Equilibrium Point Defects in Nial and Similar B2 Intermetallics Studied by Pac
نویسندگان
چکیده
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 Nivacancies 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 brittleto-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.
منابع مشابه
Structural, thermal and deformation-induced point defects in PdIn
Perturbed angular correlation of gamma rays (PAC) was applied to measure concentrations of point defects in quenched and milled intermetallic compounds. Two systems with high ordering energies were studied, PdIn and NiAl, with principal results obtained for PdIn. The defects, lattice vacancies and antisite atoms, were detected by quadrupole interactions induced at nuclei of nearby In/Cd probe a...
متن کاملPoint Defects in B2 Intermetallic Compounds
We are applying perturbed angular correlations (PAC) to study point defects near 111In probes in ordered B2 alloys. Systems studied include NiAl, CoAl, FeAl, CoGa and PdIn Ten different point defect configurations have been observed in annealed, quenched or mechanically-milled samples. The present status of this work is summarized. Systematics of hyperfine interactions of vacancy defects are th...
متن کاملEmbedded-atom potential for B2-NiAl
An embedded-atom potential has been constructed for the intermetallic compound B2-NiAl by fitting to both experimental properties and ab initio data. The ab initio data have been generated in the form of energy-volume relations for a number of alternative structures of NiAl and Ni3Al, as well as for Ni and Al. The potential accurately reproduces the basic lattice properties of B2-NiAl, planar f...
متن کاملPyDII: A python framework for computing equilibrium intrinsic point defect concentrations and extrinsic solute site preferences in intermetallic compounds
Point defects play an important role in determining the structural stability and mechanical behavior of intermetallic compounds. To help quantitatively understand the point defect properties in these compounds, we developed PyDII, a Python program that performs thermodynamic calculations of equilibrium intrinsic point defect concentrations and extrinsic solute site preferences in intermetallics...
متن کامل