Rapid solidification of undercooled Cu–Ge peritectic alloy

Bulk samples of Cu–14.4 wt% Ge peritectic alloy has been undercooled by up to 200 K (0.166 T_L) with a glass fluxing technique. The solidified microstructures are mainly characterized by α-Cu dendrites plus ζ phase which forms in the interdendritic areas within the whole undercooling regime. With the increase of undercooling, both the secondary arm spacing of primary α-Cu dendrite and the layer thickness of the peritectic ζ phase decrease. The primary trunk and secondary arm of α-Cu dendrites show round shape under small undercooling condition, whereas they keep a good dendritic shape within a large undercooling regime, indicating that the peritectic reaction proceeds for a relatively longer period of time in the former case. The volume fraction of peritectic ζ phase increases with undercooling, but that of α-Cu dendrite shows a decreasing tendency. Furthermore, drop tube experiments were also performed to reveal the competitive nucleation and growth mechanisms of primary α-Cu dendrite and peritectic phase ζ. Calculations based on the current dendritic growth model are made to analyze the crystal growth kinetics during the rapid solidification of undercooled Cu–Ge peritectic alloy.     

Macrosegregation and thermosolutal convection-related freckle formation in directionally solidified Sn–Ni peritectic alloy in crucibles with different diameters

Different from other alloys, the observation in this work on the dendritic mushy zone shows that the freckles are formed in two different regions before and after peritectic reaction in directional solidification of Sn–Ni peritectic alloys. In addition, the experimental results demonstrate that the dendritic morphology is influenced by the temperature gradient zone melting and Gibbs–Thomson effects. A new Rayleigh number (Ra_P) is proposed in consideration of both effects and peritectic reaction. The prediction of Ra_P confirms the freckle formation in two regions during peritectic solidification. Besides, heavier thermosolutal convection in samples with larger diameter is also demonstrated.     

Stability of remelting and solidification interfaces of triple-phase region during peritectic reaction at lower speed

Peritectic reaction was studied by directional solidification of Cu-Ge alloys. A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction. Under different growth conditions and compositions, different growth morphologies of triple junction region are presented. For the hypoperitectic Cu-13.5%Ge alloy, as the pulling velocity (v) increases from 2 to 5 μm/s, the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable. However, for the hyperperitectic Cu-15.6%Ge alloy with v=5 μm/s, the nonplanar remelting interface near the trijunction is presented. The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.     

Phase selection of peritectic phase in undercooled Nd-based superconducting oxides

The congruent growth of a peritectic phase, NdBa₂Cu₃O_7−δ (Nd123), from undercooled melt below a peritectic temperature (T_P) is discussed from the viewpoint of phase selection theory. This is based on competitive growth in the peritectic system under a free growth condition. Although the results of calculations based on the phase selection theory indicate the achievement of congruent growth, they also suggest that the Nd123 phase grows preferentially over the Nd₄Ba₂Cu₂O₁₀ phase (Nd422) even above T_P. A seeding experiment to clarify the temperature dependence of the phase appearance, however, revealed that the volume fraction of Nd123 drastically increased when seeding was carried out at a temperature below T_P. This suggests that the steady growth of the Nd123 phase is unstable above T_P. The measured growth velocity and volume fraction of the Nd123 phase agreed well with the theoretical calculation below the critical temperature, where the interface temperature of the Nd123 phase during steady growth remains below T_P. Achievement of congruent growth of the Nd123 phase results in the steady and preferential growth of the Nd123 phase over the Nd422 phase with lower growth velocity accompanied by the rejection of solute atoms.     

Rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys

Rapid solidification experiments, including laser remelting, melt-spinning and wedge casting, were carried out to investigate the rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys containing up to 9.0 at% Ag. For comparison, Bridgman solidification experiments of the same alloys were also carried out for growth velocities ranging from 0.02 to 4.82 mm/s, which were lower than that of 12–54.5 mm/s for laser remelting and that in the order of 10² mm/s for melt-spun samples. Optical images and transmission electron microscopy (TEM) showed that instead of the typical structure consisting of primary dendrites of ϵ surrounded by peritectic η, a two-phase plate-like η+ϵ with (or without) primary dendrites of ϵ was observed in Zn–3.1, 4.4, 6.3 and 9.0 at% Ag alloys when the growth velocity was higher than a critical value. It was found that the higher was the alloy concentration, the higher was the critical growth velocity for the formation of fully two-phase plate-like η+ϵ. From the TEM micrographs, the volume fraction of ϵ in the fully two-phase plate-like η+ϵ increased from 0.09 to 0.50 with increase in alloy concentration from 3.1 to 6.3 at% Ag. A plausible analysis was proposed to interpret the dependence of microstructural transitions on the growth velocity in Zn–3.1 to 9.0 at% Ag alloys, that is, primary dendrites of ϵ in a matrix of peritectic η→two-phase plate-like η+ϵ with primary dendrites of ϵ→ fully two-phase plate-like η+ϵ.     

Enhanced dendrite fragmentation through the peritectic reaction in TiAl-based alloys

Grain size evolution for γ-TiAl based alloys with aluminum content ranging from 43 to 52 at.% were cast at constant superheat and characterized. Experiments show that the sooner the peritectic phase transition occurs, the finer are the equiaxed grains. An accurate estimate of the number of surviving dendrite fragments and fragmentation rate is performed, which can be used to predict the position of the columnar-to-equiaxed transition in the cast ingots. For alloys solidifying with the β pro-peritectic primary phase, the fragmentation rate significantly increases when the onset of the peritectic reaction is earlier. A mechanism based on the peritectic reaction is proposed for the observed grain refinement and is discussed on the basis of the fluid flow, mushy zone permeability and the influence of the peritectic reaction.     

Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite (γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2-λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.     

Influence of hexagonal to orthorhombic phase transformation on diffusion-controlled dendrite evolution in directionally solidified Sn–Ni peritectic alloy

The hexagonal to orthorhombic (HO) transformation from β-Ni₃Sn₂ (hexagonal) phase to α’-Ni₃Sn₂ (orthorhombic) phase was confirmed in directionally solidified Sn–Ni peritectic alloys. It is shown that the remelting/resolidification process which is caused by both the temperature gradient zone melting (TGZM) and Gibbs?Thomson (G?T) effects can take place on secondary dendrites. Besides, the intersection angle between the primary dendrite stem and secondary branch (θ) is found to increase from π/3 to π/2 as the solidification proceeds. This is the morphological feature of the HO transformation, which can change the diffusion distance of the remelting/ resolidification process. Thus, a diffusion-based analytical model is established to describe this process through the specific surface area (S_V) of dendrites. The theoretical prediction demonstrates that the remelting/resolidification process is restricted when the HO transformation occurs during peritectic solidification. In addition, the slope of the prediction curves is changed, indicating the variation of the local remelting/resolidification rates.     

A numerical model of peritectic transformation

A rigorous numerical model of peritectic transformation kinetics based on the solution of unsteady state diffusion equations in moving phase fields has been presented. The overall and interface mass balance equations are solved to calculate the rate of movement of the interfaces. The predictions of the present formulation show a better agreement with the experimental kinetic data from the Cd–Ag and Pb–Bi systems, compared to those of the earlier proposed models based on quasi-static interface or quasi-steady state approximations. It is predicted that the presence of coring in the primary solid (β) would not have any appreciable effect on the kinetics of peritectic transformation, especially, prior to the consumption of the liquid phase unless the product phase (α) has a restricted composition range. Furthermore, the kinetics remain practically unaltered irrespective of a linear or cored initial concentration gradient in the α-phase. However, the assumption of a linear concentration gradient in α throughout the course of the transformation may significantly underestimate the kinetics.     

