Non-conservative Antiphase Boundaries Research Articles

Планарные сверхструктурные дефекты в сплавах сверхструктуры L10

Planar superstructural defects have a great influence on the mechanical, functional properties of binary ordered alloys of the L10 superstructure based on the fcc lattice, but there is no complete analysis of their structure and energy in the literature. For the L10 superstructure alloys of the stoichiometric composition AB, the paper gives the expressions for calculating the sublimation energy and the energy of a planar superstructural defect in the model of hard coordination spheres and pair interatomic interactions. The crystal lattice tetragonality was not taken into account. The authors presented the ordered alloy structure as a union of four monoatomic simple cubic lattices, two of which are occupied by A atoms, and the other two by B atoms. This approach allows calculating the sublimation energy required for crystal evaporation. The first eight coordination spheres were taken into account in the work. The paper shows an algorithm for determining all possible geometrically different representations of the L10 superstructure with the same sublimation energy, gives an expression for finding the planes of occurrence of all possible conservative antiphase boundaries. The study identified that the conservative and nonconservative antiphase boundaries, as well as conservative and nonconservative boundaries of C-domains are observed in the binary ordered alloys of the L10 superstructure based on the fcc lattice. The algorithms described in this work make it possible to carry out a crystal-geometric analysis of planar defects in both binary and multicomponent ordered alloys with various superstructures.

Structure and energy of planar superstructure defects in X2YZ Heusler alloys

Planar superstructure defects (PSDs) have a great influence on the mechanical and functional properties of ternary Heusler alloys, but a complete analysis of their structure and energy is lacking in the literature. In this work, for Heusler alloys of stoichiometric composition X2YZ, having the L21 superstructure, expressions are given for calculating the sublimation energy and the energy of an arbitrary PSD under the assumption of hard coordination spheres and pair interatomic interactions. It is shown that for a given crystallographic orientation there cannot be more than three PSDs with different energies. An expression is given for finding the planes of occurrence of all possible conservative antiphase boundaries (APBs). It is established that in Heusler alloys there are only conservative and nonconservative APBs and there are no defects such as boundaries of C-domains or inversion APBs. The results obtained are useful for the analysis of slip systems and the configuration of magnetic domain walls of the considered family of alloys.

Investigation of changes in the order parameters near antiphase boundaries in the Cu3Au alloy

Investigation of the specific features of order-disorder structural phase transitions in ordering alloys in the presence of antiphase boundaries of conservative and nonconservative types has been performed in a computer experiment by the example of the model Cu3Au alloy. The distributions of the long-range and short-range order parameters in the planes parallel to antiphase boundaries have been obtained by the Monte Carlo method for different temperatures. The mechanisms of disordering near conservative and nonconservative boundaries in ordering alloys are revealed. It is shown that the effect of diffusion of superstructural parameters is significant near nonconservative antiphase boundaries in comparison with conservative boundaries. Obviously, nonconservative boundaries can make a relatively large contribution to the effect of positive temperature dependence of the yield strength of ordering alloys with the L12 superstructure.

Energies of conservative and non-conservative antiphase boundaries in Ti3Al: a first principles study

The energies of conservative and non-conservative antiphase boundaries (APBs) on and (0001) planes of stoichiometric Ti3Al are calculated ab initio in order to shed light on the reason for the enormous strengthening by antiphase domains (APDs). The energy of the non-conservative APB, which has never before been estimated either experimentally or theoretically, is found to be between those of the two types of conservative APBs, and therefore, the great strengthening cannot be attributed to the energy of the non-conservative APB. On the other hand, large differences are found between the calculated energies of conservative energies and those estimated from dissociation distances of superdislocations. This is similar to the differences seen in the case of the APB energies of TiAl, which are probably due to the equilibration of APBs by rearrangements of atoms in the vicinity of APBs. The calculated APB energies in Ti3Al are significantly larger than those estimated experimentally, and the APD size dependences of the critical resolved shear stress (CRSS) estimated by substituting these calculated values into the previously proposed equations are in better agreement with the actual dependence than those obtained using the experimental values. This implies that the enormous strengthening by APDs may be attributed to the quite large energy of the APBs created by shearing of APD boundaries.

Ordered domains and boundary structure in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics

Ordered domains and boundaries have been observed in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics by electron microscopy. Individual nanoscale domains were found to be well ordered with hexagonal symmetry. Electron diffraction revealed a twinned crystallographic relationship between domains. High-resolution electron microscopy showed further details of the boundary structure. Conservative twin boundaries along the (001) and (110) planes and nonconservative antiphase boundaries with a projected displacement vector of the type [001] were observed. The relationship between the energy and structure of the domain boundaries is briefly discussed.

