Speakers

Ferritic steels with a body-centred cubic (BCC) crystal structure exhibit a brittle-to-ductile transition (BDT), in which the fracture mode changes from brittle to ductile as the deformation temperature increases from low to high temperatures. Since the BDT temperature shows strain-rate dependence, the rate-controlling process can be regarded as a thermally activated process. Furthermore, because fracture ultimately involves the breaking of atomic bonds, the BDT behaviour also depends on the magnitude of the surface energy for fracture required for crack propagation. Both of these factors are influenced by alloying elements. As steels contain various alloying elements, it is important to eclucidete the influence of each element individually in order to understand BDT behaviour. For example, the addition of Ni to low-carbon steels lowers the BDT temperature, which can be explained by an increase in dislocation mobility at low temperatures governing the BDT. The addition of Mn to low-carbon steels also increases dislocation mobility, which might lead one to expect a corresponding decrease in the BDT temperature, however, the BDT temperature increases in Mn-added steels. This indicates that the Mn changes the BDT temperature via changing both dislocation mobility and the surface energy for fracture. In this presentation, the effects of alloying elements on BDT behaviour will be discussed with particular attention to changes in dislocation mobility and the surface energy for fracture.