Reactive Phase Formation Research Articles

Reactive phase formation at AlN–Ti and AlN–TiAl interfaces

Ti specimens with embedded AlN particles, as well as AlN–Ti and AlN–TiAl diffusion couples were annealed at 900 to 1100°C for up to 40 h. The microstructure and phase composition of the reacted interfaces were characterized using electron probe x-ray microanalysis (EPMA) and electron backscatter diffraction (EBSD) in scanning electron microscope and x-ray diffraction (XRD). In AlN–TiAl diffusion couples, a ternary Ti2AlN compound was formed at the interface. A more complex AlN–TiN+Ti3AlN–Ti3Al–(Ti)Al–Ti reaction zone was observed at the AlN–Ti interface. The morphology of the two-phase (TiN+Ti3AlN) layer changed with time so that the ternary nitride dominated after longer exposures. The kinetics of the reaction zone growth was two-stage, with a significant decrease of the growth rate at the second stage. Such behavior is believed to be caused by the changing morphology of the (TiN+Ti3AlN) layer accompanied by decreasing diffusivity of the components.

Phase formation during nonisothermal decomposition of the freeze-dried Bi:Pb:Sr:Ba:Ca:Cu=1.8:0.4:1.8:0.2:1.2:2.0 complex nitrate powder

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu = 1.8:0.4:1.8:0.2:1.2:2.0 was obtained by spray-frozen, freeze-drying technique. Samples of the nitrate precursor powder were placed in a heated furnace (heating rate ∼100°C/min) and extracted in air when temperature of the powdered samples attained values of 439, 495, 550, 600, 640, 647, 717, 766, 814, and 850°C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

Initial Crystallization Reactions

AbstractDuring the initial stage of phase formation reactions nucleation conditions often apply a controlling influence on the phase selection, product phase number density and grain size. In most cases the operation of nucleation control is also associated with the development of significant undercooling or supersaturation in order to initiate crystallization. Under this constraint it is common to observe different forms of alloy metastability. In fact, a hierarchy of equilibria can be identified based upon the severity of the kinetic constraints that act during transformation. One consequence of the different levels of metastability is the development of precursor reactions before the initial crystallization. In other cases a kinetic competition between different reaction modes or structures can yield a change in the transformation pathway during reaction. The appearance of kinetic transitions is one consequence of the competition between concurrent reactions and provides a valuable opportunity for kinetics analysis and modeling. In cases where sequential reactions are separated by temperature or time intervals other kinetic constraints operate to expose metastability due to diffusional growth limitations. The identification of the reaction control provides the foundation for kinetics analysis and the development of alloy design strategies. These features can be considered in terms of the observed crystallization behavior during rapid solidification and solid state interface reactions.

Periodic pattern formation in solid state reactions related to the Kirkendall effect

The periodic layered morphology for solid state reactions is shown to occur in various ternary and higher order material systems including metal/metal and metal/ceramic diffusion couples. The experimental results on different systems where the periodic pattern formation has been observed are systematized and early explanations for this peculiar phenomenon are discussed. It is shown that a special class of structures periodic in time and in space can be considered as a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state.

Phase Reactions and Diffusion Path of the SiC/Cr System

Solid-state bonding at pressureless-sintered SiC has been carried out using 25-μm Cr foil at temperatures from 1373 to 1773 K for 1.8 ks in vacuum. The formation of reaction phases and microstructures at the interface between SiC and Cr was investigated by X-ray diffraction and microprobe analysis. At the bonding temperature of 1373 K the cubic Cr23C6phase formed next to Cr, and the hexagonal Cr7C3 phase formed next to SiC. At 1473 K the cubic phase Cr3SiCx appeared additionally on the SiC side. At 1573 K the complete diffusion path was established. Upon increasing the joining temperature beyond 1573 K all the chromium was consumed, and Cr23C6 and Cr3SiC x dissolved. A layered structure consisting of SiC/Cr5Si3 C x /Cr7C3/Cr5Si3 C x /SiC occurred.

