Formation Of Equilibrium Phases Research Articles

Steady state grain size and thermal stability of nanophase Ni 3Fe and Fe 3X (X = Si, Zn, Sn) synthesized by ball milling at elevated temperatures

Ball milling of Ni 3Fe and Fe 3X (X = Si, Zn, Sn) was performed at temperatures from 23°C to 300°C. X-ray diffractometry, transmission electron microscopy, and differential scanning calorimetry were used to measure the average grain size, internal strain distribution, and thermal stability of the as-milled samples. It was found that the average grain grain size was larger at higher milling temperatures, and the strain distribution was smaller. The alloys of Fe 3Sn and Fe 3Zn were single-phase after milling, showing that the milling process suppressed the formation of the equilibrium phases that would otherwise have formed at 300°C. Upon annealing the asmilled materials, grain growth and equilibrium phase formation occurred nearly simultaneously, probably because both are controlled by atomic diffusivity. The thermal stability against grain growth and equilibrium phase formation was not affected by milling temperature. There were, however, significant differences in the stabilities of the different alloys against grain growth and phase separation, and some controlling parameters are suggested.

Structural analysis of thermal oxide layers grown on Zircaloy-4 established with the help of chemically homogeneous sputter-deposited Zr-O thin films

In the presence of oxygen, magnetron reactive sputtering of zirconium and superficial oxidation of Zircaloy-4 in a microwave post-discharge lead to the formation of equilibrium and non-equilibrium phases of the Zr-O system. These phases can be obtained in a nearly bulk state, that is with thicknesses of several micrometers, with great chemical and structural homogeneity, in the case of magnetron sputtering. They appear only locally after oxidation. A comparison of microstructural features and conditions for the appearance of the metastable oxide-type phases shows, for the two treatments, many analogies. This allows the formulation of some hypotheses about their stabilization mechanism. In particular, it is shown that deviation from ZrO 2 stoichiometry (defined by α- ZrO 2) always leads to the formation of a phase normally found at higher temperatures. Therefore, by increasing the oxygen concentration deficit, β-ZrO 2, ψ-ZrO 2 and an amorphous phase are successively encountered.

Entropy-driven formation of a superlattice in a hard-sphere binary mixture

A MIXTURE of two dissimilar species (A and B) may freeze to form a substitutionally ordered crystal, the structure of which can vary from a lattice with only a few atoms per unit cell to a complex 'superlattice'. For example, a mixture of sodium and zinc can form a solid with the AB13 structure with 112 atoms per unit cell1 (Fig. la). One might suspect that very specific energetic interactions are needed to stabilize a structure as complex as this. But recent experiments2,3 show that the AB13 structure is also formed in mixtures of spherical colloidal particles with different diameters, which interact only via simple repulsive potentials. This raises the possibility that the formation of an AB13 superlattice might be sup-ported by entropic effects alone. To investigate this possibility, we present here computer simulations of a binary mixture of hard spheres. Our calculations show that entropy alone is indeed sufficient to stabilize the AB13 phase, and that the full phase diagram of this system is surprisingly complex. Our results also suggest that vitrification or slow crystal nucleation in experimental studies of colloidal hard spheres can prevent the formation of equilibrium phases.

In situ HREM of interface interactions

We have applied in situ high-resolution electron microscopy (HREM) to the study of interface reactions, particularly in metal-semiconductor systems. There is contrasting behavior whether or not the manufactured interface undergoes a chemical reaction. The in situ technique allows determination of the reaction mechanisms on an atomic scale.Reactive interfaces are characterized by systems in which new chemical compounds are formed (e.g., silicides for metal-silicon interfaces, metal gallides and arsenides for GaAs, etc.). We found that the equilibrium phase formation is often preceded by a solid-state amorphization reaction. In situ observations allow very precise measurement of the reaction rate in a sufficient temperature range to confirm that this process is diffusion controlled. Crystallization of the amorphous material can be followed as well as the development of any crystallographic orientation relationships. A ledge growth mechanism can easily be distinguished from a random process.It might be expected that non-reactive interfaces are stable upon heating.

