Atomistic simulation for configuration evolution and energetic calculation of crack in body-centered-cubic iron
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Chong-Yu Wang International Centre for Materials Physics, Academia Sinica, Shenyang 110016, People’s Republic of China; and Central Iron and Steel Research Institute, Beijing 100081, People’s Republic of China; and Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China (Received 2 December 2005; accepted 15 June 2006)
The molecular dynamics method has been used to simulate mode I cracking in body-centered-cubic iron. Close attention has been paid to the process of the atomic configuration evolution of the cracks. The simulation shows that at low temperatures, partial dislocations are emitted before the initiation of crack propagation, subsequently forming the stacking faults or multilayer twins on {112} planes, and then brittle cleavage and extended dislocation nucleation are observed at the crack tip accompanied by twin extension. These results are in agreement with the experimental observation that twinning and fracture processes cooperate at low temperatures. Furthermore, an energetics analysis has been made on the deformation behavior observed at the crack tip. The effect of temperature on the fracture process is discussed. At the higher temperature, plastic deformation becomes easier, and crack blunting occurs. With increasing temperature, the fracture resistance increases, and the effect of the lattice trapping can be weakened by thermal activation.
I. INTRODUCTION
Some materials exhibit intrinsically brittle behavior whereas others are ductile and deformable. Most of the common structural materials, particularly the steels, exhibit both types of behavior with a brittle-to-ductile transition (BDT) at a specific temperature. A crack existing in the material may propagate as a brittle cleavage; alternatively, material at the crack tip may show some plasticity, including dislocation nucleation and dislocation mobility. Considerable attention has been devoted to the study of crack-tip processes, which is a central issue in understanding the ductile versus brittle behavior of solids. Two basic prevailing models have been proposed to assess BDT. The first is based on criteria for dislocation nucleation or emission, which is a competition between crack propagation and thermally activated generation of a single dislocation at the crack tip.1–4 The second is based on the concept of dislocation mobility: a thermally activated generation of a single dislocation mobilitycontrolled dynamic process.5–7 In addition, both experiments8–10 and simulations11,12 show that plastic deformation occurs in conjunction with crack propagation. Bond breaking and dislocation emission therefore can be a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0307 2542 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 23 Nov 2014
simultaneous phenomena at the crack tip. These observations suggest that brittle versus ductile behavior could be controlled by a combination of both models. Known as one of the most important deformation mechani
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