Quantification of Second Phase Morphology in SSCL VQP Samples

Abstract

Select heat treatment and final size strand sample sets from Phase I of the VQP have been microstructurally characterized by electron microscopy. Samples taken after each of the precipitation heat treatments had their α-Ti second phase precipitate size and volume fraction analyzed by backscattered electron imaging (BEI) in both Transmission Electron Microscopy (TEM) and Field Emission Scanning Electron Microscopy (FESEM), both coupled with image analysis techniques. Final size strand microstructure was imaged by bright field TEM. The α-Ti “ribbon” thickness and spacing were determined by a line intercept analysis. Procedures used for sample preparation, image acquisition, and image analysis, are presented. The post heat treatment second phase volume fraction was found to vary on a size scale of a few microns. As a result, the calculated volume fraction from different images from the same NbTi filament can have significant variations. FESEM has much easier sample preparation than TEM and FESEM images are not complicated by thickness effects or precipitate “ghost” images from precipitates within the thickness of the foil. Results indicate that the post final heat treatment mean precipitate diameter, as well as the final size α-Ti ribbon thickness, is similar for all VQP vendors and independent of NbTi alloy source.