Session Q28 - Positron Annihilation Defect I.
MIXED session, Friday morning, March 21
Room 4300E, Conv. Center
Mingzhong Wei, Gary S. Collins (Washington State University, Pullman.)
We present a new and improved analysis of measurements made some years ago in this laboratory. Perturbed angular correlations of gamma-rays (PAC) was used to detect 111In/Cd probe atoms in different local environments in the L1_0 structure of TiAl. Measurements were made on annealed samples having 49 to 55 atomic % Al in order to identify signals from structural antisite defects. Measurements were also made on stoichiometric samples quenched from temperatures up to 1400\degC. The PAC spectra exhibit 3 major signals with attributions and quadrupole interactions as follows: An In_Al probe with no nearby defects (ømega_1= 134 Mrad/s, \eta= 0.0); (2) An In_Al probe with 1 Al_Ti defect in the first atomic shell around the probe atom (ømega_1= 137 Mrad/s, \eta= 0.43); (3) An In_Al probe with 1 Ti_Al defect in the first shell (ømega _1= 155 Mrad/s, \eta= 0.88). The hyperfine interactions for sites 2 and 3 can be explained using a simple point-charge defect model. The site-fraction for site 2 increases with Al content in annealed samples in just the way expected for a random distribution of Al_Ti defects. Site-fractions for sites 2 and 3 both increase with quenching temperature, and the temperature dependence of the concentrations of quenched-in equilibrium defects will be reported.