Critical temperatures for radiation enhanced diffusion and metastable alloy formation in ion beam mixing

Phase formation, stability, and mixing behavior in the metallic systems NiZr, FeZr, AuZr, and PdTa have been investigated under influence of high energy heavy ion beams as a function of temperature. Our results and data from the literature support a simple model which correlates onset temperatures for radiation enhanced diffusion and equilibrium phase formation in ion beam mixing with hole (or vacancy) formation enthalpies. The model offers the possibility to predict temperature ranges in which efficient mixing occurs by radiation enhanced diffusion without formation of stable alloys.

A kinetic study on the spontaneous vitrification of Cr 40Ti 60 alloy

Water-quenched Cr 40Ti 60, high temperature bcc solid solution, vitrifies on subsequent annealing, and vitrification is confirmed by X-ray and TEM. The kinetics of the vitrification behavior in this system are investigated by means of X-ray diffraction. For qualitative analysis, using X-ray diffraction intensity, the internal standard method is used. The activation energy for spontaneous vitrification (SV) is obtained on the basis of the Johnson-Mehl-Avrami equation: it is ∼ 46.6 kcal/mol. From DTA experiments, the measured activation energy for the equilibrium phase formation of the water-quenched β-phase is 62.0 kcal/mol. It is suggested that SV precedes the formation of quilibrium phases due to the activation energy difference between the vitrification and the equilibration. It is proposed that the mechanisms of SV is closely related to the diffusion of the Cr atom in the shore range, and consequently the lattice distortion due to the motion of Cr atoms results in the vitrification.

Preparation of ductile NbAl powders for the fabrication of Nb 3AI superconductors. II

Small NbAl powder compacts and arc-melted alloys with aluminium contents up to 30 at.% were melted and rapidly quenched from the melt using a modified twin piston and a hammer and anvil technique. At calculated cooling rates between 10 5 and 10 7 K s −1 a metastable extension of the niobium solid solution field up to 27 at.% Al was reproducibly achieved. At slightly higher aluminium concentrations the quenched specimens showed the Nb 3Al phase with the A15 structure. The nucleation of the Nb 2Al phase was suppressed. Ductile NbAl powders of the composition “Nb 3Al” were prepared by application of a combined hydriding-dehydriding process at temperatures lower than 450 °C. The surface quality of the quenched splats essentially influences the hydriding procedure. The retransformation behaviour of the metastable states as a function of content and temperature was investigated. With increasing temperature the Nb 3Al phase precipitates from the strongly supersaturated niobium solid solution. After heat treatme its at temperatures lower than 700 °C formation of the Nb 2Al equilibrium phase could not be observed between 21.5 and 27.5 at.% Al.

Solid-state reaction of Ru and Al in molten Bi

There has been much recent interest in the reactions that occur when two crystalline metals inter-diffuse at low temperatures. Metastable phase formation can occur when structural relaxation into equilibrium crystalline phases is kinetically inhibittedf. These reactions are typically studied in thin—film diffusion couples formed by sequentially depositing two elemental crystalline metals. In this paper we describe a novel low—temperature reaction sequence for the formation of very fine-grain Alx Ru,1-xalloys that allows a further extension of these techniques. This reaction regime uses molten Bi as a medium for the transfer of Al solute atoms to the surface of Ru powder where reaction occurs with the formation Alx Ru1-x,intermetallic phases. The low melting point and ease of handling of Bi, its solvent ability towards Al and Ru, the absence of equilibrium phases of Bi with Al and Ru, and previous experience in our laboratory with equilibrium and metastable phase formation in this system made it attractive as a prototype for this initial study.Ru powder and Al pellets were separatedly prereacted in molten Bi at 500 C for several hours to ensure wetting of the solid surfaces. This Ru and Al and additional excess Bi were then sealed together in a fused silica tube and reacted in a vertical tube furnace. Separate such reactions were carried out at temperatures ranging from 300 to 600 C. After reaction samples were broken free from the fused silica tube and the Bi transfer medium was etched away in concentrated HNO3.

