Influence of transformation heat treatment on microstructure and defects in RHQT-processed Nb/sub 3/Al

Nb-Al jelly-roll composite wires of stoichiometric composition (25 at% Al) have been processed by Rapid Heating-Quenching (RHQ) to temperatures just above the bcc formation reaction. Transformation heat treatments going to 800(degree)C and 1000(degree)C with various initial heating ramp-up times (from 30 sec to 30 min) and followed by 800(degree)C/10 hrs were applied to samples so obtained. The microstructure and planar defect distribution were investigated by transmission electron microscopy; the concentration of planar defects was found to decrease as the initial heating rate is increased. In situ high temperature X-ray diffraction studies were used to investigate the transformation heat treatment. It is proven that less ordering of the bcc solid solution prior to conversion to A15 occurs at high heating rates.