Microstructure analysis of melt-spun Al3Ti intermetallics by XRD and EXAFS

In an effort to expand the composition range over which Al3Ti is stable, various amounts of niobium were substituted for titanium and processed by melt-spinning. Several samples were annealed both at 600 degrees C and 1000 degrees C for 24 hours. The effects of processing parameters such as wheel speed, the amount of niobium, and annealing temperatures on the structure were investigated by XRD and EXAFS. XRD showed that for all the samples the only structure present was DO22. The DO22 structure was stable even after the high temperature heat treatments. By means of EXAFS, niobium atoms were observed to occupy titanium sites in the DOM structure. Furthermore, in the unannealed samples, increasing wheel speed of the melt spinning process or the niobium concentration tended to distort the crystal structure. It was observed that Ti EXAFS had different results from the Nb EXAFS beyond their occupying similar sites, which suggested there may exist some composition zones, i.e. rich Nb zone or rich Ti zones, although the structures present were still DO22. The samples were found to experience different distortions as a function of annealing temperatures.