Kinetics of crystallization of the metastable polytype of cerium upon melt quenching

Using a mathematical model of nonequilibrium crystallization of polymorphous metals with a narrow range of stability of the high-temperature modification, we have conducted numerical analysis of the kinetics of the formation of the metastable polytype of cerium (??-Ce) that is fixed by melt quenching at cooling rates more than ??10⁵ K/s. Agreement between the calculation data and the results of X-ray diffraction phase analysis of rapidly quenched foils has been obtained at a physically reasonable value of the model parameter having the meaning of the degree of metastability of ??-Ce.     

Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation

The metastable phase Al₉Ni₂ was investigated in order to characterize the thermodynamics and kinetics of its formation. Here, Al₉Ni₂ was observed as the first phase to form in a series of sputter-deposited Al/Ni multilayer foils, but it did not form in foils with a very small bilayer period (12.5 nm). In these foils, the stable phase Al₃Ni was the first phase observed. Using differential scanning calorimetry, the heat of formation and the Gibbs free energy for the formation of Al₉Ni₂ were both calculated to be −28 kJ/mole·atom. The average activation energy for Al₉Ni₂ was determined using a Kissinger analysis and was found to vary with bilayer thickness, with an average value of 1.58 ± 0.06 eV. In light of these experimental results, a simple nucleation model based on thermodynamics and diffusional intermixing is proposed to explain why Al₉Ni₂ forms before Al₃Ni in most cases, except in foils with very small bilayers where it is absent.     

Fractography of a metastable austenite

A study of fracture development in Fe-31%Ni-0.1%C alloy has been made. Samples of this alloy, which show deformation-induced martensite transformation, were given pretreatments of shock loading and grain refining. Fine-grained samples showed higher microvoid density with decreasing deformation temperature. This was attributed to the existence of a large number of internal interfaces (martensite/martensite, martensite/austenite, and any remaining parent austenite boundary) in these fine-grained samples which may act as void nucleation sites. This trend was not so marked in preshocked samples because of their fast reaction kinetics. Thus both treatments, grain refining and shock loading, which affect the deformation-transformation behavior of the metastable austenite, also affected its fracture behavior.     

Surface and structure modification of carbon nanofibers

The micro-structure of reinforcements, including surface characteristics, plays a critical role in the properties of composite materials that are ultimately produced. The purpose of this work is to investigate the micro-structure of carbon nanofibers (CNFs) with different core structures, which are hollow (PR) and solid (MJ) fibers, as a function of two treatments: CVD and thermal. From Raman spectroscopy and XRD analyses, an increase of crystallite thickness and width was observed after a heat treatment at 2200 °C for both types of fibers. Further, there was a more significant enhancement of crystalline structure in PR fibers. Also, an increase in thermal oxidation stability for heat-treated CNFs was observed. BET adsorption isotherms showed a significant reduction of specific surface area of MJ fibers after the heat treatment and also after the CVD surface treatment, resulting from the decrease of pore volume. However, even after heat treatment, MJ fibers possessed rougher surface than did PR fibers. Further, significant spatial discontinuity was observed with TEM micrographs due to reorganization of graphene layers during heat treatment for the both heat-treated CNFs. These results suggest that surface and structure of CNFs are important for maximizing their role in the performances of nanocomposites.     

Phase transformation of metastable beta Ti alloys with tweed structure

This paper presents the finding of a microstructural abnormality generated in the metastable phase of a supersaturated state, and compares the results with ordering behavior such as premartensitic transformation. A tweed structure consisting of the metastable phase was observed by transmission electron microscopy, and spot satellites inflicted by streaks were also observed in lattice diffraction. The present report also reviews a new phase transformation from the metastable phase caused by the processes of aging, sub-zero quenching, and induced strain or stress.     

Temperature dependence of metastable alumina formation during thermal oxidation of FeCrAl foils

Industrial FeCrAl foils were isothermally oxidized during 5 hours between 850 and 1000°C in atmosphere of pure oxygen. Characterization of transition and α‐alumina phases was performed by XRD and XPS, using reference spectra obtained by various air annealing treatments of pure γ‐Al₂O₃. An original model was proposed to deconvolute XPS spectra to obtain quantification of transition alumina formation and transformation. At 850°C, oxide scales on FeCrAl consisted of transition alumina, whereas higher temperature treatments resulted in decreased amounts of transition aluminas and in increasing α‐alumina formation. At 1000°C, the highest temperature studied, the scale could be described by XPS and XRD as pure alpha.     

