Structure and phase transformations during hot deformation of Bi(Pb)2223 high-Tc superconductor: II. Texture and microstructure

Abstract

The effect of hot deformation by torsion under quasi-hydrostatic pressure at temperature T = 865°C with the twist rates of ω 1 = 5.14 · 10 -5 and ω 2 = 1.85 · 10 -4 rpm on the basal plane texture strength (factor F), the mean colony length of the matrix phase (L), the mean area (S) and the volume fraction (V) of particles of (Sr,Ca) 14 Cu 24 O 41 and (Sr,Ca) 2 CuO 3 secondary phases in a high-temperature superconducting ceramics based on the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+d {Bi(Pb)2223} phase has been studied. The correlation between all of the mentioned parameters was observed: with increasing the twist angle the F, S and L values grew reaching the maximum and then decreased. During deformation incompatible intercolony sliding led to dislocation accumulation near the colony boundaries that resulted in their local migration and growth of the colony size. The growth of particles of secondary phases was caused by their coagulation / coalescence. The decrease in the F, S and L values after reaching the maxima was caused by dynamic recrystallization. Refinement of the secondary phase particles occurred due to intergrowth of the Bi(Pb)2223 phase colonies into the particles. It was found that with increasing the twist rate from ω 1 to ω 2 the L value increased. In the case of the torsion with the twist rate of ω 2 the softening occurred only via the local migration of the colony boundaries, whereas in the case of the torsion with the twist rate of ω 1 other relaxation mechanisms, such as diffusion creep and / or dislocation sliding, presumably operated as well. This is why the colony size was found to be smaller after deformation with the lower twist rate (ω 1 ).