Structure Of Interphase Boundaries Research Articles

The role of inclusion-matrix boundaries in steels fracture processes

The features of the fracture of the interphase inclusion-matrix boundaries under various thermal-deformation and aggressive actions are investigated. The reasons and conditions for the formation of cracks as a result of decohesion of the inclusion-matrix boundaries associated with the concentration of interfacial thermal and deformation stresses are discussed. It was found that the character of decohesion of the inclusion-matrix boundaries (rupture of interatomic bonds of a nonmetallic inclusion and a steel matrix) caused by interphase thermal stresses is due to the conditions of heating and cooling, depending on the type of processing (conventional heating, laser or electromagnetic treatment), which determines the possibility of passing relaxation processes associated with the redistribution of interfacial defects within the specified boundaries. The mechanisms and features of decohesion of the interphase inclusion-matrix boundaries at different methods and temperatures of deformation are discussed. It is shown that the decomposition of the inclusion-matrix boundaries upon deformation occurs in cases of certain types of nonmetallic inclusions (oxides, spinels, some sulfides). For different types of inclusions and steels, parameters have been determined that characterize the cohesive strength of the inclusion-matrix interface depending on the loading conditions. It is shown that decohesion (fracture) of the inclusion-matrix boundary into two free surfaces can be considered as the transformation of interphase misfit dislocations into surface steps as a result of loss of conjugation. The role of inclusion-matrix boundaries in the formation of cracks of fatigue and fatigue-corrosion origin is considered. It is shown that under the influence of aggressive media and cyclic stresses, the structure of the inclusion-matrix interphase boundaries degrades, which is associated not only with the accumulation of interfacial stresses, but also with the facilitation of the penetration of surfactant atoms from the environment along these boundaries. As a result, fatigue-corrosion destruction of the inclusion-matrix boundaries occurs, and the effect of an adsorptive decrease in their strength is manifested.

Coexistence of phases and interphase boundaries in BaTiO3

A theoretical analysis of the internal structure of interphase boundaries separating domains of coexisting phases is presented for the perovskite ferroelectric BaTiO3. The temperature dependence of interphase boundary widths and surface energies are calculated and compared with the corresponding parameters for different types of domain walls existing within the ferroelectric phases of BaTiO3.

Use of explosion energy for producing consolidated metal-ceramic materials based on refractory carbides

The article presents the basic laws of chromium carbide and silicon carbide powder mixtures compaction with titanium binder by explosive pressing. The conditions that provide the density of compactions close to the density of a monolithic material are revealed. The basic regularities of the formation and features of the fine structure of interphase boundaries in powder materials based on chromium carbide and silicon carbide with titanium binder are investigated, the conditions for consolidated hard alloys obtaining at the pressing stage are identified.

Molecular dynamics simulation of migration behavior of FCC-BCC atomic terrace-step phase boundary in iron-based alloy

The martensitic transformation between the high-temperature face-centered cubic (FCC) phase and the low-temperature body-centered cubic (BCC) phase in iron-based alloys has been studied for years, which plays a critical role in controlling microstructures and hence properties of the alloys. Generally, the BCC structure martensitic phase forms from the FCC parent phase, involving a collective motions of atoms over a distance less than the interatomic distance in the vicinity of the interphase boundary. Thus the structure of interphase boundary separating the FCC and BCC phases is the key characteristics to quantitatively understanding the mechanism and kinetics of martensitic transformation. Due to the difficulty in observing the atomic motions taking place at a velocity as high as the speed of sound, the experimental investigation on the migration of FCC/BCC interphase boundary during the transformation is as yet limited. Noteworthily, molecular dynamics (MD) simulation has been applied to studying the martensitic transformation, in particular for investigating the mobility of the FCC/BCC interphase boundary in iron. However, in most of the MD studies the atomistically planar interfaces of {111}<sub>FCC</sub> // {110}<sub>BCC</sub> are considered as the initial configuration of the interphase boundary between FCC and BCC phases, which is in contradiction to the high-resolution TEM observations. In fact, the FCC/BCC interphase boundary, which is known as the macroscopic habit plane, is a semi-coherent interface consisting of several steps and terrace planes on an atomic scale. In the present work, the atomic configuration of a terrace-step FCC/BCC interphase boundary of iron is built in terms of the topological model. The MD simulation is conducted to clarify the mechanism of interphase boundary migration in the FCC-to-BCC transformation. The results show that the FCC/BCC boundary migrates along its normal at the expense of FCC phase as a result of the lateral motions of the transformation dislocations. Meanwhile, the interphase boundary maintains the stable terrace-step structure during the transformation. Further examinations reveal that the transformation dislocations move steadily at a velocity as high as (2.8 ± 0.2) × 10<sup>3</sup> m/s, affecting the migration of the interphase boundary with a constant velocity of about (4.4 ± 0.3) × 10<sup>2</sup> m/s. The effective migration velocity of FCC/BCC interface exhibits dynamic properties consistent with the characteristic features commonly observed in a displacive martensitic transformation. Additionally, the motion of transformation dislocations gives rise to the macroscopic shape strain composed of a shear component <inline-formula><tex-math id="M3">\begin{document}$ {\varGamma _{{\rm{yz}}}} = 0.349$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.png"/></alternatives></inline-formula> parallel to the boundary and a dilatation <inline-formula><tex-math id="M4">\begin{document}$ {\varGamma _{{\rm{zz}}}} = 0.053$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.png"/></alternatives></inline-formula> normal to the boundary in the MD simulation, which is close to the crystallographic calculations by the topological model.