Study of (Bi,Pb)-2223 phase formation in reaction couples

Reaction couples of either ((Bi,Pb)-2212) single crystals with `' powder or (Bi-2212) single crystals with `' powder were used to study the formation reaction of (Bi,Pb)-2223. The samples were annealed between and , in air, for 65 h. Microstructural observations were made of the different geometrical interfaces. No (Bi,Pb)-2223 platelets were formed below . Above , the (Bi,Pb)-2212 and (Bi-2212) crystals were found to break up near the interfaces with the powders, and (Bi,Pb)-2223 platelets were formed adjoining the (Bi,Pb)-2212 and (Bi-2212) crystallites. No evidence for large quantitites of liquids was found, although small, rounded particles having a liquid droplet appearance of a Bi/Pb rich phase of stoichiometry near the phase were observed on many of the (Bi,Pb)-2223 platelet surfaces. The experiments suggest that the formation of (Bi,Pb)-2223 occurs simultaneously with decomposition of (Bi,Pb)-2212 and the presence of liquid droplets, providing further support for the dissolution - reprecipitation model for phase formation.

High Resolution X-Ray Microanalysis of Nb/Al Multilayer Thin Films

Abstract A study of reactive phase formation in thin films requires the understanding of the thermodynamics and kinetics of the reactions. Previous investigations of phase formation in Nb/Al multilayers using calorimetry and x-ray diffraction have observed that despite the presence of rather large driving forces for phase formation, a pronounced nucleation event occurs. This behavior has been reported in other systems including Ti/Al and Ni/Al. The initial nature of the interface can affect the nucleation, therefore careful studies of the structure and composition of the initial interfacial region may explain the reason for a kinetically separated nucleation and lateral growth step preceding normal phase growth. In addition, examination of the interface may also lend insight into the effects of different deposition processes and can provide evidence for the formation of solid solutions or compounds that can lower driving forces available for phase formation. Finally, the cross-sectional multilayer sample provides a means of characterizing spatial resolution and isolating specimen preparation artifacts.

Reactive phase formation in sputter-deposited Ni/Al multilayer thin films

We have investigated reactive phase formation in magnetron sputter-deposited NiyAl multilayer films with a 1 : 3 molar ratio and various periodicities, L, ranging from 320 nm down to a codeposited film with zero effective periodicity. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that Ni and Al have reacted during deposition to form the B2 NiAl phase and an amorphous phase. The formation of these phases substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. Depending on the periodicity, these reactions result in the formation of NiAl3 or Ni2Al9 followed by NiAl3. Detailed calorimetric analysis reveals differences in the nucleation and growth behavior of NiAl3 compared with other studies.

In-Situ TEM Phase Formation in Cold Rolled Aluminum-Nickel Multilayers

ABSTRACTMultilayer samples of Nickel and Aluminum with an overall composition of Al-20Ni were prepared by cold rolling of elemental foils. The sample microstructures and phases were characterized by XRD, SEM and TEM/SAED, and the reactive phase formation was then examined by DSC measurements. XRD, SEM and TEM measurements show that the rolling procedure results in a decrease of the Al and Ni layer thicknesses (down to 100 nm in average) and a decrease of the grain size (down to less than 50 nm). No phase formation is observed during the cold rolling procedure. In isochronal DSC scans of the Al-Ni multilayers, the formation of the Al3Ni phase was found to be a two step reaction process due to 2-dimensional nucleation and lateral growth and a 3-dimensional phase thickening. While XRD measurements showed Al3Ni as the only phase that forms, more detailed TEM investigations of the samples after DSC treatment also showed a small amount of an amorphous Al-Ni phase, formed by a thermally activated solid state amorphization reaction (SSAR). In-situ TEM heating of the amorphous areas under the electron beam in the microscope yielded the crystallization of the amorphous phase to a B2 structure and a growth of the B2 grains up to 100 nm in size.

Characterization of reactive phase formation in sputter-deposited Ni/Al multilayer thin films using TEM

The subject of reactive phase formation in multilayer thin films of varying periodicity has stimulated much research over the past few years. Recent studies have sought to understand the reactions that occur during the annealing of Ni/Al multilayers. Dark field imaging from transmission electron microscopy (TEM) studies in conjunction with in situ x-ray diffraction measurements, and calorimetry experiments (isothermal and constant heating rate), have yielded new insights into the sequence of phases that occur during annealing and the evolution of their microstructure.In this paper we report on reactive phase formation in sputter-deposited lNi:3Al multilayer thin films with a periodicity A (the combined thickness of an aluminum and nickel layer) from 2.5 to 320 nm. A cross-sectional TEM micrograph of an as-deposited film with a periodicity of 10 nm is shown in figure 1. This image shows diffraction contrast from the Ni grains and occasionally from the Al grains in their respective layers.

