The ordering scheme in N b aluminides with the B2 crystal structure

Abstract

This research may be summarized as follows. 1. 1. The OTLs for the three alloys Nb-15Al-xTi, where x = 10,125 or 40, have been determined. The trends in site occupancy in these compounds has been identified, and the apparent sublattice compositions have been determined. 2. 2. From these OTLs, it has been shown that while both compounds containing 10Ti and 40Ti exhibit the B2 crystal structure, their ordering schemes are rather different. 3. 3. Simulation of the ALCHEMI results using the dynamical theory of electron diffraction has provided estimates of the end-points of the OTLs. The estimated long range order parameters for the alloys with Ti contents of 10,25 and 40 have been estimated to be 0.88,0.25 and 0.5, respectivley. 4. 4. In all three compounds studied, the most ordered state would correspond to the Al atoms being confined to one sublattice, and this appears to be the most significant factor influencing the stability of the B2 crystal structure adopted by these alloys.