Creation of Ceramic Composites by High Temperature Oxidation of Iron and Nickel Alloys Using Oxidative Constructing Approach

High temperature oxidation of iron and nickel alloy blanks is characterized by the formation of bilayer ceramic and cermet structures, holding the original shape of the metal blank. The composition of the resulting ceramic depends on the temperature and synthesis time and the ratio of components with different sensitivity to oxygen in the initial alloy. Dwell at a low temperature leads to stabilization of the composition of the synthesized ceramic within the outer layer owing to the concentration leveling of the spinel phase. The outer and inner layers are characterized by different morphology of the chip surface and different porosity. The inner polycrystalline layer is porous; the outer layer of the sample is monolithic. The structure of interphase boundaries of the heterophasic sample ensures the integrity of the material and provides high adhesion properties of different phases to each other. The resulting oxidative constructed composites of ferrous alloys with nickel are promising for testing as inert anodes.

Atomic Structure of Domain and Interphase Boundaries in Ferroelectric HfO2

AbstractThough ferroelectric HfO2 thin films are now well characterized, little is currently known about their grain substructure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and phase interconversion. Here, scanning transmission electron microscopy is applied to investigate the atomic structure of boundaries in these materials. It is found that orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic interphase boundaries form throughout individual grains. The results inform how interphase boundaries can impose strain conditions that may be key to phase stabilization. Moreover, the atomic structure near interphase boundary walls suggests potential for their mobility under bias, which has been speculated to occur in perovskite morphotropic phase boundary systems by mechanisms similar to domain boundary motion.

High-pressure torsion-induced phase transformations and grain refinement in Al/Ti composites

High-pressure torsion (HPT) deformation of multiphase metallic systems produces a high density of interfaces and leads to atomic mixing between the constituent phases. Here we present a study of the interphase boundary structure, grain size evolution and intermetallic phase formation during HPT deformation of a nano-crystalline Al/Ti composite. High-resolution transmission electron microscopy was used to study the structural features of the interphase boundaries. The Al/Ti interphase boundaries were found to significantly promote the generation of dislocations during deformation. After HPT deformation to a shear strain of 87, the average grain sizes of Al and Ti are 22 and 31 nm, respectively. The chemical mixing between the Al and Ti phases was enhanced by defect-mediated short circuit diffusion and dislocation shuffle-controlled plastic deformation at the interphase boundaries. The intermetallic phases formed during HPT deformation are associated with the strain energy stored by the high density of dislocations at the interphase boundaries.

Microstructure of NbTi superconducting alloy

A superconducting multifilament wire made of Nb-60 at.% Ti alloy was investigated by methods of atom probe/field ion microscopy (APFIM), three-dimensional atom probe (3DAP), high resolution electron microscopy (HREM) and computer simulation. The results of the APFIM and HREM analysis suggest, that the specimens contain two phases, a bcc structure (β-phase), and a hcp structure (α-phase). The mesascopic and nanoscale heterogeneities of element concentration in *-phase were observed. Atom probe analyses of annealed NbTi superconductors and a statistical processing of the chains of atoms, consistently registered by mass-analyses, indicated that no atomic clusters were formed in alloy prior to the onset of the second phase formation. The β-phase and niobium enriched phase have the configurations extended along the wire axis, the nanoscale heterogeneity inside the β-phases was nearly isotropic. An atomic structure of interphase boundaries has been determined using HREM and atomistic structure calculations. Interphase boundaries in Nb-Ti alloy are coherent or semi-coherent in spite of the existence of the irregular microprotrusions.The boundaries between α- and β-phases reveal the absence of the rigid-body translation and continuity of lattice planes across the interface. It was demonstrated that the method of simulation in reciprocal space is computationally efficient for investigations of the atomic structure and energy of coherent and non-coherent interphase boundaries. The extremely high critical current density in the optimized Nb-Ti can be due to the coherent structure of interphase boundaries.

