Simulation of the process of deformation-induced destruction of long-range order in alloys with an L12 superstructure

Plastic-deformation-induced destruction of long-range order in alloys with an L1₂ superstructure is considered. A mathematical model is suggested which takes into account the following mechanisms that lead to the destruction of long-range order: generation of superdislocations, generation of single dislocations, multiplication of antiphase boundaries (APBs) upon the conservative motion of dislocations, multiplication of APBs upon dislocation climb, formation of APB tubes on superdislocations, generation of point defects, and thermal ordering. A mathematical model of deformation strengthening and long-range order destruction with allowance for the change in the type of shear-forming dislocations from superdislocations to single dislocations is formulated.     

Acoustic study of martensitic-phase aging in copper-based shape memory alloys

An acoustic technique was applied to study aging of the β ₁ ^′ martensitic phase in a number of copper-based shape memory alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Be) characterized by various degrees of martensitic-phase stabilization. The nonlinear anelasticity of the martensitic phase was studied in wide ranges of temperature (7–300 K) and vibrational strain amplitude (2 × 10^?7 ?2 × 10^?4) at vibrational-loading frequencies of ～100 kHz. It was shown that aging effects of the martensitic phase can have homogeneous and heterogeneous components. The homogeneous component is associated with a change in the degree of atomic order in the crystal volume. The basic heterogeneous mechanisms of martensitic-phase aging are associated with the formation of atmospheres of point defects and local changes (which are greater than those in the crystal volume) in the degree of atomic order in the vicinity of partial dislocations and the boundaries between martensite variants. It is concluded that various stabilization properties of the alloys at hand result not only from the different diffusion properties of quenching point defects but also from the different influence of these defects on the degree of atomic order and the different features of their interaction with partial dislocations and intervariant boundaries.     

Behaviour of vacancies in dilute Fe–Re alloys: a positron annihilation study

Positron annihilation lifetime spectroscopy was used in a room temperature study of the influence of heat treatment on behaviour of vacancies in Fe_0.97Re_0.03 and Fe_0.94Re_0.06 alloys. In this experiment, the vacancies were created during the formation and further mechanical processing of the iron systems under consideration so the lifetime spectra of positrons were collected at least twice. The first samples were taken just after the melting process in an arc furnace, and the second ones were taken for the specimens annealed at 1,270 K and then cold-rolled at room temperature. After that, the spectra were measured for all studied samples after annealing at some temperatures gradually increasing from 300 to 1,270 K. It was found that vacancy-Re pairs are the dominant type of structural defects in alloys just after the melting process. In the case of alloys after a cold rolling process, the dominant type of structural defects is vacancies associated with edge dislocations. Moreover, for cold-rolled samples annealed at 473–573 K, the growth of the vacancy clusters associated with edge dislocations is observed by an increase in the mean positron lifetime. Finally, at temperatures above 573 K, vacancy clusters associated with edge dislocations as well as vacancy-Re pairs become unstable, and freely migrating vacancies sink at grain boundaries.     

Dislocations and internal length measurement in continuized crystals. I. Riemannian material space

Distributions of dislocations creating point defects are considered. These point defects are described by a metric tensor, which supplements a Burgers field responsible for dislocations. The metric tensor depends on the distribution of dislocations and defines a Riemannian geometry of the material space of a continuized crystal and thus an internal length measurement in this crystal. The dependence of the distribution of dislocations on the existence of point defects created by these dislocations is modeled by treating the Burgers field as a field defined on the Riemannian material space. Field equations, following from geometric identities, are formulated as balance equations on this Riemannian space and their source terms, responsible for interactions of dislocations and point defects, are identified.     

