Quantitative metallography with a digital computer: Application to a Nb-Sn superconducting wire

Abstract

Accurate quantitative data pertinent to the structure of solid materials at the micro size level, which are difficult or prohibitive to obtain by traditional manual measurements, are now obtained directly by a digital computer which uses a photomicrograph as the infor-mation input. The history of picture interpretation experiments at the National Bureau of Standards is reviewed. The fundamental computer operations are illustrated, together with a description of 24 image processing routines now functional at a practical level. A micrograph of a specimen of Nb-Sn superconductor wire is exhaustively analyzed. This specimen is found to contain approximately 70 percent NbaSn, nearly all of which is mutually interconnected. It is also found that in this specimen the mean free path in the NbsSn superconducting phase is only 26.5 microns. This small value results from the spongy structure of the material and numerous interruptions caused by voids and by particles of four other solid phases. The comparative importance of the several types of interruptions is measured. It is determined that small voids are the most important single cause of the short mean free path, and deduced that these voids appear to have formed mainly from the reaction during heat treatment.