To save this undefined to your undefined account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you used this feature, you will be asked to authorise Cambridge Core to connect with your undefined account.
Find out more about saving content to .
To save this article to your Kindle, first ensure email@example.com is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
It is demonstrated that some twenty representative free boundary diffusion-reaction and hydrodynamic patterning systems fall within the compass of the isothermal variational principle where the Lagrangian density is the free energy density g, and η is an appropriate order parameter. To illustrate the universality of the formulation we survey in some detail its degree of equivalence to twenty-one currently adopted or promoted procedures for resolving pattern degeneracies, and critically reappraise theoretical descriptions of grain growth, ordinary diffusion, spinodal decomposition and Ostwald Ripening and their predictive success. The importance of a hierarchy of kinetic scaling laws is discussed. The physics of fluctuations is essential to understanding.
Phase coarsening, also known as Ostwald ripening, is generally treated as a growth phenomenon in which the average particle size increases during isothermal aging. However, coarsening is a relaxation process driven by a reduction in the excess interfacial energy of a two-phase structure. This viewpoint introduces a single temporal offset parameter that relates the experimental clock to the asymptotic time scale. Analysis of ripening as a relaxation phenomenon permits accurate determination of the kinetics that is less sensitive to particle morphology. Implications of this approach on measurement and interpretation of ripening kinetics will be discussed.
The effects of long-range elastic fields on the phase separation process of three- dimensional binary alloy systems was investigated with large-scale Langevin simulations. The elastic effects incorporated in the model are the result of anisotropy and dilational misfits introduced via inhomogeneities in the elastic constants of the constituents. The domain morphology obtained is readily understandable in terms of selection criterion for the shape and/or orientation of the domains, and is based on the different shear moduli that are present in the system. Coarsening mechanisms were found to be a combination of the classical Ostwald ripening mechanism and the elastically-driven coalescence of domains. Other aspects of the coarsening process such as dynamic scaling of the structure function is presented.
This paper presents a comparison between the computer simulation dissolution model, DICTRA, and an experimental data set from the literature, Baty, Tanzilli and Heckel, 1970. This was undertaken to support research on the modeling of the weld heat-affected zone where grain growth and Zener pinning control materials properties. BTH present quantitative-microscopy data on the dissolution of 5 size classes in their 4.2μ particle sample and 4 size classes in their 0.44μ sample. The DICTRA example of the 3-cell calculation of cementite particles in steel was used as the model to simulate the BTH experiment.
An atom probe tomography characterization of the early stages of phase separation in the FeCu-Ni system has been performed. The development of the copper-enriched regions into copper precipitates has been investigated during isothermal ageing at 573 K and 673 K for times up to 10,000 h. The formation and growth of the secondary precipitates in the nickel based superalloy, Alloy 718, have been determined during isothermal ageing at 873 K. These secondary precipitates were found to be a mixture of the L12-ordered γ' phase and the DO22-ordered γ” phase.
When two alloys are placed in contact to form a diffusion couple, the microstructure that forms on heating is a function of alloy composition. This function can be illustrated on a phase diagram as an “Interdiffusion Microstructure Map” (IMM). An IMM was determined in the present work for Ni-Cr-Al diffusion couples at 1200°C. The couples consisted of one γ+γ' alloy bonded to a number of γ+β alloys. The IMM contained five fields corresponding to five different microstructures. The fields met at a five-line node, giving the map an unexpected topology. The IMM topology was deduced by considering the transition microstructures and kinetic equations that lead to the formation of interdiffusion microstructures.
The nucleation, growth and dissolution of lamellar precipitates formed due to discontinuous solid state reactions like: discontinuous precipitation, coarsening and dissolution, are reviewed. Emphasis is given on recent studies based on analytical electron microscopy in describing the microchemistry, and in situ electron microscopy for revealing the morphological features of the reactions.
Copper is a prevalent contaminant in silicon device fabrication. Since Cu has high solubility in Sn, we are exploring the use of Sn to form Cu-Sn surface alloys as a means to inhibit Cu diffusion into Si substrates. Cu/Sn/Si (111) samples were prepared by MBE at 100°C, then annealed at temperatures between 300°C and 600°C. Resultant surface structures were imaged by AFM and SEM. AES and EDX were used to investigate the composition of two differently shaped clusters that formed after annealing. Etching with dilute HCI solutions was used to seek the chemical origin of various surface morphologies. The results are compared with those from Sn/Si and Cu/Si samples. A possible model for the evolution of 3-D structures in the Cu/Sn/Si(111) system will be presented.
