Effect Of Microsegregation Research Articles

Hydrazide‐based non‐symmetric liquid crystal dimers: synthesis and mesomorphic behavior

AbstractHydrazide‐based non‐symmetric liquid crystal dimers were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X‐ray diffraction (XRD). These non‐symmetric liquid crystal dimers are evidenced to display the monolayer smectic C phase. The effects of the lateral intermolecular hydrogen bonding as well as the length of the terminal alkyl chains and the spacers on the mesophase are discussed. Our studies reveal that intermolecular hydrogen bonding between the hydrazide groups and microsegregation effect is the driving force for the formation of the monolayer smectic C structure. Copyright © 2007 John Wiley & Sons, Ltd.

Effect of Micro-Segregation on Phase Transformation and Residual Stress

Some factors affecting the residual stresses in a thick plate of hardened steel have been studied. Dilatometry shows that the expansion during phase transformation on cooling is anisotropic and that the behaviour varies with position in the plate, being most marked in the centre and less in the sub-surface regions. This dimensional anisotropy on cooling is due to micro-segregation of the alloy elements. The extent of anisotropy also depends to some extent on conditions of austenitisation and on cooling rate. From the measurements of length change in different regions estimates are made of levels of residual stress in the hardened plates.

Microsegregation Effect of Copper in Solidification of A356 Alloy

In this study, the microsegregation effect of copper during solidification of A356.2 aluminum casting alloy was evaluated by using solidification simulation and a series of experimental works. Electron probe micro analysis (EPMA) and the computational simulation revealed clearly that copper as a solute element segregates during solidification and it becomes more significant as the solidification approaches its completion. This microsegregation effect of copper is due to accumulated copper solute ahead of the eutectic interface during solidification. Consequently, the presence of a higher content of copper in A356.2 aluminum casting alloy causes influence on the solidification characteristics of the alloy and forms copper enriched phases at the last region to solidify in the microstructures.

Compositional design considerations for microsegregation in single crystal superalloy systems

This paper investigates the role of varying refractory addition levels on the microsegregation behaviour of single crystal (SX) superalloys systems during solidification. Specifically, a series of six Ni-base alloy compositions are set in a controlled manner, such that the chemical effects of Re, W, and Ru can be independently assessed. Each experimental alloy is produced in the SX condition utilizing a modified Bridgman casting process, with subsequent compositional analysis of the solidification structures via standard electron microprobe analysis (EPMA) methods. Qualitative partitioning results indicate typical SX alloy segregation behavior with elements such as Cr, Co, Re, Mo, and W all segregating towards the dendrite core regions, while the γ′ forming elements of Al, Ti, and Ta show favorable partitioning to the interdendritic γ–γ′ eutectic regions. Both Ni and Ru exhibit ideal segregation behaviour with no favorable partitioning to either phase. Quantitative EPMA results indicate that as the nominal Re level increases, the severity of microsegregation to the dendrite core regions rises dramatically for Mo, Cr, and Re. Remarkably, evidence is presented demonstrating the role that Ru plays in counteracting the microsegregation effects of both increased Re and higher overall total refractory levels.

On the Crystallization Process of Hypoeutectic Al‐6 % Cu,Unmodified and Sr‐Modified Al‐2 % Si Solidification Alloys

The solidification process of some hypoeutectic aluminum alloys was studied over a wide range of cooling rates. Microsegregation effects were characterized by the minimum concentration of solute found in primary dendrite trunks (see Figure for a typical example), using microprobe analysis, and the fraction of eutectic phases evaluated as a function of the cooling rate. Finally, a non‐equilibrium thermodynamic model of the process, which is able to explain some of the experimental observations, was developed.

Effect of microsegregation and dislocations on the nucleation kinetics of precipitation in aluminium alloy AA3003

Nucleation kinetics of the precipitation for MnAl 6 and MnAl 12 in the aluminum alloy AA3003 alloy have been investigated experimentally and theoretically. The results show that cold rolling enhances the rate of precipitation and this effect increases as the annealing temperature decreases. Micro-segregation of the solute atom Mn at the dislocation network during cold deformation is found to have a significant effect on the nucleation kinetics of the precipitation of MnAl 6 and MnAl 12 in AA3003 in addition to the effect of dislocations, which increase the nucleation site density and reduce the nucleation barrier. A model to predict the start times of the precipitation during isothermal holding is constructed by considering the effects of dislocations and recovery as well as micro-segregation of Mn on the nucleation kinetics of precipitates. The predictions of the model are in good agreement with the experiment data.

