Identification of Relaxation Maxima of Internal Friction in Zr-Nb Alloy after Nitriding, Oxidation in Air, and In Situ Carburization

Behavior of Zr-1 wt % Nb alloy after oxidation in air, nitriding in an atmosphere of process nitrogen, and in situ carburation in the course of measurements has been analyzed using the internal friction method. In this case, carbon was deposited on specimens as a consequence of thermal decomposition of vapors of diffusion oil. The pressure in the system was about 10(-2) Pa. The logarithmic decrement of damped bending oscillations was measured as a function of temperature at the frequency of 2.2-2.4 Hz on specimens in the form of a rectangular plate with the dimensions of 22 x 8 x 0.3 mm. The measurements were carried out at constant heating rate of about 4 K/min from ambient temperature to 700-750 degrees C. The obtained temperature dependences of internal friction were subdivided into partial maxima by means of a specialized program determining the temperature of relaxation maxima and calculating the effective activation energy by the Wert-Marx equation. The spectra of internal friction during consecutive measurements on one specimen changed owing to diffusion redistribution of alloy components. The parameters of relaxation maxima for carbon (153-159 kJ/mol), oxygen (203-207 kJ/mol), and nitrogen (235-238 kJ/mol) in the considered alloy were established as a function of experimental conditions. In addition, grain boundary (GB) impurity maxima due to these impurities were detected. Their activation energies (kJ/mol) were 173-179 for C-GB; 216-219 for O-GB; and 222-229 for N-GB. The impurity grain boundary maxima related to carbon, oxygen, and nitrogen were observed in each measurement series. The activation energy of grain boundary maxima themselves monotonically decreases from 202.4 to 194.5 kJ/mol in the series with oxidation in air (experiments 711-715) and remains at about 200 kJ/mol in nitrided specimens.