Interfaces in Iron-Rich Ordered Fe-Al Alloys: An Atomic-Scale Simulation Study

The aim of this paper is to investigate the low-temperature structure of interfaces (the (3 1 0) [0 0 1] symmetrical tilt grain boundary (GB) and the (3 1 0) surface) in stoichiometric ordered Fe-Al alloys with B2 and DO₃ structures. In both alloys, (i) the GBs cannot be realistically described by geometrical models, (ii) GBs and surfaces show strong segregation effects. A simple independent-defect model cannot be applied: the interactions between point defects sometimes lead to results opposite to those predicted from the formation energies of isolated point defects. The excess energies and configurations of the most stable interface variants are determined. All interfaces show a tendency to Al segregation except the B2 GB for which the most stable structure is an Fe-rich one. The interface structures are more complex in the DO₃ than in the B2 alloy, with a high multiplicity of DO₃ configurations with close energies. Finally, values of the GB and surface energies are introduced into a Griffith model of brittle fracture, in order to assess the trends of both alloys to intergranular fracture. Comparisons are also drawn with the similar Ni-Al ordered alloys.     

Deformation-induced dissolution of the intermetallics Ni3Ti and Ni3Al in austenitic steels at cryogenic temperatures

An anomalous deformation-induced dissolution of the intermetallics Ni₃Al and Ni₃Ti in the matrix of austenitic Fe–Ni–Al(Ti) alloys has been revealed in experiment at cryogenic temperatures (down to 77 K) under rolling and high pressure torsion. The observed phenomenon is explained as the result of migration of deformation-stipulated interstitial atoms from a particle into the matrix in the stress field of moving dislocations. With increasing the temperature of deformation, the dissolution is replaced by the deformation-induced precipitation of the intermetallics, which is accelerated due to a sufficient amount of point defects in the matrix, gained as well in the course of deformation at lower temperatures.     

Atomistic modelling of TiAl I. Bond-order potentials with environmental dependence

Bond-order potentials (BOPs) for L1₀ TiAl have been developed and constructed within a tight-binding framework. In addition to the usual attractive bond-energy contribution arising from the formation of covalent bonds and pairwise contribution describing the overlap repulsion and electrostatic interaction, we have included an environmentally dependent term to represent the strong repulsion experienced by the valence sp electrons in transition metals and their alloys. The latter contribution is crucial for reproducing the negative Cauchy pressures of TiAl and other transition-metal-based intermetallic compounds. The constructed BOPs have been tested in the following ways: firstly, examination of the mechanical stability of the tetragonal L1₀ lattice with respect to large deformations and other crystal structures with the same stoichiometry; secondly, calculation of the γ surface for {111} and related evaluation of the energies of stacking-fault-type defects; thirdly, calculation of energies of the γ-γ interfaces that are present in the lamellar TiAl and energies associated with the formations of point defects in TiAl. The results of all these calculations show very good agreement with various ab-initio calculations. Importantly, we find that this potential is transferable to the different bonding environment in the hexagonal D0₁₉ Ti₃Al. Hence these BOPs are suitable for atomistic study of dislocations and other extended defects not only in L1₀ TiAl but also in Ti₃Al and possibly structures with other titanium-rich stoichiometries.     

Strain-Induced Disruption of Long-Range Order due to Generation of Superdislocations in L12-Superlattice Alloys

A disruption mechanism for the long-range atomic order in alloys with the L1₂ superlattice is examined. The disruption is caused by an accumulation of superdislocations upon plastic deformation. The rate of increase of the antiphase-boundary area resulting from multiplication of superdislocations is estimated for the cases where the spacing between superpartial dislocations is either constant or strain-dependent. Equations are derived for the rate of change of the degree of long-range order as single-crystalline L1₂-superlattice alloys are subject to deformation.     

Mechanism of Strain-Induced Breaking the Long-Range Atomic Order in Alloys with the L12 Superlattice Caused by Accumulation of Dislocation Walls

The mechanism of strain-induced breaking the long-range atomic order in alloys with the L1₂ superlattice caused by rearrangement of dislocations in dislocation walls is examined. A mathematical model of strain-induced breaking the long-range atomic order caused by accumulation of dislocation walls is constructed. The dependence of the long-range order parameter on the strain degree is calculated for alloys with high- and low-energy antiphase boundaries.     

