Phase transformations induced by pulsed shock-wave loading in a single crystal of the nickel superalloy have been studied. The crystal was loaded by an impact with a steel plate (the maximum pressure on the sample surface was 100 GPa, and the pulse duration was 10 mu s) and by the deceleration of explosion products at an obstacle (the maximum pressure was 20 GPa, and the pulse duration was 10 mu s). Depending on the loading conditions, various pressure-drop gradients behind the shock front were realized. After loading at 20 GPa, the crystal has a high dislocation density; after loading at 100 GPa, a banded structure with misorientation bands directed along the < 111 > directions is formed. X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy of the crystal subjected to shock-wave loading reveal an L1(2) -> D0(22) phase transformation in it. Deformation microtwins are observed near microcracks in the D0(22) phase.

The structure of orthorhombic Ti-22% Al-26.6% Nb and Ti-25% Al-22% Nb aluminides has been studied after severe plastic deformation by shear under pressure and pulse shock loading with a steel plate. It was found that severe plastic deformation leads to an order-disorder phase transformation with the formation of fine particles of two orthorhombic phases, namely, a B 19 phase disordered for niobium and a completely disordered A20 phase. Upon shear under pressure deformation, grains of the initial phase are refined and the content of the A20 phase increases as the degree of deformation increases. At a grain size of 20 nm, the initial orthorhornbic O phase is completely transformed into the A20 phase. Upon pulse shock loading, plates of the initial O phase are retained; small particles of the B19 phase disordered for niobium are formed at the boundaries of the plates of the initial phase.

Specific features of the microstructure and the deformation behavior of the L1(0) type alloys (NiPt, FePd, CoPt, CuAu) ordered after a strong cold deformation were studied. When the disordered alloys were subjected to heavy (60-95%) drawing, the ultimate strength and plasticity of ordered alloys were 1500-2200 MPa and 25-40% respectively. Significantly, the dislocations, which were inherited from the disordered alloy, proved to be incomplete in the superlattice. As a result, they lost mobility and formed a rigid framework. A possible mechanism, by which mobility of dislocations influences recrystallization, was proposed. TEM examination allowed determining an optimal structure of the ordered alloys: fibers, cells in the fibers, and lamellae in the cells. It was found that the lamellar structure (more precisely, a set of parallel boundaries of twin-like c-domains) plays a considerable role of a damper. It is the absence of the lamellar structure in L1(2)-type alloys that makes it impossible to use this method for improving their properties. This method was compared with other known methods used to enhance the strength and plasticity. To a certain extent, the proposed method resembles the so-called trip-effect (transformation-induced plasticity). However, a diffusionless martensitic transformation, which occurs during deformation and causes formation of a two-phase structure, is used in trip steels. The structure under study resembles most textile constructional composites reinforced with a space framework. Conditions, which may provide an optimal state in other superstructures, were formulated.

The microstructures of poly- and single-crystalline samples of the FePd alloy (L1(0) superstructure) after various heat treatments have been studied. The results of tensile tests of thin ribbons of the ordered alloy are given; a nonmonotonic temperature dependence of its yield strength has been detected. A device that provides for the formation of a single c domain in a single crystal of the alloy upon ordering under constant compression is given schematically.

Specific features of the deformation behavior of intermetallics have been analyzed. A relatively general approach has been proposed, which describes the process of plastic deformation as the evolution of a dislocation population, including multiplication and transformation of dislocations. Equations of plastic deformation have been derived in the frame of the proposed approach. These equations describe an unusual deformation behavior of intermetallics taking into account thermally activated blocking of dislocations and dislocation sources. Problems, which cause discussion in interpreting experimental data on plastic deformation of intermetallics, have been formulated.

Phase transformations induced in Ni3Al, Ti3Al-based superalloy with an addition of niobium, and orthorhombic Ti2AlNb aluminides by shock-wave loading were studied. The samples were subject to an impact of a steel plate. The maximum pressure on the surface of the samples was 100 GPa. The loading pulse was 1 mus. It was shown that the L1(2)-->DO22 phase transformation took place in a single crystal of the superalloy containing 90% Ni3Al intermetallic phase. A banded structure passing through initial gamma'-cuboids and regions with twins, which were the DO22-phase, were detected. The B2-->DO19 phase transformation was revealed in the polycrystalline Ti-25.6 at. % Al-10.3 at. % Nb alloy after shock loading. Long straight dislocations, which occasionally formed slip bands, were located in grains of the DO,9 phase and fine particles of the 132 phase were present instead of interlayers. Fine particles of the B19 phase, which was disordered on niobium, were found in the polycrystalline orthorhombic Ti-25 at. % Al-22 at. % Nb alloy after loading. The particles were observed along boundaries of initial plates of the orthorhombic O-phase. These observations were compared with results obtained after quasistatic compression-shear loading and cold or hot rolling.

