Low-stability States Research Articles

Effect of Alloying on Dislocation Structure, Cracking and Fracture of Cu–Al and Cu–Mn Alloys in Low-Stability States

Effect of Alloying on Dislocation Structure, Cracking and Fracture of Cu–Al and Cu–Mn Alloys in Low-Stability States

The Role of Stacking Fault Energy of Cu–Al and Cu–Mn Alloys During Their Deformation in Low-Stability States

The Role of Stacking Fault Energy of Cu–Al and Cu–Mn Alloys During Their Deformation in Low-Stability States

Dislocation Structure Evolution in Low-Stability States of Solid Solutions of Alloys Under Their Deformation

Dislocation Structure Evolution in Low-Stability States of Solid Solutions of Alloys Under Their Deformation

Эволюция дислокационной структуры в слабоустойчивых состояниях при деформации твердых растворов сплавов

The results of a study of the evolution of the dislocation structure of polycrystals of homogeneous solid solutions in low-stability states in Cu-Mn-based alloys with a FCC crystal lattice are presented. It is found that each stage is characterized by its own deformation carrier in the form of a certain dislocation substructure (DSS), the volume fraction of which has a maximum at this stage. At the same time, at the considered stage, the carrier of deformation of the previous stage gradually disappears as the degree of deformation (ε) increases, however, a carrier of deformation of the subsequent stage appears. In a certain range of values of ε, deformation carriers characteristic of the previous, considered and subsequent stages simultaneously coexist. It is shown that transitions from some types of DSS to other types occur in some intervals of values of the degree of deformation ε. It is discovered that each stage of deformation has its own DSS - carriers of deformation. The transition from stage to stage is accompanied by the formation of new deformation carriers, which is a characteristic feature of weakly stable states of the system. Naturally, the described picture is observed because the study is carried out at such a grain size, which suggests the possibility of implementing a low-stability structural-phase state of the alloy, since the grain size acts in this case as a kind of control parameter.

Spatially Localized Oscillations in Low-Stability States of Metal Systems

Spatially Localized Oscillations in Low-Stability States of Metal Systems

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State

Influence of Composition on the Evolution of Dislocation Substructure in the Low-Stability State Polycrystalline Cu-Al Alloys under Plastic Deformation

New concepts of dislocation physics of plasticity and strength are considered using quantitative methods of transmission diffraction electron microscopy. New concepts of dislocation physics of plasticity and strength are considered using quantitative methods of transmission diffraction electron microscopy. The analysis of changes in the parameters of the dislocation substructure (DSS) is given on the example of alloys Cu-0.5 and 14 аt. % Al and the influence of these parameters on the change in the substructure of the material at a temperature T=293 K is considered. It is shown that at each stage of deformation, there are usually two substructures ("old" and "new"). The blurring of the transition from stage to stage is associated with the presence of weakly stable pre-transition structural-phase States at certain degrees of deformation of several types of substructures simultaneously, i.e., a weakly stable structural-phase state of the system. Against the background of the "old" substructure, a "new" one is born, which in the process of deformation becomes the main one, and then the "old" one, in the depths of which another substructure is formed. Experimental evidence of this regularity is obtained for FCC alloys. The presence of grain boundaries complicates the diagrams: a third substructure is formed near the grain boundaries, which corresponds to the following substructures (later) in the sequence of DSS transformations.

Critical Grain Size at Meso-Level after Deformation of Polycrystalline Metals and Alloys in Low-Stability State

The paper deals with the dislocation and dislocation-disclination substructures of polycrystalline FCC alloys Cu–Al and Cu–Mn modified by tensile deformation. Observations are performed using the transmission electron microscopy. It is shown that the grain size of the alloy structure ranging from 10 to 240 μm, can serve as a critical parameter in the low-stability state during the deformation and transition from one stage of hardening to another. The dependences are obtained for the parameters describing the defect substructure and the mean grain size. These dependences are compared with the structure and phase composition of the alloys. The critical grain size of about 100 μm is found at a meso-level. When the grain size exceeds 100 μm, the main role in defect accumulation play intracrystalline processes. At stage II, the strain-hardening coefficient does not depend on the grain size of >100 μm and rapidly increases at a grain size of <100 μm.

