The location of tantalum atoms in Ni3(Al, Ta)

Abstract

An x-ray powder diffraction method was used to determine the location of tantalum atoms in Ni₃Al. A series of Ni₃(Al, Ta) alloys were produced with tantalum contents ranging from 0.1 to 3.0 at.%. Fine powders with average particle sizes less than 80 μm were made from melt-spun ribbons by a grinding process. The values of the intensity of the (100) superlattice peak normalized to that of the (200) fundamental peak as a function of tantalum content agreed with the calculated values, assuming that tantalum atoms substitute on aluminum sites, not on nickel sites, and also assuming small amounts of anti-site defects exist in the ordered fee structure of Ni₃(Al, Ta) alloys. It is concluded from our experiments that tantalum atoms substitute for aluminum in Ni₃Al. The long-range order parameters thus calculated of the Ni₃(Al, Ta) alloys are generally above 0.84 and below 0.95, except for Ni₇₅Al_24.8Ta_0.2, at which composition the long-range order parameter is close to unity.