I. V. Khomskaya

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%.

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.

Structure of titanium subjected to dynamic channel-angular pressing

The microstructure of titanium after dynamic channel-angular pressing in two passes is studied by metallography and electron microscopy. This structure is compared to the structure of titanium after one-pass pressing. The high-rate deformation of titanium in this method consists of uniform deformation and localized deformation. Uniform deformation creates a submicrocrystalline structure. An increase in the number of passes from one to two leads to an increase in the grain-subgrain misorientation and the formation of a more homogeneous structure. Localized deformation causes the formation of adiabatic shear bands and cracks. An increase in the number of passes from one to two is accompanied by the accumulation of localized-deformation regions. The presence of regions with a martensitic structure in adiabatic shear bands in a sample deformed in two passes indicates heating of these regions above the α-β transition temperature. ω-phase particles are observed. The orientation relationships between the α and ω phases are such that the basal planes of their hexagonal crystal lattices are mutually perpendicular.

Mechanical properties and the structure of chromium–zirconium bronze after dynamic channel-angular pressing and subsequent aging

Changes in the structure and mechanical properties of the low-alloy chromium–zirconium bronze Cu–0.14% Cr–0.04% Zr have been investigated after a high-strain-rate (104–105 s–1) deformation by the method of dynamic channel-angular pressing (DCAP) and following annealings at 300–700°C. A significant increase in the mechanical properties of the investigated bronze after DCAP and after DCAP and subsequent aging at temperatures of 400–450°C has been established. Thus, compared to the initial quenched state the ultimate tensile strength increases by a factor of 2.6 and 2.8 and the yield stress, by a factor of 3.3 and 5.1, respectively, with the retention of satisfactory plasticity. It has been shown that, upon DCAP and subsequent annealings, in the low-alloyed bronze under investigation there occurs a decomposition of the α solid solution with the precipitation of nanosized particles. This leads to a significant strengthening of the bronze and to an increase in its thermal stability compared with the pure copper subjected to DCAP.

Evolution of the structure upon heating of submicrocrystalline and nanocrystalline copper produced by high-rate deformation

Methods of electron microscopy, dilatometry, and microhardness and resistivity measurements have been used to study the effect of annealing on the process of recrystallization of a mixed submicrocrys-talline+nanocrystalline (SMC+NC) structure of 99.8% copper produced by high-rate (∼105 s−1) deformation using dynamic channel angular pressing (DCAP). It has been shown that the SMC+NC structure of copper is thermally stable upon heating to a temperature of 150°C. It has been found that the ρ/ρ0 ratio of copper with an SMC+NC structure at a temperature of 4.2 K is considerably (by 5 times) higher than ρ/ρ0 of copper in the annealed coarse-grained state. This effect is due to a high concentration of defects and a high degree of dispersity of the copper structure after DCAP. Changes in the microhardness and in the resistivity (at a temperature of 4.2 K) of the SMC+NC copper after annealing characterize the level of relaxation processes.