Intermediate Metastable Phase Research Articles

Phase composition and phase transformation in Al-Si alloys hardened from the liquid state

1. The crystallization of Al−Si alloys quenched from the liquid state is accompanied by the formation and subsequent spontaneous decomposition of the metastable intermediate phases at 273–293°K. 2. The morphology of the structural components of rapidly quenched silumins is genetically connected with the habit of the corresponding intermediate phases, which are the leading ones in the process of growth of the eutectic colony, and with the nature of the solid-phase transformations during their decomposition.

Crystallization of amorphous lead-titanate prepared by a sol-gel process

Monolithic samples of amorphous lead-titanate were prepared by a sol-gel process. Samples of the dried gel were analyzed using x-ray diffraction and differential thermal analysis. Crystallization of a metastable intermediate phase was observed to begin upon annealing at 400°C for approximately 6 hours. Tetragonal PbTiO3 appeared after longer annealing times. The crystallization temperature at a constant heating rate of 8°C/min. was approximately 470°C.

Transmission electron microscopy studies of a vapour-deposited AlZr Alloy

Thin films of an Al-9.4wt.%Zr alloy have been produced using the flash evaporation technique onto Formvar-coated copper grids. The as-deposited thin films constitute a supersaturated solid solution of zirconium in aluminium. The microstructure and crystal structure of the phases in the as-quenched state as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized using transmission electron microscopy and electron diffraction techniques. When annealing was carried out at about 773 K, two metastable intermediate phases were found to form. Electron diffraction evidence suggested that these phases have a cubic structure with a = 0.85 nm for the Al 11Zr 2 phase and an orthorhombic structure with a = 0.7489 nm, b = 0.6556 nm and c = 0.8961 nm for the Al 6Zr phase. When annealing was performed for a long time, the equilibrium constitution (i.e. a mixture of aluminium solid solution and the tetragonal Al 3Zr phases) was achieved. Details of the transformation behaviour and morphology of the metastable transformation products are discussed.

Thermogravimetry under quasi-equilibrium conditions utilized in the study of the thermal dissociation of compounds

Study of the kinetics of multi-stage thermal dissociation reactions is a difficult task. Utilization of thermogravimetry under quasi-equilibrium conditions allows determination of intermediate phases, and if necessary their separation, in order to permit study of the kinetic stability separately. Parallel utilization of thermogravimetry under linear heating and quasi-equilibrium conditions allows detection of the existence of intermediate metastable phases.

Effect of irradiation particle mass on crystallization of amorphous alloys

The crystallization temperature of amorphous alloys was found to be significantly lowered by heavy ion or electron irradiation during annealing. However, only heavy ion irradiation altered the mode of crystallization. Both a binary and multi-element amorphous alloy showed this type of response to irradiation. Radiation-enhanced diffusion processes in the amorphous state can explain the increased crystallization kinetics during irradiation. Heavy ion irradiation alters the crystallization mode by causing direct transformation to the final equilibrium phase as opposed to intermediate metastable phase formation during thermal annealing or electron irradiation. The equilibrium phase is believed to nucleate directly in the displacement cascades, which only form during heavy ion bombardment.

Formation of the DO 22-type superlattice during long-range ordering to Ni 4Mo in a Ni-Mo alloy

Khachaturyan (1973, 1974) has shown that many structures including a DO22-type superlattice can form during long-range ordering to Ni4Mo and Ni3Mo. The present investigation has the objective to show, that, in agreement with the statistical thermodynamic model used by Khachaturyan, a DO22 superlattice forms as an intermediate phase during long-range ordering to Ni4Mo. The structural transitions which occur during long-range ordering to Ni4Mo in a Ni-Mo alloy were studied. It was found that a DO22-type superlattice forms as an intermediate metastable phase. 28 references.

FURTHER STUDIES ON THE PHASE TRANSITION OF LITHIUM IODATE (LiIO3) POLYMORPHISM

Further studies on the phase transition mechanism of LiIO3 at normal pressure have been made by means of DTA, heat-treatment, and powder X-ray diffraction. There are three phases of LiIO3, β,η,δ, existing relatively stably at high temperatures and they can directly melt respectively. Their melting points are as follows: phase β, 432℃; phase η, 421℃; phase δ, 416℃. The melting points, thermal processes and the exiting temperature ranges ail show that the order of their thermodynamical stability is: β>η>δ. In dry air and at room temperature, the phase ζ of LiIO3, which changes to phase β when it is heated to 335℃ at 10℃/min, stands long periods of time without any change as well as phase α and β . In the existing temperature range of α -LiIO3, phase ζ does not transform into phase α although it is heat-treated for a long time. The temperature of phase transition of ζ to β is higher than that of α to β. The existence of phase β in the phase ζ has a promotive effect on the phase transition of ζ to β just as the case of α to β. The existence of phase ζ in the phase α has also a promotive effect on α to β . The phase θ, like the phase γ. is also an intermediate metastable phase in the phase transition processes of LiIO3.