Determination of lattice mismatch in NiSi2 overlayers on Si(111)

We determined the perpendicular overlayer mismatch for type A and type B NiSi₂ samples of thicknesses ranging from about 200 to 1000 Å via bulk x-ray diffraction. An increase in the density of dislocations which are formed to release the strain at the growth temperature leads to an increase in the magnitude of mismatch and strain at room temperature. The results on the thinner type A samples show that the perpendicular overlayer mismatch is found to depend on the overlayer thickness even when no dislocations are present at the interface. This may be due to point defects in the epitaxial NiSi₂ layers.     

Etching of slip produced antiphase domain boundaries in iron-silicon alloys

A method for revealing the slip produced antiphase domain boundaries in nearly stoichiometric ordered Fe₃Si alloys by chemical etching in a solution of HF + H₂O₂ + H₂O is reported. The method was applied to determine the character of superlattice dislocations in strained crystals. Perfect superlattice dislocations were found at the beginning of deformation. Imperfect superlattice dislocations producing antiphase domain boundaries were found at higher deformations.     

Temperature characteristics of the mechanical properties and of the dislocation structure of molybdenum-rhenium alloys

The temperature characteristics of the yield strength and of the dislocation structure of supersaturated molybdenum-rhenium alloys were studied over the 77–673°K temperature range. A lesser dependence of σ_0.1 on the temperature was found to result from twinning. The mobility of screw dislocations was found to become much lower due to alloying with rhenium. An analysis of the peculiarities in the dislocation structure of the alloys under consideration here (plane pileup, screw dipoles, singularities of interaction between glissile dislocations) has led to the conclusion that the lower mobility of screw dislocations is a consequence of the lower energy of the packing defect in molybdenum alloyed with rhenium. The lattice resistance to a movement of screw dislocations (τ_p) at room temperature has been estimated from the distance between dislocations in screw dipoles and found to be τ_p≥16 kgf/mm².     

Influence of supersaturation by point defects on the shear modulus of nickel and nickel-boron alloy

The influences of supersaturation of samples by point defects on the shear modulus is investigated by the internal-friction method. It is found that, in the temperature range 0.25–0.5T_m, the shear modulus of such samples is less. A mechanism associated with increase in mobility of the dislocations on account of diffusional influx of the point defects formed on irradiation is proposed, and an expression is obtained for estimating the magnitude of the effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 53–56, January, 1984.     

Accommodation of the Lattice Mismatch at the VCx-WC Interface

A VC doped WC-Co alloy is investigated using high resolution transmission electron microscopy. The VC grain growth inhibitor induces the presence of a thin layer on the surfaces of the WC grains in contact with Co and precipitates in the corners of Co pockets. These (VW)C_x compounds adopt an epitaxial orientation relationship with regards to the (0001) base facets of the WC crystals. Due to the small difference in lattice parameters, misfit dislocations are expected in the interfaces. Unlike the thin layers where no defects are observed, two kinds of dislocations are pointed out for larger precipitates. 1/6〈112〉_VC interfacial dislocations are sometimes present while more often 1/2〈1¯10〉_VC dislocations lying above the interface in the (VW)C_x phase are visible.     

Cu3Au与Au3Cu中点缺陷的分子动力学研究——兼论L12型合金点缺陷的性质

讨论了Ｌ１２型合金中点缺陷的各种可能构型，然后运用分子动力学的方法，采用由Ａｃｋ-ｌａｎｄ等提出的Ｃｕ-Ａｕ体系的多体势函数，计算了具有Ｌ１₂结构的Ｃｕ₃Ａｕ与Ａｕ₃Ｃｕ中点缺陷的构型、形成能及缺陷体积，并进一步讨论了目前研究较为深入的Ｌ１₂型金属间化合物中点缺陷的性质 关键词：     

A New Type of Charge-Density-Wave Pinning in Orthorhombic TaS3 Crystals with Quenching Defects

