Materials Science Forum Vol. 753

Abstract: EBSD investigation of texture and microstructure evolution during a complete thermomechanical treatment of commercially pure titanium (HCP-Ti) is presented. Titanium was cold rolled to reach various degrees of thickness reduction: 20%, 40% and 60%. Next, annealing in air atmosphere was conducted at different conditions to achieve the recrystallized state. EBSD topological maps were measured on RD-TD and RD-ND surface of each sample. Strong heterogeneity of deformed titanium microstructures is described with focus on the important role of twinning mechanisms. Texture evolution in investigated titanium appears to be limited, especially in recrystallized state. However some subtle mechanisms are discussed.

Abstract: The evolution of texture is studied in high purity (~99.7%) nickel sheets with widely different starting cube texture ({001}) intensities following heavy cold rolling and annealing. For this purpose two nickel sheets with strong and weak starting recrystallization cube texture (SSCT and WSCT, respectively) prepared by Accumulative Roll Bonding and conventional rolling, respectively, followed by annealing are used as the starting materials for subsequent processing. These sheets are cold rolled to 90% reduction in thickness and annealed at different temperatures. Profuse cube oriented bands could be identified in the SSCT nickel sheet after 90% cold rolling as opposed to rather insignificant presence of cube regions in the WSCT nickel sheet. However, the WSCT nickel sheets consistently show stronger cube texture after annealing treatments as compared to the SSCT material. The failure to observe recrystallization cube texture in SSCT is attributed to the inhibited nucleation of cube grains owing to the unfavorable misorientation environment surrounding cube regions in the deformed matrix.

Abstract: The static recrystallisation behaviour of two magnesium alloys after hot rolling have been examined. The alloys chosen for study were the conventional alloy AZ31, and an alloy containing the rare earth element Gadolinium. The recrystallisation kinetics were lower for the rare-earth alloy at low annealing temperatures, but at high annealing temperatures the kinetics were higher for the rare-earth alloy. It is suggested that this change in the comparative recrystallisation kinetics is a result of the improved mobility of the rare-earth solute at higher temperatures. This affects the recrystallisation kinetics through solute partitioning to the grain boundaries. The effect of this segregation on the recrystallisation texture is also discussed.

Abstract: The evolution of the recrystallization microstructure at different temperatures and the related change in the microhardness were studied in an Al-6061 alloy after 2 turns of high-pressure torsion (HPT) by means of Vickers microhardness and EBSD analysis. The corresponding results show that HPT processing introduces a significant grain size refinement for the deformed state by comparison with the 150 µm grain size in the initial material. The microstructure after annealing at 200°C is relatively close to the as-processed one and the distribution of misorientation angles remains essentially unchanged. However, significant microstructural changes are produced with increasing annealing temperature and at the highest temperature (400°C) there is an abnormal grain growth. The variation of Vickers microhardness (Hv) after the different annealing treatments show a gradual decrease when the temperature increases.

Abstract: During abnormal grain growth, a few Goss grains grow exclusively fast and consume the matrix grains. The Goss abnormally-growing grain (AGG) has peculiar features which are irregular grain boundaries and very high frequency of peninsular grains nearby the growth front of AGG and island grains trapped inside AGG. These features might provide a clue for clarifying the mechanism of Goss AGG. The experimentally-observed microstructural feature and grain boundary characterization of Goss were approached by the solid-state wetting mechanism. In this study, observing the three-dimensional wetting morphology in serial section images of Goss AGG by EBSD, we report some direct microstrucrual evidence supporting solid-state wetting mechanism for Goss AGG. The solid-state wetting mechanism for the evolution of the Goss AGG in Fe-3%Si steel explains the microstructural features evolved during secondary recrystallization, which cannot be approached by the conventional theories based on the grain boundary mobility.

Abstract: Changes in grain size, texture and misorientation distributions have been monitored during extensive normal grain growth in 3%Si steels. The boundary misorientation distributions deviate significantly from the Mackenzie relationship. Comparisons of correlated and uncorrelated distributions show large excesses of low angle boundaries. However, these are not a result of low energy boundaries being favoured during grain growth since the deviation diminishes as growth proceeds. The effect originates in the nucleation of grains in colonies of similar orientation during primary recrystallisation. A slight tendency for promotion of 60º boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel.

Abstract: Much attention has been generated by nanocrystalline materials and their promise of enhanced properties. However, this class of materials suffers from an inherent instability with respect to thermally induced grain growth. Substantial microstructural coarsening is frequently observed in elemental samples even under ambient conditions, and more often than not, the mode of grain growth in such specimens is abnormal. Although deliberately adding an atomic species that segregates to the grain boundaries has been shown to stabilize the microstructure of nanomaterials to high fractions of the melting point, once grain growth commences, it generally proceeds in a highly abnormal manner in these samples, as well. In the case of Fe-based nanocrystalline alloys, abnormal growth appears to be intimately tied to the alpha-to-gamma phase transformation just above 900°C. For Ni-based segregation-stabilized specimens, however, there is no such allotropic transformation; nevertheless, the microstructure after growth manifests the same telltale signs of abnormality.

Abstract: Under certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate.

Abstract: Modern steelmaking technologies utilizing microalloyed steel designs have been responsible for enormous economic benefits for both the steelmaker and fabricator. What has not been acknowledged is the environmental benefit that high strength steels have produced in terms of reduced steel usage in major infrastructure projects. The judicial use of microalloying has the potential to further reduce total tonnage requirements while delivering enhanced operational performance and service life. Various projects around the world have begun to recognize these recent microalloying developments. This paper will present the grain coarsening behavior of the new generation of Nb bearing steels, which have been used in major international steel fabrication projects.

Abstract: Texture change during grain growth in Fe-3%Si non-oriented electrical steel was investigated. Cold rolled steel, 0.35mm in thickness, was annealed and recrystallized as an initial structure. Normal grain growth and abnormal grain growth occurred by additional annealing. {111} was dominant in the initial texture. However {100} component, which was not in majority in the initial structure, became stronger after normal grain growth. It was revealed that an average grain size of {100} in the initial structure was bigger than those of other components by analysis of the EBSD data,. Therefore, it is concluded that {100} strengthened after normal grain growth due to its size advantage. On the other hand, {111} components became more stronger after abnormal grain growth. It is inferred that another mechanism of the texture change worked in abnormal grain growth.