TTT Curves Research Articles

Homogeneous nucleation and growth of ice from solutions. TTT curves, the nucleation rate, and the stable glass criterion

Using water-in-oil emulsion samples to avoid, for the most part, heterogeneous nucleation, the crystallization of ice from a series of aqueous LiCl solutions has been studied isothermally, using differential calorimetry to monitor the process through the release of the heat of crystallization. The classical form of the time–temperature transformation TTT curve has thereby been directly observed for the first time for cystallizing supercooled liquids, and its movement with concentration changes has been determined. The conditions of observation are such that each ∼3 μ diameter droplet is multiply nucleated during the time of observation. At temperatures well above the ‘‘nose’’ of the TTT curve, where multiple nucleation does not occur, a new phenomenon—in which the rate of crystallization increases with increasing temperature of observation—is encountered. It seems probable that this is due to some heterogeneous process perhaps involving a reaction at the water–oil–surfactant interface to account for the positive temperature coefficient. The results in the homogeneously nucleated region correlate well with the much longer time scale studies reported recently on the basis of time-dependent conductivity measurements. Using approximate diffusivity data for water in this system, the combined crystallization data can be analyzed to yield nucleation rates which are in reasonable agreement with the results of recent small angle neutron scattering studies on this system.

TTT curves for the formation of austenite

TTT curves for the formation of austenite

Direct observation of time-temperature-transformation curves for crystallization of ice from solutions by a homogeneous mechanism

Abstract : Emulsified samples of aqueous solutions of lithium chloride have been rapidly cooled to temperatures in the vicinity of the homogeneous nucleation temperature Tg and observed isothermally in a differential calorimeter to identify the time delay before the maximum crystallization rate (detected by the release of the heat of crystallization) is achieved. The locus of the time tp to reach the maximum rate at different temperatures has the classic form of the time-temperature-transformation TTT curve described in theories of homogeneous nucleation and growth. The displacement of this TTT curve to longer times and lower temperatures can be observed by increasing solution concentration. The results correlate well with much longer time observations made recently using electrical conductivity studies of thin bulk samples near their glass transition temperatures.

Continuous cooling approximation for the formation of a glass

Abstract Non-isothermal equations describing the liquid-crystal transformation are derived using the isothermal Avrami equations. A theoretical expression for the critical cooling rate for the formation of a glass is found. Calculations based on this expression are in better agreement with experimental values than those derived from TTT (time-temperature-transformation) curves. A study performed on typical glass forming materials enables the glass forming ability (GFA) to be determined by experimentally measuring crystallization temperatures at different cooling rates which are easily accessible with commonly available technology. The behaviour of the rate constant for crystallization is also obtained from the same data in the experimental range considered. In both cases no previous knowledge of the parameters involved is needed. With some assumptions the values of the viscosity in the crystallization temperature range can be estimated. Although the study was performed for an Avrami index of 4 an extension to other values of n is made under some restricted conditions and a more general treatment is outlined.

The role of quenching stresses in the formation of metallic glasses

Many meta l l i c glasses have now been produced by rapid quenching of thin layers of l iqu id a l loys. In a l l cases the l iqu id is e i ther smashed against a cold substrate pr sheared by a rapid ly moving metal surface with the l iqu id drop or substrate ve loc i t i es v > i0 ~ on sec I . The sharp thermal gradient between the l i qu id layer and the cold metal substrate results in quench rates of the order of 106 to 107 K sec I . The smashing or shearing action is necessary fo r the rapid spreading of the l i qu id layer on the cold substrate. The effects of the quench rate on the kinet ics of glass formation ( for example, by TTT curves) have been and continue to be under extensive examination (1). However, the e f fec t of the stress f i e lds (required to spread the l iqu id on the cold substrate) on the kinet ics of c r ys ta l l i za t i on and the structure of the glassy metals have apparently not been considered in the past. In this paper, i t is argued that the quenching stresses const i tute a key element in the suppression of c r ys ta l l i za t i on .

