PSI - Issue 43

Václav Paidar / Procedia Structural Integrity 43 (2023) 3–8 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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Dislocation glide is governed by the structure of dislocation cores (Vitek and Paidar 2008): for planar cores the glide is easier contrary to the non-planar cores. This behaviour is regulated by the occurrence of stacking faults on a particular atomic plane. Stability of stacking faults depends on the acting interatomic forces, however, a critical factor is the geometry of atomic structure. Atomic planes with the larges atomic densities in hexagonal metals are the basal planes (0 0 0 1) where the a 1/3<1 -2 1 0> dislocations can be dissociated into two partials similarly to the dissociation into two Shockley partials on the (1 1 1) planes in fcc metals. The a dislocations can glide on the basal planes, however, in Ti and Zr prismatic glide is observed (Clouet, Caillard et al. 2015). 2. Stacking faults on crystallographic planes Very useful for the stability of stacking faults are symmetry arguments that are independent of chemical composition. The crystal mirror planes perpendicular to the fault plane must be also mirror planes for the generalized stacking fault energy surface (GSFES). Therefore, the displacements corresponding to the intersections of two such mirror planes indicate symmetry dictated extrema (minima, maxima or saddle points) (Paidar and Vitek 2002). On the {2 -1 -1 2} pyramidal plane Pyr. II, GSFES is not mirror symmetric contrary to that on the {1 0 -1 1} pyramidal plane Pyr. I. The {0 1 -1 0} crystal plane perpendicular to the {2 -1 -1 2} plane is not mirror symmetric. The atomic positions on the pyramidal plane II are in the corners of a rectangle forming a periodic elementary cell as shown in Fig. 1 where possible dissociation of the c+a vector is illustrated. There is only one stable stacking fault in the middle of the c+a vector denoted as Sf1.

Fig. 1. Dissociation of (c+a) dislocations into two partials (c+a)/2 on the plane Pyr. II.

On the {1 0 -1 1} pyramidal plane Pyr. I, GSFES must be mirror symmetric along the directions <-1 0 1 2> lying in the intersection of the {1 -2 1 0} crystal symmetry plane and the {1 0 -1 1} pyramidal plane. The angle between the <-1 0 1 2> direction and the c+a direction (1/3<-1 -1 2 3> or 1/3<-2 1 1 3>) is about 15 °. The atoms on the Pyr. I plane are depicted in Fig. 2-4 as larger circles. There are two not equivalent positions of the stacking faults Sf2 and Sf3 corresponding to local minima on the GSFES. Two orientations of the c+a vector, that are denoted as (c+a)L and (c+a)R , can be distinguished, namely 1/3<-1 -1 2 3> and 1/3<-2 1 1 3>, respectively .