PSI - Issue 43

Orsolya Molnárová et al. / Procedia Structural Integrity 43 (2023) 166–171 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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3. Results To characterize microstructural changes during the CSET process of the aluminum single crystal, three measured locations were used as representatives. These locations are shown in Fig. 1b. In fact, there are indicated four points, however, location 1a (Fig. 3a) is a broader area measured at locality 1 (Fig. 2). The inverse pole figures (IPF) and their maps as well as the KAM maps are shown in Figs. 2 – 5. The structure of the sample in locality 1 proves that here, the sample before extrusion is still single-crystalline (Fig. 2). However, the axis orientation of the sample in this location differs from the nominal orientation of the single crystalline billet: In contrast to  148  nominal orientation, the axis orientation of the sample in locality 1 is  115  . This orientation is preserved until the start of the extrusion (Figs. 2c, 3b). As is shown in Fig. 2b, the simple compression introduces a gradient distribution of the stress into the single crystal with lower dislocation density in the upper part and higher stress in parts being close to the mandrel. In vicinity of the mandrel the mass flow in the direction parallel to its tip is apparent. However, in the whole single-crystalline part of the sample the preferential orientation of the axis of the sample is identical (cf. Figs. 2c and 3b). Due to the heavy compression, the structure at locality 1 is of a deformed character. Further deformation of ECAP type results in breaking the large grain of the single crystal into small grains prolonged by about 30° to the direction of the deformation (locality 2, Fig. 4a). However, this inclination is not stable, it changes in the direction perpendicular to the material flow. The grain shape aspect ratio is 0.3±0.1. The size of the grains is non-uniform with an average size (equivalent diameter) of 0.7 ±0.5 μm in resp ect to the number of the grains and/or 1.5±1.0 μm in respect to the grain area. Although the sample after this SPD is highly deformed as documented by high stress (Fig. 4b), during the movement to the 2 nd ECAP-like deformation the microstructure undergoes partial dynamic recrystallization. The preferential orientation of the sample axis in this part is composed of two directions,  223  and  125  (Fig. 4c). The microstructure of the sample in locality 3 exhibits similar microstructure composed of prolonged grains in the direction of the deformation (Fig. 5a). The aspect ratio of the grains shape ( 0.35±0.1) as well as the grain size (equivalent diameter, 0.7 ±0.5 μm in respect to the number of the grains and/or 1.5±1.0 μm in respect to the g rain area) are very similar to those of the previous locality. The preferential orientations of the sample axis in the main direction of the CSET process, are composed of two components,  355  and  001  (Fig. 5c). The microstructure of the sample in this part is partially recrystallized although the dislocation density is still very high and non-localized (Fig. 5b) as would be expected in the recrystallized state. 4. Discussion During CSET processing, the sample undergoes complex severe plastic deformation ( Molnárová et al. 2021). Mainly the process of the first ECAP-like deformation is complicated as – in contrast to traditional 2D ECAP – the deformation is of radial symmetry. Therefore, it is rather complicated to make complete orientation assessments on basis of results measured using few samples after CSET processing. However, an inspection of a limited amount of the samples can also provide us with some basic information about microstructural changes in the processed material. Inspection of the strereographic projection of original orientation to that in locality 1 shows that the sample changed its orientation by rotation of 19°  110  . The determination of the Schmid factors for individual systems related to the  148  axis orientation shows variations between 0.145 and 0.930. The above-mentioned rotation corresponds to the deformation in the most convenient setting of the {111} slip plane and corresponding  110  -type of Burgers vector to the pressing the sample by the plunger, characterized by the highest value of the Schmid factor of 0.930. Despite this fact, the rotation by 19° seems to be surprisingly high and the dislocation density , which was estimated to be of about 0.5  10 15 m – 2 , does not correspond to this rotation. However, the maximum intensity of the preferred orientation is diffuse (Figs. 2c, 3b) which might somehow reduce this rotation angle. After extrusion and 1 st ECAP-like deformation, the material is highly deformed as reflected in Kernel average misorientation (locality 2, Fig. 4b). Maximum density of dislocations was estimated to be of about 2  10 15 m – 2 in this region. This complex deformation introduced high stress into the material so that dynamic recrystallization is evoked, and, in the material, there is a mix of deformed and recrystallized microstructure (Fig. 4a).