A Novel Magnetic Hardening Mechanism for Nd-Fe-B Permanent Magnets Based on Solid-State Phase Transformation

Permanent magnets based on neodymium-iron-boron (Nd-Fe-B) alloys provide the highest performance and energy density, finding usage in many high-tech applications. Their magnetic performance relies on the intrinsic properties of the hard-magnetic Nd₂Fe₁₄B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid-state phase transformation. Using modified Nd-Fe-B alloys of the type Nd₁₆Fe_bal-x-y-zCo_xMo_yCu_zB₇ for the first time it is revealed how the microstructural transformation from the metastable Nd₂Fe₁₇B_x phase to the hard-magnetic Nd₂Fe₁₄B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm⁻¹ to almost 700 kAm⁻¹. Furthermore, after thermally treating a quenched sample of Nd₁₆Fe₅₆Co₂₀Mo₂Cu₂B₇, the presence of Mo leads to the formation of fine FeMo₂B₂ precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd₂Fe₁₄B grains and so for the high coercivity. This mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd-Fe-B magnets, such as additive manufacturing.     

High-Field Transitions in ErFe11Ti and HoFe11Ti Single Crystals

The magnetization of ErFe₁₁Ti and HoFe₁₁Ti single crystals was measured in pulsed magnetic fields up to 60 T. A number of field-induced phase transitions were observed. The inter-sublattice molecular fields were found to be 60 T in ErFe₁₁Ti and 73 T in HoFe₁₁Ti.     

1/f noise in a model of intersecting phase transitions

A mathematical model for the appearance of fluctuations with a spectrum inversely proportional to the frequency (flicker or 1/f noise) upon the intersection of phase transitions is proposed. A system with a two-valley potential, whose dynamics are described by two coupled nonlinear Langevin equations that transform Gaussian δ-correlated noise (white noise) into two modes of stochastic fluctuations with spectra having 1/f ^μ and 1/f ^v frequency distributions, where μ ≈ 1 and 1.5 ? v ? 2, is considered.     

Elimination of jets formed by the interaction of detonation waves

We have considered the phenomenon of collision of detonation waves on the surfaces of aluminum and copper casings. It is shown that a cumulative jet is formed in the first case and a combination of cumulative and “three-shock” jets in the second. Experimentally checked methods of eliminating such jets are proposed. Institute of Experimental Physics, Sarov 607190. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 103–108, July–August, 1997.     

The Nucleation of a Nanoparticle of Silicon Dioxide in a Closed Domain. Computer Experiment

The method of molecular dynamics is used to study SiO₂ clusters located in cavities of spherical, cylindrical, and cubic shape. The effect of the elastically stressed state of the cell boundaries on the physical properties of nanoparticles contained in them is investigated. Given the same pattern of application of external forces, a nanoparticle in a spherical cell experiences a stronger compression than in clusters surrounded by envelopes of other geometries. The clusters assume the shape of the cavity in which they were contained. The effect of compressive forces results in the breakage of Si–O bonds and in the subsequent oxygen enrichment of the cluster surface. The behavior of nanoparticles after the elimination of the stimulation by the cavity boundaries is treated. It is only for a cluster of spherical geometry that a significant number of Si–O bonds is restored. For other clusters, the evaporation of oxygen atoms is observed.     

A Betatron Tomograph for Nondestructive Testing of Products with a Diameter up to 1000 mm Made of High-Energy Materials

Results of developing and testing a betatron-based tomograph with an energy of up to 10 MeV are presented. The tomograph allows investigating objects with a shined-though thickness of up to 250 mm in steel equivalent. In contrast to foreign analogs, a horizontal product arrangement is used; it creates the most favorable conditions for revealing such defects as cracks and delaminations, since in this position, the product is subjected to small deformations due to gravitational forces. The proposed scanning scheme has the properties of a third-generation tomograph and preserves the quality of the tomogram and the time it is gathered with a reduced number of detectors and tomograph dimensions. The results of evaluating the tomograph sensitivity with a test sample and the results of testing products containing defects are provided.

     

Nonequilibrium phase transitions in a jet of highly superheated water

Results are given of an experimental study into the dependence of integral and local characteristics of hot water flow on the value of superheating and on the kinetics of boiling in this water. Abrupt variations are revealed of parameters of a jet of boiling water such as the reactive force, the jet cone angle, and the rate of nucleation of vapor bubbles in the jet. Abrupt variations of modes of jet flow are treated as nonequilibrium phase transitions. This enables one to invoke model concepts of generation of 1/f fluctuations under conditions of nonequilibrium phase transitions in open systems. The inversely proportional dependence of power spectra of oscillation on frequency (1/f fluctuations) in a jet of highly superheated water is found experimentally both under laboratory conditions and in a commercial-scale steam generator. Characteristic scales are discussed for reducing the experimental results for reactive force to dimensionless form.