Corresponding author: Nataliya Yu. Tabachkova ( ntabachkova@gmail.com ) © 2019 Vladimir T. Bublik, Mikhail G. Lavrentev, Vladimir B. Osvenskii, Yuri N. Parkhomenko, Nataliya Yu. Tabachkova.
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Citation:
Bublik VT, Lavrentev MG, Osvenskii VB, Parkhomenko YN, Tabachkova NYu (2019) Structure formation by hot extrusion of thermoelectric bismuth chalcogenide solid solution rods. Modern Electronic Materials 5(4): 181-185. https://doi.org/10.3897/j.moem.5.4.52932
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Major advantage of extruded Bi2Te3 based thermoelectric materials is high mechanical strength compared with that of melt-crystallized materials. Mechanical properties are of special importance for thermogenerator module applications where thermogenerator branches may undergo elevated thermal stresses due to large temperature differences at the modules. Since extrusion is typically a high-temperature process the structure of extruded materials is controlled by the plastic deformation in multiple slip systems resulting in the formation of a final deformed structure. The grain orientations are predominantly such that the most probable cleavage plane orientation is parallel to the extrusion axis. Recovery processes occur simultaneously and different recrystallization stages may take place. In the latter case the deformed texture may be destroyed.
Structure evolution along the extruded rod of Bi2Se0.3Te2.7 ternary solid solution was studied with metallography and X-ray diffraction. Extrusion was interrupted for the study and so the specimen was a whole rod the initial part of which was the extrusion billet and the final part was the as-extruded material. The structure of the material is formed by competitive processes of dislocation generation and annealing. The plastic deformation energy is the highest in the extruder zone of the rod. Both the hardening processes and the texture are controlled by the plastic deformation mechanism. Plastic deformation is accompanied by generation of defects that are most likely vacancy type ones.
hot extrusion, thermoelectric material, bismuth chalcogenides, plastic deformation
The current status of research on thermoelectric materials and its main worldwide trends have been described quite thoroughly in reviews [
Structure evolution along the extruded rod of Bi2Se0.3Te2.7 ternary solid solution was studied with metallography and X-ray diffraction.
Extrusion was interrupted for the structure evolution study and so the specimen was a whole rod the initial part of which was the extrusion billet and the final part was the as-extruded material (Fig.
General appearance of extruded rod and specimen cutting scheme for X-ray diffraction study.
The rod was cut along the extrusion axis for metallographic microstructure study.
For studying the texture and grain deformation by X-ray diffraction the specimens were cut from the extruded rod perpendicular to the extrusion axis. The texture was analyzed with the method of inverse pole figures (IPF). The IPFs were plotted based on the diffraction patterns for the cross-sections perpendicular to the extrusion axis for evaluating the probability of coincidence between poles for different planes with the extrusion axis. The statistical weights of the poles were calculated with normalization by theoretical reflection intensities. Grain deformation was evaluated from Bragg peak broadening.
Figure
Figure
(a–e) Texture and (f–j) microstructure evolution along extrusion axis: (a and f) extrusion billet, (b and g) 2 cm, (c and h) 6 cm, (d and i) 10 cm and (e and j) 12 cm from extruded rod beginning.
Extrusion deformation in pressed Bi2Te2.7Se0.3 alloy billets occurs mainly by grain boundary and dislocation slip in the basal (0001) and pyramidal (1015¯) planes. Basal plane slip is preferential at any temperatures up to near the melting point due to bond anisotropy [
The concentration of defects in the grains was evaluated based on X-ray diffraction peak broadening. The X-ray diffraction peaks of the specimens cut out perpendicular to the extrusion axis were broadened due to the dislocation-induced microdeformations. Microdeformation evolution in specimens cut from different extruded rod zones is shown in Fig.
Plastic deformation in the specimens is manifested by a permanent increase in the dislocation density until the transient zone in the extruder. After a certain deformation degree the flow stress no longer depends on the deformation degree, i.e., the specimen enters the steady state flow stage. The flow stress then reaches the highest value and decreases due to the intense development of dynamic recrystallization, producing a peak in the curve. The difference between dynamic and static recrystallization is that dynamically recrystallizing grains with low dislocation densities undergo hardening during further growth because of the persistent deformation. Grain growth at the extruder output is well illustrated by the microstructure images shown in Fig.
Figure
The lattice parameter corresponding to the ternary solid solution composition Bi2Se0.3Te2.7 is only the case for the rod zone corresponding to the initial extrusion stage at which the material is still inside the die. Selected area X-ray spectral analysis shows that the composition (within the measurement error) is the same along the rod and hence the decrease in the lattice parameter is caused by defect formation during extrusion.
The minimum lattice parameter corresponds to the maximum microdeformation zone (Fig.
The structure formation pattern during hot pressing suggests that both grain deformation and texture are controlled by the type of plastic deformation. Plastic deformation is accompanied by generation of point defects that are most likely vacancy type ones. The structure develops under the conditions of competition between dislocation generation and annealing. The plastic deformation energy is the highest in the extruder zone of the rod. A structure containing a large number of dislocations and point defects forms as a result.
The work was financially supported by Russian Basic Research Fund, Grant No. 18-02-00036.