Investigating defect evolution during thermal treatment in Ni–Cr alloy using positron annihilation spectroscopy

Defects evolved during thermal treatment in high Cr content Ni–Cr alloy have been studied using Positron lifetime and coincidence Doppler broadening spectroscopy complemented with microscopic technique. The single-phase gamma-quenched specimen obtained by solutionization at 1200 °C has been thermally aged at 650 °C and 800 °C. Thermal treatment resulted in the phase transformation of the matrix through precipitation of Cr. It is seen that solutionization treatment followed by quenching resulted in the stabilization of thermal vacancies generated at 1200 °C which effectively trap the positrons. The results infer that these vacancies may form vacancy-Cr complexes which act as nucleation site for Cr aggregation. Positron lifetime and momentum distribution of annihilation electrons indicated that in the aged alloys, positrons are trapped in vacancy-like defects (vacancy clusters/vacancy-solute complex) and the interface between Cr precipitate and matrix, the latter becoming more predominant with subsequent ageing. At the highest ageing studied, vacancy-like defects are evolved within the Cr precipitates. The results indicated that positron trapping sites (vacancies and precipitate interface) are not specific to either Ni or Cr atoms which surround the trapping site in nearly equal proportion. Therefore, unambiguous identification of the defect type at each stage of thermal treatment becomes difficult. Considering positron trapping at the surface of precipitates, we have calculated positron specific trapping coefficient to be ~ 1.33 × 10−6 cm3 s−1, which is of the order estimated for large voids and estimated the number density of precipitates.