Materials Science Forum Vols. 467-470

Abstract: Microstructure evolution in alpha-beta Ti-64 alloy samples with lamellar structure deformed to a height reduction of 70% at temperatures between 450 and 800°C has been investigated. The deformation led to a distinctly globularized structure of α- and β-phase in the whole temperature interval. The dependence of globular grain size on deformation temperature is of a linear character up to the temperature of warm deformation at which formation of an SMC structure takes place. Continuous recrystallization was observed in the α-and β-phases. Different types of defects responsible for splitting of α-lamellae such as low and high angle boundaries, shear bands and twins were found. An investigation of boundary misorientation spectra in the α-and β-phases deformed to different strains at 550 and 800°C was carried out. Typical boundary misorientation spectra for single phase metals with the same lattice were obtained. The boundary misorientation spectrum depends weakly on strain and deformation temperature. The results of this study show the importance of transformation of semi-coherent interphase boundaries to non-coherent ones for globularization of lamellar microstructures.

Abstract: The recrystallization behavior of coarse-grained tantalum deformed at large strains is strongly dependent on its deformation microstructure. In this regard, a longitudinal section of a high-purity coarse-grained tantalum ingot obtained by double electron beam melting (EBM) was rolled at room temperature to thickness reductions varying from 70 to 92% followed by annealing in vacuum at 900 and 1200°C for 1 h. Microstructural characterization was performed in cold-rolled and annealed specimens using scanning electron microscopy (SEM) in the backscattered mode (BSE), electron backscattered diffraction (EBSD), and microhardness testing. Results show that recrystallization of individual grains is strongly dependent on the initial orientation. In consequence, recrystallization kinetics varies noticeably from one grain to another. Inhomogeneous in-grain and grain-to-grain spatial distributions of textures are found in the tantalum plates. Texture components belonging to the so-called g fiber ({111}// ND) are in majority in the rolling plane. The presence of tiny recovered regions in the annealed plate is associated to stable orientations like rotated cube {001}<110>, which are very resistant to recrystallization even at high annealing temperatures. EBSD results also confirm the occurrence of growth selection during recrystallization in coarsegrained tantalum.

Abstract: The recrystallization of an electron beam melted Ta-10wt%W alloy deformed by cold swaging with reductions in area of 41 and 95% and annealed between 1300 oC and 1700 oC is quantitatively described. The recrystallized volume fraction and the interface area per unit of volume are measured. From these quantities the microstructural path is analyzed. Based on the available measurements the nucleation and growth of recrystallization in this alloy is discussed. The kinetic analysis is difficult in this case due to the large initial grain size of the specimens.

Abstract: The recrystallization behavior of coarse-grained niobium depends on the nature of its deformation microstructure. In this regard, a longitudinal section of a high-purity coarse-grained niobium ingot was cold rolled to a thickness reduction of 96% followed by annealing in vacuum at 800°C for 1 h. Metallographic inspection in cold-rolled and annealed specimens was carried out in a field emission gun scanning electron microscope (FEG-SEM). Microtexture was determined by electron-backscattered diffraction (EBSD) coupled to the FEG-SEM. The use this technique has evidenced details of the boundary character and subgrain structure found in partially recrystallized regions. The early stages of primary recrystallization are associated to the presence of high-angle lamellar boundaries found in the cold-worked state. Abnormal subgrain growth has been evidenced as a viable mechanism for nucleation of recrystallization.

Abstract: Cast iron aluminides of three compositions were strongly deformed by hot extrusion and subsequently annealed. The texture development and kinetics of recrystallization were determined by local and global texture measurements. The deformation texture of Fe-10Al (A2 structure) is a <110>-fibre, Fe-35Al and Fe-50Al (B2 structure) show a <100>-<110> double fibre texture with <110> dominating Fe-35Al. The texture change with composition is due to a change in slip system. The deformed samples are partially dynamically recrystallized. The recrystallization components are aligned along the symmetry line <100>- <110> and towards <114> for Fe-10Al. For the other alloys the recrystallization texture is <111> with a tendency to <112>. The texture components do not change during static recrystallization. In general, the recrystallization texture is quite weak. The microstructure is very inhomogeneous ranging from deformed, strongly recovered to recrystallized areas. Beside primary recrystallization abnormal grain growth takes place. The heterogeneity of recrystallization makes it difficult to quantify the kinetics of recrystallization. The results are discussed with respect to the order of the alloys.

Abstract: The deformation behavior and structure changes of magnesium alloy AZ31 were studied in compression at temperatures ranging from 523K to 673K and at a strain rate of 3 x 10-3 s-1. They depend sensitively on deformation temperature. At high temperatures, grain fragmentation takes place due to frequent formation of kink bands initially at corrugated grain boundaries and then in grain interiors, followed by full development of new grains in high strain. At lower temperatures, in contrast, twinning takes place in rather coarse grains and kink bands are formed mainly in finer original ones in low strain. It is concluded that new grain evolution can be controlled by a deformation-induced continuous reaction resulting in grain fragmentation by kink bands, i.e. continuous dynamic recrystallization (cDRX). The latter is discussed comparing with conventional, i.e. discontinuous, DRX.

Abstract: The recrystallization of a Zr-2Hf alloy sheet deformed by plane strain compression at room temperature and then heat treated in the temperature range 500-650°C is studied. The microstructure, local and global crystallographic textures are investigated by EBSD and X-ray techniques. The as-deformed condition exhibits a heterogeneous microstructure composed of highly and less deformed zones, the EBSD indexing of the latter ones being more reliable. The asdeformed condition displays a (0001) < 0 1 10 > crystallographic texture. The evolution of the microstructure during recrystallization very much depends on the amount of local deformation. Recrystallization begins in highly deformed zones, new grains having two variants of texture components, {0001} < 0 1 10 > and {0001} < 0 2 11 >. Some change of preferred orientations concomitant with grain growth at 600 and 650°C has been observed with a decrease in the {0001} < 0 1 10 > component and an increase in the {0001} < 0 2 11 > component.

Abstract: Misorientation analysis, using EBSD data sets, has enabled us to constrain better recrystallization mechanisms in rocks and minerals. Observed microstructures are not explicable in terms of recovery, boundary bulging and migration alone. We have to invoke either a nucleation process (physics unknown) or grain rotations that are not related to grain or boundary crystallography. Such rotations can occur by diffusion accommodated grain boundary sliding and this mechanism explains best the microstructure and texture of recrystallized grains in some rocks.

Abstract: This is a study on texture, decrystallization, and recrystallization in rolled and heat treated semi–crystalline polymers. Experiments were conducted using wide angle Debye-Scherrer-type X– ray diffraction. Changes in crystallinity were quantitatively monitored as a function of strain and annealing time. It was observed that crystallinity drastically drops during deformation in PET. We suggest that amorphization (decrystallization) is a deformation mechanism which acts as an alternative to crystallographic slip depending on the orientation of the nanocrystalline lamellae. Heat treatment leads to the recrystallization of amorphous PET material and to an enhancement of the original texture of the deformed crystals observed before the heat treatment. We explain this phenomenon in terms of oriented nucleation where amorphous material crystallizes alongside existing crystalline lamellae.