Assessing the Size of Phase Inclusions in Ferrochrome Alloys by Means of Ultrasound Resonance

In the strategic research program on closing the nuclear fuel cycle with fast neutron reactors, nonstandard equipment must be introduced in the fuel chambers for nondestructive monitoring of materials that are critical to fast neutron reactors. In particular, ferrochromium alloys are regarded as promising for the fuel-element casings of fast neutron reactors. An unexpected effect is observed in ultrasonic spectroscopy of cylindrical ferrochromium-alloy samples by the internal-friction method, in a narrow temperature range close to 550 K on cooling at around 0.2 K/s: coupled oscillations are seen in samples with a nonuniform temperature distribution over the radius if the temperature range includes the material’s point of magnetic phase transition (Curie point). Such a sample may be regarded as a complex oscillatory system consisting of a peripheral (cooler) region and a central region in different magnetic states. Mechanical stress is present at the pulsating boundary of those regions. Such anomalous oscillation is associated with the influence of dynamic oscillatory stress on the formation of carbonitride inclusions in heat treatment and on the magnetic phase transitions in those inclusions. A theoretical description of this effect is proposed. By recording the parameters of the coupled oscillations, the size of the phase inclusions that form may be estimated. By means of these resonant oscillations, in combination with the traditional internal-friction method, metastable phase inclusions appearing in intermediate stages of structure formation in the material may be detected, and their dimensions may be estimated. This will be of great value in the primary nondestructive testing of highly irradiated alloy samples in the fuel chambers when optimizing the alloy composition for the fuel-element casings in fast neutron reactors.