Phase selection in the undercooled peritectic Y3Fe5O12 melt

Containerless solidification of peritectic Y₃Fe₅O₁₂ garnet (YIG) was examined using an aero-acoustic levitator to investigate the direct growth of YIG from the undercooled melt. Although the melt was undercooled below the peritectic temperature (T_p), YFeO₃ perovskite (YIP) was primarily solidified in the spontaneously nucleated specimen. The phase selection theory based on the competitive growth indicates that YIG never solidifies directly from the undercooled melt even when the melt is undercooled much below T_p. However, when the melt undercooled below T_p was dropped onto the copper chill plate with high thermal conductivity, YIG solidified around the chill plate as a primary phase. On the other hand, for the undercooled melt dropped onto the quartz glass chill plate with low thermal conductivity, YIP primarily solidified. These results suggest that the high cooling rate achieved by the copper chill plate prevented the nucleation of YIP and led to the direct growth of YIG, which is explained by the phase selection based on the competitive nucleation rate.     

Microstructural length scales in directionally solidified Sn-40 at.%Mn peritectic alloy containing intermetallic compounds

Sn-40 at.%Mn peritectic alloys in which both primary and peritectic phase are intermetallic compounds were directionally solidified at different growth rates (1 μm/s∼100 μm/s). Dendritic growth of primary phase has been observed even at low growth rate, and dependence of microstructural characteristic length scales on growth rate has been investigated. Liquidus slope is the dominating factor in determining non-faceted/faceted solid/liquid morphology. Experimental results show that: λ₁ = 229.64v^−0.20, λ₂ = 33.337v^−0.34, λ₃ = 39.90v^−0.378, R = 16.90v^−0.384. Besides, both λ₁/λ₂ and λ₁/λ₃ vary greatly with increasing growth rates while λ₂/R ranges from 2 to 2.3 with increasing growth rate. Both the peritectic reaction and solute distribution of intermetallic compounds during solidification influence these length scales. A modified solute distribution coefficient which is appropriate for intermetallic compounds makes calculation agree reasonably well with experimental results. The value of λ₁ is influenced more by solute distribution of intermetallic compounds, while growth of λ₂, λ₃ and R are more significantly influenced by the peritectic reaction.     

Pyroelectric and electrocaloric effect of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal

In this paper, the polarization vs. electric field hysteresis loops of 〈1 1 1〉-oriented 0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ (0.9PMN-0.1PT) single crystal at different temperatures (20-110 °C) were measured. The adiabatic temperature change ΔT of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal due to the application or withdraw of electric field were calculated through the thermodynamic relation. The largest temperature change ΔT achieves ∼1 K with only a change of 40 kV/cm electric field, the mechanism of the electrocaloric effect (ECE) is discussed for 0.9PMN-0.1PT crystal. The pyroelectric coefficient of 0.9PMN-0.1PT under bias field was calculated according to the data of hysteresis loop. The result shows that 0.9PMN-0.1PT have large pyroelectric coefficient under bias field, the largest (∂P/∂T)_E value achieves −0.5 μC/cm² K.     

Characterization and piezoelectric thermal stability of PIN–PMN–PT ternary ceramics near the morphotropic phase boundary

The phase structure and piezo/dielectric properties of xPb(In_1/2Nb_1/2)O₃–yPb(Mg_1/3Nb_2/3)O₃–zPbTiO₃ (PIN–PMN–PT x/y/z) ternary ceramics with morphotropic phase boundary (MPB) composition were investigated. It was found that Curie temperatures of choosing composition changed from 165 °C to 293 °C and the piezoelectric properties are almost the same at room temperature in PIN–PMN–PT system near MPB. The piezoelectric coefficient d₃₃ and electromechanical coupling factor k_p are higher than 440 pC/N and 60%, respectively. Temperautre and dc bias dependence of piezoelectric response for PIN–PMN–PT ceramics were measured. The usage temperature range was found to be improved, compared with PMN–PT single crystal near MPB.     

