elevated. When solidified from the melt undergoing the TI-LLST, the solidification undercooling of the primary phase Bi₂Te₃ was enlarged and its growth manner was obviously changed, resulting in the spirally curling refined branches, contrary to the orientated thick rod-like crystals solidified from the melt failed to undergo the TI-LLST. Moreover, the volume fraction of the peritectic phase increased from 30.6% to 56.4%, which implied that the phase competition behavior during the peritectic reaction was also changed.     

Basal twinning as a room temperature plastic deformation mode of chromium- and titanium-doped sapphire crystals

The degree of basal twinning caused by indentation was determined quantitatively on a series of chromium and titanium doped sapphire crystals with variours dopant contents. It depends on the dopant content significantly. The effect of titanium ions in reducing the degree of twinning is more pronounced than that of chromium dopants. This “size effect” of the dopant ions is consistent with the mechanism of basal twinning as reported by KRONBERG .     

Thermoanalytical Investigations to Understand the Dependence Between the Growth Method and Crystal Properties of Valence Changing “YbInCu4”

The solidification phase diagram for the system Yb–In–Cu is investigated in the neighbourhood of the valence changing composition YbInCu₄ by means of differential thermal analysis. It is found that the solid phase with the structure of YbInCu₄ exists within a finite composition range and that the congruent melting composition of this phase is shifted towards Yb-rich composition with respect to the stochiometric composition. Temperature dependent x-ray powder diffraction is used to detect the valence transition of the “YbInCu₄-phase”. This transition occurs at appr. 40 K for samples grown from melts with an Yb-content less than 0.7 with respect to the formula Yb_xIn_2—xCu₄ while a higher Yb-content leads to a transition temperature of about 70 K. This is seen as a consequence of a change between differently ordered structures that occurs at a certain Yb-content.     

Impurity segregation in directional solidified multi-crystalline silicon

In this paper numerical results on the impurity segregation in directional solidified multi-crystalline silicon are presented and compared with experimental results. A solute transport model has been established to predict the final segregation pattern of impurities in the ingot. The segregation is analyzed experimentally on the basis of Fourier transform infrared (FTIR) spectroscopy and glow-discharge mass spectrometry (GDMS). Precipitates were located by IR-transmission microscopy (IRM). Qualitative agreement between simulation and experiment is found. It is demonstrated how the flow pattern can influence the final solute distribution. The simulation also shows that the solubility limit of carbon and nitrogen is reached locally in the ingot and SiC and Si₃N₄ precipitates are likely to form.