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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Fig. 4. Dissociation of (c+a)L dislocations into partials b2Ld and b2Lu on the plane Pyr. I .

3. Dissociation of screw dislocation Let us examine three typical hexagonal metals, titanium, magnesium and zinc . To analyse the dislocation reactions, we need to know their shear moduli  and Poisson´s ratio  (Gale and Totemeier 2004), that are summarized in Table 1.

Table 1. Elastic constants: shear modulus  in GPa and Poisson´s ratio  metal Ti Mg Zn  45.6 17.3 41.9  0.361 0.291 0.249

Screw dislocations can more easily change the plane of dislocation glide. For their dissociation, we shall consider the stacking fault energies calculated by the density functional theory (Yin, Wu et al. 2017) listed in Table 2.

Table 2. Stacking fault energies on pyramidal planes in mJm -2 metal Ti Mg Zn Sf1 321 165 119 Sf2 134 161 341 Sf3 634 203 169

The c+a dislocations can glide on two types of pyramidal planes: Pyr. I {1 0 -1 1} or Pyr. II {2 -1 -1 2}. Since the displacement vector of the stable stacking fault Sf1 on the Pyr. II plane is about ½ (c+a) , the total Burgers vector can dissociate into two half vectors as depicted in Fig. 1. Two qualitatively different dislocation dissociations can be considered on Pyr. I plane (compare Fig. 2 with Figs. 3 and 4). First, that with large edge components of the two partials (Fig.2) when the vector (c+a)L is composed of the partials separated by Sf3. In the other case, the edge components are smaller, i.e. the dissociation of (c+a)R with Sf3 (Fig. 3) or the dissociation of (c+a)L with Sf2 (Fig. 4). All partial dislocation Burgers vectors are compared in Tables 3 and 4.

Table 3. Burgers vectors for partials b3L (Fig. 2) metal Ti Mg Zn a [A] 2.95 3.21 2.66 b3Lu screw 3.36 3.69 3.28 b3Ld screw 2.18 2.43 2.34 edge 2.13 2.32 1.94