Microstructure and mechanical properties of a β Nb–Ti based alloy

This paper presents the results of a study of microstructure and mechanical properties (tensile and creep behavior) of a multicomponent body-centered-cubic Nb–35Ti–6Al–5Cr–8V–1W–0.5Mo–0.3Hf–0.5C alloy (compositions quoted in atomic% unless stated otherwise). The double extruded material is shown to have a Van Gogh Sky microstructure that is formed as a result of the combined effects of microsegregation and deformation gradients. The effects of recrystallization heat treatments on microstructure were examined in an effort to produce a fully recrystallized microstructure. The changes in tensile properties and fracture mechanisms at different temperatures (27–704 °C) are discussed for material in fully recrystallized condition. The micromechanisms of creep deformation at 704 °C are also elucidated for the fully recrystallized condition.

A new criterion for internal crack formation in continuously cast steels

To estimate the cracking condition in continuously cast steels, a new model for critical fracture stress given from the measured critical strain has been proposed, which can take into account the brittle temperature range and strain rate. The effects of brittle temperature range and strain rate on critical strain for internal crack formation were analyzed. When the brittle temperature range and strain rate were increased, the possibility of internal crack formation increased due to the decreasing critical strain. To describe the thermomechanical property model of the mushy zone between zero strength temperature (ZST) and zero ductility temperature (ZDT), the yield criterion for porous metals, which can take into account δ/γ transformation, was used. Using the fitting equation for the measured critical strain and the microsegregation analysis, the thermomechanical behavior of the mushy zone could be successfully described by the proposed model, which incorporates the effects of microsegregation of solute elements and δ/γ transformation on hot tear during solidification at the given range of steel compositions and strain rates. A cracking criterion based on the difference of deformation energy in the brittle temperature range is proposed to explain the cracking phenomenon of whole carbon range.

SCS14オーステナイト系ステンレス鋳鋼の海水中での孔食挙動に及ぼすミクロ偏析の影響

SCS14オーステナイト系ステンレス鋳鋼の海水中での孔食挙動に及ぼすミクロ偏析の影響

A Finite Element Model for 2-Dimensional Slice of Cast Strand.

A two-dimensional thermo-elasto-plastic finite element model for the analysis of thermo-mechanical behavior of strand in continuous casting process has been developed. The model incorporates the effect of microsegregation of solute elements on hot tears using a thermo-mechanical model of mushy zone and δ and γ phases. A finite element technique for the liquid region of the slice of strand, which can take the ferrostatic pressure due to gravity force into account, was proposed. The model successfully analyzed the thermo-mechanical behavior of the solidifying shell of slab in the mold during the solidification. The calculated results, such as the deformed geometries, the temperature history, the stress distribution and the formation of air gap between solidifying shell and mold in the continuous casting process of slab, were obtained. These were compared with the reported observations.

Strength and toughness of ductile cast iron

Ductile cast iron is used for various structural components which need high strength and toughness. The evaluation of fracture toughness and an understanding of fracture mechanism are necessary for the design and use of this iron. Ductile fracture changes to brittle fracture with a lowering of the testing temperature and the transition temperature is affected by loading rate, notch shape and specimen geometry. The average value of J(AC) and J(R), that is J(mid), is proposed to define the fracture toughness of this iron. J(mid) is considered to be a reasonable measure for the fracture toughness, irrespective of loading condition and the pearlite content in the matrix. On the other hand, newly developed AD1 with greater toughness and good fatigue properties is introduced, where microsegregation effect of alloying elements and special austempering treatment are applied. Refining and increasing the stability of the retained austenite phase and avoiding the precipitation of carbide are the key factors in toughening ADI.

Dislocation pattern formation in epitaxial structures based on SiGe alloys

Single layer SiGe/Si and Si/SiGe heterostructures were grown by the molecular beam epitaxy technique with Si solid and GeH 4 gas sources. Dependence of dislocation distribution in heterostructures along the growth direction on alloy composition and layer thickness was studied using transmission electron microscopy and chemical etching/Nomarski microscopy. It has been found that misfit dislocation network evolves from flat regular to dense three-dimensional during the epitaxial growth. This process is strongly affected by alloy composition and accompanied by complex changing in dislocation density in both the layer and substrate. The phenomenon is interpreted qualitatively in terms of Ge-rich microsegregation effect on dislocation generation, propagation and multiplication.

