Segregation of solute atoms to interphase boundaries in GdNi2

Lattice locations of In-111 impurity probe atoms in intermetallic GdNi2 were studied as a function of alloy composition and temperature using perturbed angular correlation spectroscopy (PAC). Measurements were made on a pair of samples that were richer and poorer in Gd. Three nuclear quadrupole interaction signals were detected and their equilibrium site fractions were measured up to 700 degrees C. Two signals have well-defined electric field gradients (EFGs) and are attributed to In-probes on Gd- and Ni-sites in a well-ordered lattice. A third signal exhibiting strong inhomogeneous broadening was observed at low temperature in Gd-richer samples. This is attributed to segregation of the In-probes to phase boundaries (PB) of minor volume fractions of the neighboring GdNi phase. A measurement made on a stoichiometric GdNi sample exhibited the same inhomogeneously broadened signal, indicating that In-probes prefer to occupy PB and/or grain boundary sites in GdNi rather than the well-defined Gd-and Ni-sites. Changes in site fractions were reversible above 300 degrees C, indicating that probe atoms equilibrate among all lattice locations within a time period of one day. Thus, PB sites provide lower enthalpy environments for In-probes than either crystallographic site in GdNi2. Enthalpy differences between the levels were determined from measurements of temperature dependences of ratios of site fractions. The enthalpy of transfer of In-probes from the Gd- to Ni-sublattice, which is coupled to intrinsic disorder in the compound, was found to be much smaller in the Gd-richer sample than in the Gd-poorer sample. This can be explained by differing temperature dependences of intrinsic defect concentrations at the two compositions. Among those probes that remain within the GdNi2 phase, there is a temperature dependence of the ratio of site fractions of In-probes on Gd-and Ni-sites. Taking this into account, a macroscopic segregation enthalpy is derived from the measurements. Additionally, brief measurements on GdNi2 samples doped with 1 or 2 at.% Cu were carried out.