Phase Composition and Thin Structure of Steel Surface after Plasma Electrolytic Carbonitriding

Using the transmission electron microscopy of thin foils the paper studies the phase composition and fine structure of 0.18С–1Cr–3Ni–1Mo–Fe steel after plasma electrolytic carbonitriding carried out through the surface saturation with nitrogen and carbon. The steel specimens are studied before and after carbonitriding on the surface and at a ~40 μm distance from the surface. It is found that carbonitriding causes significant qualitative and quantitative changes in the steel structure. Thus, the layers of residual austenite appear along the boundaries of martensitic lamellas, and the particles of alloyed cementite and carbonitrides are observed inside the lamellas. The layers of residual austenite locate at a ~40 μm depth along the boundaries of all martensitic crystals, while inside the crystals there are only particles of alloyed cementite and carbonitride M23(C, N)6. The control parameter of changes in the structure and phase composition is the concentration of interstitial atoms of carbon and nitrogen. Changes in this concentration lead to the system deviation from thermodynamic equilibrium. The transition of system to equilibrium or quasi-equilibrium states is considered to be performed through the transition of crystal lattices, which form a gradient structural-unstable state of the matrix, intermetallics and carbonitride compounds, to a low-stability state.

Phase Formation Initiated by Irradiation of the Film (Si) – Substrate (Grade 3 Steel) System with a High-Intensity Pulsed Electron Beam

The physical principles of formation of phase composition and defect substructure of a surface alloy in a low-stability state formed as a result of irradiation of the film (Si) - substrate (Grade 3 steel) system with a high -intensity electron beam are revealed. The formation of a multiphase submicro- and nanocrystalline layer is observed. It is shown that irradiation of metals and alloys with a pulsed electron beam in a regime of the surface-layer melting followed by high-rate crystallization frequently gives rise to the formation of local low-stability regions exhibiting a tendency towards the formation of metastable states up to amorphization of the material. A treatment regime is identified, which allows increasing the material microhardness and wear resistance by about 3 and 7.5 factors, respectively.

The Influence of Modification on Crystal Lattice Stability of Austenite in Stainless Steel

Using the methods of electron diffraction microscopy and X-ray diffraction analysis, the influence of alloying of the austenitic steel, Grade 110H13, with chromium and vanadium, as well as high-melting, ultrafine-grained TiO2, ZrO2 powders and Na3AlF6 cryolite on its structural-phase state and microstructure is investigated. It is shown that the matrix of non-modified steel is completely austenitic and consists of an iron-based solid solution and the interstitial (C, N, O and other) and substitutional (Cr, V and other) atoms simultaneously. Alloying with chromium and vanadium changes neither its phase composition nor defect structure, while alloy modification results in qualitatively new structural features: γ → e-transformation, high-intensity microtwinning, defect structure changes, and a sharp increase in the scalar dislocation density. The features of the deformation-induced microtwinning and e-martensite plates identified in the modified steel promote revealing additional microtwin systems in the matrix γ-phase, which result in structural changes making it possible to classify it as a γ′-phase. It is found out that an introduction of modifying additions gives rise to the following sequence of structural-phase transformations: γ→γ′→(γ′ +e). The experimental data obtained demonstrate that as a result of modification the crystal lattice transits into a low-stability state. This transition is accompanied by marked structural-phase changes consisting in the formation of several microtwin systems and γ → e-transformation. These structural-phase changes in the modified steel are due to the crystal-lattice transition into the low-stability state, followed by new structural-phase alterations.

Structural-Phase Low-Stability States of BCC-Intermetallic Compounds with APB Complexes

Using a mono-nickel aluminide (NiAl) as an example, the influence of APB complexes (a pair of shear APBs along the direction and a pair of APBs along the direction) on the low-stability pre-transitional states of BCC-intermetallic compounds is investigated by the Monte Carlo method. It is shown that in the region of the low-stability states of this compound the formation energy of a complex of thermal APBs is higher than that of a complex of shear APBs. The contribution of APBs into disordering is essential up to the structural-phase transformation temperature. The most significant factor for the long-range ordering in the system is the appearance of a defect in the form of an APB itself, while the differences in the APB types and planes of their occurrence do not so essentially affect the long-range order behavior with the temperature variations. A system with structural defects is obviously less ordered compared to a defect-free system. The presence of a defect in the form of an APB promotes disordering of the system at lower temperatures: the degree of ordering starts to decrease in the case of thermal APBs at a lower temperature compared to the case of shear APBs. In the NiAl alloy with a complex of APBs, the first distortions of the structural order invariably appear near the Al–Al boundary. In the alloy with a complex of shear APBs, the distortions of the structural order are observed only in the regions where the boundaries cross. The presence of antiphase boundaries affects the alloy stability during heating. It is shown that the process of disordering is accompanied by smearing of the boundaries and their faceting.

