G. Gottstein

Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60

Abstract The mechanisms of plastic deformation and dynamic recrystallization (DRX) in a Mg–5.8% Zn–0.65% Zr alloy were studied by compression tests at temperatures between 423 and 723 K and at strain rates ranging from 10−5 to 10−1 s−1. It was shown that the mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature. Low-temperature DRX (LTDRX below 473 K) was associated with the operation of twinning, basal slip and (a+c) dislocation glide. In the intermediate temperature range (473–523 K) continuous DRX (CDRX) was observed and associated with extensive cross-slip due to the Friedel–Escaig mechanism. At temperatures ranging from 573 to 723 K both bulging of original grain boundaries and subgrain growth were the operating DRX mechanisms and controlled by dislocation climb.

Work hardening in heterogeneous alloys—a microstructural approach based on three internal state variables

Abstract A new work-hardening model for homogeneous and heterogeneous cell-forming alloys is introduced. It distinguishes three internal state variables in terms of three categories of dislocations: mobile dislocations, immobile dislocations in the cell interiors and immobile dislocations in the cell walls. For each dislocation population an evolution law is derived taking into account dislocation generation, annihilation and storage by dipole and lock formation. In particular, these rate equations take into account the number of active glide systems and, thus, introduce texture in the model in addition to the Taylor factor. Microstructure is represented by the dislocation cell structure as well as second-phase particles, which may undergo changes by precipitation and Ostwald ripening. Interaction of mobile dislocations with the microstructure is taken into account through an effective slip length of the mobile dislocations. For the same set of parameters, the predictions are in excellent agreement with measured stress–strain curves of both a precipitation-hardened aluminium alloy (Al–4.16 wt% Cu–1.37 wt% Mg, AlCuMg2) and a precipitation-free model alloy (Al–0.35 wt% Cu–0.25 wt% Mg), the composition of which corresponds to the matrix of the two-phase alloy.

Necklace formation during dynamic recrystallization: mechanisms and impact on flow behavior

Abstract The aim of the current study was to investigate the nucleation mechanisms of new grains during DRX and the deformation behavior of a necklace structure. The investigations were conducted on Ni 3 Al, because Ni 3 Al develops a distinct necklace structure during dynamic recrystallization DRX. Local orientation measurements were conducted to determine misorientations between new recrystallized grains and their parent grains. DRX was set off by strain induced bulging of prior grain boundaries. Additionally, the formation of new grains by recrystallization twinning was observed. With progressing DRX the orientation coherency of DRX grains with the matrix grains diminished rapidly, and the texture tended to randomize. The strain rate sensitivity indicated superplastic flow in the recrystallized volume. The deformation behavior changed significantly, when these soft regions formed a contiguous 3D network along the original grain boundaries. A new model for the flow curve is proposed that accounts for the percolation character of necklace structures.

Influence of triple junctions on grain boundary motion

Abstract The paper is dedicated to the steady state motion of the grain boundary systems with the triple junctions. The main features of one of the systems where the steady state motion is possible are considered. In the experimental part the special technique of in-situ observations and recording of the triple junction motion and the results of the experiments on the tricrystals of Zn are described comprehensively. It was shown, in particular, that the described method makes it possible to study the motion of a grain boundary system with a triple junction and, what is of importance to measure its mobility. The shape of the moving half-loop in the tricrystal fits the theoretically calculated. The transition from the motion controlled by the triple junction kinetics to the boundary kinetics is observed. By this is meant that the triple junction along with the other structural defects can drag the boundary motion, or, conversely, and their role and properties should be taken into consideration in theories of grain growth.

Simulation of primary recrystallization using a modified three-dimensional cellular automaton

A modified three-dimensional cellular automaton algorithm has been developed to simulate primary static recrystallization of cold-worked metals. The driving force for the nucleation and growth of recrystallized grains is the volume free energy due to the stored dislocation density of the deformed matrix. Growth terminates upon impingement. The code allows the introduction of specific conditions for recovery, nucleation and grain boundary motion. The model is capable of simulating kinetics, microstructure and texture development during recrystallization. The modification of the cellular automaton approach allows for the simulation of an orientation dependent growth rate. A novel algorithm minimizes the computational load and computer memory requirement.

