THE LOCATION OF TANTALUM ATOMS IN NI3(AL,TA)

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 Ni75Al24.8Tao.2, at which composition the long-range order parameter is close to unity. © 1990, Materials Research Society. All rights reserved.