Multi-phase Nanocrystalline Research Articles

Electrochemical characterization of galvanically coupled single phases and nanocrystalline NdFeB-based magnets in NaCl solutions

Galvanic interaction between intermetallic phases occurring in NdFeB-based permanent magnets was investigated for the purpose of understanding the electrochemical behaviour of nanocrystalline hot pressed Nd14Fe80B6 and Nd12Dy2Fe73.3Co6.6Ga0.6B5.6 magnets in 0.5 M NaCl solution of pH 8.4. The microstructure of multiphase nanocrystalline magnets was characterized using XRD, SEM and EDX techniques. The electrochemical behaviour was assessed using zero resistance amperometry and potentiodynamic polarization measurements. From the electrochemical data obtained, it was concluded that the Nd-rich phase/ferromagnetic (φ)- phase couple generated the highest galvanic current density. The strength of the galvanic coupling effect was typically controlled by the oxidation rate of Nd-rich phase, which is the least polarizable. Pitting and intergranular corrosion were detected on the surface of nanocrystalline magnets and found to be dependent on each other, since intergranular corrosion processes nucleated on the pit walls and propagated from there. Crystallographic tunneling was found to be the way of both pit and intergranular corrosion propagation. Co and Ga additions generally increase localized corrosion resistance of NdFeB-based magnets due to the significant reduction in the strength of galvanic coupling effect between magnet phases.

The structure and magnetic properties of rapidly quenched and annealed multi-phase nanocrystalline Nd 9Fe 91− xB x ribbons

Melt-spun and annealed Nd 9Fe 91− x B x ribbons with 0 ≤ x ≤ 9 have been studied by means of X-ray diffraction, thermomagnetic analysis and magnetic measurements. The metastable phase diagram up to 780°C has been plotted. A metastable phase of the TbCu 2 structure has been identified in the ribbons of binary composition and those containing up to 7 at.% B. The Curie temperature of this phase was found to be strongly affected by both boron content and annealing temperature. The structure of exchange coupled nanocrystallines of Nd 2Fe 14B. α-Fe and Fe 3B was realized by means of short-time annealing. The annealed isotropic Nd 9Fe 84B, ribbons had a remanence of 100.5 emu g −1, an intrinsic coercivity of 4.5 kOe, and an energy product of about 13 MG Oe. The details of structure formation, as well as the criteria and conditions for the effective exchange coupling, are discussed.

US 4,808,649 Plasticizers for PVC

Based on a higher order refined beam model, damping vibration analysis of multi-phase viscoelastic nanocrystalline nanobeams on visco-Pasternak foundation in carried out accounting for nano-grains and nano-voids sizes. For the first time, a contribution of nonlocal and couple stress effects is applied for damping vibration analysis of nanocrystalline nanobeams. In fact, couple stress theory considers grains micro-rotations. While, nonlocal elasticity theory considers long-range interactions between the particles. Viscoelastic medium is described as infinite parallel springs as well as shear and viscous layers. Hamilton’s principle is employed to derive the governing equations and the related boundary conditions which are solved applying an analytical approach. The frequencies are compared with those of nonlocal and couple stress based beams. It is observed that damping frequencies of a nanocrystalline nanobeam are significantly influenced by the grain size, grain rotations, porosities, interface, damping coefficient, surface energy, nonlocality and structural damping.