EQUILIBRIUM-POINT DEFECTS IN TIAL STUDIED BY PAC

Site fractions of dilute In-111/Cd-111 probe atoms in different lattice locations in quenched, equi-atomic TiAl are measured using perturbed angular correlations of gamma rays (PAC). The vacancy-free substitutional site fraction, observed via its nuclear quadrupole interaction, is observed to decrease with increasing quenching temperature, reflecting an increase in the thermal defect concentration. Assuming that the thermal defect is the Schottky defect, the law of mass action, and a binding energy of 0.2 eV between vacancies and the In probe, the experimental temperature dependence is analysed to yield a formation enthalpy of 4.7(4) eV and entropy of 25(4) k(B) for the Schottky defect. The vacancy concentration on one sublattice is given in terms of these parameters by the expression [v] = exp(S-F/2k(B)) exp(-E(F)/2k(B)T), and is found, for example, to have the value 1.4% at 1350 degrees C, a large value.