Radiation-enhanced diffusion in nickel-10.6% chromium alloys

Abstract

Results of investigations of the diffusion rate of nickel-10.6% chromium alloys after plastic deformation, after quenching from 700°C and from 1030°C, and during irradiation with 18 MeV protons and 1.85 MeV electrons are reported. The diffusion rate is measured by means of the electrical resistivity which increases with increasing degree of short range order. It was found that the characteristic temperature below which short range order develops is T t = 550°C. Below about 400°C the atomic mobilities of the component atoms of the alloy are so small that no further increase in the degree of short range order is found in due laboratory times. The activation energy for self-diffusion was determined after quenching from 700°C to Q SD = 2.88eV. For the migration activation energy of vacancies a value of E 1 V M = 1.18 e V was obtained after quenching from 1030°C. For the migration activation energies of interstitials and vacancies values of E 1 I M = 1.04 e V and E 1 V M = 1.16 e V are derived from results of measurements of radiation enhanced diffusion, respectively. These values decrease with increasing high energy particle flux. It was further found that a vacancy diffusion mechanism is rate-determining during irradiation for the increase of the degree of short range order. Interstitials have to jump about 150 times more often than vacancies for the degree of short range order to increase by the same amount. The characteristic temperature for interstitial cluster formation is T t = 300°C. Above this temperature radiation-induced interstitials and vacancies annihilate mainly by pair recombination. Below this temperature interstitials also annihilate at sinks which are formed during irradiation so that the concentration of vacancies increases with irradiation time. Their migration activation energy is approximately obtained in a straight-forward way from the experimental data. Above about 380°C the radiation enhanced diffusion rate is surprisingly much smaller than the thermal diffusion rate. The quasi-dynamic vacancy concentration built up during irradiation is much smaller than the thermal vacancy concentration.