Defect formation during oxygenation was investigated in a bulk melt-processed superconductor. Oxygenation at 550C resulted in the formation of tweed-like twins, plus some micro-cracks. A non-uniform phase transition occurred in specimens which were annealed at 450C, together with micro-cracking that was caused by O-induced stress. It was suggested that there was a critical stress at which the stress-relief mode changed from twin formation to crack formation. Below this critical value, stress was relieved by the formation of twins. Above the critical value, stress was relieved by the generation of micro-cracks.

H.G.Lee, B.H.Ryu, J.J.Kim, I.H.Kuk, G.W.Hong: Materials Letters, 1996, 27[1-2], 27-32