Freezing diagrams: Part I. Development and Implications for glass formability

Prediction of equilibrium and metastable phase formation for a particular system, after cooling from the liquid state, is possible by the development of afreezing diagram. the freezing diagram is constructed by consideration of kinetic as well as thermodynamic factors. The kinetic factors divide the diagram into three zones. These are: (1) where long-range diffusion can occur; (2) where short-range diffusion only is possible; and (3) where no significant diffusion is possible in the liquid ahead of the solid-liquid interface. The solid-liquid interface temperature,T1, determines which diffusion process occurs.T1 is the vertical axis of the diagram plotted against composition. the microstructure that results is predicted by the thermodynamic possibilities in each kinetic zone. In the first zone, the equilibrium phase diagram is applicable. The second zone only allows massive transformations to single-phase structures, and their limits are defined byTo lines. The third zone is below the glass transition temperature, and any liquid retained to this zone is quenched-in and becomes a glass. The development, significance, and limitations of the freezing diagram are discussed.

On the Stability of Hetastable and Amorphous Pvd-Compound Films

ABSTRACTIn using low temperature physical vapour deposition (PVD) techniques new metastable and amorphous materials can be deposited. Structural order in a coating is produced largely by the mobility of the adatoms. Low mobility does not allow the formation of equilibrium phases and metastable and/or amorphous structures are observed. These coatings have sufficient thermal stability for tools, and for both wear- and corrosion-protection. Examples are given within the systems Al-O-N, Ti-Al-N and Cr-Al-N and the properties, crystalline and thermodynamic state of the coatings are characterized. An example of decomposition phenomena is given in metastable thin coatings by the Ti-Zr-N system.

Structural transformations occurring in flame-sprayed Ni60Nb40 alloy coatings during heating in the presence of oxygen

During the heating of flame-sprayed Ni60Nb40 alloy coatings in the presence of oxygen the process of crystallization of their amorphous matrix occurs at T > 750°K through the formation of the niobium oxide Nb2U5 and precipitation of nickel. Decomposition of the amorphous matrix is accompanied by a decrease in length of the coatings. The oxidation of niobium at 750–890°K prevents the formation of the equilibrium phases of the Ni-Nb system during the crystallization of the amorphous structure. During heating in the 890–1110°K range the metastable Ni8Nb phase is present in the coatings.

Advanced layer material constitution

The increasing knowledge and understanding of the interrelations between the constitution, microstructure and properties of materials are the basis for the development of so-called advanced materials. In particular, ceramic bulk materials, metastable structures and special coatings for various applications are material groups of high interest. The background for developments in these areas is the constitution of multicomponent systems and the adjustment of special microstructures. Critical assessments and compilations of phase diagrams, the so-called maps of materials, are undertaken with strong emphasis worldwide. In many cases, however, the conditions employed in low temperature coating techniques do not allow the formation of equilibrium phases. Metastable structures and amorphous solid solutions are frequently observed. These are frozen in up to temperatures of about 0.3 T m (where T m is the melting temperature). As a consequence new metastable materials can be used in hard material coating systems (melting points of about 2500–3300°C) up to 700–1000°C. The possibility of predicting the constitution of multicomponent layer materials is a subject of special interest. This would support finally a specific layer material development. It should be pointed out that the ideas discussed are not restricted to hard coatings but have to be considered for all kinds of functional coatings.

Thermodynamic activities at 1273 K in the system cupric oxide-magnesium oxide using a solid-state galvanic cell

