The Nb 3Sn grain structure evolution in bronze-processed multifilamentary superconducting wire

Abstract

Two schemes have been proposed to explain the observed Nb 3Sn layer morphology in bronze-processed superconducting wire. One scheme attributes the morphology to the competition between nucleation and growth of Nb 3Sn grains at the niobium interface while the other ascribes it to a breakup of the columnar grains. Microstructural observations show that the latter scheme operates. Columnar grains develop very early during layer growth. With subsequent layer growth they retain approximately constant length. Since the layer continues to grow this requires that the columnar grains formed earlier in the reaction break-up. This process is responsible for the observed equiaxed grain morphology. The few dislocations observed in the columnar or equiaxed grain regions are present to accommodate the crystallographic mismatch between grains. Recrystallization is not indicated since fine grains are not observed. The specific mechanism for the columnar grain break-up remains unclear; however, two classes of processes have been excluded, polygonization and recrystallization.