Phase Evolution Studies Research Articles

A calorimetric study of phase evolution in a WE43 Mg alloy

A DSC investigation has been performed on a Mg–RE–Y–Zr (RE=rare earth) technical alloy WE43. Hardness trend during isothermal treatments has been correlated to the calorimetric traces evolution and to the forming β phases with its precursors. Oversaturation of solute elements occurs at temperatures higher than 150±C, on cooling at room temperature after the anneals. Activation energies, found under non-isothermal conditions on artificially aged samples, suggest a slow transformation velocity, while the hardness response is relatively fast.

Microscopic and Spectroscopic Studies of Phase Evolution in LiFePO4

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Combustion synthesis and characterisation of lanthanum hexa-aluminate

Lanthanum hexa-aluminate (LHA) is considered a promising material for thermal barrier coatings for gas turbine applications as well as oxidation resistant coatings for fibres in oxide based ceramic composites. Combustion synthesis has attractive advantages such as simple experimental setup, low cost, short reaction time, energy savings due to exothermic reaction, better control of stoichiometry and fine powder size. The present study involves combustion synthesis of LHA using lanthanum and aluminium nitrates as oxidising precursors, and urea as fuel. The precursor salts were dissolved in a minimum amount of deionised water and the solution was heated in a mantle heater to temperatures of around 300–350°C, at which combustion takes place with rapid evolution of gas. The LHA powder was also prepared by a precipitation route for comparative study. The as synthesised powders were calcined at up to 1450°C and subjected to X-ray diffraction for phase evolution studies. Product powders were then characterised. The influence of the lanthanum/aluminium molar ratio in the starting composition and the effect of temperature on the formation sequences of lanthanum monoaluminate (LMA) and LHA and on the conversion of LMA to LHA are discussed.

Synthesis and characterization of lanthanum strontium manganite

Abstract La0.85Sr0.15MnO3 (LSM) powder has been prepared by co-precipitation. Phase evolution studies by DTA, TGA and XRD indicated initiation of LSM formation above 750 °C. Densification behaviour of LSM powder with and without pore former was different. Sintered LSM was characterized for electrical conductivity, thermal expansion and microstructural development.

Structural investigations of non-stoichiometric solid solutions (Bi2O3)1−x(M2O3)x (M=Y, La, Pr, Nd, Gd and Dy)

The structural investigations of the non-stoichiometric solid solutions (Bi2O3)1−x(M2O3)x (M=Y, La, Pr, Nd, Gd and Dy) have been carried out. Polycrystalline samples have been synthesized using ceramic technique and the synthesis parameters have been optimized for each dopant concentration. The structural phase evolution studies with dopant concentrations have been performed using X-ray powder diffraction data over a wide range of compositions 0.10≤x≤0.40. The unit cell parameters have been obtained from the analysis of the data.

Tma study of phase evolution in Nd-Fe-B material during mechanical milling and annealing

Phase evolution of the matrix Nd/sub 2/Fe/sub 14/B magnetic phase in a pre-alloyed Nd/sub 15/Fe/sub 77/B/sub 8/ alloy lump during mechanical milling (MM) and subsequent annealing was investigated mainly by thermomagnetic analysis (TMA). TMA is also extended to an examination of the structural features of the Nd/sub 2/Fe/sub 14/B grains in mechanically milled and annealed material. The Hopkinson effect in the TMA of the milled and annealed material is also discussed in relation to the grain structure and magnetic properties. The Nd/sub 15/Fe/sub 77/B/sub 8/ alloy can be amorphized completely by a high-energy MM. On annealing of the amorphous material, fine /spl alpha/-Fe crystallizes first from the amorphous phase. These fine /spl alpha/-Fe crystallites react with the remaining amorphous phase afterwards, leading to crystallization of Nd/sub 2/Fe/sub 14/B with ultra fine grain structure. The milled and annealed Nd/sub 15/Fe/sub 77/B/sub 8/ alloy shows an unusual TMA curve with magnetization maximum just below T/sub c/ of the Nd/sub 2/Fe/sub 14/B phase. This is attributed to a Hopkinson effect associated with the ultra-fine grain structure of the Nd/sub 2/Fe/sub 14/B phase in the milled and annealed material.

