Atomistic simulation of fracture in Ni 3 Al
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Chong-Yu Wang International Centre for Materials Physics, Academia Sinica, Shenyang 110016, China; Central Iron and Steel Research Institute, Beijing 100081, China; and Department of Physics, Tsinghua University, Beijing 100084, China
Tao Yu Central Iron and Steel Research Institute, Beijing 100081, China (Received 24 September 2007; accepted 4 January 2008)
The molecular dynamics method has been used to simulate mode I cracking in Ni3Al. Close attention has been paid to the process of atomic configuration evolution of the cracks. The simulation results show that at low temperature, the Shockley partial dislocations are emitted before the initiation of the crack propagation, subsequently forming the pseudo-twins on (111¯) planes in crack-tip zone, and then the crack cleavage occurs. The emitting of the Shockley partial dislocations accompanies the crack cleavage during the simulation process. At the higher temperature, the blunting at the crack tip is caused by the [110] superdislocations emitted on (100) plane. The present work also shows that the dipole dislocations on (111¯) planes in the 1/2[110] dislocation core can be formed.
I. INTRODUCTION
A physical understanding of dislocation nucleation from a stressed crack tip is a fundamental aspect of the brittle or ductile nature of a solid. A crystal is said to be intrinsically brittle if an existing crack is able to propagate along a crystallographic plane in a cleavage manner when the solid is under stress. On the other hand, the crystal is considered to be ductile if the crack-tip extension is accompanied by plastic deformation. In earlier studies, Kelly1 proposed that a solid would be either ductile or brittle depending on the ratio of the shear strength to the tensile strength. The dislocation emission from a crack tip has been studied by Rice and Thompson,2–4 using the Peierls–Nabarro model5 of a straightline dislocation. By introducing the unstable stacking energy (␥us) to characterize dislocation nucleation, Rice and Thompson4 suggested that the ratio of ␥us to surface energy (␥s) could be a simple parameter determining the brittle-to-ductile transition (BDT). Later, Zhou et al.6 proposed a new BDT criterion for materials that did not depend on the surface energy but contained only the unstable stacking energy. It is well known that nickel-based superalloy is one of the most important structural materials for advanced air-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0192 J. Mater. Res., Vol. 23, No. 6, Jun 2008
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craft turbine blade. It consists of a Ni-based matrix with a dispersion-strengthening phase, i.e., the ordered intermetallic precipitate particles of Ni3Al. In the singlecrystal superalloy, the Ni3Al volume fraction can reach 70% or even higher. In the past two decades, the mechanical properties of Ni3Al has been the subject of extensive experimental study and theoretical modeling. The strength anomaly was investigated by several groups
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