Myrjam Winning

On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals

Abstract Aluminum bicrystals with symmetric 〈1 1 2〉 tilt boundaries and misorientations of 8.7° (small angle), 15.4° (transition), and 31.5° (large angle) were deformed in a channel die experiment in order to study the influence of misorientation on the deformation at grain boundaries. Samples were characterized by strain measurements and microtexture mappings. The experiments were compared to crystal plasticity finite element simulations. We studied strain heterogeneity at the macroscopic and at the microscopic level. Even macroscopically homogeneous areas showed microscopic heterogeneity in the form of bands of different sets of glide systems. We observed clear effects of the grain boundary misorientation on the deformation kinematics close to the boundaries. The 8.7° grain boundary did not show any orientation change which was interpreted in terms of free dislocation penetration. In contrast, the 15.4° and 31.5° bicrystals showed orientation changes which were attributed to dislocation pile-ups.

Mobility of low-angle grain boundaries in pure metals

The mobility of low-angle grain boundaries in pure metals is reviewed and several theoretical treatments are provided. The approach that provides the best agreement with the available experimental data is one in which the mobility is controlled by vacancy diffusion through the bulk to (and from) the dislocations that comprise the boundary that are bowing out between pinning points. The pinning points are presumed to be extrinsic dislocations swept into the boundaries or grown in during the prior processing of the material. This approach yields a mobility that is constant with respect to misorientation angle, up to the transition to the high-angle regime. For small misorientations of the order 1°, however, the mobility appears to increase with decreasing misorientation angle.

In-situ observations of coupled grain boundary motion

Coupled grain boundary migration and sliding were observed in-situ in Al bicrystals with <112>- and <100>- symmetric tilt grain boundaries by using X-ray diffraction. The motion of the grain boundaries was induced by a shear stress that allows the grain boundaries to move parallel as well as perpendicular to the applied stress. The activation parameters for the coupled motion were determined and compared with experimental results of the pure migration process under the influence of a mechanical stress. The measured ratios of the displacements of the migration and sliding were also compared with results of simulations of the coupled grain boundary motion.

Thermal Stability of ECAP Processed Pure Cu and CuZr

Abstract Samples of pure Cu and Cu with 0.17 wt.% Zr were processed by equal-channel angular pressing up to 12 passes with a 90° die angle using route Bc. Microstructure evolution after deformation and subsequent heat treatment of this severely deformed material was investigated. The deformed and annealed states were characterized by electron back scatter diffraction, transmission electron microscopy, optical microscopy and microhardness tests. The microstructural changes were compared to the behavior of specimens produced by conventional cold rolling. The annealing phenomena were analyzed with regard to recrystallization and grain growth mechanisms. It is shown that the microstructural change during annealing of equal-channel angular pressing deformed specimens has to be associated with discontinuous recrystallization. CuZr samples showed a significantly higher thermal stability against recrystallization than pure Cu samples. This is demonstrated to be primarily due to impurity drag.

ECAP Processed Copper during Deformation and Subsequent Annealing

Microstructure and texture evolution of pure copper (99.95%) after Equal-Channel Angular Pressing (ECAP) and subsequent heat treatment were investigated. Initially the material was subjected up to twelve passes with a 90° die angle using route Bc. This resulted in an equivalent strain of 13.8. After deformation the samples were annealed at different temperatures. The deformed and annealed states were characterized by using the crystallographic texture analysis, EBSD measurements and microhardness tests.

3D Tomographic EBSD Measurements of Heavily Deformed Ultra Fine Grained Cu-0.17wt%Zr Obtained from ECAP

Obtaining knowledge on the grain boundary topology in three dimensions is of great importance as it controls the mechanical properties of polycrystalline materials. In this study, the three dimensional texture and grain topology of as-deformed ultra fine grained Cu-0.17wt%Zr have been investigated using three-dimensional orientation microscopy (3D electron backscattering diffraction, EBSD) measurements in ultra fine grained Cu-0.17wt%Zr. Equal channel angular pressing was used to produce the ultra fine grained structure. The experiments were conducted using a dual-beam system for 3D-EBSD. The approach is realized by a combination of a focused ion beam (FIB) unit for serial sectioning with high-resolution field emission scanning electron microscopy equipped with EBSD. The work demonstrates that the new 3D EBSD-FIB technique provides a new level of microstructure information that cannot be achieved by conventional 2D-EBSD analysis.

A Texture Component Model for Predicting Recrystallization Textures

The study presents an analytical model for predicting crystallographic textures and the final grain size during primary static recrystallization of metals using texture components. The kinetics is formulated as a tensorial variant of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The tensor form is required since the kinetic and crystallographic evolution of the microstructure is described in terms of a limited set of growing (recrystallizing) and swept (deformed) texture components. The number of components required defines the order of the tensor since the kinetic coupling occurs between all recrystallizing and all deformed components. The new method is particularly developed for the fast and physically-based process simulation of recrystallization textures with respect to processing. The present paper introduces the method and applies it to the primary recrystallization of low carbon steels.

Texture Evolution during Grain Growth under the Influence of Mechanical Stresses

In order to study the influence of mechanical stress fields on the kinetics and texture evolution of grain growth, experiments were performed on high purity aluminium. Samples were annealed under the influence of different mechanical stresses. The temporal evolutions of grain sizes and of macrotexture were analysed in ependence on the applied stress. The results show that mechanical stresses can change the kinetics of grain growth and slow down the increase in the grain size. Also effects on the texture evolution were observed and shall be discussed.

Influence of Grain Boundary Mobility on Microstructure Evolution during Recrystallisation

The paper introduces first investigations on how low angle grain boundaries can influence the recrystallisation behaviour of crystalline metallic materials. For this purpose a three-dimensional cellular automaton model was used. The approach in this study is to allow even low angle grain boundaries to move during recrystallisation. The effect of this non-zero mobility of low angle grain boundaries will be analysed for the recrystallisation of deformed Al single crystals with Cube orientation. It will be shown that low angle grain boundaries indeed influence the kinetics as well as the texture evolution of metallic materials during recrystallisation.

Recrystallization and Grain Growth in Ultra Fine Grained Materials Produced by High Pressure Torsion

Investigations of the microstructure of materials processed via severe plastic deformation methods such as high pressure torsion (HPT) and their recrystallization behaviour is of great interest as they are capable of producing ultra fine grained material (UFD) with good mechanical properties.