Atomic Defects and Disorder in Mechanically-Milled Intermetallic Compounds

Local configurations of point defects were detected with atomic-scale resolution in mechanically milled PdIn and NiAl through nuclear quadrupole interactions they induce at neighboring sites of In-111 probe atoms, using the technique of perturbed angular correlation of gamma rays (PAC). For PdIn, signals were detected which could be attributed to Pd vacancies, Pd antisite atoms, and In vacancies, as in previous studies of annealed or quenched PdIn. In addition, a new signal has been observed which is attributed to high-energy In antisite atom defects next to the In probe. Concentrations of Pd vacancies and In antisite atoms were determined from measured site fractions and monitored as a function of milling time. For PdIn, milling in a WC vial under argon using a Spex 8000 mill leads to a Pd vacancy concentration that increases from zero and saturates after 30 minutes at a value of 3.5(5) at. pct. Such large vacancy concentrations make a contribution to the stored excess enthalpy of about 4.4 kJ/mol. The In antisite atom concentration reached 4 at. pct after two hours of milling, providing evidence of gradual disordering of the B2 structure. Spectra for NiAl exhibit larger vacancy site fractions attributed to binding with the impurity In probe. (Author)