Giant electrostrains accompanying the evolution of a relaxor behavior in Bi(Mg,Ti)O3–PbZrO3–PbTiO3 ferroelectric ceramics

Extremely enhanced electrostrains (up to 0.39%) were surprisingly observed in (0.67 ? x)Bi(Mg_0.5Ti_0.5)O₃–xPbZrO₃–0.33PbTiO₃ (BMT–xPZ–PT) ternary solid solutions, possibly resulting in BMT–xPZ–PT ceramics having great potential for large-displacement actuator applications. The generation of giant strains was found to be closely associated with the evolution of a weak relaxor behavior from diffuse-type BMT–PT binary ferroelectrics, during which the domain switching is actively facilitated owing to a change in the dynamics of the polar nanoregions from a static state to a dynamic state. It can be also attributed to a ferroelectric nature of the evolved relaxors in PZ substituted BMT–PT ceramics instead of a dipole glass freezing state. These judgements were reasonably supported by a couple of measurements, including strains vs. electric field, Raman scattering, dielectric spectroscopy and the time- and electric-field-dependent polarization. The present study can provide a general approach towards an appropriate compositional design for large electrostrains in BMT-based and related systems.     

Peritectic coupled growth

Even though peritectic coupled growth (PCG) has been often discussed in the literature, it remains poorly understood in comparison to eutectic coupled growth (ECG). We report the results of a combined experimental and numerical study that clearly establishes the existence of PCG in directional solidification of Fe–Ni alloys. Furthermore, the results shed light on the similarities, as well as the differences, between PCG and ECG. The main findings are that: (i) a necessary condition for PCG is a G/v ratio close to or above the critical value for plane front growth of both solid phases, (ii) both lamellar and fibrous PCG is observed and the transition between the two morphologies correlates with the degree of asymmetry of solid volume fractions, as for ECG, (iii) lamellar PCG can be stable even though the slope of the undercooling-spacing relation is negative, in agreement with recent experimental and numerical findings for ECG, (iv) the stability of PCG is limited at both small and large spacings by short-wavelength oscillatory instabilities, whereas the stability of ECG is limited at small spacing by a known long-wavelength instability associated with lamellar elimination, and (v) PCG and ECG can be initiated by different mechanisms. In addition, cellular non-isothermal PCG is found when the G/v ratio is below the limit for plane front growth of the primary phase and above that for plane front growth of the peritectic phase. The transition from isothermal PCG to cellular PCG with decreasing G/v is discontinuous (sub-critical). Cellular PCG is characterised by diffusion-coupling between cells of one phase and nearly plane front of the other under the constraint of mechanical equilibrium at the triple junction.     

Semiconducting polymers based on electron-donating bithiophene and electron-accepting 5,5′-bithiazole units for organic thin-film transistors

New type of π-conjugated polymers composed of electron-donating bithiophene and electron-accepting 5,5′-bithiazole were prepared by organometallic polycondensation using a zerovalent nickel complex, which have charge-transfer (CT) structures in the polymer backbone. The number average molecular weights (M_n) of the obtained polymers, PT2Z2-C8 and PT2Z2-H, were 45000 and 20000, respectively, estimated from GPC measurements. Head-to-head (HH) arrangements of 5,5′-bithiazole units leads to good solubility of the polymers in common organic solvents. Ionization potentials (IPs) of PT2Z2-C8 and PT2Z2-H were measured to be 5.15 and 5.20 eV, respectively. UV–vis peaks of the polymer films corresponding to the π–π* transition were shifted to longer wavelengths over 70 nm relative to those obtained for the corresponding chloroform solutions. PT2Z2-C8 is considered to have a coplanar π-conjugated backbone conformation, as judged from UV–vis data and XRD data. OTFTs fabricated using the polymer PT2Z2-C8 as a channel material show a typical p-type behavior and provide a mobility of 0.001 cm²/Vs with a low off-state current (of about 1 pA).     