Observation of crystallization in Fe80B20 metallic glass by DSC and Moessbauer spectroscopy

The crystallization behavior of the Fe80B20 metallic glass ribbon (MG2605) has been observed by using isothermal differential scanning calorimetry, DSC, and Moessbauer spectroscopy. The crystallization kinetics follow an Avrami-type relation with an activation energy of 3.04∓0.1 eV., and characteristic time exponent, n = 3.38∓0.1. Eutectic phase separation of α-Fe and tetragonal Fe3B is observed as the first stage of crystallization. The structure of the amorphous phase has been evaluated in terms of the distribution of the hyperfine field, obtained from the Moessbauer spectra by Window's method1. The intermediate metastable crystalline phases and final stable crystalline phases produced from the initially amorphous Fe80B20 have also been characterized.

Metastable phases in vapour-deposited Al-Ru alloys

Vapour deposition of two Al-Ru alloys containing 10 and 15wt% Ru resulted in the formation of supersaturated solid solutions. Electron microscopy and electron diffraction techniques have been used to characterize this as-quenched structure and also those formed on subsequent decomposition. Elevated-temperature (650–800 K) annealing treatments resulted in the formation of a metastable intermediate phase. Electron diffraction evidence suggested that this phase can be assigned a simple cubic structure either of the Mo3N2-type witha=0.42 nm or of the Mg2Zn11-type witha=0.84 nm. Further annealing led to the formation of the equilibrium AI and AI6 Ru phases. The morphology and structural features of transformation products and the crystal structure details of the metastable intermediate phase have been discussed.

すず‐ニッケル合金めっきの熱安定性に及ぼす合金組成の影響

Transformation upon heating of electrodeposited metastable tin-nickel alloys, prepared from pyrophosphate baths, was studied by means of differential thermal analysis and X-ray diffraction. Metastable tin-nickel alloys were obtained as a single phase (NiSn) in the composition range from 40 to 55at.% Sn. Alloys containing 49-55at.% Sn were transformed exothermically into mixed or single equilibrium phases of Ni3Sn2+Ni3Sn4 or Ni3Sn4. Tin-nickel alloys with higher tin content had lower transformation temperature; e.g., they were transformed at below 100°C for 55at.% Sn, whereas at 340°C for 49at.% Sn. Tin-nickel alloys containing 40-44at.% Sn were transformed to a single phase of Ni3Sn2 at ca. 300°C with no distinct heat evolution, corresponding to the transformation. Tin-nickel alloys (47at.% Sn) electrodeposited from fluoride baths were transformed at 360°C into another supersaturated metastable phase. The intermediate metastable phase was decomposed into mixed equilibrium phases of Ni3Sn2+Ni3Sn4 at 600°C.

Kinetic Model of Zeolite Paragenesis in Tuffaceous Sediments

AbstractThe sequence of mineral reactions involving zeolites and other authigenic phases in tuffaceous sedimentary rocks can be explained by growth- and dissolution-reaction kinetics. Kinetic factors may determine the specific authigenic phases which form and the temporal and spatial constraints on the solution composition during irreversible dissolution and growth reactions in glass-bearing rocks. The glass phase generates a high level of supersaturation with respect to a variety of aluminosilicates in the pore fluid. The sequence of assemblages formed during a series of metastable reactions resembles an Ostwald step sequence. Metastable reactions occur because formation of less stable phases such as gels, clays, and disordered zeolites may lower the total free energy of the glass-bearing system faster than the growth of the stable assemblage including ordered feldspars, quartz, and micas. Eventually, after a series of steps, the most stable silicate assemblage for the bulk composition, temperature, and pressure may form. However, the formation of intermediate metastable phases can delay the attainment of equilibrium by as much as tens of millions of years.

Precipitate Anisotropy in RSR 197 Superalloy

With the possible exception of Au-Cu alloys, no ordering system has been explored as intimately as the Ni-rich Ni-Mo system. The ordered intermediate metastable phases that form in this binary have been extensively reviewed. One binary intermediate, the Ni3Mo metastable DO22 phase, also appears in RSR 197 alloy (Al-13, Mo-9, Ta-2, Ni at.%) and in many other Ni-Mo-Al-X alloys as well. The most obvious difference (beyond compositional differences) between Ni-Mo binaries and the higher order systems is the presence of a second phase, γ', Ni, Al, a Dl2 superlattice, in addition to the disordered γFCC phase. This stable Dl2 phase is present before the intermediate metastable DO22 phase and has an due to the high ratio of Mo partitioned to the γ phase, in contrast to most γ/γ' superalloys where . The presence of this Dl2 phase has interesting effects on the precipitation morphology and crystallography of the subsequently formed DO22 phase.

