PSI - Issue 23

Joakim Nordström et al. / Procedia Structural Integrity 23 (2019) 457–462 Joakim Nordström / Structural Integrity Procedia 00 (2019) 000 – 000

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elongation at RT, L-RT, is higher than that at - 196°C, L -CT. This indicates that the higher elongation at CT is solely attributed to the homogeneous plastic deformation. In table 1, it can also be found that the contraction of the fractured specimen at RT (70.3%) is larger than that at CT (66%), though it has experienced a higher homogeneous plastic deformation. This is the first observation ever since that a material with a larger homogeneous plastic deformation elongation has a smaller relative contraction.

Figure 1: Deformation and fracture behaviour of Alloy 625: a) tensile test curves for both RT and CT and b) the specimens as virgin specimen and tested at both RT and CT.

Table 1: Tensile properties at RT and CT

Rp 0.2 ( MPa )

Rm ( MPa )

Temperature

A (%)

Z (%)

Room temperature

370 550

792

69.0 89.4

70.3 66.0

- 196°C

1134

3.2 Deformation and strengthening mechanisms To study the above extraordinary deformation behavior of the Alloy 625, the microstructure and texture in the tested specimens were evaluated by EBSD and presented as inverse pole figure maps, see Figure 2. For the material with as received condition (Fig. 2a), the grains are randomly oriented and near isotropic. For the specimen tested at RT, the grains have been rotated and are oriented mainly towards <111> and to some extent also in the <100> direction (Fig. 2b). For the specimens tested at CT, the grain oriented towards <111> or <111> fibre texture is now dominant (Fig. 2c). In the specimen tested at - 196°C, mass nano deformation twins (red lines in the grain) can be observed. These twin boundaries will act as obstacles for further dislocation movements, which consequently will increase the strength. This indicates that formation of texture and nano-twins in the grains are the strengthening mechanisms. As known, the formation of deformation twins will increase the ductility of the material, which is called twinning induced plasticity (TWIP) (Grassel O et. al, 2000). The above discussion shows that high strength and high ductility of Alloy 625 tested at CT has its origin in TWIP and texture formation, besides normal dislocation slip – the most common RT deformation behavior.