Standard enthalpies of formation of some carbides, silicides and germanides of cerium and praseodymium

The standard enthalpies of formation of some congruent-melting compounds in the binary systems Re---X, where Re Ce, Pr or La and X C, Si or Ge have been determined by direct-synthesis calorimetry at 1473 ± 2 K. The following values of ΔH_f^o are reported: CeC₂, −25.4 ± 1.4 kJ (mol atom)⁻¹; CeSi₂, −60.5 ± 2.0 kJ (mol atom)⁻¹; CeSi, −71.1 ± 3.3 kJ (mol atom)⁻¹; Ce₅Ge₃, −73.4 ± 2.3 kJ (mol atom)⁻¹; CeGe_1.6, −75.6 ± 1.9 kJ (mol atom)⁻¹; PrC₂, −29.4 ± 1.6 kJ (mol atom)⁻¹; PrSi₂, −61.5 ± 1.7 kJ (mol atom)⁻¹; PrSi, −78.1 ± 1.9 kJ (mol atom)⁻¹; Pr₅Ge₃, −70.4 ± 2.3 kJ (mol atom)⁻¹; PrGe_1.6, −81.7 ± 1.7 kJ (mol atom)⁻¹; LaSi₂, −56.8 ± 2.5 kJ (mol atom)⁻¹. The results are compared with earlier experimental data, with predicted values from Miedema's semiempirical model, and with available data obtained for Sn and Pb compounds by Borzone et al., by Palenzona and by Palenzona and Cirafici.     

Mechanism and conditions of the chessboard structure formation

The observations of the pseudo-periodical chessboard (CB) microstructure in metal and ceramic solid solutions indicate that this is a general phenomenon. We propose a theory and three-dimensional (3-D) computational modeling explaining the origin of the CB microstructure in the cubic → tetragonal decomposition. The 3-D modeling demonstrates that the formation of two-phase CB structures is contingent on the formation of a compositionally stabilized precursor state with the tweed structure that is spontaneously formed at the initial stage of the transformation. The modeling has shown that this tweed structure is a distribution of spatially correlated tetragonal nanodomains whose spatial arrangement has the CB topological features. This precursor tweed structure serves as a template for the precipitation of the equilibrium cubic phase. The CB-like tweed template channels the microstructure evolution towards the two-phase CB structure whose complex and detailed 3-D geometry is in excellent agreement with electron microscopic observations. The thermodynamic analysis and obtained evolution sequences allow us to formulate the necessary thermodynamic, structural and kinetic conditions for the CB structure formation. Reasons for its relative stability are discussed. It is also shown that the coherency between the cubic and tetragonal phases comprising the CB structure produces the stress-induced tetragonality of the cubic phase, orthorhombicity of the tetragonal phase, and rotations of cubic phase rods. These effects should diminish and disappear upon lifting of coherency.     

Conditions for the formation of new oxide within oxide films growing on metals

When both cation and oxygen ion defects are mobile within a growing oxide film there exists the possibility that new oxide may be formed within the film. The conditions under which this can occur are examined for several combinations of cation and anion defect. The results show that the growth of oxide within the film is very much dependent upon the defect structure of the oxide. The implications of the results for stress generation and oxide formation at grain boundaries are discussed.     

Conditions for quasicrystal formation from mechanically alloyed Zr-based glassy powders

The absence of quasicrystalline phase formation in mechanically alloyed Zr₅₇Ti₈Nb_2.5Cu_13.9Ni_11.1Al_7.5 glassy powders with respect to melt-spun ribbon having the same composition has been investigated by analyzing the influence of mechanical deformation, oxygen contamination and chemical composition on quasicrystal (QC) formation. The results suggest that oxygen affects the formation of QCs by selectively reacting with zirconium thus driving the chemical composition of the glassy phase out of the range suitable for QC formation. This hypothesis has been verified by appropriately adjusting the composition of the powders. The addition of an adequate amount of zirconium changes the crystallization behavior inducing the formation of the quasicrystalline phase as the first crystallization product. This implies that if a particular short-range order is necessary for the formation of QCs, it can be achieved also by solid-state processing and therefore the presence of a specific quenched-in short-range order is not a mandatory prerequisite for QC formation.     

Microstructure and formation mechanism of cerium conversion coating on alumina borate whisker-reinforced AA6061 composite

Cerium conversion coatings, formed on alumina borate whisker-reinforced aluminum composite with pre-treatment for different time, were characterized by SEM/EDX, TEM and electrochemical method. The structure and formation mechanism of the coatings were investigated in detail. It was found that the pre-treatment prior to coatings has strong influence on formation, growth and morphology as well as granular size of coatings. Two typical coating morphologies existed at the pre-treated substrate surfaces. Cracks only presented on thick coatings because of the higher deposition rate and the greater inner stress. Cracks preferably produced at the whisker/matrix interface and the grain boundary of matrix.