Nondestructive analysis of phase evolution and microstructure development in Ag/(Bi,Pb) 2Sr 2Ca 2Cu 3O x composite superconductor by 25 keV transmission X-ray diffraction

X-ray diffraction (XRD) measurements were made on silver-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x (Ag/Bi-2223) composite superconductor using 25 keV X-rays produced by an insertion device beamline at a third generation synchrotron. These measurements, performed on fully sheathed multifilament-type Ag/Bi-2223 wire specimens, provided high resolution diffraction patterns that revealed (1) the phases present in the cores of the superconducting filaments and (2) the nature and quality of the Bi-2223 grain colony texture. In addition to the Bi-2223 phase, we were able to detect and to monitor specimen-to-specimen variations in the relative amounts of Bi-2212, Bi-2201, and the “3221” phase. The scattering geometry used (X-ray beam perpendicular to the Ag/Bi-2223 conductor rolling direction) resulted in the detection of several remarkably intense diffraction lines of Bi-2223, most notably the (2 0 0)/(0 2 0) pair, that were highly useful for recording pole figure maps evidencing the contiguous fiber texture microstructure of the Bi-2223 grain colonies. The nondestructive nature of the 25 keV transmission XRD method offers many advantages for the comprehensive study of phase evolution and microstructure development in Ag/Bi-2223 composite conductors.

Real-time/in situ diffraction study of phase and microstructural evolution in sputtered β-Ta/Ta2O5 films

The evolution of texture, grain size, and phase content in sputtered β-Ta/Ta2O5 films was studied and controlled in situ using a laboratory-based real-time x-ray diffraction system. The films were dc magnetron sputter deposited using sputter gas (Ar) pressures ranging from 2 to 20 mTorr. To collect diffraction data during growth, a grazing incidence x-ray scattering (GIXS) configuration was necessary. Because this configuration complicated the interpretation of texturing data, complete Schultz geometry pole figures were also used to characterize the films after growth, and the relationship between pole figure and GIXS data was examined. The film composition was evaluated using sputter profile x-ray photoelectron spectroscopy and secondary ion mass spectroscopy. It was found that the degree and rate of (002) out-of-plane texturing was systematically stronger in films grown at lower Ar pressures (< 10 mTorr). These films were predominantly β-Ta in composition and had relatively large grains (∼50 nm in diameter). Films grown using progressively higher Ar pressures had increasing amounts of Ta2O5, developed a (002) out-of-plane texture more slowly, and were nanocrystalline/amorphous in nature. Data from depositions where the Ar pressure was changed during growth showed that the deposition environment at the instant of layer formation determined the phase content, degree of texture, and grain size in the developing film; the characteristics of previous layers had little bearing on subsequent growth. This shows that tailored multilayer β-Ta/Ta2O5 films may be created and monitored in situ.

The First Million Years of the Sun: A Calculation of the Formation and Early Evolution of a Solar Mass Star

We present the first coherent dynamical study of the cloud fragmentation phase, collapse, and early stellar evolution of a solar mass star. We determine young star properties as the consequence of the parent cloud evolution. Mass, luminosity, and effective temperature in the first million years of the proto-Sun result from gravitational fragmentation of a molecular cloud region that produces a cluster of prestellar clumps. We calculate the global dynamical behavior of the cloud using isothermal three-dimensional hydrodynamics and follow the evolution of individual protostars in detail using a one-dimensional radiation hydrodynamic system of equations that comprises a correct standard solar model solution, as a limiting case. We calculate the pre-main-sequence (PMS) evolutionary tracks of a solar mass star in a dense stellar cluster environment and compare it to one that forms in isolation. Up to an age of 950,000 yr, differences in the accretion history lead to significantly different temperature and luminosity evolution. As accretion fades and the stars approach their final masses, the two dynamic PMS tracks converge. After that, the contraction of the quasi-hydrostatic stellar interiors dominate the overall stellar properties and proceed in very similar ways. Hence, the position of a star in the Hertzsprung-Russell diagram becomes a function of age and mass only. However, our quantitative description of cloud fragmentation, star formation, and early stellar evolution predicts substantial corrections to the classical, i.e., hydrostatic and initially fully convective models: at an age of 1 million yr, the proto-Sun is twice as bright and 500 K hotter than according to calculations that neglect the star formation process.

Phase evolution, microstructure and transport property of(Bi, Pb)-2223/Ag tapes prepared using powders of varying particle sizedistribution

(Bi, Pb)-2223/Ag sheathed tapes were prepared using precursor powderswith different particle size distributions which were obtained by grinding thecalcined powder for different durations. DTA results revealed that thereaction temperature of the precursor powder depends on the particle size andis at a minimum for the finest powder. Phase evolution studies, transport currentmeasurement at 77 K with and without the application of an external field,microstuctural examination etc, were carried out in order to evaluate therelative performance of the tapes. The results showed that the particle sizehas significant effects on the reaction kinetics, ultimate phase fraction,microstructure and transport critical current density. For the tapesfabricated using coarser powder, the reaction kinetics were found to be slowerand the microstructure contained smaller grains with many misalignments andvoids. On the other hand use of extremely fine precursors yielded tapes with alarge fraction of secondary phases due to the degradation of the crystallinityof the precursor and a consequent change in the reaction sequence. The tapeprepared using a precursor with an average particle size of 3-4 µm showedthe best results with respect to JC, high-TC phase fraction,microstructure and JC-B characteristics.

