Unconventional Nonequilibrium Phase Stabilization in FeNi Alloy Examined by Positron Annihilation Spectroscopy

Abstract

The equilibrium FeNi alloy prefers face centered cubic (fcc) phase whereas the formation of metastable phases through certain low‐temperature synthesis methods is obscure. FeNi alloy exhibiting nonequilibrium body centered cubic (bcc) structure beyond 5 at% Ni is obtained through chemical reduction at 453 K with noticeable lattice shrinkage. Positron lifetime analysis and Doppler broadening spectroscopy are employed to probe the correlation between defects and phase stability. The mean lifetime of the chemically synthesized alloys is larger than 300 ps, implying an average of 7‐vacancy cluster. The modified two‐state trapping model reveals defect concentration in the range of , on par with irradiated alloys. Significant deviation in the high‐momentum component between the bulk and chemically synthesized samples is correlated to the changes in 3d electrons. The Fe and FeNi alloy uncover significant contraction in d‐band of Fe which facilitates bcc phase stabilization above 5 at% Ni through cluster vacancy formation.

Influence of vacancy type defects on the exchange integrals in ferromagnetic metals: a Monte-Carlo-based approach

“…The real crystal system is susceptible to defects like vacancies and impurities depending on the nature of the synthesis method and exposure to harsh environments. The positron annihilation lifetime analysis of Fe and Ni [17] reported recently has unveiled the presence of large cluster vacancies with an average of around 10 vacancies in a cluster. For materials exhibiting RKKY-type interaction, Habibi et al [39] showed that vacancies suppress short and long-range interactions, thereby diminishing atomic-scale oscillations and Friedel oscillations over distance.…”

“…The presence of vacancies and their configuration in 3d metals and alloys reported [17][18][19] could increase the spin fluctuations as the nearby moments are less bound to their neighbors [16]. The density functional theory (DFT) typically considers a dimensionally smaller unit cell with lesser number of atoms to study the material properties.…”