Fine structure of interphase boundaries in hard alloys of the chromium carbide–titanium system

The structure of interphase boundaries in hard alloys fabricated by the explosive compacting of the powder mixtures of chromium carbide (Cr3C2) and titanium is investigated. It is established by electron microscopy that similar boundaries have thicknesses of about 100 nm, over the extension of which a smooth variation in chromium and titanium contents is observed at the almost identical carbon concentration. The boundary structure is nonuniform over the thickness, notably, a layer with a thickness of 5–7 nm and an amorphous structure is revealed between two crystalline interlayers. It is shown that the revealed layers are the layers of specific “boundary phases” not corresponding to any phase of the equilibrium phase diagram of the Cr–C–Ti system.

Diffusion-controlled growth rate of stepped interfaces.

For many materials, the structure of crystalline surfaces or solid-solid interphase boundaries is characterized by an array of mobile steps separated by immobile terraces. Despite the prevalence of step-terraced interfaces a theoretical description of the growth rate has not been completely solved. In this work the boundary element method (BEM) has been utilized to numerically compute the concentration profile in a fluid phase in contact with an infinite array of equally spaced surface steps and, under the assumption that step motion is controlled by diffusion through the fluid phase, the growth rate is computed. It is also assumed that a boundary layer exists between the growing surface and a point in the liquid where complete convective mixing occurs. The BEM results are presented for varying step spacing, supersaturation, and boundary layer width. BEM calculations were also used to study the phenomenon of step bunching during crystal growth, and it is found that, in the absence of elastic strain energy, a sufficiently large perturbation in the position of a step from its regular spacing will lead to a step bunching instability. Finally, an approximate analytic solution using a matched asymptotic expansion technique is presented for the case of a stagnant liquid or equivalently a solid-solid stepped interface.

Fine Structure of Interphase Boundaries in Hard Alloys of the Chromium Carbide–Titanium System

The structure of interphase boundaries in hard alloys fabricated by explosive compacting of the powder mixture of chromium carbide (Cr 3 C 2 ) and titanium is investigated. It is established by electron microscopy that these boundaries have a thickness of the order of 100 nm, along which, a smooth variation in the chromium and titanium content with the almost constant carbon content is observed. The structure of boundaries is nonuniform over the thickness: a layer 5–7 nm thick with an amorphous structure is detected between two crystalline interlayers. It is shown that these layers are the layers of specific «boundary phases» matching none phases of the equilibrium phase diagram of the Cr–C–Ti system.

Structure of boundaries in composite materials obtained using explosive loading

We have presented the results of studying the fine structure of interphase boundaries for a number of composite materials obtained by methods of explosive welding and explosive compacting of powder mixtures. Joints of different metals (titanium-low-carbon steel, copper-tantalum) and metals with refractory carbides (chromium carbide-titanium) have been investigated. Under welding, pairs differed from each other by the type of interaction. It has been found that, in these composites, interphase boundaries exhibit a final thickness on the order of 200 nm, throughout which the composition of the material changes gradually from a composition that corresponds to one of the components of the composite to a composition that corresponds to the second component. It has been shown that the structure of interphase boundaries is complex. With the limited solubility of components along boundaries, two fairly thick crystalline interlayers are detected, the total thickness of which is equal to the total thickness of the boundary; between the interlayers, there is a thin (to 5–7 nm in thickness) interlayer with a crystalline or amorphous structure.

Structure and stability of the interface between magnetic and conventional fluids. Model of a three-component medium

The structure of a flat interphase boundary between a magnetic suspension and a conventional immiscible fluid is investigated within the framework of the model of a three-component medium taking the dependence of the free energy of the system on the concentration gradients into account. It is shown that for certain values of the constitutive parameters the bulk magnetic particle concentration increases significantly inside the interfacial layer, i.e., the particles are significantly adsorbed on the interface. The dependence of the surface tension on the magnetic field strength is determined. It is shown that for certain problem parameters this dependence qualitatively corresponds to that obtained experimentally and described in the phenomenological theory developed by Golubyatnikov and Subkhankulov in 1986. In the case of strong particle adsorption the dependence of the surface tension on the magnetic particle concentration on the phase interface is significantly nonlinear. A refined model of the interface as a two-dimensional continuum with surface magnetization is constructed. Constitutive equations, conditions on the interface, and necessary stability conditions are obtained.

On the formation of a refractory skeleton in composite materials. A review

Papers on theoretical and experimental investigations into the formation of a refractory skeleton in composite materials have been analyzed. It has been shown that the critical parameters that describe the refractory skeleton are the contiguity of particles and dihedral angle. In the theory of the formation of a refractory skeleton certain results have been achieved in the establishing of interrelationship between the values of dihedral angle, contiguity, concentration of phases, and the conditions for the skeleton formation. The composition and structure of interphase boundaries of the refractory skeleton have been described and the thickness of the particles contact zone has been substantiated. Various technologies that allow one to actively influence the formation of the refractory skeleton have been considered and ways of the further investigations in the field have been suggested.