Schmahl and Minzl [1] examined the phase relationships in the system CuO-MgO by the X-ray method. The phase relations at 1273K are characterized by two terminal solid solutions, the MgO-rich rock salt phase and the CuO-rich tenorite phase which are separated by three regions, i.e. the region in which the rock salt phase and a compound of CuO and MgO, the guggenite phase, coexist; the single-phase guggenite region over a varying composition range and a two-phase region in which the guggenite and the tenorite coexist. The activity-composition relations for CuO and hence the phase-boundary limits were determined at 1273 K by directly measuring the oxygen pressures in equilibrium with the phases in the presence of Cu20. Navrotsky [2] determined the activities of CuO at 1273 K by measuring the oxygen pressures for the equilibrium-phase assemblages. The activities of MgO were calculated by a Gibbs-Duhem integration. The activity-composition relations obtained by Navrotsky [2] agree well with those determined by Schmahl and Minzl [1] for the rock salt phase and the two-phase region containing the rock salt phase and guggenite. No direct measurements of activity of MgO have been reported before. In the present work, a solid-state galvanic cell with MgF2 as the solid electrolyte was used to measure MgO activities at 1273 K in the equilibrium phases of the CuO-MgO system. The suitability and reliability of this technique to measure MgO activities in solid solutions containing MgO are well documented [3-6]. Magnesium fluoride, for making the solid electrolyte pellets and electrodes, was prepared according to the method of Taylor and Schmalzried [3]. The electrolyte was prepared by pressing the powder in a cylindrical die. The green pellet was then sintered in an atmosphere of CO2-free dry oxygen gas at 1323 K for 4 h. Samples of the desired composition were weighed from dried reagent-grade CuO and MgO and ground under acetone. The equilibrium phases were prepared by sintering the oxide-mixture pellets at 1273 K in a stream of CO2-free dry oxygen gas. The product was ground and sintered again at 1273K for 30 to 72h. The formation of true equilibrium phases was confirmed by the X-ray method using CuK~ radiation. The apparatus used in the present investigations was similar to that described earlier [7]. The performance of the polycrystalline MgF 2 as the solid electrolyte was checked by operating the cell:

Phase formation in co-deposited metallic alloy thin films

Phase formation in co-deposited thin films of Pd-Rh, Cu-Ag and Cu-Sn has been examined. Depending on the substrate temperature, either equilibrium or non-equilibrium phases are formed. It is postulated that the formation of multiphase structures in co-deposited alloy thin films is controlled by the diffusional breakdown of fully intermixed depositing atoms, so that three kinetic regimes are observed: (i) at low substrate temperatures the surface mobility is insufficient for the decomposition of the fully intermixed depositing atoms and the films contain non-equilibrium single-phase structures; (ii) with increasing substrate temperature decomposition to non-equilibrium two phase structures is observed; and finally (iii) with a further increase in substrate temperature the atomic mobility at the surface is sufficient to allow the full atomic rearrangements necessary for the formation of equilibrium phases. By relating the distance an atom can move on the surface during deposition to substrate temperature and deposition rate, it hau proved possible to account for the temperature ranges of the above deposition regimes. A hypothesis is put forward that relates the non-equilibrium phases formed in the first kinetic regime to free energy against composition diagrams at the temperature of the substrate, and this provides the basis for understanding the complex non-equilibrium phase formation found in the Cu-Sn system.

The effect of boundary conditions in ion mixing of multilayered NiSi samples

In ion mixing of metal-semiconductor systems, bilayer samples initially lead to the formation of equilibrium phases. However, in multilayered samples where the average composition is fixed, ion irradiation at high doses leads to metastable (or amorphous) phase formation. The metastable phase appears to form at a critical ion dose, apparently without the formation of any compounds at subcritical doses (at least not reported in the literature). The objective of this study is to investigate the effect of individual layer thickness of NiSi multilayered samples on ion induced reactions. Four types of NiSi samples were used: (i) Ni( ~ 700 Å)/Si〈100〉, (ii) two layers of Ni( ∼350 Å) interposed with two layers of Si( ∼ 360 Å) on SiO 2, with an average composition of Ni 1.65 Si (this composition is in the two phase region of the phase diagram), (iii) three layers of Ni( ∼ 240 Å) interposed with three layers of Si( ∼ 260 Å) on SiO 2, and (iv) five layers of Ni( ∼140 Å) interposed with five layers of Si( ∼145 Å) on SiO 2. It was found that the mixing efficiency increases with the number of layers in the samples. Amorphous phase formation was observed only in samples with six and ten individual layers at high doses. An equilibrium phase of Ni 2 Si does appear to be detectable by X-ray diffraction in all types of samples at low doses. Based on this observation, and to a first order approximation, the boundary condition imposed by multilayered samples is similar to that imposed by bilayer samples for ion mixing at low doses. As mixing continues under ion irradiation the boundary conditions start to differ for these two types of samples. For bilayer samples, Ni 2Si grows with dose, as reported in the literature. For multilayer samples, amorphous phase formation is possible when uniform mixing is achieved since the average composition of these samples is in a two phase region of the phase diagram. The relationship between layer thickness, mixing efficiency and amorphous phase formation is discussed.