Microstructure of Ti-48 at. % Al-1 at. % V samples prepared by pulsed volume pressing by various regimes such as casting into copper or steel molds using additional gas pressure and casting into a steel mold without pressure was studied by X-ray diffraction, transmission electron microscopy, and optical microscopy. Neither texture typical of deformed alloys nor dendritic structure typical of cast TiAl alloys were found in the Ti-48 at. % Al-l at. % V alloy prepared by the method used. It was found that a high-strength state of the Ti-48 at. % AI-l at. Clr V alloy cast in the copper mold under gas pressure can he provided by a specific combination of structure characteristics. This alloy consists of two ordered phases such as TiAl (gamma) having an L1(0)-type structure and Ti3Al (alpha (2)) having a D0(19)-type structure. The alloy has a polycrystalline structure close to the oriented lamellar one. The high yield strength of the alloy prepared by the method used displays the presence of a rigid deformation mode. The alloy structure is characterized by uniform equiaxed grains consisting of gamma and alpha (2) lamellae of certain orientations. The average grain size is approximate to 40 mum. The alpha (2) lamellae are nan ow (0.02-0.06 mum) and uniformly distributed.

An ordered CuAu alloy without domain boundaries was mechanically tested at temperatures from -196 to 385 degreesC. The temperature dependence of the yield strength was shown to he anomalous. The dislocation structure of the alloy deformed to 3% at different temperatures was examined by electron microscopy. Specific features of superdislocations that art, responsible for the positive temperature dependence of the yield strength were detected. Various models of the anomalies observed are discussed.

The process of plastic deformation is described as the evolution of a dislocation ensemble that is determined by dislocation multiplication and dislocation transformations with allowance for the fact that all these processes occur against the background of elastic stress fields generated by the dislocations themselves. A set of equations is written, which includes the equations of balance for the densities of dislocations of various types and an equation that describes the operation of dislocation sources. This latter equation is to a certain extent similar to the well-known equation for the growth of a population, but contains quantities characteristic of the dislocation ensemble. By developing the approach proposed in our previous work (Phys. Met. Metallogr., 1998, vol. 86, no. 3, pp. 240-249) and introducing a threshold stress that determines the onset of plastic flow, we suggest a simple way to allow for the smearing of this threshold. As a result, we are capable of considering some extreme variants of the deformation behavior of the system which are related to either the high initial density of dislocations density or the appearance of a yield point effect (yield "tooth") in flow curves. Theoretical curves are shown to agree well with the experimental data that were obtained (in particular for the upper and lower yield stresses in semiconductors) upon changes in temperature, deformation rate, and initial dislocation density. In terms of the suggested scheme, the fact has also been taken into account that a certain, also smeared, stress should be reached in some substances for plastic flow to be started, which is necessary to block dislocation sources. The account of this circumstance permits one to describe the anomalous temperature dependence of the yield stress in intermetallic compounds.

An anomalous temperature dependence of the yield stress in the ordered CuAu alloy was detected. Blocked <101] superdislocations were observed in the region of the anomalous trend from room temperature to 300 degrees C. Blocking of single dislocations was not detected at these temperatures. The comparison with the known data on the TiAl intermetallic was carried on. The causes of the nonmonotonous temperature dependence of the yield stress with two extremum points were analyzed. The set of possible states of dislocations in TiAl was discussed. A unified scheme with account of mutual dislocation transitions was proposed for the description of the deformation behavior over the whole temperature interval. Specific features of the dislocation structure of CuAu and TiAl were explained. The connection between the brittleness of TiAl and dislocation transformations was discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.

A comparative analysis of the results of prestraining experiments is carried out for intermetallics. The role played by both blocking of dislocation sources and the formation of a dislocation framework is considered. A new mechanism, which is associated with Kear-Wilsdorf barriers and which initiates the blocking of dislocation sources, is proposed. The operation of this mechanism is shown to depend on the temperature. As a result, a new slip system, which is inoperative at the high temperature step, can be switched on at the low-temperature step. The switching-on of the new slip system provides for the sharp decrease in the yield stress in accordance with its anomalous temperature behaviour.

