Identification of relaxation maxima of internal friction in the Zr – Nb alloy after nitriding, oxidation in air, and in situ carburization

Abstract

The internal friction method was used to study the behavior of the Zr – 1 wt. % Nb alloy after oxidation in air, nitriding in an atmosphere of technical nitrogen, and in situ carburization during measurements. In this case, carbon was deposited on the samples during measurements due to thermal decomposition of vapors of diffusion oil. The pressure in the system was about 10–2 Pa. The temperature dependence of the logarithmic decrement of damped bending vibrations with a frequency of 2.2 – 2.4  Hz was measured on specimens in the form of a rectangular plate 22 × 8 × 0.3 mm in size. The measurements were carried out at a constant heating rate of about 4 K/min from room temperature to 700 – 750 °C. The obtained temperature dependences of internal friction were divided into partial maxima using a special program. The program determines the temperatures of the relaxation maxima and calculates the effective activation energy using the Werth — Marx formula. The internal friction spectra in successive measurements on one sample changed due to the diffusion redistribution of the alloy components. The parameters of the relaxation maxima for carbon (153 – 159 kJ/mol), oxygen (203 – 207 kJ/mol) and nitrogen (235 – 238 kJ/mol) in the alloy under study, depending on the experimental conditions, have been established. Grain-boundary (Gb) impurity maxima due to these impurities were also found. The following activation energies were obtained: 173 – 179 kJ/mol for C-Gb, 216 – 219  kJ/mol for O-Gb and 222 – 229 kJ/mol for N-Gb. Impurity grain-boundary maxima associated with carbon, oxygen, and nitrogen were observed in each series of measurements. The activation energy of the grain-boundary maxima proper decreases monotonically from 202.4 to 194.5 kJ/mol in a series with oxidation in air (experiments 711 – 715), and remains about 200 kJ/mol in nitrided samples.