Papers by Author: Stefan Zaefferer

Abstract: 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.

Abstract: Microstructural evolution during the annealing of cold rolled Mg, Mg-1.5Nd and Mg-3Y sheets has been examined. The experimental results show a significant difference in recrystallization kinetics and grain growth between pure Mg and Mg-RE alloy sheets. Pure Mg sheet shows rapid recrystallization and grain growth, whereas recrystallization is considerably retarded in the Mg-RE alloys. Although recrystallized grains which are triggered at shear bands in the cold rolled pure Mg sheet show a relatively weak texture with a basal pole split into the sheet rolling direction, rapid grain growth is accompanied by re-strengthening of the basal-type texture. In contrast, a weak texture appears in the early recrystallization stage in Mg-RE alloys and is retained during annealing due to retarded recrystallization and grain growth.

Abstract: The relevance of EBSD-based investigations for statements on the macroscopic or mesoscopic behavior of materials is critically relying on the statistical representativeness of the data. Particularly, the statistical reliability of the EBSD-based results (e.g. texture, phase fraction or grain size) remains an open question since the areas observed by the EBSD technique are quite small compared to XRD techniques. It has already been shown that covering larger areas and probing more grains with the help of large step sizes is beneficial in terms of representativeness [1]. On the other hand, small step sizes are beneficial in terms of grain reconstruction and data clean-up. However, step sizes significantly smaller than the average grain size of the material lead to either covered areas or number of probed grains being too small to be representative or to very large datasets and correspondingly long measurement times. In this contribution, the benefits of a new mapping technique [1] that joins the advantages of large and small step size measurements will be demonstrated. The representativeness of the EBSD datasets obtained by classical and this new mapping techniques were compared by calculating the pole figure symmetries of a TRIP steel. The results show that the proposed mapping technique significantly improves the reliability and representativeness of EBSD-based texture measurements.

Abstract: The role of rare earth addition on the microstructure and texture during recrystallization of cold rolled sheets is investigated by a comparative study of pure Mg, Mg-3Y and Mg1.5Nd sheets. In pure Mg, nucleation occurs mainly at shear bands which results in a texture weakening. The basal-type texture re-strengthens rapidly during grain growth of the pure Mg sheet. In contrast, in the Mg-RE alloys the weaker texture formed during early recrystallization strage is retained during further annealing due to retarded grain growth. Uni-axial tensile and Erichsen tests show that ductility and sheet formability are significantly improved by addition of rare earth elements.

Abstract: The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth.

Abstract: The influences of rare earth elements addition on the crystallographic texture and microstructural evolutions are examined during rolling and annealing of Mg-sheets. In case of Nd or Y additions, dynamic recrystallisation is suppressed such that the deformed microstructure is observed after hot rolling with relatively large strain per pass. Cold rolled binary Mg-Nd alloy sheet shows strong texture with splitting of the basal poles in the rolling direction, however, the texture intensity decreases significantly during the recrystallisation annealing. From the comparison of deep drawing behaviours between commercial ZE10 and AZ31 sheets, it is observed that the addition of the rare earth elements and accompanying texture changes result in the improved formability.

Abstract: A fine grained and recovered structure with densely formed fine particles has been created by means of a thermomechanical process (TMP) in Fe3Al (bcc derivative structure)-based alloys. The TMP consists of the following two parts; the first part is grain refinement and the second the creation of recovered structure stabilized by fine particles. It was found in the first part that coarse particles are required in the process of deformation to refine grain size by particle stimulated nucleation (PSN) mechanism. Fine M2C type carbide particles were densely precipitated in the second part. These particles were observed to inhibit the growth of subgrains formed around coarse particles during annealing at 700°C. This result suggests that if the fine particle density remains high, the recovered structure can be maintained at 700°C.

Abstract: A Goss-oriented single crystal was cold rolled up to 89 % thickness reduction, and subsequently annealed at 550°C or 850°C. During deformation most of the initially Goss-oriented material rotated into the two symmetrical {111}<112> orientations. In addition, Goss regions were observed related to microbands or microshear bands. Goss regions in microshear bands formed during straining, whereas Goss regions between microbands were retained from the initial Goss orientation. The recrystallisation texture for annealing temperatures of both 550°C and 850°C is characterised by a Goss texture. However, the origin of the Goss recrystallisation nuclei appeared to be different for the different annealing conditions. In the material annealed at 550°C, the Goss texture originated from the Goss regions in the microshear bands. In contrast, for an annealing temperature of 850°C, the Goss grains between the microbands are likely to form recrystallisation nuclei.

Abstract: Recrystallisation behavior was studied in two Fe3Al-based alloys containing both large and fine particles with a different fine particle dispersion level using high-resolution SEM and EBSD. High misorientation of 15-30° was created around large particles after a hot rolling process in the two alloys. The kinetics of recrystallisation were, however, considerably retarded in the alloy containing dense fine particles. It was observed that the growth of subgrains created around the large particles was inhibited by the presence of the fine particles. This result clearly suggests that when the particle density (Ns) is high relative to local stored energy (E) around large particles, nucleation can be completely hindered. As the Ns/E level decreases, nucleation may occur and the kinetics of recrystallisation might be determined by both the nucleation rate and the growth of nuclei into a matrix with fine particles.

Abstract: A Co-20at%Ni polycrystal produced by electrodeposition was studied in its detail using orientation microscopy. By analyzing the local crystallographic texture, grain morphology and the grain boundary character on three distinct sections of the film, we have obtained a complete understanding of its microstructure. The microstructure and grain morphology is very complex, consisting of grains elongated in deposit direction, clusters of coarse and fine grains and further very fine structures. The deposition parameters generate a strong (1120)//ND texture and the columnar grain reveals inner orientation gradients along the growth direction