Dorte Juul Jensen

Development of microstructure in FCC metals during cold work

The evolution in microstructure and local crystallography is described, with emphasis on the behaviour of medium to high stacking fault energy FCC metals deformed at low temperature. This evolution is analysed within a common framework of grain subdivision on different size scales. The largest scale is the macroscopic subdivision of crystals or grains, which will be presented using the behaviour of single crystals as an example. Subdivision on smaller scales is demonstrated for polycrystals, with emphasis on the effects of strain and grain orientation on the evolution in microstructure and local crystallography. This leads to an introduction of structural parameters and their analysis using a scaling hypothesis. A key finding is the correlation between structure and crystallographic orientation, which allows a slip pattern description by standard crystal plasticity models leading to an analysis of the relationship between slip pattern and microstructure. These findings are briefly related to the macroscopic properties of deformed metals.

Modelling flow stress anisotropy caused by deformation induced dislocation boundaries

Models have been developed for the combined effect of texture and microstructure on the flow stress anisotropy of metals containing dislocation boundaries with a macroscopic orientation with respect to the sample axes. These are the Taylor and the Sachs models modified to include the anisotropic critical resolved shear stress from the dislocation boundaries. The model predictions have shown that the presence of dislocations in an idealized configuration has a significant effect on the anisotropy caused by the crystallographic texture. These model predictions have been tested for a number of materials parameters. Modelling results have finally been compared with measurements of flow stress anisotropy in aluminium sheets cold rolled 18%, and good agreement has been found.

Deformation and Recrystallization Textures in Commercially Pure Aluminum

The deformation and recrystallization textures of commercially pure aluminum (99.6 pct) containing large intermetallic particles (FeAl3) are measured by neutron diffraction, and the orientation distribution functions (ODF’s) are calculated. Sample parameters are the initial grain size (50 and 350 μm) and the degree of deformation (15 to 95 pct reduction in thickness by cold-rolling). The textural results are compared with microstructural observations and good correlations are found. The intermetallic particles may act as nucleation sites giving nuclei with a wide spread of orientations. Thereby the particles can have a randomizing effect on the textural development during recrystallization. In specimens deformed at medium degrees of deformation the randomizing effect of particles is maximum. At lower and higher degrees of deformation the effect of particles is less as other nucleation sites become more effective. In general, the randomizing effect of particles is limited due to a low growth rate of nuclei of random orientation compared with nuclei of other orientations.

The role of grain size and strain in work hardening and texture development

Polycrystalline copper (99.999 pct) having four different grain sizes (from 4 to 220 μm) was strained in tension at room temperature to true plastic strains of 0.05, 0.10, 0.20, and 0.30. The initial texture of the materials was determined by neutron diffraction, as were the deformation textures. Both inverse pole figures and calculated TaylorM factors were then derived from the data. In general, it was observed that the texture strengthens at increasing strain and that the effect of grain size on this development is not very pronounced. The measured change in the volume concentration of the (111) texture component was compared to that obtained from a model simulation, and in general, the experiments and the simulations agreed well. The effect on the flow stress of the initial texture, and on the texture which develops during straining, could be accounted for on the basis of TaylorM factors calculated from the experimental results, and it was found that there is an effect of texture on the flow stress. The flow stress at strains above yield, expressed as a modified Hall-Petch relationship, was not greatly affected by corrections toM induced by strain and grain size.

Fast Texture Measurements Using a Position Sensitive Detector

A technique for fast texture determination by neutron diffraction is described. With the technique a complete texture analysis requires from 15 to 45 minutes measuring time and the kinetics of the development in single texture components can be studied with a time resolution of the order of seconds. It is shown how these two measuring principles can be used for in-situ kinetic investigations of recrystallization. Independent of speed, texture measurement by neutron diffraction has an advantage in improved statistics which is examplified by a series of measurements on the early stage of texture development in copper and brass.

Effects of Nuclei Clustering on Recrystallization Kinetics

In this work, it is shown that the contiquity ratio, Crr, defined as the fraction of the total grain boundary area of recrystallized grains shared by other recrystallized grains, is a useful experimental parameter for describing the clustering of recrystallization nuclei in partially recrystallized microstructures. Two experimental examples are presented where anisotropic impingement behavior, i.e. non random recrystallized grain distributions, is thought to be important. In the experimental examples, the deviation of Crr from Crr = Vv which is a characteristic of a random distribution, is clearly shown. The results justify the need for these cases to be modelled, as had been done, by impingement patterns other than the simple random nucleation behavior so often postulated without proof in recrystallization kinetics studies.

Deformation induced dislocation boundaries: Alignment and effect on mechanical properties

Abstract The dislocation boundaries formed during cold-rolling of FCC metals have been reported to have a preferred macroscopic direction with respect to the sample axes. However, boundaries have also been reported to form on crystallographic slip planes. The directions of the boundaries formed on crystallographic slip planes are investigated using a newly developed model for formation of such boundaries. It is concluded that these boundaries also lie in the preferred macroscopic direction, i.e. the entire dislocation structure is highly anisotropic. The impact of the anisotropic dislocation structure on the flow stress anisotropy is illustrated by comparison of experimental data and model calculations which take the combined effects of the anisotropic dislocation structure and texture into account.

Textural and Microstructural Evolution during Cold-Rolling of Pure Nickel

High purity nickel (99.99%) with a grain size about 100 {mu}m has been deformed by cold-rolling from 37% to 98% reductions. The deformation microstructures and the crystallographic texture have been characterized using transmission electron microscopy and neutron diffraction, respectively. The microstructural evolution has been described within a general framework which consists of a grain subdivision by dislocation boundaries on a finer and finer scale with increasing strain. The influence of this deformation pattern on the texture development is discussed.

Importance of Local Structural Variations on Recrystallization

Effects of local variations in the deformation microstructure on subsequent recrystallization are discussed and illustrated by three examples. The three examples consider local variations on different length scales and are: 1. Effects of local variations in the deformation microstructure on the formation of protrusions on migrating boundaries. 2. Effects of an inhomogeneous spatial distribution of second phase particles on growth. 3. Effects of stored energy and orientation variations on recrystallization kinetics.

Effects of Initial Parameters on the Development of Cube Texture during Recrystallization of Copper

A series of oxygen free high conductivity copper samples with different initial grain sizes, cold rolling conditions and storage times as well as slightly different impurity contents was used to investigate the effects of these initial parameters on the development of cube texture during recrystallization. For rolling reductions of 90% and 95%, cube textures with volume fractions between 3% and 50% were observed. Higher rolling reduction led to a stronger cube texture. Cube texture development is very sensitive to the initial grain size before rolling. In general, fine grained material gives a strong cube texture after recrystallization, and the requirement on fineness of the grain size may vary for materials with different purity. Large sample widening during rolling can largely inhibit the development of cube texture after recrystallization. Neither storage time, nor the slight change in impurity content had large effects in the present investigation.