B. A. Greenberg

Formation of vortices during explosion welding (titanium-orthorhombic titanium aluminide)

The possibility of cladding commercially pure titanium by a plate of orthorhombic titanium aluminide has been investigated. The bimetallic joints of orthorhombic titanium aluminide (Ti-30Al-16Nb-1Zr-1Mo) with commercially pure titanium have been obtained by explosion welding. It has been found that the weld joint investigated had a multilayer structure consisting of a zone of continuous deformation observed in both materials, a zone of titanium recrystallization, and a transition zone near the interface. Wave formation and formation of isolated vortex zones have been observed. It has been found that upon explosion welding the bonding of the surfaces is effected via melting and subsequent mixing (in the zone of vortices) and the transfer of particles of one metal into another with the formation of particle tracks (outside the zone of vortices). A possible scenario of the formation of the vortex zone in the melt with a subsequent eutectic decomposition is proposed. The structure of the vortex zones was found to consist of an ultrafine mixture of α and β grains (both phases are disordered) with the grain size changing in the limits of 50–300 nm. The regions of transition from the vortex zone to the region of continuous deformation of the aluminide and to the recrystallized zone of titanium have been investigated.

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: I. Theory

A new concept of the possibility of thermoactivated blocking of superdislocations in the absence of external stresses is suggested. A factor that is related to the internal structure of a superdislocation and initiates its blocking due to the creation of a certain effective force has been revealed. Irrespective of the way in which the rearrangement of the superdislocation occurs and of the fine details of the barrier structure, the role of the above factor is determining. Conditions have been formulated under which the formation of barriers becomes possible without loading after a preliminary deformation while their destruction becomes impossible. Based on an analysis of slip on the cubic system, factors have been clarified that are responsible for the ambiguousness of the attempts of restoring the energy of antiphase boundaries from the width of superdislocations in some cases. The mechanisms of dislocation blocking in intermetallic compounds and semiconductors are compared.

Microheterogeneous Structure of Local Melted Zones in the Process of Explosive Welding

The dispersed structures formed in the process of explosive welding and solidification after melting were investigated in areas near the interface. It was shown that melting can be initiated by particles flying away as a result of granulating fragmentation. This is the fastest process during explosive welding, which is similar to fragmentation in conventional explosions with the formation of fragments but occurring in the presence of a barrier. The reaction between the particles and their environment may lead to local heating sufficient for melting. This is confirmed by the observation of numerous particles of the refractory phase within the local melted zones. In the absence of mutual solubility of the initial phases, the solidified local melted zones are to a certain extent analogous to colloidal solutions of immiscible liquids. Correlations between the typical temperatures were obtained that determine the conditions for the formation of various types of colloidal solutions.

Deformation Behavior of Intermetallics: Models and Experiments

A sufficiently general thermally activated mechanism for extension of dislocations in some preferential direction was proposed. This mechanism comprises a necessary step of dislocation transformations that lead to blocking. The reasons for blocking of different types of dislocations in different materials are diverse. A new concept was developed concerning the possibility of thermally activated blocking of superdislocations in the absence of external stresses. Some experiments with single crystals of Ni3(Al, Nb) were performed. They included no-load heating after preliminary low- or high-temperature deformation. It was found that the initial dislocation structure, which included curvilinear dislocations, transformed to a set of long rectilinear blocked superdislocations after no-load heating. The experimental results confirmed theoretical assumptions on the possibility of thermally activated transformations of superdislocations to indestructible barriers in the absence of external stresses.

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: II. Observation of blocked superidislocations upon heating without stress

Experiments have been performed which reveal that heating in the absence of an external stress after a preliminary both low-temperature and high-temperature deformation of intermetallic compounds leads to a fundamental change in their dislocation structure. For the investigation, [251] single crystals of Ni3(Al, Nb) have been used. The low-temperature deformation was performed at −196°C; the high-temperature deformation, at 800°C. It has been found that the initial dislocation structure consisting of curvilinear dislocations was changed upon heating without a load by a set of rectilinear blocked dislocations. It has been shown that upon heating after a preliminary low-temperature deformation the barriers present in the structure belong to the cubic cross-slip plane, whereas upon heating after high-temperature deformation, to primary cubic slip planes. It has been found that the decisive effect on the blocking of superdislocations upon heating without stress comes from one of the dislocations that compose the superdislocation, namely, either a superpartial dislocation in the case of low-temperature deformation or a simple partial dislocation in the case of high-temperature deformation. The concept of the possibility of thermoactivated blocking of superdislocations in the absence of external stresses suggested in part I of this work [Phys. Met. Metallogr. 102, 61–68 (2006)] has been confirmed experimentally.

Interface relief upon explosion welding: Splashes and waves

For copper-tantalum, aluminum-tantalum, and magnesium-titanium joints the character of the interface relief has been investigated in different regimes of explosion welding at, below, and above the lower limit of weldability. It has been found for the first time that protrusions on the plane interface have the shape of splashes. This shape is unusual with allowance for the fact that protrusions arise from the solid phase that experienced no melting. Upon transitioning to the region below the lower limit, the number of splashes decreases and they prove to be insufficient to provide weldability. Upon transitioning to a region somewhat above the lower limit, the interface becomes quasi-wavy and inhomogeneous and, in some places, in addition to the wavy surface, splashes are also observed. Splashes and waves have been observed simultaneously for the first time. Models that describe possible variants of their interrelation have been proposed.

Electron-microscopic examination of the transition zone of aluminum-tantalum bimetallic joints (explosion welding)

A study of the structure of an aluminum-tantalum joint and a comparison of this structure with the structures of iron-silver and copper-tantalum joints have revealed the following processes of the interpenetration of the materials that occur during explosion welding: the formation of protrusions, the injection of particles of one material into the other, and the formation of zones of local melting. Regardless of the mutual solubility of the metals being welded, two types of fragmentation occur, i.e., (1) a granulating fragmentation (GF), which includes the formation, explosion-governed (EG) dispersion, and partial consolidation of particles, and (2) the fragmentation that is usually observed during severe plastic deformation. It is important that this traditional fragmentation is not accompanied by the formation and EG dispersion of particles. This feature allows one to easily distinguish these types of fragmentation (traditional and GF fragmentation).

The first observation of dislocation blocking in pure metal without external stress

Self-blocking of c + a edge dislocations at second-order pyramidal slip in magnesium single crystals whose axis is parallel to the c axis has been found. Self-blocking is confirmed by the dislocation extension along a selected direction in the absence of external stress. Transmission electron microscopy (TEM) images of (c + a) dislocations extended in the

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: IV. Thermoactivated straightening of dislocations along a preferred direction in TiAl

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