Microstructures and surface step-induced antiphase boundaries in epitaxial ferroelectric Ba0.6Sr0.4TiO3 thin film on MgO

Ba 0.6 Sr 0.4 TiO 3 thin films were epitaxially grown on (001) MgO substrates using pulsed laser ablation. Cross-sectional and plan-view transmission electron microscopy have been employed to study the microstructures and the interface behavior of the as-grown thin films. The 110-nm-thick Ba0.6Sr0.4TiO3 thin films have a flat surface and sharp interface. The entire thin film has a single-crystal cubic structure with an interface relationship of (001)Ba0.6Sr0.4TiO3//(001)MgO and 〈100〉Ba0.6Sr0.4TiO3//〈100〉MgO with respect to the substrate. The 6.4% lattice mismatch between Ba0.6Sr0.4TiO3 and MgO was completely released at the interface by forming equally spaced misfit dislocations with a distance of ∼3.2 nm. High-resolution transmission electron microscopy investigation shows that the initial grown layer of the film is the TiO2 monolayer. The growth models of “substrate surface-terrace induced defects” for perovskite on a rock-salt system have been developed to understand the as-grown features where the conservative and nonconservative antiphase boundaries can be formed.

Observations and analyses of dislocations and stacking faults in the massive γm phase in a quenched Ti–46.5 at.% Al alloy

The defect structures in the massively formed gamma (γm) grains in a Ti–46.5 at.% Al alloy, rapidly quenched from the high-temperature α-phase field, have been studied using transmission electron microscopy (TEM). The results reveal that the defect structures are composed of dislocations, stacking faults and antiphase boundaries intimately associated with dislocations or stacking faults. Contrast analysis indicates that both 1/2<110] and 1/2<101] unit dislocations were present in γm phase, with the latter linked by highly curved non-conservative antiphase boundaries. Comparison of experimental and computer simulated TEM images established that wide stacking faults, which are created by the dissociations of 1/2〈101〉 unit dislocations, lie on {111} planes and are bound by b=1/6〈121〉 Shockley partial dislocations of all possible types. In addition, antiphase boundaries are found to commence or terminate on the stacking faults at the partial dislocations with b=1/6<121], but not those with b=1/6<112]. Based on the observations and subsequent analyses, a model for the formation of these defects—involving the occurrence of an intermediate disordered f.c.c. phase during the α→γm massive transformation—is proposed.

TEM investigation of the microstructure and defects of CuZr martensite. Part II: Planar defects

In Part II of these two sequential papers the details of two different planar defects are presented and discussed in view of their relation with the martensitic transformation. It is shown that the lattice displacements and habit planes of the defects can be explained by assuming non-conservative anti-phase boundaries existing in the parent B2 phase, that is, prior to martensitic transformation. The main difference between both defects is the sequence of shuffled (001) planes which remains unchanged in the first type but skips one plane in the second type. The microtwin sequence is also clearly affected by the second type.

A study of antiphase boundaries and “223” planar faults in epitaxial YBa 2Cu 3O 7 films by high resolution electron microscopy

The atomic structure of antiphase boundaries in YBa 2Cu 3O 7 thin films has been characterized by means of high resolution transmission electron microscopy. Two kinds of antiphase boundaries were found, conservative and nonconservative. The nonconservative antiphase boundaries can be related to either intrinsic or extrinsic faults. The antiphase boundaries usually end at planar defects in the a-b plane. In addition, “223” planar faults have been observed. This type of defect includes both an extra Y and an extra O layer inserted parallel to the a-b plane at the z= 1 2 position of the YBa 2Cu 3O 7 structure.

Novel faulted structures in rapidly solidified Fe-37 at.% Al-15 at.% Mo alloy

Fe-37 at.% Al-15 at.% Mo has an equilibrium two-phase structure consisting of an ordered B2 (FeAl-based) matrix with Mo 3Al as the second phase. Rapid solidification by chill-block melt-spinning suppresses the formation of the Mo 3Al phase, giving instead single phase B2-FeAl supersaturated with Mo and containing a network of faults revealed by TEM. Imaging with B2 superlattice reflections revealed an anisotropic antiphase domain contrast while imaging with fundamental (b.c.c.) reflections showed typical stacking fault or thin precipitate contrast for the same faults. The faults were identified as a/2〈111〉{100} type B2 antiphase boundaries. It is proposed that the formation of such anisotropic and non-conservative antiphase boundaries is related to the retention of excess Mo within the B2 matrix by rapid solidification.

