Papers by Author: Y.Y. Tse

Abstract: The relationship between the deformation orientation distribution function (ODF) and the primary recrystallised ODF in cold and warm rolled metals, is not a simple mathematical transformation from one to the other, but is through thermally activated processes occurring in the deformation microstructure. In BCC metals the mature rolling microstructure consists of cells, microbands and shear bands on a length scale of fraction of a micron, to deformation and transition bands at the grain scale, when this is of the order of 10 or more microns. There is evidence that grain boundary regions are sometimes distinct from grain interiors. Wherever there is a relatively sharp change in either orientation or microstructure such locations are potential sites of recrystallisation nuclei. In this paper the results of a systematic investigation of the development of microstructure in rolled interstitial free (IF) steel using both transmission and scanning electron microscopy are presented. It is shown how the dislocation mesh structure, formed at the earliest stages of rolling, develops into the mature microstructure consisting of cells, microbands and shear bands. Deformation heterogeneities in the microstructure, known to be of vital significance in the recrystallisation process are associated with the α and γ fibre components of the rolling texture. Shear band thickening and α grain fragmentation are also considered, since both processes can produce recrystallisation nuclei, which in the α fibre case can reduce desirable mechanical properties.

Abstract: IF steel was homogeneously cold rolled between 30-95% reduction in thickness. The global cold rolling textures showed a gradual strengthening of both stable α and γ components with increasing reduction until ~80% after which γ remained effectively unchanged but α components intensified until 95% reduction of thickness. Deformation Banded (DB) and also fragmented microstructures were found exclusively in γ grains up until about 85% reduction after which DB was unexpectedly detected in α grains, becoming significant after 95% reduction. This is in sharp contrast with the rather undifferentiated microstructures found in α grains at low to medium levels of deformation. At lower reductions the annealing texture was a weak α, but the γ component increased with rolling strain and became dominant at ~80% rolling deformation. A peak type γ recrystallisation texture with orientations ranging from {554}<225> to {111}<123> was found in the 95% rolled sample. In addition to this a {411}<148> component began to intensify, reading 5R at 95% reduction. Microstructural analysis showed that DB provided the lattice curvature for nucleation in the α fibre.

Abstract: In an investigation of nucleation of recrystallisation in an Interstitial Free steel it was found that new crystals were almost always contained within the rolled-out hot band grain envelopes and were mostly equiaxed. At a later stage they grew and had an aspect ratio of 2:1 but at the completion of recrystallisation were again equiaxed. This is explained by the notion that nucleation occurs relatively frequently in certain grains, that these nuclei have very similar orientations and are thus orientation pinned within the solute and precipitate containing envelopes of the hot band grains. Provided the misorientation is small the impinged group are capable of spheroidisation provided the driving force across the pinned boundary is sufficient to overcome the pinning, because, by definition, this pinned boundary is of high angle character. The theory, as it is presented as coalescence, relies on a form of Östwald ripening and therefore provides a possible explanation of why grain growth kinetics obeys a time exponent of between 1/2 and 1/3. A similar observation of high aspect ratio grains has been made many times in the case of cold rolled copper which forms cube texture. Again, nuclei are formed in the cube bands, but these are prevented from lengthening because of orientation pinning. However, when the length of a group of such impinged nuclei is sufficient, spheroidisation will produce equiaxed grains.