Cascade of Peritectic Reactions in the B-Fe-U System

The solidification paths for UFeB₄, UFe₃B₂ and UFe₄B, ternary compounds, situated along the U:(Fe,B) = 1:5 line in the B-Fe-U phase diagram, are proposed based on x-ray powder diffraction measurements, differential thermal analysis, heating curves and scanning electron microscopy observations complemented with energy and wavelength dispersive x-ray spectroscopies. The compounds melt incongruently and are formed by peritectic reactions. The present work demonstrates the existence of a cascade of peritectic reactions along the U:(Fe,B) = 1:5 composition line, establishes peritectic temperatures and proposes an isopleth diagram along this line.     

X-ray diffraction and dielectric studies across morphotropic phase boundary in (1 − x) [Pb(Mg0.5W0.5)O3]-xPbTiO3 ceramics

We present here the results of comprehensive X-ray diffraction and dielectric studies on several compositions of (1 − x)[Pb(Mg_0.5W_0.5)O₃]-xPbTiO₃ (PMW-xPT) solid solution across the morphotropic phase boundary. Rietveld analysis of the powder X-ray diffraction data reveals cubic (space group Fm3m) structure of PMW-xPT ceramics for the compositions with x ≤ 0.42, tetragonal (space group P4mm) structure for the compositions with x ≥ 0.72 and coexistence of the tetragonal and cubic phases for the intermediate compositions (0.46 ≤ x ≤ 0.68). Temperature dependence of the dielectric permittivity above room temperature exhibits diffuse nature of phase transitions for the compositions in the cubic and two phase region while the compositions with tetragonal structure at room temperature exhibit sharp ferroelectric to paraelectric phase transition. The PMW-xPT compositions with coexistence of tetragonal and cubic phases at room temperature exhibit two anomalies in the temperature dependence of the dielectric permittivity above room temperature. Using results of structural and dielectric studies a partial phase diagram of PMW-xPT ceramics is also presented.     

Stress-induced structural changes in La-doped BiFeO3–PbTiO3 high-temperature piezoceramics

High-temperature piezoelectric polycrystalline ceramics of the system (1 ? x)(Bi_1?yLa_y)FeO₃-xPbTiO₃ (BF–PT), which are mixed phase in their consolidated state, have been investigated by in situ neutron diffraction during the application of uniaxial compressive stress. It is suggested that the achievable strain in BF–PT is largely generated by straining of the rhombohedral phase. The results of the neutron diffraction measurements are compared and discussed with respect to the measured macroscopic ferroelastic constitutive behavior for various compositions of BF–PT.     

Ferroic transitions in the multiferroic (1 − x)Pb(Fe1/2Nb1/2)O3–xPbTiO3 system and its phase diagram

The results of the first comprehensive study of ferroic phase transitions as a function of temperature and compositions in the mixed multiferroic (1 ? x)Pb(Fe_1/2Nb_1/2)O₃–xPbTiO₃ (PFN–xPT) system are reported. Temperature-dependent powder synchrotron X-ray diffraction studies confirm unambiguously an interferroelectric transition between monoclinic and tetragonal phases for x ? 0.10. The tetragonal phase of PFN–xPT for x ? 0.10 and x > 0.10 is found to transform to the cubic phase on heating above room temperature. All these transitions are accompanied by distinct anomalies in the temperature dependence of dielectric constant. Our magnetization studies reveal that the nature of the magnetic phase transition changes across the morphotropic phase boundary (MPB) composition of PFN–xPT such that for tetragonal compositions with x > 0.08 there is only one magnetic transition, whereas two antiferromagnetic transitions are observed for monoclinic compositions with x < 0.08. These studies have enabled us to construct a phase diagram of PFN–xPT for the first time.