Microstructural and compositional transients during accelerated directional solidification of Al-4.5 wt pct Cu

The transient behavior of mushy-zone velocities, primary dendrite arm spacings, and microsegregation effects have been investigated for an Al-4.5 wt pct Cu alloy by instantaneous velocity changes in a standard Bridgman furnace. After suddenly imposed velocity changes, the mushy-zone velocities, dendrite arm spacings, and compositions exponentially adjust to new steady-state values. Good agreement was found between the transient mushy-zone positions and velocities and predictions from the theoretical model of Saitou and Hirata. The primary dendrite arm spacings appear to adjust to changed velocity conditions about as rapidly as the mushy-zone velocity adjusts. Steady-state arm spacings agree very well with corresponding steady-state data from the literature. However, the observed composition profiles in the dendrite core and the interdendritic liquid appear to adjust more slowly than the corresponding adjustment of the mushy-zone velocity and arm spacings. Our observation of the sluggish response of the compositional profiles is consistent with an estimated Lewis number \((Le = \frac{k}{{c_p \rho D}})\) of 9.4 × 103 for the aluminum-copper system. The diffusivity of heat, thus, greatly exceeds the diffusivity of solute in this system. These results indicate that testing for the steady state during directional solidification experiments by looking for constant primary dendrite arm spacings can lead to errors, since the microsegregation profiles adjust more slowly than the spacings. It is suggested that constancy of composition also be tested for critical experiments investigating steady-state microsegregation effects.

Modelling of melting and solidification behaviour during laser spot welding

It is well known that rapid solidification and fast cooling rates occur in pulsed laser spot welds. It is particularly difficult, but important, to determine various solidification parameters in terms of development of novel microstructure, prevention of welding defects, and improvement in mechanical properties. Therefore, in this study, a heat conduction and solidification model considering the effects of microsegregation and latent heat is developed to provide a reasonably good knowledge of rapid melting and solidification behaviour in such a spot weld. It was confirmed that the latent heat would greatly affect thermal histories during solidification and growth rates of cellular dendrite tips. Also, microsegregation models on the basis of the pseudobinary phase diagram are proposed for rapid solidification to obtain a better knowledge of thermal histories and growth rates of cellular dendrites. These models could be utilised to reveal variations in a molten pool and a mushy zone region during spot welding, and consequently to reduce cracking in laser spot welds of commercially available aluminium alloy.

Modelling of melting and solidification behaviour during laser spot welding

It is well known that rapid solidification and fast cooling rates occur in pulsed laser spot welds. It is particularly difficult, but important, to determine various solidification parameters in terms of development of novel microstructure, prevention of welding defects, and improvement in mechanical properties. Therefore, in this study, a heat conduction and solidification model considering the effects of microsegregation and latent heat is developed to provide a reasonably good knowledge of rapid melting and solidification behaviour in such a spot weld. It was confirmed that the latent heat would greatly affect thermal histories during solidification and growth rates of cellular dendrite tips. Also, microsegregation models on the basis of the pseudobinary phase diagram are proposed for rapid solidification to obtain a better knowledge of thermal histories and growth rates of cellular dendrites. These models could be utilised to reveal variations in a molten pool and a mushy zone region during spot welding, and consequently to reduce cracking in laser spot welds of commercially available aluminium alloy.

A Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic Modeling

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-e model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in terms of the effects of macro- and microscale inhomogeneities of concentration and temperature on free radical polymerization kinetics. The observed nonidealities in terms of minima in the initiator consumption curves and multiple steady states are explained on the basis of competing turbulent transport and chemical kinetics. Microsegregation effects are shown to be negligible in comparison to macrosegregation effects. Given the fact that the CFD model is based on reactive scalar and energy balances without adjusted parameters in the three-dimensional flow field of the entire reactor, it is tentatively concluded that commercial-scale LDPE vessel reactors can have significant macrosegregation effects beyond a certain steady-state adiabatic operating temperature that is specific to the initiator being used.

