Microstructure development in bronze route Nb3Sn and (Nb,Ta)3Sn multifilamentary composites

Electron microscopy has been used to characterize the microstructure of commercial binary and ternary Al5 multifilamentary composites. TEM sections have been prepared using ion beam milling from samples reacted for various times at temperatures in the range 650- 800°C. Layer growth kinetics have also been investigated using a combination of SEM and low temperature inductive measurements. The presence of a band of columnar grains adjacent to the refractory core is confirmed in all cases and is particularly marked at the lower temperatures. As layer growth proceeds, the columnar structure develops into an equiaxed grain structure; the mean grain size depends on the core composition. On the basis of this microstructural investigation and precise measurements of filament volume and length changes during reaction, it is proposed that the mechanism for microstructure modification is stress driven grain boundary sliding. A model describing this process is summarised.