Metallography of ϵ-phase precipitation in dilute zinc-copper alloys

Precipitation of the ϵ-phase from supersaturated η-solid solutions of Cu in Zn has been studied for aging temperatures between room temperature and 300°C in alloys with Cu contents between 0.3 wt% and 2.5 wt%. The first process to occur was discontinuous precipitation at the grain boundaries, and in alloys with up to 1.2 wt% Cu this was the only type of precipitate to form. Alloys with copper contents ≥ 1.6 wt%, when aged at temperatures ≥ 150°C formed a Widmanstätten precipitate of ϵ-phase after the discontinuous precipitate formed. As the aging temperature approached that of the solvus, spheroidization of the precipitate took place. In the case of an alloy containing 2 wt% Cu, the results are summarized in the form of a TTT curve. In coarse grain-sized specimens intragranular (Widmannstätten) precipitation was accelerated, and within the colonies of precipitate the presence of voids was observed. This effect arises from the volume change associated with the formation of the ϵ-phase.

鉄系合金の非晶質形成能におよぼす半金属の影響

The purpose of the present work is to evaluate the glass forming ability of iron-based alloys containing phosphorus, carbon and boron by measuring the critical cooling rate for glass formation. Measurments of the quenching rate from melt are made by using a modified piston-anvil type equipment. The cooling rate to obtain a perfectly amorphous phase is determined from the examination of the microstructure of a quenched sample by X-ray analysis. The degree of supercooling resulting from the measurements revealed that it is a practical quantitative indicator by which critical cooling rates can be estimated. The minimum critical cooling rate is obtained at about 20 at% metalloids for Fe-P and Fe-B alloys. The correlation of metalloids to the critical cooling rate of iron is obtained for Fe-20 at% metalloids alloys. Glass formation of iron-metalloids alloys is affected mainly by the liquidus temperature and the viscosity of supercooled liquid, using calculated TTT curves. Finaly the effect of metalloid elements on the glass forming ability of iron is discussed from the stand point of interaction of atoms in liquid.

The kinetics of transformation in Zn-Ai superplastic alloys

We report herein on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures, and it has a relatively high strength at ambient temperature (∼345 MPa). TTT curves for the alloy are presented, and the corresponding microstructures obtained at the various transformation temperatures are reported. Also, the results of Jominy endquenched tests are reported and the corresponding continuous cooling kinetics are compared to the isothermal kinetics. The alloy was observed to decompose by two distinct mechanisms, depending upon the degree of supercooling. At temperatures just below the eutectoid, it decomposes into a lamellar microstructure, whereas at larger undercooling it decomposes into a coherent two-phase mixture. The interlamellar spacing and colony size are reported as a function of transformation temperature, and shown to follow expected trends. Since neither the lamellar nor coherent microstructure is superplastic, thermomechanical methods of producing a superplastic structure are discussed.

The dynamics of transformation interfaces in steels—II. Transformations in FE-C-MO alloys at intermediate temperatures

High-temperature high-voltage electron microscopy has been used to observe directly the growth of ferrite in iron-carbon-molybdenum alloys at intermediate transformation temperatures, characteristic of the bay in the TTT curve. Alloy carbide fibres, rows of carbide precipitates and less organized carbide dispersions have been observed to form at moving α-γ interfaces. In addition, the growth of heavily dislocated ferrite has been observed as a seemingly competitive mode of transformation. The possible significance of these phenomena, as they may influence transformation rates in these alloys, is discussed.

Thermal stability of amorphous Cu6oMe40alloys (Me=Cd, Zr)

AbstractThe formation and stability of splat-cooled metallic glasses of compositions Cu6oMe40 and Cu6oMe40 were investigated by the X-ray diffraction method and calorimetry. The TTT curves (time-temperature-transformation) and activation energies of crystallization, obtainedfrom differential scanning calorimetry curves arepresented. The thermal stability of these glasses and the ability of copper-base alloys toform glasses are explained in terms of alloying components.

An analysis of the kinetics of the β → α phase transformation in plutonium

Although a great deal of information has been published on the isothermal transformation characteristics of the β → α reaction in plutonium, the kinetics of the process have hitherto not been analysed in detail. Recently, a model has been proposed to account for the veining phenomenon in unalloyed plutonium. This model required for its success, that the mechanism of the β → α transformation changed in the vicinity of normal ambient temperature. It was proposed that the higher temperature reaction was one requiring thermally activated diffusion, while the lower temperature reaction was diffusionless. In order to test the validity of the model, all the suitable isothermal transformation data pertaining to the β → α phase change have been analysed. These data have been considered in the context of the form of the TTT curves which they give, the activation energy of the transformation and the temperature dependence of the exponent n derived from the equation: log log[ 1 (1 − y) ] = n log t + const , where y = fraction transformed at time = t. The analysis of the available evidence confirms that the mechanism of the transformation changes in the vicinity of normal ambient temperature. However, the kinetic evidence alone is insufficient to enable the nature of the two reaction mechanisms to be unambiguously determined.

