Nature of high-angle grain boundaries in metals subjected to severe plastic deformation

The evolution of structural parameters of deformation fragments and dynamically recrystallized grains during severe plastic deformation in a Bridgman chamber as the number of complete revolutions in torsion at room temperature increases is analyzed using transmission electron microscopy and electron backscattered diffraction. It is found that the formation of a significant amount of high-angle grain boundaries in the structure is caused not by deformation fragmentation but by the low temperature dynamic recrystallization.     

Mechanism of strain hardening and dislocation-structure formation in metals subjected to severe plastic deformation

The equations of dislocation kinetics are used to theoretically analyze the mechanism of strain hardening and the formation of fragmented dislocation structures in metals at large plastic strains. A quantitative analysis of the available data on aluminum and an aluminum-magnesium alloy shows that strain hardening at large plastic strains and the formation of fragmented dislocation structures are related to the interaction and self-organization of geometrically necessary dislocations (GNDs). On the microscale, the source of the GNDs is a locally nonuniform plastic deformation induced by a dislocation-density gradient in dislocation-cell boundaries.     

Distinctive features of plastic strain localization under severe plastic deformation of metals

Mechanical testing is performed and the structure of zirconium and aluminum predeformed by ∼450% using multiaxial forging (MAF) and equal channel angular pressing (ECAP) is investigated. Tensile loading tests of the severely deformed specimens exhibited their tendency to necking, with the ductility of the material in the neck, however, being superior to that in the neck of initial coarse-grained specimens. The results of the experiments imply that a fundamental stage of plastic flow of solids under severe plastic deformation (SPD) is the formation of cellular-banded structure and strain localization in the fine-grain bands. This considerably retards further deformation-induced refinement of the structure by SPD, and also results in the rapid formation of a fracture neck in the materials with this structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 43–49, November, 2007.     

Surface effects in plastic deformation of metals

It is now accepted that the appearance of slip bands on the surface of a plastically deformed metal is evidence that the deformation is not homogeneous but is concentrated on relatively few atomic planes. Recent microscopical experiments have suggested that this conclusion is only valid in the later stages of deformation and that the first fractional per cent of strain is much more nearly homogeneous. Theories to account for both these stages of deformation are examined in the light of microscopical evidence.

The validity of conclusions about internal processes based on experiments on the surface is discussed; it is shown that the surface finish affects not only the appearance of internal processes but also the processes themselves.

In cases where the deformation is not homogeneous the balance of evidence is that it is also not continuous in time: instead, slip on an active slip plane tends to a limit which is reached either gradually or suddenly depending on the nature of the metal and the conditions of stress. The same processes which stop slip on the active planes produce general hardening of the metal. However, slip can restart on or near to former slip planes as a result of mechanisms activated by temperature and stress, and can, in favourable cases, continue until fracture. Therefore slip bands, the sources of hardening, are also places of weakness.     

Modern aspects of plastic deformation in metals

After a historical survey of crystal plasticity the rôle of dislocations in plastic deformation of metals is outlined. The theory of plastic deformation in metal single crystals before the impact of transmission electron microscopy is described. Recent experimental results on glide and workhardening in single crystals of b.c.c. metals are reviewed and explained by a simple dissociation model of the screw-dislocation cores. A relative success of this explanation together with calculations of atomic structure of dislocation cores support the conclusion that different structures of screw dislocation cores are responsible for both differences and similarities between plastic behaviour of f.c.c. and b.c.c. metals. Further developments in the field of metal crystal plasticity by modern experimental techniques (transmission electron microscopy of deformed crystals in the stress-applied state, magnetic studies of dislocations in ferromagnetic crystals) and by atomic calculations of defect configurations are discussed.Na Slovance 2, Praha 8, Czechoslovakia.Invited paper presented on a plenary session of the First European Conference on the Physics of Condensed Matter organized by the Board of the Condensed Matter Division of E.P.S., in Florence on 14–17 September 1971 (Chairman: Prof. S. F. Edwards; Schuster Laboratory, University of Manchester).     

Study of the heat treatment influence on the formation of nanostructured state in bulk titanium nickelide alloys subjected to severe plastic deformation

The influence of annealing on bulk samples of Ti_49.4Ni_50.6 alloy subjected to severe plastic deformation by torsion under high pressure has been studied by transmission electron microscopy and X-ray diffractometry. It is found that a homogeneous nanocrystalline state is formed in the bulk samples after annealing.     

Evolution of mechanisms of plastic deformation in porous metals

An experimental study was made of the structure of deformed porous iron with a porosity P ranging from 4 to 40% within the range of strains up to fracture. Measurements are made of grain-boundary slip and rotations of structural elements and their individual contributions to the total strain as a function of P and . It is established that there is a change in the dominant mechanisms of plastic deformation with an increase in porosity, this change being connected with a change in the topological characteristics of the system and corresponding to the transition from intergranular slip to a group of mechanisms that ensures movement of the grains as a whole. The established laws are analyzed on the basis of the concept of structural levels of deformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–105, January, 1996.     

Plasticity limit of metals during hot plastic deformation

A model of plasticity limit has been derived in the condition of hot plastic deformation, where dynamic recrystallization takes place, through the ratio between the rate of grain boundary sliding and the overall deformation rate. If fracture occurs preferentially at the grain boundaries we can replace the grain boundary deformation through the energy needed to cause fracture and express the temperature influence on the deformation stress. The plasticity limit is then the function of Zener-Hollomon parameter and deformation stress, where the exponent of deformation stress has a value of –4·3.     

Hardening and softening during plastic deformation of hexagonal metals

The influence of temperature on the deformation behaviour of single crystals and polycrystals is investigated. The temperature dependences of the critical resolved shear stress, work hardening rates, the stress at the onset of stage C., the yield stress and maximum stress are reported. Possible deformation mechanisms concerning hardening and softening during plastic deformation of hexagonal metals are discussed.     

Dislocation evolution in titanium during surface severe plastic deformation

Surface mechanical attrition treatment (SMAT) is an innovative technique which can produce nanocrystalline (nc) layers of several tens of micrometers thickness on surfaces of metallic materials. In this work, the grade structures of commercially pure titanium (CP Ti) processed by SMAT was studied intensively, and the microstructure observations indicated that the dislocation evolution could be separated into three steps: (1) formation of dislocation tangles; (2) formation of dislocation bands; and (3) dynamic recrystallization of dislocation bands until the formation of nc Ti.     

Microstructure fragmentation in Fe-O alloy during severe plastic deformation

The microstructure evolution in Fe-O alloy during mechanical ball milling is considered. It is shown that severe plastic working leads to the formation of a nanocrystalline structure. High internal stress facilitates an increase in the density of grain boundaries.