�ν− fion𝐼𝐼𝐼𝐼=Understanding the Brittle-to-ductile Transition Based on Shielding Theory The fracture toughness of crystalline materials is enhanced by plastic deformation at the crack tip. This phenomenon is known as the brittle-to-ductile transition (BDT). This BDT behavior is strain-rate dependent, and the transition temperature increases as the deformation rate increases. Several ideas have been proposed for a mechanism behind this toughness enhancement. The value of activation energy obtained from the strain rate dependence of BDT temperature is almost equal to the value of activation energy of dislocation motion, which leads to the understanding that BDT is controlled by a dislocation migration rate. The applied stress intensity factor during crack propagation, or in other words, the macroscopic fracture toughness KIC, is given by the following equation. where μ, ν, and γ are shear modulus, Poisson's ratio, and surface energy for fracture, respectively. The pioneering studies on BDT based on the dislocation theory were mainly conducted using model materials such as silicon single crystals. The presenter has extended the stress shielding theory to steels and investigated their BDT based on the stress shielding theory. It was demonstrated that the addition of nickel to interstitial free (IF) steels lowers the BDT temperature, which can be explained by considering that the addition of nickel increases the dislocation mobility at low temperatures. The addition of Mn to IF steels also increases dislocation mobility. This leads one to expect that the addition of Mn also causes a decrease in BDT temperature, however, the BDT temperature increases for adding Mn in steels instead. In the fracture surface after brittsteels was transgranular, whereas that in the Mn-added steels was intergranular. From these results, it can be concluded that the addition of Mn has a stronger effect of deteriorating toughness by lowering the fracture surface energy than improving low-temperature toughness by increasing dislocation mobility, and that Professor, Department of Materials, Faculty of Engineering, Kyushu University, Japan Abstract Mn addition acts to increase the transition temperature. Session 1 ｜Prof. Masaki Tanaka completed PhD at Kyushu University in 2005. He was appointed as a professor to the Department of Materials, the Faculty of Engineering at Kyushu University after working as a postdoctoral research fellow at the Department of Materials, University of Oxford in UK, Post-doctoral researcher, Assistant Professor, and Associate Professor at the Department of Materials Science and Engineering, Kyushu University. His current research interest is mechanical properties (mainly deformation and fracture) of crystalline materials such as steels, silicon, titanium alloys and so on, based on dislocation theory. NIMS Award Symposium 2023Masaki Tanaka Deformation and Fracture4μγ1−Σkd, le fracture, the initial fracture in the Ni-added Invited Talk S1-3 25
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