Structure defects in rare-earth-iron carbides with the hexagonal Th 2Ni 17 structure type: A study by means of high resolution electron microscopy

Rare-earth iron carbides ( RE) 2Fe 17C x have the hexagonal ( hP38) Th 2Ni 17 structure from terbium to lutetium and for yttrium, provided that the carbon concentration is below the value of the hexagonal-to-rhombohedral transformation point. High resolution electron microscopy reveals structure defects which change the site symmetry and therefore the magnetocrystalline anisotropy of the rare-earth sublattice on a local scale. Under suitably chosen imaging conditions, the rare-earth atom columns can be identified as white dots in the high resolution images. Less bright or even absent dots at some particular positions of the rare-earth columns are interpreted in terms of partially disordered atom columns with a mixed occupation of rare-earth atoms and iron dumbbells. Image simulations confirm this hypothesis of local disorder. Conservative antiphase boundaries are observed. Defect areas containing successive nonconservative antiphase boundaries are identified as local slabs of material with the ThMn 12 structure type.

High-resolution structure imaging of stacking sequences in Y2Fe17Cx(0·6x1·6) magnetic material

Abstract High-resolution electron microscopy has been used to investigate planar crystal defects in magnetic materials of the type Y2Fe17C x (x0·6), which have the rhombohedral Th2Zn17 type of structure. The basal plane stacking sequences are imaged in the [100] orientation. Edge-on and inclined coherent twin boundaries are observed, the latter giving rise to a characteristic coincidence site pattern. The high-resolution images further reveal a high density of conservative and non-conservative antiphase boundaries, which can mutually combine into an antiphase boundary cross-link. Non-conservative antiphase boundaries are also seen to make cross-links between successive twin boundaries. In local areas, a periodic antiphase boundary structure with Y3Fe29C x , stoichiometry is observed. Y2Fe17C x compounds resulting from an annealing treatment at low temperature exhibit an order-disorder phenomenon, based upon the rhombohedral structure. High-resolution structure imaging allows us to confirm the microdomain model that was previously derived on the basis of electron diffraction results only. However, some particular restrictions on the disordered stacking are further derived from the high-resolution images. In Gd2Fe17 compounds without C, an identical order-disorder phenomenon is observed depending on the annealing temperature. The relation of the observed defects with respect to coercivity mechanisms and magnetic anisotropy is discussed.

Formation process of periodic non-conservative antiphase boundary structure in L-Pd5Ce

The low temperature phase of Pd5Ce (L-Pd5Ce) has a one-dimensional long period superstructure (1D-LPS) derived from Ll2. The periodic antiphase boundaries (APBs) are parallel to (110) planes and have a shift vector of 1/2[110]. Hereafter, the indices are referred to the basic lattices of Ll2 As insertion of the APB causes a change in composition, such an APB is called “non-conservative”. Then, a domain size M depends upon the Ce concentration in the alloy. It was found that M increases also with temperature. The temperature dependency of M is attributed to a change of the degree of order within the antiphase domains. In this work, morphology of the non-conservative APBs is observed to clarify the formation process of the 1D-LPS.The alloy of Pd-16.7 at%Ce was prepared by arc melting in argon atmosphere. Disc specimens made from the alloy ingot were first held at 985 K for 260 ks and quenched in iced water to obtain the state of M=∞ or Ll2, followed by annealing for various lengths of time. The annealing temperature was 873 K where the equilibrium value for M is about 3 in unit of (110) lattice spacing of Ll2. Observation was carried out using microscopes JEM-2000FX, JEM-4000EX (HVEM Lab., Kyushu Univ.) and JEM-2000EX (Dept. of Mater. Sci. Tech., Kyushu Univ.).

Two-dimensional long period structures in Cu-Pd. A study of the mechanism of the transition from a one-dimensional LPS to a two-dimensional LPS

Abstract The nucleation and formation mechanisms by which two-dimensional long period structures (2D-LPS) are generated from one-dimensional long period structures (1D-LPS) in Cu3Pd alloys are analysed. A series of alloys with very specific heat treatments was prepared to permit an analysis of different static steps in the transition from 1D-LPS to 2D-LPS. This analysis is based on studies by high-resolution electron microscopy and electron diffraction, which were essential to substantiate a crystallographic model for the transformation mechanism. The proposed mechanism ascribes the formation of the 2D-LPS to the propagation of parallel sets of hairpin-shaped configurations of non-conservative antiphase boundaries. Their progression in a direction perpendicular to the 1D-LPS boundaries gives rise to the 2D-LPS. Displacement vector analysis, and detailed observations of boundary geometries, support the proposed crystallographic model. The nucleation of the transformation proceeds in two ways, both related ...