In-situ small-angle neutron scattering was performed on Ni-(10 to 12) at.% Ti polycrystals and single crystals at temperatures between 870 and 1270 K. During decomposition metastable precipitates of cuboidal shape form with preferred alignement along <100>. The Ti concentration of these precipitates is 17(1) at.% (between 870 and 950 K). Metastable precipitates precede the formation of platelets of the stable η phase even outside the hitherto accepted metastable miscibility gap.
Titanium aluminide alloys based on the intermetallic γ (TiAl) and α2 (Ti3Al) phases are being considered as light-weight materials for high-temperature applications in advanced energy conversion systems. However, for such applications the material suffers from insufficient creep resistance at the intended service temperature of 700°C. The paper reports an electron microscope study of diffusion controlled mechanisms which apparently cause the degradation of the strength properties. The processes lead to significant structural changes involving the formation of extended ledges and recrystallization. The driving forces of these mechanisms probably arise from non-equilibrium phase compositions and significant coherency stresses occurring at the interfaces.
A multilayer sample of amorphous SiC (α-SiC) and polycrystalline Ni was prepared by ionbeam sputtering, and was used as a model, prototype system to study the stability of metal contacts with SiC against interdiffusion reactions near the interface. The modulation wavelength is around 80 nm and with equal thickness of α-SiC and Ni layers. By use of XRD, TEM and energy dispersive spectroscopy (EDS), information on the structural evolution and composition distribution during heat treatment was obtained. The diffusion of Ni into the SiC layer, and Si and C into the Ni layer appears to take place concurrently during the annealing process. An amorphous reaction layer was formed during the heat treatment that is distinct from the α-SiC. The increase in thickness of the reaction layer followed a parabolic time dependence in the initial stage, until further reaction was limited by the precipitation of a graphite layer near the original α-SiC/Ni interface. A metastable intermediate phase and a NiSi phase were identified in the original Ni layer due to the diffusion of Si and C based on the decomposition of the α-SiC.
A growth model starting from a surface and in presence of mobile nonreactive localized impurities has been studied for the statistical physics of the microstructure and the surface roughness through simulation. The model is based on the Eden model. The microstructure is described at the mesoscopic (particle) scale level. A dynamical repulsion effect on the front with the mobile particles has been investigated numerically. The presence of such mobile particles shifts the usual percolation transition for three-dimensional (3D) systems from 0.65 corresponding to static hindrances, to the value of 0.8. The particle pushing leads to an aggregation phenomenon with self-organizing particles near and after the interface. This leads to the formation of front facets, competition between faces of different orientations and surface roughening.
The internal stress of TiCux-alloy films on alumina substrates was measured in situ under UHV-conditions as a function of substrate temperature and stoichiometry using a cantilever beam technique.
The stress vs. thickness curves of alloy films with TiCu0.5, TiCu, TiCu2 and TiCu3 stoichiometry indicate that the film growth is amorphous at 130°C and polycrystalline at 350°C substrate temperature, which was confirmed by TEM-investigations. Furthermore TED-results of alloys deposited at 350°C imply that the TiCux-alloy-phases formed at that temperature are Ti2Cu, γ-TiCu, TiCu3 and TiCu4.
The influence of substrate temperature on the growth of TiCux-alloy films (x = 1) was investigated in more detail. At substrate temperatures below 300°C the stress vs. thickness curves indicate the formation of amorphous alloy films, again confirmed by TEM- and TED-analysis. At substrate temperatures around 300°C a strong tensile stress contribution above a mean thickness of about 20 nm indicates a change in the growth mode and thus microstructure of the alloy film. The corresponding TEM- and TED-micrographs show quasi single-crystalline films. Finally at 350°C substrate temperature the stress vs. thickness curves indicate once again a change in the film growth mode, the film structure is polycrystalline and the formation of γ-TiCu is deduced fore TED-results.
The mechanisms of eutectoid decomposition are important because of the widespread use of alloys which exhibit a eutectoid transformation. However, the transformation remains a matter of controversy for a number of reasons. The Ti-Co system is examined in order to test current understanding of eutectoid decomposition.
The high temperature beta phase can transform on cooling into several products - alpha plates, a non-lamellar mixture of alpha and Ti2Co which can be designated bainite, lamellar pearlite, spheroidal pearlite and Ti2Co allotriomorphs. In this study the high temperature beta phase was decomposed in hypoeutectoid, eutectoid and hypereutectoid alloys, for a range of undercoolings. The microstructure and crystallography of the transformation products was characterised by reflected light microscopy, SEM, XRD and TEM. The classification of the transformation products is discussed.
