Equilibrium point defects in NiAl and similar B2 intermetallics studied by PAC

Point defects in NiAl and other structural intermetallics have been studied using perturbed angular correlation of gamma rays (PAC), by which signals are detected from antisite atoms and/or lattice vacancies in the first few atomic shells of probe atoms. For samples of quenched NiAl having 50 to 54 at.% Ni, site-fractions of Ni-vacancies (proportional to the vacancy concentration) were found to be independent of composition, indicating that the equilibrium, high-temperature defect is the Schottky vacancy pair and not the so-called triple defect. An effective formation enthalpy of 1.11(4) eV was measured for Ni-vacancies after equilibrating and quenching samples from temperatures in the range 700-1400 C. The formation enthalpy of the Schottky pair is two times larger. Quenched-in vacancies exhibit striking, novel behavior at low temperature: vacancies start to become mobile at about 350 C in 15-minute anneals, as observed by trapping at the probe atoms, but are only able to anneal out appreciably at temperatures of about 700 C. This behavior is attributed to different mobilities of Ni and Al vacancies. Recent data suggests that the Al-vacancy becomes thermally activated at about 350 C, converting into a Ni-vacancy and Ni-antisite atom when it comes near the probe atom. The Ni-vacancy becomes mobile only at about 700 C. Thermal activation of motion of quenched-in and confined vacancies at low temperature, observed here for the first time, offers an explanation for the well-known transition from brittle-to-ductile mechanical behavior in NiAl near 300 C. The explanation is supported by a study of vacancy interactions with Zr solutes that is briefly described. Results for other B2 intermetallics are also briefly described.