Microstructure controlling by heat treatment and complex processing for Ti 2AlNb based alloys

The phase evolution and microstructure controlling process were studied for Ti 2AlNb based alloys. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Phase evolution studies showed that depending on the heat treatment schedule, Ti 2AlNb based alloys may obtain various constituent phase, including B2, α 2, O phases. This study indicated that substitution of Nb with Ta improved the B2 transus temperature. Heat treatments in different phase fields produced different microstructures. It was difficult to remove the prior large B2 grain boundary when solid soluted above B2 transus temperature. In order to optimize the microstructure, a complex processing method was proposed: prior heat treatment was applied to obtain a fine and homogenous O+B2 lath structure, followed by thermomechanical processing at suitable temperature. By means of this method, duplex microstructure with equiaxed α 2/O particles and fine O phase laths in B2 matrix, and without prior large B2 grain boundary, was secured. A systematic means of microstructure controlling was suggested.

Fabrication of (Bi,Pb)-2223/Ag–Cu alloy sheathed tapes: optimization of Cu composition in the sheath with respect to the Cu stoichiometry in the system

(Bi,Pb)-2223/Ag–Cu alloy sheathed tapes with varying Cu compositions (3, 6, 9 and 12 at.%) in the sheath have been fabricated by powder in tube (PIT) technique using rolling as the deformation step. Two precursor powders with Cu stoichiometries of 3.0 and 3.2 were used to prepare (Bi,Pb)-2223/Ag–Cu alloy sheathed tapes. Phase evolution study by means of XRD and critical current density ( J c) measurements at 77 K, 0 T have been used to elucidate the performance of the tapes and to determine the optimum Cu doping level in the sheath with respect to the Cu stoichiometry in the system. It was found that the level of Cu doping in the sheath is dependent on the Cu stoichiometry in the system. A doping level of 6 at.% is found to be the optimum for the Cu stoichiometry of 3.0 in the system with regard to the Bi-2223 phase growth, the ultimate Bi-2223 phase fraction and the critical current density ( J c). Alloy sheathed tapes using the precursor with a Cu stoichiometry of 3.2 is found to yield inferior superconducting properties especially when the doping level of Cu exceeds 3 at.%.

Petrographical analysis of calcium aluminate cement mortars: Scanning electron microscopy and transmitted light microscopy

The samples studied for this paper were obtained from experimentally induced carbonation and alkaline hydrolysis of calcium aluminate cement (CAC) mortars. The mineralogical composition of phases present was studied from X-ray diffraction (XRD) spectra. Scanning electron microscopy (SEM) and transmitted light microscopy (TLM) were used to obtain some morphological (size and shape) and fabric data of mineralogical phases to improve the interpretation of damage processes. SEM is a useful tool to examine the evolution of mineral phases in cavities and discontinuity zones. The interpretation of the degree of carbonation and the distribution of carbonated phases in the samples, the presence of C 3AH 6 and CAH 10, as well as possible alterations of aggregates are well differentiated and defined using TLM method. The main objective of the present paper is to validate the use of both microscopic techniques (TLM and SEM) for the study of phase evolution in the CAC mortar during curing and degenerative processes.

Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys

Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α 2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α 2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α 2+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α2 (ordered close-packed hexagonal D019 structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti2AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α2+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α2+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α2/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.

Monoclinic 2212-BSCCO single crystals: Phase transformation, twinning, evolution, superconductivity and doping studies

An orthorhombic, superconducting to monoclinic, non-superconducting phase transformation in Bi 2Sr 2CaCu 2O y is overviewed. Substitutional doping studies suggest that the transformation is dominated by oxygen diffusion in the vicinity of the CuO 2 rather than in the BiO layers.

