Fracture morphology. In order to in

Fracture morphology. In order to investigate the fracture mode, specimen was firstly fracturedalong a typical longitudinal crack, and the fracture morphology was presented in Fig. 3. It can beseen that the dendrite shape in the crack surface was coarse, and fine secondary dendrite armscovered with oxide film were observed as circled in Fig. 3 (a). As measured, the thickness ofcolumnar grains was determined to be ~0.63 mm in the cracked region, and was only ~67 % of thethickness compared to columnar grains in the un-cracked region. The relationship of SDAS (λ) andcooling rate () can be represented as λ  68860.[4]. The SDAS measured on the cracksurface (200 um below the strip surface) was ~9- ~10μm, larger than the spacing of 4 to 5μm in thenormal region.Fig. 4 shows the fractographs of a fracture beneath longitudinal crack and intentionallyfabricated fracture. It can be seen that the intentionally fabricated fracture below the interfacialregion was composed of dimples, indicating normal ductile fracture. But some oxide film wasobserved in the tip of dendrite on the fracture beneath the longitudinal crack. So it can be estimatedthat the crack was formed in the roll-casting process.