Superconducting Tl-Pb-Ba-Sr-Ca-Cu-O(Ag) thick films (5-20 μm) prepared using a commercial spray pyrolysis system and 2-zone furnace annealing

A spray pyrolysis route to superconducting Pb-, Sr-, and Ag-substituted Tl-Ba-Ca-Cu-O thick films has been developed. First, a precursor powder with the stoichiometry Pb/sub 0.46/Ba/sub 0.40/Sr/sub 1.52/Ca/sub 1.86/Cu/sub 3.00/O/sub x/(Ag/sub /0/sub .37/) (PBSCCO) was synthesized with the particle size of this powder being subsequently reduced to 4-6 /spl mu/m using a ball mill. Next, this powder was mixed with ethyl cellulose binder and sprayed with an airbrush using an N/sub 2/ carrier gas, polycrystalline Y/sub 2/O/sub 3/:ZrO/sub 2/ (YSZ) as single-crystal MgO, SrTiO/sub 3/, NdGaO/sub 3/, and LaAlO/sub 3/ substrates were mounted on a hot plate with typical growth temperatures of 80-100 /spl deg/C. Growth times of /spl sim/10 min provided films which were 5-20 /spl mu/m in thickness. After an intermediate O/sub 2/ anneal to remove the organic binder, these films were subjected to a flowing 2-zone thallination process. Very well c-axis oriented Tl/sub 0.70/Pb/sub 0.36/Ba/sub 0.39/Sr/sub 1.58/Ca/sub 1.98/Cu/sub 3/O/sub x/ (TP-1223) phase material was obtained for films grown on single-crystal LaAlO/sub 3/. Films grown on all other substrates surveyed in the study gave rise to impurity phase formation and/or incomplete reaction. The PBSCCO and TP-1223 films were characterized by /spl theta//2/spl theta/ and /spl omega/-rocking curve X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), variable temperature magnetic measurement, and inductively coupled plasma atomic emission spectroscopy (ICPAES). >

Reactive Phase Formation in Thin Films: Evolution of Grain Structure

AbstractIt is a well known fact that the properties and performance of polycrystalline materials, including polycrystalline thin films, are strongly affected by the grain structure. Therefore, in treating reactive phase formation in these films, it is (or it will inevitably be) necessary to quantify the grain structure of reactant and product phases and its evolution during the course of the reaction. Theoretical models and the conventional view of thin film reactions, however, have been largely extensions, to small and finite dimensions, of theories and descriptions developed for bulk diffusion couples. These models and descriptions primarily focus on the growth stage and to a much lesser extent on the nucleation stage. Consequently, these models and descriptions are not able to treat the development of product phase grain structure. Recent calorimetric investigations of several thin film systems demonstrate the importance of nucleation kinetics (and hence nucleation barriers) in product phase formation and provide quantitative measures of the thermodynamics and kinetics of formation of the product phases, thereby allowing some degree of comparison with reaction models. Furthermore, microstructural investigations of thin-film reactions demonstrate the non-planarity of the growth front and highlight the role of reactant-phase grain boundaries. In this paper, a summary of these experimental studies and recent theoretical treatments, which combine nucleation and growth in an integrated manner, is presented, with particular emphasis on the Nb/Al system. These experiments and models lead to a new view of reactive phase formation and grain structure evolution as one in which the latter is an integral part of the former. Based on this view, directions for future research are discussed.

Reactive Phase Formation in Sputter-Deposited Ni/Al Thin Films

ABSTRACTWe have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer thin films with a 3:1 molar ratio and periodicities ranging from 2.5-320 nm. In addition, we studied the transformation of a codeposited film of the same composition. We find that an amorphous phase has already formed during deposition, and that the extentof formation of this phase increases with decreasing periodicity. The first crystalline phase then nucleates from this amorphous phase upon annealing. The formation of the amorphous phase considerably reduces the driving force and explains why during subsequent reactions nucleation kinetics become important. We obtain Ni2Al9 as the first product phase during heat treatment in some cases before NiAl3 occurs. For films with modulation periods larger than 40 nm, formation of NiAI3 is a two stage process as reported earlier, with the first stage being due to nucleation and growth to coalescence of NiAl3 grains, and the second stage being the growth of NiA13 normal to the initial interface until the reactant phases are consumed.

