Transformation processes in deformed Al-30 wt.% Zn alloy containing metastable precipitates

The Al-30wt.%Zn alloy containing metastable α′_R or α′ precipitates was cold rolled to investigate the effect of plastic deformation on transformation processes during the subsequent ageing at 200 °C. Transmission electron microscopy and X-ray diffraction studies proved that the deformation leads to the rapid decomposition of deformed α′_R and α′ precipitates and enhances also the decomposition of α′ precipitates formed from α′_R to α′ transformation. The deformation does not, however, change the growth rate of α′ particles. It leads, therefore, to the decrease of critical sizes of α′ particles for the α′ to β transformation. The increasing degree of deformation enhances the density of nucleation sites for the β formation on α/α′ interfaces and leads to the decrease in sizes of stable β precipitates. The acceleration of transformation processes is associated with the partial relaxation of stresses introduced by the applied deformation.     

Melting behavior of the oriented β-phase polypropylene texture crystallized by the temperature slope method

Isotactic polypropylene consisting of uniaxially oriented P-phase lamellae was crystallized in a temperature gradient. The β → α transition was investigated by simultaneous measurements with differential scanning calorimetry (DSC) and X-ray diffraction using synchrotron radiation (SR). To compare the transition mechanism, the β-phase sample was deformed by rolling it along the direction of the crystallization. During rolling, the β-crystal is deformed by interlamellar and interchain slip, which induces c-axis-oriented molecules along the rolling direction. The melting behavior is changed by the rolling deformation. For the as-grown β-crystal, the DSC thermogram has three peaks: the β-melting endotherm at 150°C, an exotherm by recrystallization into the °-form, and the endotherm at 167°C caused by melting of the recrystallized α-form. After the rolling deformation, the β-endotherm is extinguished by the successive exotherm. Simultaneous X-ray measurements reveal that the β → α transition is shifted to a lower temperature and that the recrystallized α-form has a c-axis-orientation caused by the rolling deformation. In the process of the β→ α transition, higher-order lamellar structure is developed earlier than formation of the crystalline structure. In this study, the heating phenomena, such as the β α transition and thickening of the β- and α-lamellae, are consistently explained by a mechanism involving melting and subsequent recrystallization.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

Solvus line of the metastable rhombohedral phase in the Al-Zn system for Al-rich alloys

The X-ray diffraction on single crystals and on coarse-grained foils of Al-Zn alloys was used to determine the solvus temperatures of the metastable rhombohedral α′ _R -phase. The dissolution of α′ _R -phase was studied simultaneously by application of DTA technique. Results obtained from different heat-treatments of the investigated alloys were critically interpreted in terms of the nucleation theory. The dissolution of large α′ _R -particles was necessary to obtain the correct α′ _R -solvus line unaffected by the reversion of the solid solution below the solvus temperatures.     

Transition of β to α phase in isotactic polypropylene

In samples of isotactic polypropylene, the influence of the nucleation agent chinacridine permanent red E3B on the crystallization rate and content of α and β modifications and on the rate and mechanism of the β → α transition was investigated using x-ray diffraction, DSC microcalorimetry, and electron microscopy. It was found that, though E3B increases the content of the β phase, it is also a very efficient nucleation agent for the α-phase crystallization. The β → α transition occurring above 140°C could be characterized by a gradual melting and annealing of the β phase, accompanied by a surface crystallization of the α phase on lateral faces of β lamellae by a mechanism similar to cross-branching of α lamellae.     

The formation of rhombohedral phase during the slow cooling of the Al-Zn solid solutions

The decomposition of Al-Zn solid solutions (from 20 to 60 wt. % of zinc) during slow cooling was studied by differential thermal analysis. The X-ray diffraction and electron microscopic investigation of structural changes were carried out on samples heat-treated simultaneously with DTA-samples. The pronounced peaks on DTA-curves were explained by the formation of metastable rhombohedral α′ _R -phase. This decomposition of supercooled Al-Zn solid solution starts at temperatures corresponding to the nose of the C-shaped T-T-T curve for the beginning of transformation.     

Phase behavior and nucleation kinetics of Octaphenylcyclotetrasiloxane

Phase behaviour and certain aspects of phase-transformation kinetics for Octaphenylcyclotetrasiloxane (OPCTS) have been determined by means of differential scanning calorimetry and optical microscopy. Three solid phases have been observed: 3 (stable at room temperature), 2 (stable above 76.6°C), and 1 (stable above 189.5°C). The 3-and 2-phases are optically anisotropic and mechanically brittle; the 1-phase is optically isotropic and soft or plastic. Although the 3-phase is thermodynamically unstable above 76.6°C, its transformation to the 2-phase is sufficiently slow that the 3 → 1 transition can be studied. The transition temperatures (and corresponding latent heats) for the 3 → 2, 3 → 1, 2 → 1, and melting transitions are: 76.6°C (～2.9 KJ/mol), 186.0°C (47.3 KJ/mol), 189.6°C (43.76 KJ/mol), and 204.9°C (1.95 KJ/mol). The entropy of melting (ΔS_m = 0.491 R) is one of the lowest measured for any solid. The low-melting entropy, isotropy, and plasticity of the 1-phase lead to the conclusion that it is an orientationally-disordered crystal mesophase (plastic crystal).