A new approach to the description of the plastic deformation processes has been developed. The dislocation ensemble is examined as a certain population, an evolution of which is defined by a multiplication of dislocations as well as their transformations, such processes occurring on the background of elastic stress fields created by the dislocation ensemble itself. The non-linear equation for the change of dislocation density with time due to the operation of sources is proposed. It is analogous to the well-known equation for the population growth, but it contains specific values, such as critical stress and characteristic time required to switch-on the dislocation sources. It is with this last value that is connected the possibility of a fast or slow mutual adjustment of the dislocation density and external stress. On this basis, an explanation was proposed for the non-monotonous stress-strain dependence observed in certain cases at the transition from elastic to plastic deformation. An analysis of the deformation curves at various values of the system parameters was performed with an account of the dislocation transformations in titanium aluminides. The peculiarities of plastic deformation for TiAl, Ti3Al and their lamellar structure alloys are described.

The effect of induced anisotropy of the yield stress of intermetallics is presumed to exist. This effect may be detected if high-temperature prestraining and cooling are followed by deformation at different orientations. The anticipated anisotropy is due to the fact that the rigid regular dislocation framework, which has a high density of dislocations and is inherited upon cooling, proves to be transparent for the subsequent plastic flow at certain orientations and opaque at other orientations. After the same high-temperature step the induced anisotropy will also show up as different shapes of the temperature dependence of the yield stress drying the further deformation realized at different orientations. Special experiments have been suggested to defect the induced anisotropy of the yield stress.

The evolution of the microstructure of cold-deformed CuAu alloy is analyzed as a function of temperature and annealing time. Different temperature ranges were determined where the behavior of the microstructure formation during ordering of the alloy differs. The influence of the typical lamellar structure of CuAu on recrystallization was studied. Transmission electron micrographs suggest that a cooperative ‘tandem-mechanism’ involving ordering and dislocation resaturation operates and contributes to the formation of new grains. A comparison with other ordered alloys having the L10, L12 or B2 superstructure is conducted in the paper. The role played by the immobilization of dislocation networks in retardation of recrystallization is also considered.

The intermetallics have been synthesized from Ti3Al-and TiAl-composition powders by using the spherical shock waves and the structures was studied. The shock wave was formed by means of the spherical charge of explosive which surrounded a steel ball (capsule of conservation) with the powder mixture inside. It was revealed that the synthesized intermetallics had the increased microhardness as compared with materials obtained by traditional methods. The role of iron particles, injected during the synthesis from the capsule of conservation, in the formation of structure was found out. The existence of a great variety of phases, including metastable and nonequilibrium ones, was shown.

The paper deals with the analysis of the two-step deformation of intermetallics where the yield stress possesses a temperature anomaly. Different schemes of deformation were studied. The principal differences between the schemes are whether the transition from one step of deformation to the other is accompanied by increasing or decreasing temperature and whether the said transition takes place at a high or low degree deformation. A unified approach to the description of the two-step deformation is proposed. The approach considers that for the plastic flow to occur, it is necessary to overcome, on the one hand, the thermally active blocking of dislocation sources and, on the other hand, elastic stress fields of the existing dislocation structure.

The paper presents analysis of the structures formed in the intermetallic compound materials synthesized from Ti3Al- and TiAl-composition powders using the spherical shock-waves method. The shock-wave front was predetermined by location of explosive on a steel sphere of conservation capsule which contained a mixture of Al and Ti. The role played in the formation of structure, by disperse iron fragments penetrating into the bulk of the material during synthesis from the conservation capsule, has been established. Complex investigation has revealed quite a variety of phases, including metastable and non-equilibrium ones. The intermetallics synthesized by this method are shown to have enhanced microhardness as compared with materials one can obtain using conventional techniques.

Experiments on prestrained intermetallics with a temperature anomaly of the yield stress are analysed theoretically. It is discussed in which eases the behaviour of intermetallics resembles, despite the aforesaid anomaly, the behaviour of usual metals in Cotrrell-Stokes experiments. Conditions are formulated under which a sharp drop of the stress (macrojump) in intermetallics can be observed with decreasing a temperature that accompanies the transition from one step of deformation to another. Specific features of the dislocation structure at different steps of deformation are analysed. To produce a dislocation structure of a particular type, it is proposed to perform special experiments involving various modifications of the two-step deformation scheme. It is planned to add more steps of deformation connected with a repeated heating or, alternatively, with a different orientation of the single crystal.