A. E. Kheifets

High-strain-rate deformation of titanium using dynamic equal-channel angular pressing

Titanium samples were deformed using equal-channel angular pressing (ECAP). Structural changes upon the uniform and localized high-strain-rate deformation and specific features of the nucleation and propagation of cracks have been studied. A geometrical method of determining the amount of uniform shear strain upon equal-channel angular pressing has been suggested. The method is based on a metallographic examination of the spatial orientation of structural components. The localized deformation leads to the appearance of adiabatic-shear bands. Two band systems are formed: longitudinal and transverse, arranged at an angle to the longitudinal. The occurrence of recrystallization inside the bands indicates local heating of the material to 770–870 K. Specific features of the structure of the adiabatic-shear bands arising in this method of deformation is their large width (to 100 μ m) and a multilayer structure.

Structure and microhardness of chromium-zirconium bronze subjected to severe plastic deformation by dynamic channel-angular pressing and rolling

Bulk samples of chromium-zirconium bronze have been subjected to severe plastic deformation by two methods, namely, high-strain-rate dynamic channel-angular pressing (DCAP) and quasistatic deformation by rolling. After deformation and additional aging using metallography and electron microscopy, the structure has been investigated and the microhardness of the samples has been measured. It has been shown that the high-strain-rate deformation by DCAP is of a periodic character. It has been established that, in the investigated bronze subjected to DCAP, in four passes, the structure of dynamic polygonization is predominantly formed, which is accompanied by processes of aging. Upon the rolling, cells of deformation origin and a structure with randomly distributed dislocations and numerous extinction contours are formed.

Structure of chromium-zirconium bronze subjected to dynamic channel-angular pressing and aging

Structure changes in chromium-zirconium bronze upon high-speed deformation and subsequent annealing have been studied using the methods of metallography and electron microscopy and microhardness measurements. Deformation was performed by the method of dynamic channel-angular pressing in one and three passes. The deformation creates a submicrocrystalline structure and increases the microhardness by 2.4 times. Aging additionally increases the microhardness by 10%. During annealing at temperatures of 400–700°C, aging and recrystallization take place. At early stages of aging, nanosized particles pin dislocations, thus hampering the formation of recrystallization centers. At subsequent stages, chromium particles impede the migration of large-angle boundaries, thereby blocking the development of recrystallization. At the beginning of aging process in the deformed structure, chromium particles are coherent with the copper matrix and have an fcc structure. Upon the coarsening of the particles in the recrystallized structure, they acquire a bcc structure that is typical of chromium.

Structure formation in copper during dynamic channel-angular pressing

The structural changes in copper samples (99.90% Cu) subjected to severe plastic deformation using a shock loading technique have been studied. The samples are extruded through two or three channels disposed at an angle of 90°. The microstructure of the copper samples changes under the simultaneous action of high-rate deformation and a high temperature. A relation between the dynamic pressing parameters and the specific features of the structure forming in the samples is established. Dynamic pressing is shown to cause substantial grain refinement (by three orders of magnitude) in copper after two-pass extrusion.

Structure of titanium after dynamic channel angular pressing at elevated temperatures

Dynamic channel-angular pressing of titanium at a temperature of 500°C has been performed. An increase in the temperature prevented the formation of cracks and adiabatic-shear bands that usually occur upon pressing at room temperature. As a result of dynamic pressing of titanium at 500°C, there is obtained a structure which represents a dispersed mixture of fine recrystallized grains and unrecrystallized regions (duplex structure). The formation of recrystallized grains is caused by a local increase in the temperature in the sites of deformation localization and is a mechanism of relaxation of accumulated stresses. The recrystallized grains are grouped into inclined extended shear bands (large-scale relaxation) and into short bent chain of clusters, which are located between inclined bands (small-scale stress relaxation). The unrecrystallized regions consist predominantly of elongated subgrains formed as a result of deformation and dynamic polygonization. The microhardness of titanium in the recrystallized regions is equal to 1980 MPa, that in unrecrystallized regions, to 2150 MPa.

Structure and mechanical properties of titanium subjected to high-rate channel angular pressing and deformation by rolling

The macro- and microstructure have been analyzed and the tensile mechanical properties have been measured for commercial titanium subjected to dynamic channel angular pressing (DCAP) at high temperatures using one or two passes, as well as to additional warm rolling and low-temperature annealing. The structure of titanium after DCAP at a high temperature consists of a dispersed mixture of fine recrystallized grains (1 to 2 μm in size) and deformed nonrecrystallized regions. The deformed regions have a subgrain structure with sub-grains 200–300 nm in size. After the second pass, the size of the recrystallized grains becomes less by two times as compared to their size after one-pass DCAP, the subgrains in the deformed regions acquire a more equiaxed shape, and the microstructure becomes more uniform. The warm rolling of the samples subjected to DCAP at high temperatures increases the total density of dislocations and provides a high level of internal stresses. After two-pass DCAP at 530°C, the ultimate strength of titanium was 650MPa and the relative elongation was 19%. Additional rolling to 50% at 300°C and low-temperature annealing increases the ultimate strength to 790 MPa, while the relative elongation is retained at a high level of 15%.

