The U-Nb-Al ternary system: experimental and simulated investigations of the phase equilibria and study of the crystal structure and electronic properties of the intermediate phases

Abstract The phase relations in the ternary U-Nb-Al system were established for the whole concentration range for 900 K and 1200 K. They were derived from quenched samples annealed at 900 K for three months and at 1200 K for two months by using x-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analyses. The formation of the two ternary phases, UNb 2 Al 20 and U 6 Nb 4 Al 43 was confirmed. Both phases form by peritectic reaction, at 1200(5) K and at 1262(5) K for UNb 2 Al 20 and U 6 Nb 4 Al 43 respectively. Single-crystal structure refinements confirmed that UNb 2 Al 20 adopts the CeCr 2 Al 20 type (cubic, F d 3 ¯ m ) and that U 6 Nb 4 Al 43 crystallizes with the Ho 6 Mo 4 Al 43 type (hexagonal, P 6 3 / mcm ). The isothermal sections are characterized by (i) the extended homogeneity ranges due to mutual exchange between U and Nb and between Nb and Al in the bcc -phases (γU and Nb), at 1200 K, and (ii) a substantial ternary extension with limits of about 6(1) to 10(1) at.% U at 900 K and 1200 K respectively, of the σ-phase (Nb 2 Al, tetragonal, P 4 2 / mnm ). The modeling of these isothermal sections was carried out by the Calphad method. The liquidus projection and the invariant reactions along with their corresponding temperatures were calculated using our optimized database. The electronic properties of both intermediate compounds were measured by means of dc -susceptibility, specific heat and resistivity measurements in the temperature range 2–300 K, revealing for UNb 2 Al 20 enhanced Pauli paramagnetism and complex magnetic features for U 6 Nb 4 Al 43 at low temperature. For the latter compound a composite behavior involving long-and short-range orderings, resulting from intra and inter U dimeric chains along the c -axis is suspected below 12 K.