Electron microscope study of the effect of deformation on precipitation and recrystallization in copper-titanium alloys

Structural changes occurring during aging of deformed as-quenched copper-titanium alloys were studied by means of hardness measurements and thin foil electron microscope examination. It was demonstrated that the average subboundary misorientation tends to increase with increasing strain values. The development and microstructure of deformation bands and their role in precipitation and recrystallization are discussed. Depending on the degree of deformation and aging temperature nucleation of either recrystallized grains or discontinuous precipitates was observed. After deformation lower than 30 pct the formation of recrystallization nuclei proceeds according to the strain induced boundary migration mechanism. After higher deformation recrystallization nuclei are formed mostly within deformation bands. A schematic TTT diagram is proposed, illustrating the sequence of various phase transformations that occur in the alloys investigated. The tendency of TTT curves shifting with increasing deformation, and a range of proposed heat treatment are discussed. It is shown that in deformed alloys spinodal decomposition occurs at aging temperatures lower than those in the underformed ones. Coarsening of modulations is affected by heterogeneous α' and β' phases precipitation, which is followed by discontinuous precipitation of α and β' phases of determined crystallographic relationship.

オーステナイト・ステンレス鋳鋼の極低温における組織安定性と機械的性質に及ぼす合金元素の影響

A study was made of the effects of alloying elements on the δ-ferrite formation, athermal and deformation-induced martensitic transformation and mechanical properties of SCS13 & SCS14 austenitic stainless steels at cryogenic temperature.The results obtained are as follows:1) Emperical equation for δ-ferrite formation can be expressed as a function of Ni equivalent (Δ) as: δ(%)=-2.17×(Δ)+4.242) Athermal martensite of Fe-18Cr-Ni alloys formed at -196°C shows a good agreement with that of calculated values according to the equation proposed by Magee. Martensite content of cast alloys is greater than that of wrought alloys for the same Ni content, because of larger grain size of cast alloys.3) With increasing Ni, Mn and N content of metastable austenite, the tensile strength lowers due to stabilization of austenite.0.2% proof stress at 20°C does not change by Mn addition, however increases by N addition.Elongation increases with increasing Ni and N content, and shows the maximum by Mn addition at about 6% Mn.4) Impact value at -196°C lowers by Mn and N addition, but does not change by Ni addition.5) Relationship between δ-ferrite and impact value at -196°C can be expressed as: vE=-1.25×(δ%)+2.286) Nv values of δ-ferrite are from 3.31 to 3.70 and that of bulk is 2.85. Judging from the obtained TTT curve for δ to σ transformation, δ-ferrite easily transforms to σ phase during water-quenching from 1100°C.7) The formation of σ phase more than 1% extremely deteriorates the impact values.

Production of superplastic zinc–aluminium alloys

AbstractThe production of superplastic properties in Zn–Al alloys involves control of casting, homogenization, quenching, transformation, and thermomechanical treatment to produce the required fine-grained equiaxed microstructure with a particular combination of superplastic and conventional properties. The optimum production route is determined mainly by the proportions of zinc and alumimum and the presence of other alloying elements, as they affect the shape and position of the TTT curve, the compositions of the phases, and their relative abundance. Results are presented which indicate the effect of processing variables and composition on the combination of superplastic with room-temperature tensile, and other conventional mechanical properties. While the majority of work concerns the production of sheet, the process routes for extruded rod and preforms will also be discussed. The impact of these on the use of superplastic-forming techniques for component manufacture will be indicated.