System (PbI2)x – (BiI3)1-x of two layered semiconductors PbI2 and BiI3 with various composition were synthesized and grown both as bulk single crystals (by vertical Bridgman method) and films (by thermal vacuum deposition). Grown composites were tested by different techniques (X-ray difflaction analysis, optical transmission, I – V measurements, response to X-ray pulse excitation). Property versus composition showed nonlinear behavior of measured parameters. The structural model of ordered structures formation in this system, which was based upon local electrical neutrality of unit cells, was worked out. It has been experimentally demonstrated that a formation of ordered structures is possible within (PbI2)x-(BiI3)1-x system, which is consistent with the suggested model. The possibility of crystal structures formation as ordered sets of BiI3 and PbI2 layers was also checked.
The influence of Fe on the microstructure of Al2O3-MgO-Fe2O3 spinel is described in this work. Heating a mixture of initial oxides in an electric furnace at 1400°C for 15 hrs produced the spinel. A ternary diagram for concentration ranges: Al2O3 (5-75 mol%), MgO (10-80 mol%), and Fe2O3 (3-70 mol%) summarize the constitution of this spinel. Amount, composition and weight ratios of each phase were determined by X-rays powder diffraction.
Microstructure of specimens was characterized by scanning electronic microscopy and Xray spectrometry. Distribution, size, shape of phases and pores were examined. Hardness measurements of spinel completed this study.
Direct current (d.c.) magnetron sputtering was applied to polycrystalline Si growth and yielded a uniform Si film at a deposition temperature below the glass softening point. The approach involved the deposition of a thin Ni film over SiO2 prior to Si sputtering. The interaction of a fine-grained metallic Ni with an atomic Si provided by a sputtering gun resulted in the formation of Ni silicide at the Ni-Si interface immediately after the onset of Si deposition. The phase composition of the nickel silicide was controlled by the temperature of deposition and the Si-to-Ni concentration ratio. The silicide region exhibited a layered structure where the bottom layer consisted of the mixture of several NixSiy phases and the top layer represented the pure NiSi2 phase. This Ni disilicide layer provided the nucleation sites for the epitaxial Si crystal growth. As a result, the polycrystalline silicon film consisted of 0.1-0.5µm grains with preferred (11) or (110) orientation without an indication of an amorphous phase. The carrier lifetime of 11 µs indicated good electrical properties which make the film potentially applicable to fabrication of thin film transistors and solar cells. The Ni prelayer thickness in the 5-100nm range dramatically influenced the crystal size and preferential crystal orientation. Possible mechanisms responsible for the nickel silicide induced grain growth of silicon and the correlation between the Ni silicide and silicon microstructure is discussed.
A multilayer structure of Co/a-Si/Ti/Si(100) together with Co/Ti/Si(100) is applied to investigate the process and mechanism of CoSi2 epitaxial growth on a Si(100) substrate. The experimental results show that by adding an amorphous Si layer with a certain thickness, the epitaxial quality of CoSi2 is significantly improved. A multi-element amorphous layer is formed by a solid state amorphization reaction at the initial stage of the multilayer reaction. This layer acts as a diffusion barrier, which controls the atomic interdiffusion of Co and Si and limits the supply of Co atoms. It has a vital effect on the multilayer reaction kinetics, and the epitaxial growth of CoSi2 on Si. The kinetics of the CoSi2 growth process from multilayer reactions is investigated.
By analogy to reactive deposition epitaxy and titanium interlayer mediated epitaxy experiments, an attempt has been made to constrain the supply of reactants to the reaction interface in the solid phase reaction between Fe and Si. The goal being to change the normal phase formation sequence by using a suitable diffusion barrier, so that β-FeSi2 forms directly. Both Fe-V and Fe-Zr diffusion barriers were used to constrain the supply of the two reactants during Fe-silicide formation. Measurements with these barriers, show first phase formation of β-FeSi2, but direct formation of 3-FeSi2 as first phase has not been observed. In the case of the Fe-V diffusion barrier it was shown that the use of the diffusion barrier resulted in smoother layers of β-FeSi2 than could be formed by direct reaction of Fe on Si. In the case of the Fe-Zr barrier it is found that the barrier fails structurally at high temperatures. While it does prohibit Fe diffusion at low annealing temperatures, significant Si diffusion occurs prior to ε-FeSi formation.
We discuss the rather scattered measurements of the lattice parameters for C49 TiSi2, which are reported in literature, along with new and accurate X-ray diffraction measurements and ab-initio calculations. Both agree in indicating that the density of the metastable C49 structure cannot be much smaller than the one for the polymorphic C54 phase, as it is commonly reported. We conclude by demonstrating that only in the case of such a smaller difference in density between the two phases, the elastic strain contribution to the nucleation energy of the C54 structure in the C49 matrix can be neglected. The estimation of the critical radius strongly depends on this issue.