ON THE UNSTABLE STACKING CRITERION FOR IDEAL AND CRACKED COPPER-CRYSTALS

The unstable stacking criteria for an ideal copper crystal under homog eneous shearing and for a cracked copper crystal under pure mode II lo ading are analysed. For the ideal crystal under homogeneous shearing, the unstable stacking energy gamma(us) defined by Rice in 1992 results from shear with no relaxation in the direction normal to the slip pla ne. For the relaxed shear configuration, the critical condition for un stable stacking does not correspond to the relative displacement Delta = b(p)/2, where b(p) is the Burgers vector magnitude of the Shockley partial dislocation, but to the maximum shear stress. Based on this re sult, the unstable stacking energy Gamma(us) is defined for the relaxe d lattice. For the cracked crystal under pure mode II loading, the dis location configuration corresponding to Delta = b(p)/2 is a stable sta te and no instability occurs during the process of dislocation nucleat ion. The instability takes place at approximately Delta = 3b(p)/4. An unstable stacking energy Pi(us) is defined which corresponds to the un stable stacking state at which the dislocation emission takes place. A molecular dynamics method is applied to study this in an atomistic mo del and the results verify the analysis above.