Diminishing in the concentration of quenching defects during thermocycling of orthorhombic TaS₃ samples in the temperature range below the Peierls transition temperature T < T_P is observed. It makes it possible to study the character of pinning of the charge density wave by these defects. A number of fundamental differences from pinning by ordinary local pinning centers—impurities and point defects—have been found. We conclude that quenching defects are extended (non-local) objects (presumably, dislocations) that can diffuse from the crystal during low-temperature thermocycling due to their strong interaction with the charge density wave, which is intrinsic for Peierls conductors. The presence of these defects leads to a previously unknown non-local type of the charge-density-wave pinning that acts on T_P and the threshold field for the onset of charge-density-wave sliding, E_T, differently in comparison with the local pinning centers.

     

Strained and strain-relaxed epitaxial Ge1-xSnx alloys on Si（100） substrates

Epitaxial Ge1-xSnx alloys are grown separately on a Ge-buffer/Si(100) substrate and directly on a Si(100) substrate by molecular beam epitaxy (MBE) at low temperature. In the case of the Ge buffer/Si(100) substrate, a high crystalline quality strained Ge0.97Sn0.03 alloy is grown, with a χmin value of 6.7% measured by channeling and random Rutherford backscattering spectrometry (RBS), and a surface root-mean-square (RMS) roughness of 1.568 nm obtained by atomic force microscopy (AFM). In the case of the Si(100) substrate, strain-relaxed Ge0.97Sn0.03 alloys are epitaxially grown at 150°C-300°C, with the degree of strain relaxation being more than 96%. The X-ray diffraction (XRD) and AFM measurements demonstrate that the alloys each have a good crystalline quality and a relatively flat surface. The predominant defects accommodating the large misfit are Lomer edge dislocations at the interface, which are parallel to the interface plane and should not degrade electrical properties and device performance.     

Ni3Al and Ni3Ge. Energy of plane surface defects and abnormal temperature dependence of flow stress

A significant temperature anomaly of the yield point is observed in Ni₃Al and Ni₃Ge alloys with the L1 ₂ superstructure. The temperature anomaly is caused by the thin structure of sliding superdislocations and plane defects in L1 ₂ entering the superdislocation structure. The temperature and concentration dependences of the parameters characterizing in the best way the temperature strengthening in Ni₃(Al_{1 ? x} , Ge _x ) alloys are investigated. It is shown that the alloying Ge element leads to more pronounced manifestation of the temperature strengthening effect in Ni₃(Al_{1 ? x} , Ge _x ).     

The differential geometry of elementary point and line defects in Bravais crystals

Since line defects (dislocations) and point defects (vacancies, self-interstitials, point stacking faults) in Bravais crystals can mutually convert, only theories which comprise these two sorts of defects can be closed in the sense of general field theory. Since the pioneering work of Kondo and of Bilby, Bullough, and Smith it is clear that differential geometry is the appropriate mathematical tool to formulate a field theory of defects in ordered structures. This is done here on the example of the Bravais crystal, where the above-mentioned defects are the only elementary point and line defects. It is shown that point defects can be described by a step-counting procedure which makes it possible to include also point stacking faults as elementary point defects. The results comprise two equations with the appropriate interpretation of the mathematical symbols. The point defects are step-counting defects and are essentially described by a metric tensorg, which supplements the torsion responsible for the dislocations. The proposed theory is meant to form a framework for defect phenomena, in a similar way that Maxwell's theory is a framework for the electromagnetic world.     

Interaction of dislocations and deformation hardening of the ordered CuAu alloy

The critical configurations of combinations of various types of dislocations in ordered alloys with L1₀ superstructure are analyzed numerically. The characteristics of the dislocation combinations, as well as the partial resistance to motion of dislocations, owing to interaction of dislocations, are calculated for polycrystals and single crystals with different orientations of the ordered alloy CuAu.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 21–26, August, 1982.We thank V. I. Syutkina for her constant interest in this work and for valuable discussions.