PHASE DIAGRAM EFFECTS IN RAPID THERMAL PROCESSING OF REBa2Cu3O7-delta

Recent investigations of rapid thermal processing of REBa2Cu3O7-delta by drop tube melting, laser surface treatment and shock compaction have shown that the microstructure resulting from these processes is a complex mixture of non-equilibrium phases. While it is tempting to ascribe the variations in microstructure to variations in quench rates between the processes, we will demonstrate that with the exception of extremely rapid solidification from above the liquidus, the microstructure and phase distribution results from the heating rather than the cooling part of the curve. Since all of the above processes induce non-uniform heating, different parts of the sample reach different maximum temperatures. The maximum temperature reached, and the time spent near that maximum, uniquely determine the starting state of the quench. If the entire sample is not above the liquidus the sluggishness of the peritectic reactions results in similar microstructure regardless of the quench rate. Upon annealing the final microstructure is much more dependent on the number of nucleation sites than on the type of site resulting in a uniform REBa2Cu3O7-delta microstructure over a broad range of processing parameters.