A numerical study of the combined effects of microsegregation, mushy zone permeability and fllow, caused by volume contraction and thermosolutal convection, on macrosegregation and eutectic formation in binary alloy solidification

Numerical simulations of the columnar dendritic solidification of a Pb-20 wt% Sn alloy in a square cavity cooled from one side and fed by a rectangular riser are reported. Overall macrosegregation patterns predicted using Scheil and lever-rule type microsegregation models are found to be similar, although the predicted eutectic fraction is significantly higher with the Scheil-type model. The choice of mushy zone permeability function significantly affects the predicted number, length and orientation of segregated channels. The inclusion of shrinkage-driven flow leads to the prediction of the well-known inverse macrosegregation pattern. However, macrosegregation caused by thermosolutal convection readily masks the inverse segregation. The microsegregation models predict different solid concentrations and eutectic fractions, leading to different solid density distributions which, in turn, cause differences in the extent of contraction-driven flow.

Relative Importance of Macro- and Micromixing in Turbulent, Reacting Jets

The averaged convective diffusion equations which describe turbulent mixing of reactants are frequently unsolvable because incomplete micromixing with nonlinear reaction rates leads to unclosed covariance terms. A solution which does exist for an instantaneous second order reaction is used to estimate the bounds on the errors introduced by neglecting the incomplete micromixing in a free turbulent jet. The results indicate that macrosegregation dominates microsegregation in the jet; i.e., the nonhomogeneity of the mean concentrations slows the reaction much more than the fine scale inhomogeneity. The maximum effect of microsegregation is to decrease conversion by 12%. Since the decrease should be less for finite speed reactions, the naive closure which sets the covariance equal to zero may give useful conversion estimates. However it would be overreaching to assume this to be true with the multiple reactions where selectivity is the important issue

Macro‐ and microsegregation of Zn in bridgman‐grown CdZnTe

AbstractOne of the remaining problems in the use of CdZnTe material as substrates in liquid phase epitaxy (LPE) of CdxHg1‐xTe (CMT) layers is that of variation in lattice matching, i.e. Zn content, across substrates. This wil become increasingly important in the future as larger focal plane arrays of infrared detectors are required. The basic Bridgman growth process for CdTe/Cd0.96Zn0.04Te has been extended by applying the accelerated crucible rotation technique (ACRT). A marked reduction in axial Zn segregtion is seen in 50 mm diameter ACRT material, but this effect is smaller in the case of 75 mm diameter crystals. Radial variations in Zn content are small in both sizes of crystal, demonstrating the benefits obtained from ACRT stirring. Both macro‐ and microsegregation effects have been studied in these crystals in an attempt to understand the growth mechanism. Zinc distributions have been assessed by near‐infrared transmission, X‐ray lattice parameter measurements, atomic absorption spectrometry (AAS) and Auger electron spectrometry (AES). The last technique was used for the microsegregation studies, while AAS provides the absolute calibration for Zn content. Comparisons with segregation behaviour found in the literature will be given. It will be shown that the low temperature gradient and low growth rate lead to a degree of supercooling in the first‐to‐freeze region and this leads to significant Zn segregation in both radial and axial directions. As the crystals reach full diameter, the radial variation is decreased, presumably by the action of the ACRT, and axial segregation is also reduced.

Microsegregation of Silicon in Iron-Carbon Alloys

Residual microsegregation of silicon in hypoeutectic Fe-C alloys was shown to depend on the ratio of silicon to carbon. Concentration regions where direct and inverse microsegregation of silicon develops were established experimentally in the Fe-C-Si system. The behaviour of the third component changes in a similar manner in other systems, featuring a positive slope in the liquidus isotherms such as Fe-C-AI, Fe-C-Cu, and Fe-C-Ni. The concepts of para- and meta- equilibrium are defined, it being shown how geometric thermodynamic methods were employed to establish the equilibrium of the liquid and solid phases which was chosen as the criteria for describing the absence of silicon microsegregation in an alloy. X-ray spectrum analysis of the micro-segregation zones revealed that rules of dendritic segregation established for ternary alloys are applicable to real multi-component Fe-C alloys containing Si, Al, Cu and Ni within the range which is typical for unalloyed steels and irons. The peculiarities of silicon eutectic segregation, in relation to the valley geometry of double eutectics having stable and metastable phases, were considered. Finally, the microsegregation effect of major components, primarily silicon and manganese, on the formation of both structure and properties of irons and unalloyed steels is discussed.