Site preference of ternary Fe addition in Ni₇₅Al₂₅

Abstract

An extensive investigation of structure, surface morphology and composition in Ni_75-xFe_xAl₂₅ alloys with nominal composition x = 0, 5, 10 and 20, deliberately 'prepared' in different states of site disorder, has been carried out by x-ray diffraction, scanning electron microscopy, x-ray fluorescence (energy dispersive absorption of x-rays) and inductively coupled plasma optical emission spectroscopy. The results of an elaborate analysis of the present x-ray diffraction data on Ni_75-xFe_xAl₂₅ (Ni-deficit) alloys and those reported previously on Ni₇₅Al_25-xFe_x(Al-deficit) alloys vindicate the recent theoretical prediction that the composition of the alloy plays a decisive role in determining the site preference of Fe by clearly demonstrating that Fe has nearly exclusive Ni (Al) site preference in the Ni-deficit (Al-deficit) alloys and thereby resolves a long-standing controversy surrounding the site preference of the ternary Fe addition in the γ '-Ni₃Al phase in the Ni-Al phase diagram. However, contrary to the recent theoretical claim that entropy promotes Al site preference of Fe in Ni-deficit alloys, we find that Fe exclusively occupies the Ni sites in those Ni-deficit alloys that have a greater degree of site disorder.