Application of ultrasonic resonance oscillations to estimate the size of phase inclusions in samples of chrome – iron alloys

During the implementation of strategic research program of the Technology Platform “Closed Nuclear Fuel Cycle with Fast Reactors”, hot cells are fitted with original non-standard equipment for primary post-reactor non-destructive studies of critical fast reactor materials. These materials include chrome-iron alloys, considered as promising for fuel rod cladding of fast neutron reactors. During the investigation via the ultrasonic spectroscopic method of internal friction of cylindrical samples from chrome-iron alloys in a narrow temperature range near 550 K at their cooling with rate of about 0.2 K/s, we have revealed an anomalous effect from the point of view of oscillations classical theory. Its peculiarity consists in the occurrence of coupled oscillations in samples with a nonuniform temperature distribution along the radius, in case the interval of temperature variation contains the point of the magnetic phase transition of the material. Such a sample can be considered as a complex oscillatory system consisting of peripheral (cooler) and central areas in different magnetic states, with a pulsating interface, on which mechanical stresses act. It was estimated that this anomalous mode of oscillations is associated with the influence of dynamic vibrational stresses on formation and magnetic phase transformations in carbonitride inclusions occurring during heat treatment of alloys. A theoretical description of this effect is proposed. It is shown that by registering the parameters of coupled oscillations, it is possible to estimate the sizes of the resulting phase inclusions. The discovered features and established regularities of resonance oscillations in combination with the traditional method of internal friction can be used to detect metastable phase inclusions occurring at intermediate stages of the material structure, and to estimate their sizes. It will undoubtedly be useful in the case of primary non-destructive tests in hot cells of heavily irradiated samples of these alloys when optimizing their composition for fuel rod claddings of fast neutron reactors.