Nano-Grained Structure Induced by Mechanical Twinning during Multi-Directional Forging and the Mechanical Properties

A bulky SUS316 austenitic stainless steel (SUS316) was multi-directionally forged (MDFed) at 77 K and 300 K up to a cumulative strain of = 6 at maximum. With increasing cumulative strain, the grains were subdivided by mechanical twinning and martensitic transformation. Especially, mechanical twins accelerated grain fragmentation by subdivision of the initial grains and by intersection of the previously formed twins during MDF. The intersection of twins caused finally evolution of packet grains, which were composed of lamellar structured twins. The packet size and the lamellar twin spacing decreased down to 35 nm and 15 nm by MDF to  = 6 at 77 K. The average grain size achieved was estimated to be about 10 nm. Twinning appeared more frequently and uniformly at 77 K than at 300 K. Tensile test at 300 K revealed ultimate tensile strength of 2.1 GPa and fracture strain of about 0.2. The fracture strain, however, appeared to be constant over ∑Δε = 2.4 independent of cumulative strain. The observed excellent balance of strength and ductility of the nano-grained SUS316 is discussed in relation with the effects of twins on grain fragmentation and mechanical properties.

Structural Change during Cold Rolling of Electrodeposited Copper

Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20˚ of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.

CBED Analysis of YBa2Cu3-xAlxOy single Crystals

Single crystals of YBa2Cu30y were grown from melting mixture of yttria, barium carbonate and copper oxide contained in alumina crucibles. Crystals grew as platelets with the (001) broad surface. The use of alumina crucibles for crystal growth resulted in incorporation of aluminum to varying extents in crystals of composition YBa2Cu3-xAlx0y, x=0-0.23. Microstructural and microchemical characterization of single crystals were carried out by transmission electron microscopy along with electron dispersive x-ray analysis. Convergent beam electron diffraction (CBED) technique was used to determine crystal symmetry and evaluate symmetry-modification due to twins or local inhomogeneities. Critical voltage effects were studied with respect to oxygen stoichiometry and aluminum concentrations.The microstructures of YBa2Cu3-xAlx0y single crystals show large twin. domains (Figure la), lamellar twin structures (Figure 2a) and {110}-type tweed pattern modulations (Figure 3a) aparently depending on aluminum concentration. In selected area diffraction patterns splitting diffraction spots along the [110] direction were observed from twined structures and <110> star-shaped streakings around the diffraction spots from tweed structures.