Abnormal grain growth in YBa2Cu3O7-x high-Tc superconducting ceramics during high-temperature deformation

D.B. Kabirova1, V.V. Pavlova2, M.F. Imayev3
1Institute for Metals Superplasticity Problems RAS, 39 Khalturin St., 450001, Ufa, Russia
2Ufa State Aviation Technical University, 12 K. Marx St., 450008, Ufa, Russia
3Bashkir State University, 32 Z. Validi St., 450076, Ufa, Russia
The formation of the bimodal structure in YBa2Cu3O7−x high-Tc  superconducting 
ceramics during high-temperature deformation is considered.
Abnormally coarse grains of Y123 ceramics with (a) rectilinear 
form    and   (b)   curvilinear   form  is shown on the figure . Bimodal microstructure is formed in high-Tc superconducting ceramics YBa2Cu3O7-x deformed by torsion under pressure at 1008oC. The appearance of second peak on grain length distribution is associated with formation of abnormally coarse grains with length L up to 300-500 μm. Cooling rate has almost no effect on the density of abnormal grains n and L, indicating that such grains are formed during deformation. The L value increases monotonically with increasing twist angle α, while n increases only up to α = 25 °, and then drops. The value of α above which n decreases sharply corresponds to the formation of strong texture with a factor F ≥ 0.95. Formation of abnormal grains can be explained by strain-enhanced grain growth during deformation. Intergranular sliding produces damage zones, the healing of which takes place due to the local migration of grain boundaries. Apparently, abnormal grains appear in places of most intense intergranular sliding. As the texture increases the degree of freedom of movement of grains decreases, so it decreases the possibility of accommodation of sliding of neighboring grains. This leads to increase in the number of damage zones and strain value in them that is accompanied by the growth of n and L. After it reaches a certain critical level of texture (F≈0.95 at α≥25o) the possibility of stress relaxation due to local migration of initial grains is exhausted, and dynamic recrystallization of abnormal grains begins. Therefore, when α≥ 25o the n value is decreased sharply.
Received: 30 October 2016   Revised: 02 November 2016   Accepted: 14 November 2016
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