The Heusler Phase Ti25(Fe50 − xNix)Al25 (0 ≤ x ≤ 50); Structure and Constitution

Alloys with composition Ti25(Fe50 − xNix)Al25 (0 ≤ x ≤ 50) were investigated employing electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD). For TiFe2Al, in situ neutron powder diffraction (ND) was used for the inspection of phase constitution covering the temperature range from 27 °C (300 K) to 1277 °C (1550 K). Combined Rietveld refinement of ND and XPD data for TiFe2Al revealed that Fe atoms occupy the 8c site in space group \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ Fm\bar{3}m ,$$\end{document} Ti with a small amount of Al sharing the 4a site, and the remaining Ti and Al atoms adopting the 4b site. This structural model was successfully applied in the refinement of all alloys Ti25(Fe50 − xNix)Al25 (0 ≤ x ≤ 50). Partial atom order exists on the Fe-rich side while complete order is observed for the Ni-rich side. Profiles recorded by in situ neutron powder diffraction for TiFe2Al in the range of investigated temperatures show two phases, namely Heusler phase and MgZn2-type Laves phase. Diffraction peaks from the Heusler phase dominate the profiles at lower temperatures but at higher temperatures the MgZn2-type Laves phase is the main phase. No CsCl-type phase was found in the alloy in the investigated temperature range. The thermal expansion coefficient of TiFe2Al is 1.4552 × 10−5 K−1.