Grain boundary distribution and texture in ultrafine-grained copper produced by severe plastic deformation

Abstract Microstructure, micro- and macrotexture and grain boundary distributions, which had been generated by severe plastic deformation (using the equi-channel angular technique) were studied. Grain refinement from 23 to 0.21 m accompanied by drastic changes of texture and GB distribution occurred during deformation. More than 90% of grain boundaries were found to be high-angle boundaries. The fractions of low-angle and twin boundaries were considerably lower than in the original state of the material. Generally, the GB distribution tended towards a random misorientation distribution. The formation of this type of microstructure can be explained by the concurrent action of dynamic recovery and a rotational mode of severe plastic deformation.

On the effect of purity and orientation on grain boundary motion

The influence of impurities on grain boundary mobility in a Σ7 (38.2° ) and in an off-coincidence boundary (40.5° ) was investigated. The grain boundary mobility was found to strongly depend on grain boundary crystallography and material purity. The measured concentration dependence of activation enthalpy and preexponential mobility factor did not comply with predictions of traditional impurity drag theory. An extended impurity drag theory is presented that takes into account interaction of the adsorbed atoms in the boundary. This theory predicts a concentration dependence of the activation enthalpy. For the Σ7 boundary it can explain qualitatively the frequently observed high values of preexponential mobility factor and activation enthalpy. The compensation temperature was found to depend on composition.

The effect of triple-junction drag on grain growth

Abstract Current theories of grain growth presume that grain boundary migration is the rate-limiting step, and either explicitly or implicitly assume that triple junctions can always move with sufficient speed to accommodate the changing positions of the grain boundaries. Following from some recent observations of triple-junction drag effects in tricrystals of zinc and in molecular dynamics models, an analytical theory is developed to explore the effects of triple-junction drag upon grain growth, for a two-dimensional solid. The theory is developed in the framework of the Von Neumann–Mullins formulation, and demonstrates that drag effects operating exclusively at the triple junctions result in a retardation of grain growth. The stability of six-sided grains in the isotropic, drag-free case of the Von Neumann–Mullins analysis is successively extended to grains of 6± N sides, where N increases with the strength of the triple-junction drag.

Triple junction drag and grain growth in 2D polycrystals

Abstract The process of grain growth in 2D systems is analyzed with respect to the controlling kinetics: from solely boundary kinetics, when grain growth in a polycrystal is determined by the Von Neumann–Mullins relation, to exclusively triple junction kinetics, when grain growth is governed by the mobility of triple junctions. It is shown that in the “intermediate” case, when the driving force for grain boundary motion and the characteristic mobility are grain boundary curvature and grain boundary mobility, respectively, a limited mobility of triple junctions essentially influences grain boundary motion. The Von Neumann–Mullins relation does not hold anymore, and this is the more pronounced the smaller the triple junction mobility. In the case where grain growth is determined by the mobility of grain boundary triple junctions (triple junction kinetics) all grains are transformed into polygons in the course of grain growth. Grain growth would cease if all grains assumed the shape of regular polygons, not only hexagons like in the Von Neumann–Mullins case. The only exceptions are triangles: they collapse without transforming into a polygon. The respective relation for the rate of a change of grain area under triple junction kinetics is obtained and discussed with regard to microstructure evolution.

The evolution of recrystallization textures in body centred cubic metals

Abstract A model of the evolution of recrystallization textures for b.c.c. metals was suggested based on the ODF and EBSD analysis of the texture change during annealing of cold rolled steel. The model is composed of two principles: first, the maximum principal stress direction in the deformed grain becomes the minimum Youngs modulus direction in the recrystallized grain; second, a {110} plane, parallel to the maximum principal stress direction (hence, minimum Youngs modulus direction), is taken for variant selection. The current model is successfully applied to describe the evolution of recrystallization textures in various b.c.c. transition metals such as tantalum, molybdenum and tungsten.