Dislocations unstable stacking energy

What we have learned is that dislocation nucleation will occur once 4>i np) reaches its maximum allowable value. This idea is depicted graphically in fig. 11.19 where it is seen that instability to dislocation nucleation occurs when (Siip) = Yus, where yus is a material parameter that Rice has christened the unstable stacking energy. This idea is intriguing since it posits that the competition between cleavage and dislocation nucleation has been reduced to consideration of the relative values of two simple material parameters, both of which admit of first-principles determination, and relevant geometrical factors. 

One of the achievements of the PN theory is that it provides a reasonable estimate of the dislocation size. The optimal size of the dislocation core, characterized by the value of is a result of the competition between the two energy terms in Eq. (10.20), as shown schematically in Fig. 10.9 If the unstable stacking energy Yus is high or the elastic moduli K are low, the misfit energy dominates and the dislocation becomes narrow ( is small) in order to minimize the misfit energy. 

Fig. 12(a) further shows that the 110 unstable stacking-fault energy is systematically smaller than the 211 fault energy for all pressures up to 400 GPa in Ta and Mo, and up to 230 GPa in V. This has important implications for the motion of a/2 < 111 > screw dislocations on 110 and 211 slip planes. At all... 

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