Magnetic Acoustic Emission in Ferromagnetic Materials. 3: Effect of Structural Changes on Magnetic Acoustic Emission (Review Article)

This part of the review is devoted to effects of structural changes in multiply alloyed ferromagnetic materials on parameters of magnetoelastic acoustic emission.     

Correlation between magnetic properties and the hardness of powder steels

Density and carbon content are studied for their effect on strength (hardness) and magnetic (coercive force, saturation magnetization) properties of powder steels ZhGr1 and ZhGr1D3. It is shown that the hardness of articles made of these steels may be determined indirectly by measuring two magnetic characteristics.Physicotechnical Institute, Russian Academy of Sciences, Izhevsk. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 67–69, January–February, 1994.     

Effect of the Angular Distribution of Cathode Ions on the Current Flow in a High-Current Vacuum Arc

The effect of the angular distribution of cathode ions on the parameters of a vacuum arc is investigated. It is shown that, at current densities characteristic of a high-current arc, collisions of flows of fast ions from different cathode spots may result in a high value of the ion temperature at the cathode plasma boundary. The boundary in current density of the region of a stable current flow as a function of the width of angular distribution of cathode ions is determined. It is found that, in the region of supersonic flow of ions, the permissible value of the current density drops rapidly as the angular distribution width increases, and in the subsonic region, it rises rapidly. An expression is obtained for the maximum possible ion sound velocity and for the equivalent critical ion velocity. Two- and three-fluid hydrodynamic models are used to perform calculations that illustrate characteristic distributions of parameters in the discharge gap. Analysis of the calculation results makes it possible to discuss the characteristic features of current flow in a vacuum discharge.     

Elastic and Magnetoelastic Properties of an Iron–Carbon Powder Steel

The resonance frequency, internal friction, and emf of the double conversion are measured in samples of ZhGr steels of the variable density and carbon content by means of the resonance electromagnetic-acoustic conversion method. For the contactless excitation, the close correlation between the density (porosity) and the resonance frequency of the ultrasonic vibrations is obtained. It is shown that the magnetic state of the samples affects the internal friction. The dynamics of the field dependences of the emf is investigated with allowance for the internal friction in variation of the carbon content. This makes it possible to determine the optimum interval of the field for estimating this parameter.     

Internal Demagnetization Factor for Porous Ferromagnets in Remagnetization Process

Computational and experimental methods of determining the internal demagnetization factor of porous ferromagnetic materials are compared. The effect of the porosity, the shape of pores, and the magnetizing field strength on the demagnetization factor of model porous materials are considered. It is established experimentally that the internal demagnetization factor is subjected to hysteresis when the porous materials are remagnetized along the magnetic hysteresis loop.     

Mechanical Alloying of Immiscible Elements in the Fe–Mg System

Mechanical alloying of immiscible elements in the Fe–Mg system (32 at % Mg) is achieved by grinding in a planetary ball mill. The process is studied by x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. The results attest to the formation of supersaturated solid solutions of Mg in -Fe. The highest Mg content of the solid solutions is 5–7 at %. Mössbauer results are used to evaluate the changes in the hyperfine magnetic field at Fe nuclei associated with the presence of one Mg atom among the nearest neighbors and next nearest neighbors of Fe in the solid solutions: H ₁ = –1760 kA/m and H ₂ = –800 kA/m, respectively. Increasing the Mg content of the starting mixture reduces Mg solubility in Fe. Thermodynamic analysis indicates that the driving force for the formation of solid solutions may be associated with the excess energy of coherent interfaces in the Fe–Mg nanocomposite resulting from mechanical alloying. The elastic strain arising from the lattice mismatch between Fe and Mg facilitates incorporation of Mg into -Fe. Above a certain Mg content, no coherent interfaces are formed, and the thermodynamic driving force for Mg dissolution disappears. As a result, the system becomes immiscible.