An atomic model of crack tip deformation in aluminum using an embedded atom potential
- PDF / 5,638,236 Bytes
- 12 Pages / 593.28 x 841.68 pts Page_size
- 36 Downloads / 252 Views
M. S. Daw, S. M. Foiles, and M. I. Baskes Sandia National Laboratory, Livermore, California 94551-0969 (Received 29 July 1989; accepted 9 October 1989) The atomic configuration at the tip of a mode 1 crack in aluminum is modeled by means of molecular dynamics calculations using an embedded atom potential. This potential intrinsically incorporates many-body contributions. This paper is concerned with the characteristics of the atomic displacement fields in comparison to the linear elastic predictions and dislocation emission phenomena. Three crack/crystal orientations are examined in which the crack plane-crack propagation directions are (010)-[100], (Tl0)-[110], and (IlO)-[lll]. The first two models behaved in a brittle fashion as dislocation emission did not occur for reasons associated with the use of periodic boundary conditions parallel to the crack front. For the models which remained atomically sharp, the positions of the atoms near the crack tip in equilibrium configurations are different from the linear elastic predictions but, to first order, retain an rm dependence, with smaller K, and with the origin displaced behind the physical crack tip. This near tip region is also observed to be elastically softer than in the far field. Dislocation emission readily proceeds in the (110)-[lll] model by the sequential emission of partials with attendant nonzero uz displacements. The blunting is characterized by the creation of two corner defects that separate as emission occurs and relaxation of the strains in the region initially confronted by the crack tip. Additional features of the results are discussed.
I. INTRODUCTION
The manner in which the atomic configuration at the tip of a crack in a solid evolves as load is applied is an issue of considerable importance because it is central to understanding the factors controlling the onset of crack extension and, therefore, toughness. As discussed by Thompson1 and others,2'3 this evolution is usually not directly coupled to the applied load but is screened from it to varying degrees by other sources of internal stress associated with dislocations, phase transformations, microcracks, etc. Nevertheless, the effectiveness of these screening operations, and of other effects such as blunting that alters the basic character of the local crack tip stresses, are dependent on various atomicscale events at the crack tip, especially the emission of dislocations and cleavage separation. The competition between these particular events has been examined using simple atomic models and linear elastic fields of defects by Kelly, Tyson, and Cottrell4 and in further detail by Rice and Thompson5 and Lin.6 There is competition in the sense that dislocation emission prior to cleavage may inhibit cleavage which encourages high toughness and ductile behavior. Brittle cleavage fractures (and low toughness) are the consequence of the alternative. These analyses provide an explanation for J. Mater. Res., Vol. 5, No. 2, Feb 1990
http://journals.cambridge.org
Downloaded: 13 Mar 2015
the ductile
Data Loading...