Phase transformations in uranium, plutonium, and neptunium

The actinide metals uranium and neptunium are characterized by three allotropes, and plutonium has six solid phases with two of the allotropes having complex, low symmetry, crystal structures. Further, many phase changes in these metals are accompanied by a large specific volume change and a large enthalpy change. Phase transformations in uranium and plutonium have been studied extensively but very little experimental work has been conducted to understand the phase transformations in neptunium. The low temperature transformations occur isothermally and exhibit simple TTT curves similar to many alloys. The kinetics in the metals are controlled more by purity and thermal history than by any inherent feature of the metals. Mechanisms in the pure metals are inadequately understood, and the crystal-lographic relations are complex. The β → α transformations in uranium and plutonium are diffusional near the α β equilibrium temperature, and they are both reminiscent of shear transformations at low temperatures. Skip transformations have been studied in these metals and the role of grain size on transformation behavior has been investigated more than in other metals. Accordingly, the roles of metal purity, thermal history, microstructure, mechanical properties, and thermodynamic variables are discussed, particularly as they might be related to phase transformations in other systems. Generalities have been difficult to establish for plutonium and uranium alloys. Among well-known metals the zirconium and titanium transformations form the closest analogs to the uranium transformations, but there is no obvious analog to the plutonium transformations.

Kinetics of the low-temperature phase transformation of ammonium iodide

The temperature-dependent specific electrical conductivity of polycrystalline, polymorphic NH4I samples was measured in the temperature range between −170°C and +25°C. The conductivity in the ranges of the tetragonal →CsCl-type and CsCl-type→NaCl type transformations can be explained by the formal kinetics of a diffusion-controlled nucleation and growth process. An estimation of the activation energy of the formation of the second-and first-phase nuclei in the respective parent phases by the evaluation of TTT curves yielded energy values of 0.004–0.05 and 0.0001–0.024 eV.

Influence of al, co, and si upon the kinetics of the proeutectoid ferrite reaction

The effects ofca. 3 at. pct of Al, Si, or Co upon the kinetics of grain boundary ferrite allotriomorph formation (and thus upon hardenability) relative to those in Fe-C alloys of comparable carbon content were evaluated. All three alloying elements displace the TTT curve for the initiation of transformation to shorter times at the higher reaction temperatures. Both aluminum and silicon increase the parabolic rate constant for allotriomorph thickening,α, relative to that in their counterpart Fe-C alloys; the influence of cobalt uponα, if any, is appreciably less. In the Fe-C-Al and Fe-C-Si alloys, thickening proceeds noticeably less rapidly than volume diffusion control (as assessed by Atkinson’s analysis of the growth of an oblate ellipsoid) allows. In the Fe-C-Co and Fe-C alloys, the average calculated and experimental α’s are in better agreement but, evidently as a result of the presence of dislocation facets at a broad face of allotriomorphs, some allotriomorphs actually thickened more rapidly than calculated. The substantial scatter inα in all alloys was also attributed to these facets. Indirect determinations indicated that all three elements increased the rate of nucleation of ferrite allotriomorphs.

A metallographic study of phase transformations in dilute mischmetal-magnesium alloys

The sequence of phase transformations occurring in a mischmetal —5 wt.% Mg alloy has been studied. The observed microstructures can be explained on the basis of a pseudobinary phase diagram given herein. The morphology of proeutectoid crystals can be described by the Dubé morphological classification system and the kinetics of the eutectoid reaction are summarized by a TTT curve. A lamellar morphology could not be obtained by phase decomposition from a previously quenched specimen.

The electrical conductivity and the phase transformation of thallous iodide single crystals

The temperature-dependent specific electrical conductivity of zone-refined TlI single crystals was measured in the temperature range between 400 and 600 °K. Repeated measurements of the same crystal samples resulted in annealing effects in the course of which metallic-grey precipitates were formed. The conductivity in the transformation range can be explained by the formal kinetics of a diffusion-controlled nucleation and growth process. An estimation of the activation energy of the formation of the second-phase nuclei in the parent phase by the evaluation of TTT curves yields energy values of 0.2 to 0.4 eV.

The metallographic preparation of some mischmetal alloys

The sequence of phase transformations occurring in a mischmetal —5 wt.% Mg alloy has been studied. The observed microstructures can be explained on the basis of a pseudobinary phase diagram given herein. The morphology of proeutectoid crystals can be described by the Dubé morphological classification system and the kinetics of the eutectoid reaction are summarized by a TTT curve. A lamellar morphology could not be obtained by phase decomposition from a previously quenched specimen.