Long-period structures in the Nb–Ga system

The alloys NbGa3 - x form composition-driven long-period superstructures derived from the D022 by the introduction of non-conservative antiphase boundaries along (001) planes. The displacement vectors of successive antiphase boundaries are alternately ¼ [201] and ¼ [021]. In alloys with a composition which deviates less from NbGa3 than Nb5Ga13 successive antiphase boundaries all have the same displacement vector, either ¼ [201] or ¼ [021]. The alloy Nb5Ga13 is the first material which has a long-period superstructure based on the alternation of two types of antiphase boundaries.

High resolution electron microscopy of long range ordered alloys

In this paper a survey is given of the application of high resolution electron microscopy and selected area electron diffraction to the study of long range ordered alloys. The characteristic features of the diffraction effects and of the imaging of ordered alloys are discussed. Group theoretical considerations allow prediction of microtexture of such alloys.A number of simple alloy structures are imaged and it is shown, by means of simulated images, that under suitable diffraction conditions the columns of minority atoms are represented as bright dots. The columns of majority atoms are only visible at very small thicknesses. Subsequently a number of one- and two-dimensional long period superstructures derived from these simple structures are analysed, using high resolution micrographs. Finally the observed microtexture of the same alloy superstructures is compared with the predictions of group theory. Strongly non-conservative antiphase boundaries are found to dissociate inapb’s which are as close to conservative as is compatible with thefcc lattice.

A new long period superstructure with ideal composition Au35Mn13 derived from the DO22 structure, studied by means of high resolution electron microscopy and electron diffraction

A new long period superstructure with ideal composition Au35Mn13 is studied by means of high resolution electron microscopy combined with selected area electron diffraction and optical diffraction of the high resolution images. The superstructure is derived from the DO22 structure by the introduction of non-conservative anti-phase boundaries on (302) planes with a displacement vector 1/4 [201]. The relationship with other superstructures of the DO22 structure is discussed.

Lattice relaxation at non-conservative antiphase boundaries in Ni3Mo

Non-conservative antiphase boundaries in the intermetallic alloy Ni3Mo have been studied by electron microscopy. It is shown that if a small excess of molybdenum is present along the boundary, a relaxation of the lattice occurs along the interface. This gives rise to a small supplementary displacement vector ϵ, the total displacement vector being R ϵ. The direction and the sense of ϵ have been determined from diffraction experiments; the supplementary displacement is equivalent to a small expansion of the boundary regions. An analysis of the structural model confirms the results. Nichtkonservative Antiphasengrenzen wurden in der intermetallischen Legierung Ni3Mo durch Elektronenmikroskopie untersucht. Es wird gezeigt, das eine Relaxation des Gitters in Richtung der Grenzflache auftritt, wenn ein geringer Uberschus von Molybdan vorhanden ist. Dies fuhrt zu einem kleinen zusatzlichen Verschiebungsvektor ϵ, wobei der Gesamtverschiebungsvektor R ϵ ist. Die Richtung und das Vorzeichen von ϵ wurden aus Beugungsexperimenten bestimmt; die zusatzliche Verschiebung ist gleichbedeutend mit einer geringen Ausdehnung der Grenzbereiche.

Long-period order in vanadium carbide

Abstract Periodic arrangements of anti-phase boundaries (APBs) have been detected by electron diffraction and microscopy in long-range ordered crystals of vanadium carbide near to the composition V 6 C 5, The APBs are non-conservative and are believed to accommodate excess vacancies or carbon atoms in response to deviations in crystal composition from V 6 C 5. The atomic arrangement at non-conservative APBs has been deduced from diffraction contrast analysis of isolated APBs and their observed configurations in crystals containing non-periodic distributions. The observed variation in spacing of the long-periodic structure is consistent with analyses of the average crystal compositions. A mechanism of formation of the long-period structure has been deduced from observations on partially transformed crystals.

On short range order and micro-domains in the Ni 4Mo system

Electron diffraction and field ion microscopy have been used to investigate the nature of short range order in as quenched Ni 4Mo. The results indicate that the as quenched state consists of an assembly of micro-domains possessing the six orientational variants of the Ni 4Mo superstructure. The 1 1 2 0 diffuse peaks have been attributed to the occurrence of non-conservative anti-phase boundaries within the micro-domains. A similar argument is used to account for the occurrence of 1 1 2 0 peaks in the disordered α-phase at other compositions in the NiMo system.