Papers by Author: Lev Polyakov

Abstract: Annealed oxygen-free and tough-pitch copper samples have been processed by equalchannel angular pressing (ECAP) by route BC. The samples included 8 x 8 mm section pieces and a 40 mm diameter bar. Thermal stability was assessed based on the changes in the standard mechanical properties (conventional yield strength, tensile strength, elongation, proportional elongation and contraction) after annealing at different temperatures for 1 hour. Thermal stability of the same grade of material has been found to be different for different batches and to depend on the structural conditions of deformed material. The zone of thermal stability for copper of the two grades of interest does not depend on the material’s chemical composition.

Abstract: Annealed commercially pure copper and titanium were processed by equal-channel angular pressing (ECAP) using routes BC and C. Pressed samples had a square section with a side length of 8 mm. Anisotropy was analyzed based on the conventional yield strength at compression in three mutually perpendicular directions. The tests were conducted with the as-received materials and after different number of ECAP passes. For copper, which is isotropic in the as-received state, considerable anisotropy manifests itself after the first pass and holds out after the subsequent passes. Anisotropy of as-received titanium holds out at further pressing and makes about 25 percent. Anisotropy after all the passes is similar both for room and elevated temperature. Anisotropy manifests itself during pressing in both routes, BC and C, although its behavior differs for the different routes.

Abstract: During the last decade it has been shown that severe plastic deformation (SPD) is a very effective for obtaining ultra-fine grained (UFG) and nanostructured materials. The basic SPD methods are High Pressure Torsion (HPT) and Equal Channel Angular Extrusion (ECAE). Recently several new methods have been developed: 3D deformation, Accumulative Roll Bonding, Constrained Groove Pressing, Repetitive Corrugation and Straightening, Twist Extrusion (TE), etc. In this paper the twist extrusion method is analyzed in terms of SPD processing and the essential features from the “scientific” and “technological” viewpoint are compared with other SPD techniques. Results for commercial, 99.9 wt.% purity, copper processed by TE are reported to show the effectiveness of the method. UFG structure with an average grain size of ~0.3 μm was produced in Cu billets by TE processing. The mechanical properties in copper billets are near their saturation after two TE passes through a 60º die. Subsequent processing improves homogeneity and eliminates anisotropy. The homogeneity of strength for Cu after TE is lower than after ECAE by route BC, but higher than after ECAE by route C. The homogeneity in ductility characteristics was of almost of inverse character. The comparison of mechanical properties inhomogeneity in Cu after TE and ECAE suggests that alternate processing by ECAE and TE should give the most uniform properties.

Abstract: Titanium VT1-0 was processed by four passes of equal-channel angular pressing (ECAP) using two routes, BC and C. Pressed samples had a square section with a side length of 8 mm. Mechanical properties at tension (tensile strength, conventional yield strength and elongation) were determined at 9 points across the sample using small-size specimens, 1.5 mm in diameter, cut out along the pressing direction. Heterogeneity in the mechanical properties across the sample was determined based on the value of the variation coefficient. One can observe that heterogeneity in the mechanical properties generally tends to decrease with the number of ECAP passes.