The kinetic behavior of the 3 → 2 phase transformation, as determined by DSC, supports a theoretical model taking homogenous nucleation as the rate-determining process.     

Microdisperse Iron Silicide Structures Produced by Implantation of Iron Ions in Silicon

Iron implanted and subsequently annealed n-type Si(111) was studied by conversion electron Mössbauer spectroscopy for phase analysis and Auger electron spectroscopy for sputter depth profiling and element mapping. During implantation (200 keV, 3 × 10¹⁷ cm^?2, 350°C) a mixture of β- and α-FeSi₂ is firmed and after the subsequent annealing (900°C for 18 h and 1150°C for 1 h) a complete transition to the β- and the α-phase can be detected. The as-implanted profile has Gaussian shape and is broadening during annealing at 900°C to a plateau-like profile and shows only a slight broadening and depth depending fluctuations of the iron concentration after the 1150°C annealing. With scanning Auger electron spectroscopy the lateral iron and silicon distribution were investigated and show for the sample annealed at 900°C large separated β-FeSi₂ precipitates which grow due to the process of Ostwald ripening. At 1150°C additionally coalescence of the precipitates occur and a wide extended penetration α-FeSi₂ network structure is formed.     

α to β Phase Transformation and Microestructural Changes of PVDF Films Induced by Uniaxial Stretch

The phase transformation from α to β poly(vinylidene fluoride) (PVDF) through a stretching process at different temperatures was investigated. Samples of originally α-PVDF were stretched uniaxially at different temperatures at draw ratios from 1 to 5. The stretched samples were studied and characterized by infrared spectroscopy, scanning electron microscopy, and differential scanning calorimetry. The maximum β-phase content was achieved at 80°C and a stretch ratio of 5, but the samples still showed 20% of the original α-phase. Accompanying the phase transformation, an orientation of the polymer chains was observed. The stretching process also influenced the degree of crystallinity of the polymer. Poling of the samples also improves the α- to β-phase transformation.     

Phase Transition in β-nucleated Isotactic Polypropylene Induced by Combination of Annealing and High Pressure

The effects of annealing and high pressure on the microstructure of β-nucleated isotactic polypropylene (iPP) were investigated. Annealing treatment was carried out at different temperatures from 90–170°C under different pressure conditions, that is, atmospheric pressure and high pressures of 150, 250, and 350 MPa, respectively. The microstructure of the specimens was comparatively investigated through using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Different from the common annealing treatment under atmospheric pressure, which induces the single-phase transition from metastable β-iPP to the more stable α-iPP (β→α) depending on the annealing temperatures, the application of high pressure induces not only the phase transition of β→α but also the phase transition of β→γ. Furthermore, the thermostability of β-iPP was improved greatly under high pressure. The higher the pressure applied, the smaller was the degree of the transition from β-iPP to α-iPP and/or γ-iPP. This work reports, for the first time, a novel crystalline morphology composed of β-iPP and γ-iPP.     

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu₂(MoO₄)₃ during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ～550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.     

Study on electrical conductivity and phase transition in singly doped BIPBVOX (Bi2V1−xPbxO5.5−x/2) solid electrolyte

Samples of bismuth lead vanadium oxide (BIPBVOX) (Bi₂V_1–xPb_xO_5.5–x/2) singly substituted system in the composition range 0.05 ≤ x ≤ 0.20 were prepared by sol–gel synthesis route. Structural investigations were carried out by using a combination of differential thermal analysis (DTA) and powder X-ray diffraction (PXRD) technique. Energy dispersive X-ray spectroscopy analysis (EDXA) of doped samples was carried out to predict the sample purity and doping concentration. Transitions, α?β, β?γ and γ′?γ were detected by XRD, DTA and variation in the Arrhenius plots of conductivity. The ionic conductivity was measured by AC impedance spectroscopy. The solid solutions with composition x ≤ 0.07 undergo α?β phase transition, at 329 °C and β?γ phase transition at 419 °C. The highly conducting γ′-phase was effectively stabilized at room temperature for compositions with x ≥ 0.17 whose thermal stability increases with Pb content. At 300 °C, the highest value of conductivity 6.234 × 10^?5 S cm^?1 was obtained for composition x = 0.15 and at 600 °C the highest value of conductivity 0.65 S cm^?1 is observed for x = 0.17. AC impedance plots reveal that the conductivity is mainly due to the grain contribution to oxide ion conductivity.     

