Materials Science Forum Vols. 558-559

Abstract: In order to elucidate the predominance of Goss grains after SIBM in electrical steel sheets, Goss, D-Cube and {111}<112> grains after temper rolling of 5 and 9% reduction were observed by TEM. In 5% strain the amount of dislocations in Goss grains was the smallest of the three orientations. In 9% strain dislocations in Goss grains were distributed more heterogeneously than the other two types of grains. It is considered that {111}<112> grains have large amounts of dislocations owing to high Taylor factors and the differences of microstructures between Goss and D-Cube grains are due to orientation stabilities. Goss grains are speculated to be easy to recover and therefore they are predominant after SIBM.

Abstract: The nucleation of new grains during annealing in high purity single crystals of Cu-8%wt. Al alloy with initial (112)[111] orientation has been characterized by detailed transmission electron microscopy (TEM) orientation measurements. The samples were channel-die deformed and then recrystallized to form a substructure composed of a mixture of deformation twins, shear bands (SBs) and the recrystallized grains. Substantial progress in understanding the mechanism of texture transformations at the early stages of recrystallization was possible thanks to orientation mapping using TEM. Microtexture analysis of partly recrystallized samples indicates a simple 25-40o (<111> or <112>) relation, most frequently observed during the early stages of recrystallization between single nuclei and one of the two as-deformed groups of components (twins or matrix). As recrystallization proceeds, recrystallization twinning develops strongly and facilitates rapid growth of the recrystallized phase. The first and higher generations of twins systematically tend to obscure the initial crystallographic relation between the deformed state and the recrystallization nuclei.

Abstract: The relations between the active slip systems within the deformed state and the orientations of the first formed nuclei of recrystallized grains have been studied in strongly deformed samples of high purity aluminium single crystal and polycrystalline aggregates. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM-FEG/EBSD and semi-automatic measurements in TEM. Investigations of the direction of growth indicate the privileged role of {111} planes in the initial stages of recrystallization. The grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the most active {111} slip planes as shown clearly in the stable and structurally homogeneous Goss{110}<001> oriented crystallite. In the case of polycrystalline material the influence of grain boundaries and their triple junctions on the crystallographic aspects of nucleation is discussed.

Abstract: Two low carbon steels alloyed with 0.48wt% and 0.78wt% Cr were warm rolled to 65% reduction at 640°C. Annealing was carried out at 710°C to achieve a range of recrystallized volume fractions up to 100%. Texture analysis was performed by X-ray diffraction and Electron Back Scattering Diffraction. During the initial stages of recrystallization, more recrystallized grains nucleated at shear bands than at grain boundaries in the 0.48wt%Cr steel, whereas this was not the case in the 0.78wt%Cr steel. This is associated with a decrease in the propensity to form shear bands due to the lower amount of C in solid solution in the high-Cr steel. Additionally, the nuclei showed a preference to develop the same fiber as the deformed region within which they nucleated. In both steels, an increase in the annealing time led to the deterioration of the ND (or γ) –fiber and a strengthening of both the RD (or α) –fiber and Goss component. In the fully recrystallized samples, the ND component was somewhat stronger in the steel with the higher Cr content.

Abstract: During casting and homogenisation of aluminium the microstructural fundament for further processing is made. Particle structure (dispersoids and primary particles), grain structure and level of elements in solid solution govern the mechanical and annealing properties of the material. In 3xxx-alloys, Mn in solid solution and Mn-containing dispersoids formed during homogenisation play an important role in controlling the recrystallization behaviour of the material [e.g. 1, 2, 3]. Other elements, such as Si, will have an influence on the formation of dispersoids [4, 5]. Hence, to control the annealing behaviour of the material, it becomes important to control the particle structure. In the present investigation, an AA3103 alloy, and modified versions of this alloy, have been investigated. Various homogenisation treatments have been performed and the resulting material has been studied. Electrical conductivity has been measured and microstructural investigations have been carried out.

Abstract: A high-purity Al alloy and a supersaturated Al-0.3wt.% Sc alloy were accumulative roll bonded (ARB) at 200 °C to generate 0.5 mm gauge sheet consisting of 32 alternating layers of Al and Al(Sc). The material was subsequently annealed for 6h at 350 °C. The deformation and annealed microstructures were investigated using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The deformation microstructure composed primarily of lamellar bands of unequal fineness with shear bands and deformation bands being additional substructural features in the Al(Sc) layers. The high strain deformation generated Al layers containing lamellar boundaries separated by a large fraction of high angle grain boundaries, thereby creating the ideal microstructure for continuous recrystallization. Annealing of the as-deformed material generated a hybrid microstructure consisting of alternating layers consisting of ~20 0m grains produced by continuous recrystallization (Al layers) and a lightly recovered substructure (Al(Sc) layers); the latter were highly resistant to recrystallization due to precipitation of nanosized Al3Sc particles during annealing.

Abstract: PM 1000 is a nickel-based oxide dispersion strengthened (ODS) superalloy used for high-temperature applications. The primary recrystallization of a <100>-fiber textured coarsegrained oxide dispersion strengthened nickel-based superalloy (PM 1000) has been investigated. The annealing behavior of this alloy is quite complex. Even when annealing is performed at high homologous temperatures (e.g. 0.9 Tm, Tm is the melting point), recrystallization is partial. In order to understand such a behavior, the microstructure of specimens in both the as-received, deformed, and annealed conditions has been imaged in detail using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron backscatter diffraction (EBSD). In the annealed state we observe a significant volume fraction of tiny crystals in the interior of the recovered grains. These tiny grains are elongated and grow mostly along the existing low angle dislocation boundaries (anisotropic growth). In the present paper we propose a twinningassisted nucleation mechanism to clarify their origin during recrystallization.

Abstract: Two kinds of radiation-induced structural changes were observed in U-Mo/Al dispersion fuel: radiation-induced recrystallization of U-Mo fuel particles and radiation-induced amorphization of interaction products. During irradiation, U-Mo fuel showed refined microstructures of submicron-size grains due to dynamic recrystallization, occurring initially from pre-existing grain boundaries. The interaction products formed by interdiffusion between the U-Mo particles and Al matrix in U-Mo/Al dispersion fuel transformed from crystalline to amorphous during irradiation. In this paper we deal with both of the phenomena simultaneously.