Detection and quantification of D03 chemical order in Fe-Ga alloys using high resolution X-ray diffraction

Fe-Ga alloys have exhibited significant increases in magnetostriction as a function of Ga content, reaching a maximum near the solubility limit of Ga in BCC alpha-Fe (A2). At this limit, the magnetostriction is dependent on the thermal history of the sample and the formation of long-range chemically ordered D0(3) phase. However, due to nearly identical unit cells of A2 and D0(3) phases and similar atomic scattering factors of Fe and Ga, detection and quantification of phase mixtures in Fe-Ga alloys is problematic. This work demonstrates the use of high resolution powder X-ray diffraction in detecting the presence of two-phase mixtures in Fe-Ga alloys. Peak splitting of the primary reflections is resolved, irrespective of whether or not superlattice reflections from the ordered structure are detectable. This peak splitting is clearly discernible from K alpha(1)-K alpha(2) splitting at moderate and high 2 theta values and texturing of the samples does not interfere or mask the peak splitting. Quantitative analysis showed Fe-19.5 at.% Ga was 100% A2 when quenched from high temperatures, but was a two-phase mixture, 67% A2 and 33% D0(3) by volume, when slow cooled. Conversely, Fe-22 at.% Ga alloy was a two-phase mix, 40% A2 and 60% D0(3) when quenched and 100% D0(3) when slow cooled. (c) 2005 Elsevier B.V. All rights reserved.