Transmission electron microscopy of Ca oxide nano- and microcrystals in α-tricalcium phosphate prepared by sintering of β-tricalcium phosphate

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to study the porous and non-porous α-tricalcium phosphate (α-Ca₃(PO₄)₂, α-TCP) prepared through a sintering procedure at 1200–1400 °C of β-tricalcium phosphate (β-Ca₃(PO₄)₂, β-TCP). The interpretation of experimental and calculated X-ray and electron diffraction patterns showed that the final product at 1400 °C was primarily α-TCP but roughly 3.0–8.0 wt.% of the starting β-TCP phase and up to 8.0 wt.% of CaO were in the final product. TEM images and electron diffraction patterns showed that the CaO phase – formed by decomposition of TCP – exists as micron-sized areas of various oriented nanocrystals embedded into the bulk α-TCP material and also as self-standing spherulite particles of a few microns in size. Surprisingly, formation of CaO from TCP decomposition occurred at temperatures below those predicted from the phase diagram of the CaO–P₂O₅ system.     

Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3-SrO ceramics

A metastable hexagonal R-phase is revealed in polycrystalline In₂O₃-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 Å in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at T _a ? 300°C saturates dangling bonds between the R-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the R-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with g = 1.875. In addition, the sample in this state acquires a high resistivity (ρ ～ 10⁶ Ω cm). Relaxation at T ? 300°C after annealing at T _a > 300°C disrupts bonds between the strontium-enriched clusters of the R-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the R-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with g = 2 with the line with g = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state (ρ ～ 100 Ω cm).     

Investigation of the reconstructive phase transition between metastable (α) and stable (β) modifications of the NH<Subscript>4</Subscript>LiSO<Subscript>4</Subscript> crystal

Crystals of ammonium lithium sulfate NH₄LiSO₄ in α and β modifications are studied, and conditions of their nucleation and growth are determined. The α modification of NH₄LiSO₄ and α → β phase transitions are investigated using polarized light microscopy, x-ray diffraction, and differential scanning calorimetry in the temperature range 80–530 K. It is found that, depending on the conditions of growth and storage, there exist two temperature ranges (T_{α → β} ≈ 340–350 and ≈440–450 K) in which the crystals can undergo an α → β reconstructive phase transition. The enthalpy of this transformation depends on the symmetry of the final phase. In the former case (340–350 K), the reconstructive phase transition leads to rapid destruction of the sample. In the latter case (440–450 K), the crystal structure undergoes a slow transformation (recrystallization) without noticeable distortions. The results obtained indicate that no structural phase transition occurs in the α modification of NH₄LiSO₄ at 250 K.     

X-rays investigation of the α→β phase transition in the GexSi1−x solid solutions at high pressure

We have investigated the α → β phase transition in the solid solution Ge_xSi_1?x using a diamond anvil cell and the energy dispersive x-ray diffraction technique. The transition pressure p_T obtained from samples with x=0, 0.345, 0.543, 0.774, 0.897 and 1 shows a clear nonlinear dependence on the Ge concentration x; p_T is always larger than expected on the basis of a linear interpolation between the p_T-values of pure Si and Ge. In addition to that we have obtained the volume of the α-phase (diamond structure) and of the β-phase (white- tin structure) at p_T. The first one is linear in x, whereas the second one exhibits a nonlinear x-dependence. These results are discussed on the basis of recent pseudopotential calculations by Soma et al.     

H+对高压处理后的β-LiIO3转变为α相的作用

室温下经过不同的高压(最高为80kbar)作用后的β-LiIO₃的X射线衍射实验表明:1)受压后的β-LiIO₃在大气中发生的β→α相转变中,高压的作用是促使α相晶核形成。成核率N与压力P的关系接近于N∝e^kp(k=26.8/bar);2)受压力处理后的β-LiIO₃中的α相量在大气中的明显增加,可归结为大气中水分子在α相晶核表面上解离产生的氢离子引起了α晶核的迅速长大,这一论断受到了H⁺注入产生类似效应的支持。 关键词：     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

Investigation of the optic modes in VH0.51 by neutron spectroscopy

We have measured the frequencies of the optic modes of VH_0.51 at different temperatures in the α-, β- and ε -phases. In the β -phase a new mode was detected at 220 meV. The threefold splitting in the ε-phase is consistent with an off center location of Hydrogen. The frequencies are well reproduced by a force constant model with an axially symmetric metal Hydrogen interaction.     

Effects of molecular weight on the perforation impact behavior of injection-molded plaques of α-and β-phase isotactic polypropylenes

This study was devoted to the instrumented falling weight impact (IFWI) behavior of injection-molded a-and β-phase isotactic polypropylene (iPP) homopolymers. The perforation impact response of iPP with various melt flow indices (MFIs), and thus molecular weight (MW) characteristics, was studied at two different temperatures (T = 23°C and T =?40°C) and incident impact speeds (v_inc = 5 and 10 m/s). The impact resistance of β-iPP was superior to the α-modification. The absolute resistance to perforation increased with increasing MW or decreasing MFI, whereas the relative toughness improvement between the β-and α-iPPs followed an opposite tendency. The molding-induced skin-core morphology did not affect practically the out-of-plane response of the impacted plaques. Changes in the fractograms (viz. force-time curves) under various experimental conditions were traced to variations in the failure mode, showing a competition between radial and circumferential cracking with respect to the clamping ring. In the case of the more ductile β-iPP. circumferential cracking was favored.