Origin of subgrain structure within a domain in melt-processed ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The microstructures of melt-processed ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ with different cooling rates were studied by polarized light optical microscope. The present study concentrated mainly on the origin of subgrain structure within domains, where there exist controversies in the literature, as due to the polygonization process in the solid state or during the crystallization process. The present study strongly supports the view that subgrains result from the solidification instabilities during crystallization from the melt rather than due to the polygonization process in the solid state.