Phase Transformations on Aging Cu-Al-Ni β Phase Alloys

ABSTRACTThe phase transformations occurring on quenching and subsequent aging of a Cu-Al-Ni β phase alloy have been studied using transmission electron microscopy, x-ray diffraction and optical microscopy. Quenching produces an ordered solid solution, β1, based on the DO3structure of Cu3A1. X-ray and electron diffraction show satellite diffraction peaks. On aging, these satellite peaks disappear and the parent phase undergoes a martensitic transformation. It is suggested that the parent phase undergoes a process of phase separation through the mechanism of spinodal decomposition before the martensitic transformation occurs. The martensitic transformation shows isothermal kinetics for which a transformation diagram is presented. Prolonged aging results in the formation of equilibrium phases.

Equilibrium and nonequilibrium phase transformations of quaternary solutions based on a physiologic support medium containing NaCl, dimethyl sulfoxide, and glycerol

Insight into the circumstances attending the freezing and thawing of biological materials suspended in cryoprotective solutions might be expected to result from knowledge of the colligative properties and glass behavior in systems based on typical physiological media, NaCl, and important cryoprotective agents. Differential thermal analysis has been used to determine phase diagram relationships in the aqueous-rich region of the quaternary system composed of a complex physiological support medium (Eagle's minimum essential medium), NaCl, and two cryoprotective compounds (glycerol and dimethyl sulfoxide). Thermograms revealed behavior corresponding to the primary crystallization of ice from sample solutions as well as the glass transition and devitrification of nonequilibrium amorphous phases. In most quaternary solutions, equilibrium-phase formation in the form of a pseudoeutectic transformation was inhibited and the formation of a metastable amorphous phase was observed as the final mode of solidification.

Formation of metastable and equilibrium phases in the decomposition of the β solid solution in Zr alloys

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Structure and properties of Al-Mg-Li-Zr alloys

1. The addition of zirconium to alloy 01420 leads to metastable zirconium phase and formation of dispersed equilibrium phases S (and possibly γ) characteristic of Al-Mg-Li alloys. Along with zirconium contained in the solid solution, these phases evidently inhibit recrystallization. 2. Models were plotted of the changes in the mechanical properties and the aging effect of Al-Mg-Li-Zr alloys in relation to the magnesium and lithium concentrations. 3. The larger aging effect with increasing lithium concentrations in the Al-Mg-Li-Zr alloys investigated is due to an increase in the density of δ′ precipitates (Al3Li).

Change in electrical resistivity on ageing for Al-Cu alloys containing Cd or In

Change of clectrical rcsistivity during ageing was measured on Al-Cu alloys containing Cd or In, and the other, further containing Mn as an additional element. Some differences were found in the effects of additional elements on change in electrical resistivity during ageing.It is well known that the addition of Cd, In, or Sn accelerates the rate of artificial ageing, but retards the rate of natural ageing. As expected from the above fact, the temperature at which abrupt decrease of resistivity took place in isochronal ageing was lower in ternary alloys than in binary. Electrical resistivity continued to fall up to about 310°C under the conditions of the present experiments and there was found no difference between the two kinds of ternary alloys (Al-Cu-In and Al-Cu-Cd alloys) in that range of temperature. Electrical resistivity change in Al-Cu-In alloys for isochronal ageing at above 310°C was similar to that in binary alloys, but was different from that in Al-Cu-Cd alloys. The increase in electrical resistivity occurred at above 310°C, but the degree of increase was larger in the latter than in the former. In the temperature range from 310 to 400 and tens of °C, electrical resistivity could be explained by the combined effect of an increase due to re-dissolution of intermediate phase and a decrease due to precipitation of equilibrium phase. Therefore, the results of the present experiments suggest that the formation of equilibrium phase would be more difficult in Al-Cu-Cd alloys than in Al-Cu-In alloys.At the early stage of isothermal ageing, an increase in electrical resistivity occurred at temperatures lower than 150°C in Al-Cu-In alloys. The above both temperature were rather lower than those claimed by Holmes et al. These temperatures can be elevated to some extent by the addition of Mn to the ternary alloys, though the rate of artificial ageing was retarded.