Generalized Ginzburg-Landau functional calculations of the structure and energy of the coherent interphase boundaries between austenite and martensite in iron

The earlier-developed methods and experimental data on the phonon spectra and thermodynamics of fcc and bcc iron are used to construct a generalized Ginzburg-Landau functional for microscopic investigations of the austenite-martensite phase transformations in iron. This functional is used to calculate the structure and properties of a plane austenite-martensite interface for arbitrary orientations of this interface at various temperatures. The transformation parameter profiles in the interface region are calculated. The interface width is found to substantially exceed the interatomic distances. The interfacial surface energy is 500–800 erg/cm2, depends strongly on the orientation, and has a sharp maximum when the interface is parallel to a close-packed plane.

The composition and structure of interphase boundaries of III–V semiconductors formed in a liquid medium

By the example of indium antimonide and gallium and indium arsenides, a possible mechanism of the formation, composition, and structure of a surface phase layer and a near-surface (transition) crystalline layer with a disturbed structure is considered for semiconductor compounds after chemical treatment. Assuming the maximum allowable deviation from the stoichiometric ratio of the components within the homogeneity range of compounds and diffusion, a possible thickness of the crystalline near-surface layer is estimated. The theoretical calculations were experimentally verified using X-ray diffraction.

Interphase boundary structure and accommodation mechanism of lenticular martensite in Fe–Ni alloys

This study investigates the interphase boundary structures of lenticular martensites in Fe–33Ni and Fe–31Ni, and discusses the accommodation mechanism for transformation strain. Despite the large difference in the macroscopic morphology of the interphase boundary, the nature of interfacial dislocations in both alloys is almost the same; there are two sets of screw dislocations with a/2 [ 1 ¯ 1 ¯ 1] M and a/2 [1 1 ¯ 1] M Burgers vectors on the interphase boundary. On the basis of the array of interfacial dislocations, the shear planes of dislocations with a/2 [ 1 ¯ 1 ¯ 1] M and a/2 [1 1 ¯ 1] M Burgers vectors were determined to be (1 1 2) M (=(1 0 1) A) and ( 1 ¯ 1 2) M (=(0 1 1) A). Two sets of interfacial dislocations accommodate the atomistic misfit strain due to the change in the stacking sequence and the shape of parallel close packed planes. The macroscopic transformation strain and elastic strain energy of lenticular martensite are also reduced by the presence of the observed lattice invariant deformations.

Influence of crystallography and bonding on the structure and migration of irrational interphase boundaries

Interphase boundary structure developed during precipitation from solid solution and during massive transformations is considered in diverse alloy systems in the presence of differences in stacking sequence across interphase boundaries. Linear misfit compensating defects, including misfit dislocations, structural disconnections, and misfit disconnections, are present over a wide range of crystallographies when both phases have metallic bonding. Misfit dislocations have also been observed when both phases have covalent bonding (e.g., US:β US2 by Sole and van der Walt). These defects are also found when one phase is ionic and the other is metallic (Nb:Al2O3 by Ruhle et al.), albeit when the latter is formed by vapor deposition. However, when bonding is metallic in one phase but significantly covalent in the other, the structure of the interphase boundary appears to depend upon the strength of the covalent bonding relative to that in the metallically bonded phase. When this difference is large, growth can take place as if it were occurring at a free surface, resulting in orientation relationships that are irrational and conjugate habit planes that are ill matched (e.g., ZrN:α Zr-N by Li et al. and Xe(solid):Al-Xe by Kishida and Yamaguchi). At lower levels of bonding directionality and strength, crystallography is again irrational, but now edge-to-edge-based low-energy structures can replace linear misfit compensating defects (γm:TiAl:α Ti-Al by Reynolds et al.). In the perhaps still smaller difference case of Widmanstatten cementite precipitated from austenite, one orientation relationship yields plates with linear misfit compensating defects at their broad faces whereas another (presumably nucleated at different types of site) produces laths with poorly defined shapes and interfacial structures. Hence, Hume-Rothery-type bonding considerations can markedly affect interphase boundary structure and thus the mechanisms, kinetics, and morphology of growth.

General discussion session of the 2004 hume-rothery symposium on “the structure and diffusional growth mechanisms of irrational interphase boundaries”

Details on the general discussion session of the 2004 Hume-Rothery Symposium on "The Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries" is presented. The symposium was held on Mar 17, 2004 at the Charlotte Convention Center in Charlotte NC.

Influence of crystallography and bonding on the structure and migration of irrational interphase boundaries

Influence of crystallography and bonding on the structure and migration of irrational interphase boundaries