Site preferences of indium impurity atoms in intermetallics having Al3Ti or Al3Zr crystal structures

Abstract

Site-fractions of indium impurity probe atoms occupying up to three inequivalent Al-sites in Al 3 Ti, Al 3 V and Al 3 Zr phases were measured using perturbed angular correlation of gamma rays (PAC). Sites were identified via characteristic nuclear quadrupole interactions. Ratios of site-fractions were measured in thermal equilibrium in the range 600 to 1,210 K. Arrhenius plots of the ratios were fitted with thermally activated expressions, yielding differences in vibrational entropies and siteenthalpies. Enthalpy differences were greatest for Al 3 Zr, ∼0.22 eV, and smaller for Al 3 Ti and Al 3 V, which is correlated with the excess volume of the transition-metal atom over the Al-atom. Vibrational entropy differences were small, in the range 0 to −0.25 k B .The site preference of dilute 111 In impurities in the Laves phase GdAl 2 was previously studied as a function of composition and temperature [1]. Occupied Gdand Al-sites were determined through measurements of quadrupole interactions using the method of perturbed angular correlation of gamma rays (PAC). In that study, 111 In impurity probe atoms were observed to switch between sites of different elements. The ratio of site fractions of indium solutes on Gd-and Al-sites was found to be thermally activated, with solutes transferring from the Gd-to Alsublattice with increasing temperature. Transfer takes place heuristically through the reaction In Gd + Al Al ⇔ In Al + Al Gd , in which Al Gd is an antisite atom that is either created or destroyed. Applying the law of mass action to the reaction leads to [In Al ][Al Gd ])/[In Gd ] = exp(−G tr /k B T), in which G tr is the free energy of transfer

Diffusion and Equilibration of Site-Preferences Following Transmutation of Tracer Atoms

“…In thermal equilibrium the ratio of site fractions of In-parent probes on the two sublattices of an A 3 B phase is thermally activated according to 11/19 in which f  and f  are site fractions of In probes on the R-and Pd-sublattices, A is a prefactor and Q transf is an activation enthalpy for transfer of probes between the two sublattices. Such activation enthalpies have been measured previously in this laboratory for 111 In probes in GdAl 2 [17], Ga 7 Pd 3 [19], and Al 3 Ti and Al 3 Zr [20]. Experimentally determined values of Q transf are shown for R= Ce, Pr, Nd, Sm, and Eu phases in Fig.…”

The Solubility of Indium in Liquid Gallium Supercooled to 12 K

“…Using PAC, one can measure electric field gradients at nuclei of the probe atoms. Lattice locations of the probes can be identified when the different sites occupied have different point symmetries (see, e.g., [5,6]). Diffusional jumps of the probe atoms between sites having different symmetries leads to relaxation of the quadrupole interaction that can be fitted to obtain the jump frequency (see, e.g., [7,8]).…”

Experimental and ab initio study of the electric field gradients at Ta and Cd impurities in TiAl3

“…From now on the term EFG will be used to refer to V zz . The presented investigation, together with the previously reported one done by Saarland University Group [11] and Washington University Group [12], could be of special importance for modeling newer generations of alloys with superior properties.…”

“…The large In impurity in 2(a) Ti-site would be surrounded by 12 smaller Al atoms. Actually the PAC measurements in TiAl 3 with 111 In probes indicate the indium solute location in Al sublattice with strong preference of 4(d) site [12].…”