Undercooling Behavior of Liquid Metals

ABSTRACTThe undercooling of liquid metals is observed often, but is restricted by the catalysis of the most potent heterogeneous nucleation site in contact with the liquid. An experimental approach that yields large undercoolings involves the slow cooling from the melt of a dispersion of stabilized fine (5–20μ) liquid droplets. Droplet studies have extended measurements of the undercooling limit for low melting point metals to the range of 0.3 to 0.4 Tm. In droplets, surface coating and size characteristics usually limit the undercooling and control the uniformity of crystallization behavior. A nonuniform undercooling behavior can produce microstructural variations in powder samples with respect to metastable phases and solute segregation, but these variations can be diminished for isenthalpic solidification of hypercooled liquids. In alloys the trend in nucleation temperature can follow a similar pattern as the composition dependence of the liquidus. With highly undercooled droplets and even at modest ΔT (ΔT ≤ 0.lTm) with known heterogeneous sites, structural modifications such as extended solid solutions and metastable intermediate phases can be generated which are similar to those produced by rapid quenching techniques. These features highlight the importance of a high liquid undercooling as well as a high cooling rate in promoting effective rapid solidification processing treatments.

A kinetic study of the system [formula omitted

A time-temperature study has been made of the system: γ- AlOOH (boehmite) Al 2O 3 . The isothermal TTT diagram produced demonstrates the course of the transformation sequence, which involves a number of metastable transition alumina intermediate phases. This type of treatment allows calculation of kinetic parameters for the various transformation processes, and these may be correlated with the mechanisms operating. Kinetic aspects of variations in microstructure and crystal structure have also been studied. In particular, the progress of cation migration processes involved in the transformation γ → δ-Al 2O 3 has been followed in terms of the continuous variation of the spinel subcell parameters.

Annealing behaviour of amorphous ?- phase alloys of FeW and MoCo

Crystallization in sputter deposited, amorphous FeW and MoCo of μ-phase composition alloys has been studied using transmission electron microscopy (TEM), electrical resistivity and X-ray diffraction. The amorphous alloys exhibit high thermal stability and transform directly to the equilibrium μ-phase with no intermediate metastable phase formation. In FeW, a tungsten-iron solid solution also forms as the alloy composition is actually in a two-phase (μ + solid solution) region. In both alloys, the crystallites form on a very fine scale and the μ-phase is heavily faulted. The crystallization behaviour may be indicative of a marked similarity between the short range structure of the amorphous phase and the μ-phase structure. Changes in electrical resistivity during crystallization are in substantial agreement with the structural changes observed through TEM and X-ray analysis.

Infrared Intensity Measurements of Cryodeposited Thin Films of NH3, NH4HS, H2S, and Assignments of Absorption Bands

Interferometric infrared spectral transmission measurements from 60 to 10 000 cm−1 were made for thin films of NH3, NH4HS, and H2S cryodeposits, and recorded as a function of thickness and temperature from 88 K to evaporation points. Integrated band intensities were measured and are reported for all major absorptions. Assignments for lattice and internal fundamental modes are discussed. Particular attention is devoted to the various reported phases of solid NH3, and it is concluded that three phases are supported by experimental evidence: the stable cubic phase, a low temperature amorphous phase, and an intermediate temperature metastable phase. Results and changes of the spectra as a function of temperature will be summarized. Intercomparisons of the spectra obtained for the three cryodeposits are made.

Decomposition studies of a liquisol quenched Al-33 at.% Mg alloy

X-ray and electron metallographic studies were undertaken on an aluminum-33 at.% magnesium alloy rapidly quenched from the melt by the “gun” technique. A small supersaturation of Mg in Al and two metastable intermediate phases were detected. These phases were found to display complex decomposition behavior on annealing at temperatures up to 300°C.

Phase transformations in the carbon system

1. It is suggested that the diamond-graphite transitions can proceed either directly or through intermediate metastable phases. 2. The conditions required for thin-layer graphite-to-diamond transition are determined by the depth and mutual orientation of the layers.

The role of Fe<inf>3</inf>B compound in the crystallization of Fe-B metallic glasses

An amorphous Fe-B system was studied in composition range from 12.5at%B to 24.9at%B by magnetic measurements and Mössbauer spectroscopy. It was found that samples of low B content exhibit crystallization in two stages while in the case of higher B concentration a single stage crystallization occurs. In most cases during the crystallization an intermediate metastable crystalline phase Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B is formed, which decomposes into α-Fe+Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> B at higher temperatures. The temperature stability of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B compound reaches its maximum at about 20% B concentration. A tentative explanation to this experimetal fact is presented.

Crystallization of amorphous Te-20 at% Sn alloy

Amorphous Te-20 at% Sn alloy was obtained by rapid cooling from the liquid state. Phase transitions occurring during continuous heating of amorphous films were studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. It was found that the crystallization of the amorphous phase begins at 382 K and proceeds via nucleation and growth of metastable phases MS1 and MS2. The first of these phases was assigned the primitive cubic structure with lattice parameter a=3.2A. The second phase, being a Te (Sn) solution, was assigned the hexagonal structure with lattice parameters a=4.45A and c=5.85A. During heating at 410 K the remaining amorphous phase decomposed into a mixture of Te crystals together with metastable phase MS3, which probably has the ZnS type structure with lattice parameter a=6.05A. Within the temperature range 450 to 550 K the MS1 phase was transformed into SnTe, and the MS2 phase into Te. The metastable intermediate phase MS3 decomposed only near the solidus temperature, and the alloy attained its equilibrium structure.