Structural-Phase Low-Stability States of BCC-Alloys with APB-Complexes in the Course of an Order-Disorder Phase Transition

Using the Monte Carlo method, the influence of the APB-complexes (a pair of shear-induced APBs along the direction and a pair of thermal APBs along the direction) on the low-stability states of β-brass is investigated during an order – disorder phase transition in the CuZn alloy as an example. It is shown that the formation energy of a complex of thermal APBs is higher than that of a complex of shear APBs. The contribution of APBs into disordering is essential up to the temperature of the order – disorder phase transition. The most significant fact for the long-range order in this system is the appearance of a defect in the form of an APB; the difference in the types of APBs and the plane of their occurrence do not exert a significant influence on the long-range order behavior with the temperature variations. The types of antiphase boundaries do affect the structural-energy characteristics of the system at the temperatures below that of the phase transition. It is obvious that a system with structure defects is less ordered than a defect-free system. The presence of a defect in the form of an APB favors the onset of disordering at lower temperatures: reduction of ordering in the alloy begins in the case of thermal APBs at lower temperatures compared to the case of shear APBs. In the CuZn alloy with a complex of thermal APBs along the direction the first distortions of the structural order invariably appear near the Zn–Zn boundary. In the alloy with a complex of shear-induced APBs along the direction the distortions of the structural order are only observed in the regions where the boundaries intersect. The presence of antiphase boundaries influences the alloy stability during heating. The CuZn alloy without distortions of structural defects is more stable than that with APBs. It is shown that the process of disordering is accompanied by the smearing and faceting of the boundaries.

Structural-Phase Features of the Order – Disorder Transition in an Fcc-Alloy with В2 Superstructure in the Presence of a Complex of Thermal Antiphase Boundaries

Using the Monte Carlo method, it is shown that the presence of a dual defect in the form of a pair of thermal antiphase boundaries in an ordered FCC-alloy with В2 supersructure (CuZn alloy taken as an example) gives rise to considerable structural-phase variations in the system during the order – disorder transition compared to the defect-free system. The availability and character of the observed peculiarities largely depend both on the temperature and the distance between thermal antiphase boundaries. It is underlined that the boundaries of the Cu–Cu and Zn–Zn types differ both in linear dimensions and in the degree of ordering of near-theboundary regions. For a boundary of the Cu–Cu type, this region is smaller and less ordered in contrast to linear dimensions and ordering degree of the Zn–Zn type boundary. At low temperatures, linear dimensions of the near-the-boundary disordered regions increase with the temperature, while the overall order in the system decreases. In the range of low-stability states of the system, the dimensions of the near-the-boundary disordered regions are maintained at approximately 10 interplanar distances for a boundary of the Cu–Cu type and about 12 interplanar distances for a boundary of the Zn–Zn type. It is noted that ordering degrees in these regions become similar, antiphase boundaries become smeared and get faceted; the first disordered regions invariably appear in the vicinity of the Zn–Zn type boundary.

Structural State of a Weld Formed in Aluminum Alloy by Friction Stir Welding and Treated by Ultrasound

The experimental data on structural state of an aluminum alloy, AlMg6, in the weld zone formed by friction stir welding are analyzed in order to evaluate the effect of its subsequent ultrasonic treatment. It is found that the crystal lattice transits into a low-stability state as a result of combined heat-induced and severe shear deformation. This transition is accompanied by considerable structural-phase changes that are manifested as an increased lattice parameter of the solid solution. This increase is caused by both high values of internal stresses and increased concentration of Mg atoms in the solid solution due to essential dissolution of the β-Al2Mg3 particles with the content of manganese higher than that in the matrix. This is accompanied by high-intensity diffusion and relaxation processes due to the low-stability state of crystal lattice (inhomogeneous stresses) in the weld zone.