Monte Carlo and mean-field studies of phase evolution in concentrated surfactant solutions

A two-dimensional lattice model, originally introduced by Granek et al. [J. Chem. Phys. 101, 4331 (1994)], is used to demonstrate the intricate coupling between the intramicellar interactions that determine the optimal aggregation geometry of surfactant molecules in dilute solution, and the intermicellar interactions that govern the phase behavior at higher concentrations. Three very different scenarios of self-assembly and phase evolution are analyzed in detail, based on Monte Carlo studies and theoretical interpretations involving mean-field, Landau–Ginzburg, Bethe–Peierls, and virial expansion schemes. The basic particles in the model are ‘‘unit micelles’’ which, due to spontaneous self-assembly or because of excluded area interactions, can fuse to form larger aggregates. These aggregates are envisaged as flat micelles composed of a bilayerlike body surrounded by a curved semitoroidal rim. The system’s Hamiltonian involves one- through four-body potentials between the unit micelles, which account for their tendency to form aggregates of different shapes, e.g., elongated vs disklike micelles. Equivalently, the configurational energy of the system is a sum of micellar self-energies involving the packing free energies of the constituent molecules in the bilayer body and in rim segments of different local curvature. The rim energy is a sum of a line tension term and a 1D curvature energy which depends on the rim spontaneous curvature and bending rigidity. Different combinations of these molecular parameters imply different optimal packing geometries and hence different self-assembly and phase behaviors. The emphasis in this paper is on systems of ‘‘curvature loving’’ amphiphiles which, in our model, are characterized by negative line tension. The three systems studied are: (i) A dilute solution of stable disklike micelles which, upon increasing the concentration, undergoes a first-order phase transition to a continuous bilayer with isolated hole defects. An intermediate modulated ‘‘checkerboard’’ phase appears under certain conditions at low temperatures. (ii) A system of unit micelles which in dilute solution tend to associate into linear micelles. These micelles are rodlike at low temperatures, becoming increasingly more flexible as the temperature increases. Upon increasing the concentration the micelles grow and undergo (in 2D) a continuous transition into nematic and ‘‘stripe’’ phases of long rods. At still higher concentrations the micellar stripes fuse into continuous sheets with line defects. (iii) A system in which, already in dilute solution, the micelles favor the formation of branched aggregates, analogous to the branched cylindrical micelles recently observed in certain surfactant solutions. As the concentration increases the micelles associate into networks (‘‘gels’’) composed of a mesh of linear micelles linked by ‘‘T-like’’ intermicellar junctions. The network may span the entire system or phase separate and coexist with a dilute micellar phase, depending on the details of the molecular packing parameters.

Solution sol-gel synthesis and phase evolution studies of cordierite xerogels, aerogels and thin films

A sol-gel process employing silicon alkoxide, chelated aluminum sec-butoxide, and magnesium acetate as starting precursors, was used to synthesize cordierite xerogels, aerogels and thin films. The xerogels were prepared using normal drying conditions, while supercritical drying conditions were employed to synthesize the aerogels. Thin films were grown by spin coating polymerized sols on (100) silicon substrates. The aerogel and xerogel powders as well as thin films were studied for phase evolution and phase stability using X-ray diffraction. All three forms indicated the formation of the μ-cordierite phase at 900°C. In addition, the aerogels and thin films showed evolution of other phases. The initiation of the μ → α cordierite transformation was observed in the temperature range 1000–1100°C for both the xerogels and thin films, while μ-cordierite obtained from the aerogels showed a much higher stability and transformed to α-cordierite only at 1200°C.

Low-temperature chemical routes to formation and IR properties of lanthanum sesquisulfide (La2S3) ceramics

Lanthanum sulfide (La2S3) exists in three different allotropic forms in the crystalline state. The low-temperature orthorhombic phase (α) transforms to the intermediate tetragonal phase (β) which further undergoes transformation to the high-temperature cubic phase (γ). The intermediate tetragonal phase (β) is an oxysulfide with possible use as a phosphor, and the cubic phase (γ) of La2S3 shows potential for application as a far IR (8–14 μm) window material. We report metal-organic synthesis routes for the preparation of the β- and γ-phases of La2S3 by processing modifications and choice of precursor chemistry. The alkoxide-derived precursors and the transformed sulfide powders were characterized for morphology, microstructure, and thermal stability. Phase evolution studies were also conducted, and the results are discussed in the context of formation of the β- and γ-phases. Results of densification, microstructure, and infrared transmission characteristics are also presented.

Phase diagram of implanted [formula omitted] system

A systematic study of nitride phases formation in iron implanted with nitrogen was performed using conversion electron Mössbauer spectroscopy (CEMS). The samples of α-iron (armco) were implanted at room temperature with 100 keV N2+ at doses ranging from 5×1016 to 6×1017 N at./cm2. After implantation each sample was isothermally annealed in vacuum at temperatures ranging from 150 to 500°C in 50°C steps, for 1 h. The CEMS measurements allowed us to identify the iron nitride phases formed by implantation. The study of phase evolution due to annealing allowed us to derive a new phase diagram for the implanted FeN system.