The interaction between dislocations and intergranular cracks
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R. Thomson Center for Materials Science, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 20 July 1990; accepted 9 October 1990)
The elastic interactions of dislocations and intergranular cracks in isotropic materials have been studied. In the first part of the paper, a model based on the Rice-Thomson theory is established under which the conditions for dislocation emission and crack propagation can be described in terms of an emission surface, cleavage surface, and loading line in the local /c-space associated with a mixed mode intergranular crack. For a given crack, the local &-field changes with the emission of dislocations from the crack tip, which alters the balance of cleavage and emission. In the second part, we present experimental results of in situ TEM observations of intergranular cracks in molybdenum. Alternating brittle crack propagation and dislocation emission is observed. The number of emitted dislocations corresponding to each crack propagation is in good agreement with the calculated values.
I. INTRODUCTION
Intergranular fracture has been studied for many years. It is well known that grain boundaries act as fracture paths and as special sites for the accumulation of impurities which are believed to be the major cause of intergranular embrittlement. Over the years, research on intergranular fracture has concentrated on various types of segregation-induced intergranular embrittlement, such as temper-embrittlement (P, Pb, Sn, etc. in steels, etc.), hydrogen-embrittlement (H in Ni), and embrittlement of some alloys (Bi in Cu). It is believed that the Griffith criterion can be applied to intergranular brittle fracture after simple modifications, which can be expressed in terms of the critical strain energy release rate, Jc Jc = 2y = 2ys - yb (1) where y is the fracture energy, ys the surface energy, and yb is the energy of the pre-existing grain boundary. Since both ys and yb are affected by concentration of impurities, one can expect that fracture energy may change greatly with impurity segregation. However, for most engineering materials, it is rare to have pure brittle intergranular fracture unless extremely high impurity levels occur. Therefore, brittle propagation of intergranular cracks combined with plastic deformation is the general finding. McMahon and Vitek1 suggested that by extending the Orowan equation to intergranular fracture, the fracture energy, as well as the critical strain energy release rate, can be rewritten as Jc = 2y = 1ys - yb + yp = 2y' + yp 314 http://journals.cambridge.org
(2)
J. Mater. Res., Vol. 6, No. 2, Feb 1991 Downloaded: 05 Apr 2015
where y' = ys - \yb represents the cohesive energy of the grain boundary and yp is the plastic work associated with dislocation motion during the propagation of the intergranular crack. In Eq. (2), it is implied that yp is a material parameter in the same sense as ys and yb, and is independent of crack geometry, loading condition, etc. As a macroscopic and qualitative criterion, Eq. (2) is accept
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