LOW-STABILITY PRE-TRANSIONAL STATES, PHASE ORDER-DISORDER TRANSION AND STRUCTURAL TRANSFORMATION IN Cu3Au ALLOY

The low-stability states and structural peculiarities in the phase transition area of the Cu3Au binary alloy are investigated. The thermal reconstruction characteristics of the binary alloy in case of its stoichiometric composition deviation were revealed on the micro, nano and macro structural scales. It is shown that in the vicinity of the structural phase transition of the Cu3Au alloy at the 25 at. % Au domain the low-stability states arise which exhibits the whole range of abnormal phenomena responsible for the system transformation inducement.

Macroscopic Correlation of Deformation Events in Low-Stability State of the Crystal Lattice and Spectral Density of Acoustic Emission Signals

It is shown that in a low-stability crystalline medium oscillations of a standing wave activate elementary deformation shears in a certain volume, which is related to the standing wave length and determines correlation at the macroscopic scale. It is also established that the correlated deformation shears generate acoustic signals satisfying the coherency condition, whose interference results in a unit acoustic signal of anomalously high amplitude. An analysis of the low-frequency spectrum of acoustic emission from aluminum under conditions of its high-temperature plastic deformation indicates that its discrete pattern is controlled by re-distribution of the vibrational energy of the primary acoustic signal over the resonant vibrations of the resonator standing waves.

Macroscopic Correlation of Elementary Deformation Events in a Low-Stability Crystal Lattice of FCC-Metals

A discrete pattern of the low-frequency acoustic emission spectrum under conditions of high-temperature plastic deformation of aluminum is analyzed. It is attributed to re-distribution of vibrational energy of the primary acoustic signal over resonant vibrations of standing waves of the resonators. In a low-stability crystal medium, standing-wave oscillations initiate elementary deformation displacements in a certain material volume. The linear dimensions of this volume are related to the length of the standing wave, thus determining the macroscopic scale of correlation. The correlated deformation displacements in turn generate acoustic signals, whose interference results in the formation of a single acoustic signal of abnormally high amplitude. In a low-stability state of the crystal lattice, activation of the elementary plastic shears could result from a combined action of static forces, thermal fluctuations and dynamic forces of standing acoustic waves.

High-Temperature Plastic Deformation and Acoustic Emission of Aluminum in a Low-Stability State

It is found out that under conditions of high-temperature mechanical loading of aluminum in a low-stability state, deformation is manifested as discrete macroscopic changes. An analysis of the activation energy of the temperature-dependent deformation and acoustic emission demonstrates that the period of deformation buildup is accompanied by the diffusion-controlled processes, giving rise to a stepwise accumulation of deformation and quasi-periodic transmission of high-amplitude acoustic-emission signals. The activation volume of an elementary deformation event is increasing exponentially with temperature, indicating an increased scale level of cooperative atomic displacements and formation of a local low-stability state or crystal-lattice instability. The macroscopic manifestation of the sharp deformation change (jump) serves as an evidence of correlation between the elementary events of deformation.

Structural-Phase Transformations in the Cu3Pt5 Alloy During Atomic Ordering

1,5 Using the Monte Carlo approach, it is found out that a complex structural-phase transformation is observed in the Cu3Pt5 alloy in the course of an order – disorder transition, which is a combination of atomic ordering and structural transformation. Cooling of the alloy gives rise to horizontal sections appearing in the temperature curve of configurational energy, which correspond to different structural states of the alloy, the difference between their energies being very small. This is an indication of low stability of these states with respect to structural-phase mutual transformations. In this case, structure defects, e.g., antiphase boundaries, significantly affect the structural-phase state of the entire system. Thus, structure defects predetermine a channel for a transition in this low-stability state. Under thermal cycling, there are hysteresis loops in the temperature dependence of the configurational energy in the vicinity of the order – disorder transition in the low-stability state of the alloy: during heating and cooling the system undergoes differing structural-phase states. At low and high temperatures, its atomic and phase structures are quite well defined, while at intermediate temperatures they are principally indefinite. Under conditions of thermal cycling, the phase structure in the region of low-stability states exhibits peculiar features: a principal difference between the structural-phase states during heating and cooling.