Lamellar subdivision during accumulative roll bonding of a titanium interstitial free steel

Titanium alloyed interstitial free steel was processed by means of accumulative roll bonding (ARB) in order to obtain an ultrafine grained structure. Ten consecutive rolling passes were applied at 480 degrees C with a nominal reduction of 50% per pass and an intermediate annealing treatment of 5 min. at 500 degrees C. A total true strain was obtained of epsilon(vM) = 8.0 which corresponds to an accumulated reduction of 99.9%. Orientation imaging microscopy was used to evaluate textures and microstructures. A pronounced lamellar structure was observed until the 5(th) pass with an incidence of high angle grain boundaries predominantly parallel to the rolling direction. After the 6(th) pass (M = 4.8) an increased fragmentation perpendicular to the rolling direction starts to develop in site of the lamellar microstructure with an average spacing of approximately 1 mu m. From the 7 pass onwards (epsilon(vM) >= 5.6) a random high angle grain boundary distribution develops which results in a more equi-axed ultrafine microstructure after the 9(th) pass (epsilon(vM) = 7.2) with an average gain width of 200 nm. As the rolling is carried out without lubrication, the surface areas display a slightly more fragmented structure than the midlayer sections and typical shear texture components are present in these surface zones (< 110 >//ND and < 211 >//ND fibre). Although the sheets are stacked upon each other after each subsequent pass, the shear strain microstructural and textural features are rapidly decomposed in the midlayer in each subsequent rolling pass which is clearly revealed by the cross sectional orientation scan on the composite sample. Hence it cannot be concluded that the surface shear strain significantly contributes to the gain fragmentation in the bulk volume of the composite sample.