On the ternary Laves phases Ti(Mn 1− xAl x) 2 with MgZn 2-type

Abstract

Alloys with composition Ti(Mn 1− x Al x ) 2 ( x Al = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.55, 0.6, and 0.67) were studied by X-ray powder diffraction (XPD), optical microscopy, electron probe microanalysis (EPMA) and electronic density functional theory. EPMA and XPD defined an extensive solid solution of up to x Al = 0.64 at 900 °C. Structure determination from Rietveld refinements of X-ray powder diffraction data for Ti(Mn 1− x Al x ) 2 (0 ≤ x Al ≤ 0.60) and from single crystal X-ray counter data for x Al = 0.2 revealed a C14-MgZn 2-type Laves phase, where Ti atoms fully occupy the 4 f sites, whereas Mn and Al atoms share the 6 h and 2 a sites in various ratios. By means of density functional theory, the structural stabilities and site preferences of nine ternary compositions, Ti(Mn 1− x Al x ) 2 ( x Al = 0, 0.125, 0.25, 0.375, 0.500, 0.625, 0.750, 0.875, 1.00), were further calculated for a large number of structural models. The derived Al content dependent structural stabilities, lattice parameters, and site occupations as well as enthalpies of formations are in nice agreement with experimental results. Based on the calculated data, we analyzed the occupation behaviour of Al atoms substituting for Mn atoms at the 2 a and 6 h sites: up to concentrations x Al < 0.375 Al atoms prefer the 6 h sites, whereas for 0.375 < x Al < 0.625 Al atoms prefer the 2 a sites. The calculations furthermore revealed that the Al atoms at the 6 h sites exhibit a trend of getting paired.