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

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 1022 m−3$\left(10\right)^{22} \left(\text{ m}\right)^{- 3}$ , 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.