Theoretical Considerations about Phase Growth and Phase Formation

AbstractThe first part of this paper constitutes a summary account of reactive phase formation as seen from a purely kinetic point of view. In the longer second part a more detailed analysis of phase formation based on microscopic, and calorimetric observations is presented. It is shown that these phenomena also tend to be dominated by kinetic factors. This explains why purely kinetic considerations (which should always prevail in any case at some point) usually provide a fairly accurate account of the growth of new chemical phases forming on a solid substrate, regardless of the physical state (solid, liquid or gas) of the second reactant.

Amorphous and Crystalline Phase Formation in Ni/Al Multilayer Thin Films

ABSTRACTWe have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer films with a 1:3 molar ratio and various periodicities ranging from 320 nm to a codeposited film with an effective periodicity of zero. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that a reaction which results in the formation of an amorphous phase has taken place during the multilayer deposition process. This reaction substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. The mode of transformation for a film with 10 nm periodicity was investigated, in detail, by applying the Johnson-Mehl-Avrami analysis to data obtained from isothermal and constant heating rate differential scanning calorimetry, in combination with electron microscopy studies of the transformation microstructure.

The effects of sulphurizing agent on the formation of calcium lanthanum sulphide

For calcium lanthanum sulphide (CLS) formation, the effect of the sulphurizing agent is studied. CS 2 and sulphur are used as sulphurizing agents to sulphurize the lanthanum and calcium carbonate precursors which are formed by a coprecipitated method. With CS 2 as a sulphurizing agent, the intermediate product formed is LaS 2 whereas when sulphur is employed, the La 2O 2S was the intermediate product. If LaS 2 is formed first, it is not difficult to complete the sulphurizing reaction for CLS at a low temperature (700°C) for a short soaking time (3 h). However, if the La 2O 2S is formed first, the phase formation reaction of CLS should be completed at a high temperature (800–1050°C) for a long soaking time in order to dissociate the LaO bond.

Reactive Phase Formation in the Diffusion Zone between Si<sub>3</sub>N<sub>4</sub> and Ni-Cr Alloys

The interaction between dense Si3N4 and Ni-Cr-alloys at 1398K was investigated. Reactive phase formation can be explained by assuming a N2-pressure at the interface. To understand the thermodn. and diffusion kinetics in this system direct nitriding of Ni,Cr(Si)-alloys from the N2-gas atm. was performed and a thermodn. description of the nitrogen behavior was attempted by using the Thermo-calc databank system. The \up hill diffusion of nitrogen towards the front of nitride ppts. can be predicted. [on SciFinder (R)]

Experimental Investigation of Reactive Phase Formation at Interfaces by Ion Beam Techniques

Experimental Investigation of Reactive Phase Formation at Interfaces by Ion Beam Techniques

Investigating the Role of Grain Boundaries in Interface Reactions

ABSTRACTThe paper is built around a nonexhaustive review of the literature on the role of grain boundaries in reactive phase formation. Examples are chosen to illustrate these effects in silicide and oxide growths, and later on in metal-metal interactions. A short section deals with the effect of grain boundaries and grain boundary adsorption of impurities on the kinetics of growth and on the morphology of the growing layer. Some attempts at understanding the mechanisms of phase growth from the tracking of isotopes are briefly analyzed.

Preparation and characterization of mullite-zirconia composites from various starting materials

Abstract Mullite-zirconia composites were prepared from three kinds of combination of starting materials such as mullite-zirconia (MM), alumina-silica-zirconia (RS3) and alumina-zircon (RS2). Phase formation reaction, sinterability and microstructures of these samples were investigated. Sinterability was good in all the samples and they reached almost full density by firing at around 1520°C. The microstructure of the MM sample was composed of two different types of mullite grains, which were elongated large grains and equiaxed small grains, and intergranularly dispersed zirconia grains. That of the RS3 sample was composed of only equiaxed mullite grains and intergranularly dispersed zirconia grains whereas that of the RS2 sample was composed of irregularly shaped mullite grains and round-shaped zirconia grains, which were distributed intragranularly and intergranularly. The chemical composition of mullite was examined from the length of the lattice parameter of the a-axis. Only mullite in the RS2 samples was found to be apparently richer than 60 mol% in alumina composition, but it approached to 60 mol% Al2O3 by annealing at 1570°C.