Mapping defects during phase transformation in high Cr content NiCr solid-solution through positron trapping

Abstract

Understanding the role of point defects towards hardening –and influence on secondary phase evolution demands a systematic investigation of defects during aging. In the present work, we investigate defects present/evolved during thermal aging in high Cr content NiCr solid-solution using positron annihilation lifetime and coincidence Doppler broadening spectroscopy. Thermal aging at the studied temperature results in phase transformation of the matrix primarily by discontinuous precipitation leading to Cr precipitation as revealed from field-effect scanning electron microscopy. Positron annihilation results reveal that non-equilibrium thermal (excess) vacancies are predominant defects in the studied alloy. These vacancies are retained in the alloy due to the formation of vacancy-solute complex. During solutionization treatment, the excess vacancies are largely recovered attaining an equilibrium vacancy concentration. On the other hand, excess vacancies are not recovered during thermal aging and stable up to 24 h aging period. In addition to vacancy-solute complex, dislocations are evolved at higher aging times and suggested to be present at the interface of Cr precipitate and matrix. Under the consideration of positron trapping model, the estimated atomic concentration of vacancies (vacancy-solute complex) in the phase transformed NiCr is ~ 10−5 and the dislocation density is ~ 4 × 109 cm−2.