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 Pdln, 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.%. Such large vacancy concentrations make a contribution to the stored excess enthalpy of about 4.4 kJ mol(-1). The In-antisite atom concentration reached similar to 4 at.% after 2 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.