Deformation- and temperature-related processes that occur upon the collapse of a thick cylindrical shell made of steel 20

An experiment has been performed on the collapse of a thick steel cylindrical shell into a continuous cylinder under the action of a sliding detonation wave. The process of the collapse has been recorded via X-ray photography, and it has been found that the time of collapse in one section is equal to 30 μs. The average degree of deformation is 77% and the rate of deformation is 104 s−1. The structure of steel 20 in the transverse section of the cylinder consists of three zones. In the outer zone, the initial ferrite-pearlite structure changes under the effect of compressive shock wave and localized shears. The shock wave leads to the formation of a high-pressure ɛ phase and twins. Upon the subsequent inertial collapse of the shell, substantial shear deformations arise in the surface layer, which are localized in directions located at angles of 60° to the cylindrical surface. The structure of the middle zone changes under the action of severe plastic deformation, which occurs predominantly in the radial direction. The deformation leads to the appearance of an internal pressure and to an increase in the temperature. As a result of the action of three factors (pressure, temperature, and deformation), the temperature of the formation of austenite decreases by several hundred kelvins. In the free ferrite, an α → γ transformation occurs and quenching takes place following a subsequent sharp decrease in pressure (barothermic quenching). The pearlitic regions suffer plastic deformation. The microhardness of the steel with this structure is equal to the microhardness of quenched steel. The structure of the third, i.e., central, zone, changes under the action of a significant increase in temperature caused by the further increase in the degree of deformation. The complete transformation of ferrite into austenite occurs at the center of this zone, which means that the temperature in this zone reaches 850–900°C or greater. The microhardness decreases to values typical of annealed steel.

Study of the structure and mechanical properties of submicrocrystalline and nanocrystalline copper produced by high-rate pressing

Evolution of the structure of pure copper (99.8 %) in the course of severe plastic deformation performed by the new method of dynamic channel-angular pressing (DCAP) and the mechanical properties of the arising mixed SMC+NC structures have been studied. The rate of deformation of the material during DCAP is ∼105 s−1, the duration of the process of deformation is 500 μs, the pressure does not exceed 2 GPa. It has been established that in the course of twofold-fourfold pressing the grains and subgrains of copper become refined by three orders of magnitude. It has been found that a decrease in the inner radius of the profile of the zone of intersection of the channels in the die (R) from 7 mm to 0 leads to the formation in the bulk samples of an SMC+NC structure consisting of grains and subgrains with a size from 50 to 350 nm. It is shown that the high-rate deformation with the use of the DCAP method increases the copper hardness by a factor of 2–2.2; the strength grows by a factor of 1.4; and the plasticity remains high. The estimation of the variation of the value of shear along the cross section of the samples upon DCAP showed that in the scheme with R = 7 mm the relative shear is γ ≤ 1.65; in the scheme with R = 0, γ = 1.8-2.0. In the case of a scheme with R = 0, the DCAP leads to a considerable (in comparison with the initial state) increase in the strength without loss of plasticity even upon single-pass pressing.

The ω-phase formation in titanium upon deformation under pressure

The ω-phase formation in titanium of the VT1-00 grade upon deformation under pressure has been investigated by X-ray diffraction analysis and diffraction electron microscopy. The deformation was effected by two methods: shear under a pressure of 6 GPa in Bridgman anvils and high-strain-rate equal-channel angular pressing at a pressure of about 2 GPa. Upon deformation under a pressure of 6 GPa, the ω phase is formed as grains that are isolated or clustered into groups. Upon deformation at a pressure of 2 GPa, this phase arises in the form of nanosized particles that are orientationally related to the α phase. After deformation by shear under pressure using one revolution of the anvils, new grains of the α phase up to 2–3 μm in size have been detected. The grains are nearly free of dislocations and have wavy boundaries. The origin of these grains is tracealle the reverse ω → α phase transformation that takes place upon pressure release and occurs via the “normal” rather than martensitic mechanism, at the expense of migration of the inter-phase boundaries. Upon heating, the reverse ω → α transformation at 100°C does not yet begin, whereas at 220°C the transformation proceeds almost completely. A temperature distribution in the titanium sample upon shear under pressure at a rate of 0.3 rpm has been calculated; according to this distribution, the maximum temperature rise is 12 K.

Electron microscopic investigation of aging in the Cu–0.06% Zr alloy

The decomposition of supersaturated solid solution in the Cu–0.06 wt% Zr alloy has been investigated. Upon aging of the initially quenched alloy the homogeneous precipitation of particles is dominating. The decomposition begins from the precipitation of a metastable copper–zirconium phase, the particles of which have the shape of nanodimensional disks. An increase in the aging temperature results in the formation of coarser rodlike particles of the Cu5Zr equilibrium phase. Aging of the deformed alloy is characterized by the predominance of the heterogeneous precipitation of particles at subboundaries and dislocations, and the decomposition begins at a lower temperature. The particle size is less by an order of magnitude than that in the quenched state. The precipitation of nanodimensional particles at dislocations retards the formation of recrystallization centers.