PSI - Issue 23

Solveig Melin et al. / Procedia Structural Integrity 23 (2019) 137–142 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.2. Influence from grain boundary and crystallographic orientation

To determine the influence of a grain boundary, the axial stress and the dislocation density were determined as function of axial strain for all three cases. The results are shown in Fig. 3. In all cases the maximum stress, just prior to plastic initiation, dropped significantly as compared to single crystal beams, cf. Fig. 2a and Fig. 3. Also the strains at plastic initiation, ε xi , and at rupture, ε xf , were lowered in the presence of a grain boundary. The values are listed in Table 1. It was found that repeated hardening and softening occurred in all cases in higher degree than for the single crystals, cf. Fig. 2. By comparing the stresses and dislocation densities in Figs 3a-b it can be seen that no dislocations exist prior to the first drop in the stress curve, indicating the onset of plasticity. However, in Fig. 3c a different behavior was observed, with several small drops in the stress curve at low strains and formation of dislocations prior to the first large drop in the stress curve. This will be discussed in Chapter 3.3. From Fig. 3 it can also be seen that a general trend is that, as the stress curve has a drop then, simultaneously, the dislocation density increases, and when the stress increases, the dislocation density decreases. Similar trend has also been observed by Zhan et al. (2011) and Yaghoobi and Voyiadjis (2016), among others.

Fig. 3. Stress (blue line) and dislocation density (green line) vs. strain for: a) [100]-[110], b) [100]-[111], c) [110]-[111] grain boundaries.

Table 1. Axial strain at plastic initiation, ε xi , and at rupture, ε xf . geometry ε xi

ε xf

[100] single crystal [110] single crystal [111] single crystal

0.0875 0.0625 0.0650 0.0300 0.0289 0.0275

0.4050 0.5925 0.3400 0.1400 0.2795 0.2275

[100]-[110] grain boundary [100]-[111] grain boundary [110]-[111] grain boundary

Also the different types of dislocations formed during deformation in the cases of presence of a grain boundary were studied. Here the dislocations are grouped into mobile dislocations and immobile dislocations. The amount of different types of dislocations, together with the total amount of dislocations consisting of mobile, immobile and Other dislocations, are seen in Fig. 4. It can be seen that in the early stages of plastic deformation the majority of the dislocations are mobile for all cases. At higher strains, more immobile dislocations are formed in the beams, especially for the case in Fig. 4c where, at high strains, there exists more immobile than mobile dislocations, a phenomena not observed for the two other cases.