Electron holography of Nd–Fe–B nanocomposite magnets

Magnetization distribution in Nd–Fe–B nanocomposite magnets was investigated by electron holography, using a new pole piece apparatus dedicated to observations of nanocrystalline ferromagnetic materials. The exchange coupling between the magnetically soft and hard grains of 20–30 nm was experimentally verified by this microscopic study with improved resolution.     

Succession law of Nd–Fe–B alloys with different coercivities

In order to establish a succession law of every phase in sintered Nd–Fe–B magnets marked as 38/38H/38SH/38 UH, four alloys of Nd33-xDyxAl0.7Nb0.6Cu0.1B1.05 Fe bal.(at%) were investigated after smelting processing, sintering processing, high-temperature tempering processing, and highand low-temperature tempering processing. It is found that the four phases: the Nd2Fe14B matrix phase, Nd-rich phase,B-rich phase, and defect phase can be inherited by means of the subsequent processing. These phases might have the special constitution and appearance in the different states.Magnetic properties also have succession law. In every processing except smelting one, the values of remanence and maximum energy product hardly alter, but the value of coercive force increases gradually.     

Influence of Zn Content on Microstructure and Tensile Properties of Mg–Zn–Y–Nd Alloy

In the present study, the effect of Zn content on the microstructure and deformation behavior of the as-cast Mg–Zn–Y–Nd alloy has been investigated. The results showed that as Zn content increased, the volume fraction of secondary phases increased. Moreover, the phase transformation from W-phase to W-phase and I-phase occurred. In the as-cast state,W-phase exists as eutectic and large block form. When Zn content increases to 6 and 8%(wt%), small I-phase could precipitate around W-phase particles. Additionally, the effect of Zn content on the tensile properties and deformation behavior varies with the testing temperature. At room temperature, the tensile strength increases with Zn content, whereas the elongation increases initially and then decreases. At 250 °C, as Zn content increases, the tensile strength decreases initially and then increases slightly, whereas the elongation decreases. At 350 °C, the elongation increases with Zn content,whereas the tensile strength decreases initially and then increases slightly.     

Serrated flow and tensile properties of a Mg–Y–Nd alloy

Serrated flow in WE54 alloy during tensile deformation is reported. The observed static strain ageing effect and negative strain rate sensitivity suggest that the serrated flow is due to interactions between dislocations and solute atoms, known as dynamic strain ageing (DSA). The tensile properties are analysed in relation to the DSA effect.     

Hot deformation behavior and microstructure evolution of a Mg–Gd–Nd–Y–Zn alloy

The hot deformation behavior of homogenized Mg–6.5Gd–1.3Nd–0.7Y–0.3Zn alloy was investigated during compression at temperatures of 250–400 ℃ and at strain rates ranging from 0.001 to 0.100 s~(-1). Microstructure analyses show that the flow behaviors are associated with the deformation mechanisms. At the lower temperatures(250–300 ℃), deformation twinning is triggered due to the difficult activation of dislocation cross-slip. Dynamic recrystallization(DRX) accompanied by dynamic precipitation occurs at the temperature of 350 ℃ and influences the softening behavior of the flow.DRX that develops extensively at original grain boundaries is the main softening mechanism at the high temperature of 400 ℃ and eventually brings a more homogeneous microstructure than that in other deformation conditions. The volume fraction of the DRXed grains increases with temperature increasing and decreases with strain rate increasing.     

Effect of Ga addition on the microstructure and magnetic properties of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B powder

Microstructural evolution of hydrogenation–disproportionation–desorption–recombination processed Nd_12.5Fe_72.8Co_8.0B_6.5Ga_0.2 powder has been investigated in relation to coercivity development during the desorption recombination process. Coercivity increases when residual NdH₂ is completely dehydrogenated and the decomposed Nd is segregated at the grain boundaries of the Nd₂Fe₁₄B phase. Three-dimensional atom probe (3DAP) analysis indicates the segregation of Ga at the grain boundary. The reason for the enhancement of the coercivity by the trace addition Ga is discussed based on the 3DAP results.     

Optimisation of Nd–YAG laser welding of superduplex stainless steel by parameter design

Abstract

Many researchers have found the welding of super duplex stainless steel UNS S32760 (Zeron 100) by high power CO₂ laser to be an unacceptable process, because the austenite/ferrite (γ/α) ratio in the laser weld bead normally deviates significantly from the optimum balance of 50 : 50. To date, the best γ/αratio in a laser weld reported in the published literature was ～30 : 70. The high percentage of ferrite would result in lower strength and poor corrosion properties. The present investigation was aimed at achieving a near 50 : 50 γ/αratio laser weld using the Nd–YAG laser. These lasers have more process parameters that can be varied, compared to CO₂ lasers. Three different kinds of output power waveform, continuous, sine, and square waves, together with many other parameters, were investigated. The Taguchi method was used for parameter design, to reduce the number of experiments. Based on the Taguchi approach, optimum process parameters were determined and laser weld beads that consisted of 55 : 45 γ/α were achieved.     

The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd–Fe–B sintered magnets

We discuss the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations. Scanning electron microscopy and electron probe microanalysis showed that a (Nd,Dy)₂Fe₁₄B shell formed in the outer region of Nd₂Fe₁₄B grains while its thickness decreased from the surface to the center of a cube-shaped sample. Atom probe tomography showed that the Dy content at grain boundaries (GBs) was close to that in the (Nd,Dy)₂Fe₁₄B shell. High-temperature annealing (at 900 °C) of a GB diffusion processed magnet led to the disappearance of the GB layers, which resulted in a substantial reduction in coercivity. This suggests that both the (Nd,Dy)₂Fe₁₄B shell and the Nd-rich GB phase layer are required microstructural features for the coercivity enhancement by the GBDP.     

Characterization of the High-Strength Mg–3Nd–0.5Zn Alloy Prepared by Thermomechanical Processing

Magnesium alloys based on Nd and Zn are promising materials for both aviation industry and medical applications.Superior mechanical properties of these materials can be achieved by thermomechanical processing such as extrusion or rolling and by aging treatment, which can significantly strengthen the alloy. The question remains especially about the connection of texture strength created in the alloys based on the specific conditions of preparation. This work focuses on the Mg–3 Nd–0.5 Zn magnesium alloy prepared by hot extrusion of the as-cast state at two different temperatures combined with heat pre-treatment. Extrusion ratio of 16 and rate of 0.2 mm/s at 350 and 400 °C were selected for material preparation. The structures of prepared materials were studied by scanning electron microscopy and transmission electron microscopy. The effect of microstructure on mechanical properties was evaluated. Obtained results revealed the strong effect of thermal pre-treatment on final microstructure and mechanical properties of extruded materials. The Hall–Petch relation between grain size and tensile yield strength has been suggested in this paper based on the literature review and presented data. The observed behavior strongly supports the fact that the Hall–Petch of extruded Mg–3Nd–0.5 Zn alloys with different texture intensities cannot be clearly estimated and predicted. In addition, Hall–Petch relations presented in literature can be sufficiently obtained only for fraction of the Mg–3Nd–0.5 Zn alloys.     

Microstructure and mechanical properties of Mg–Zn–(Nd)–Zr alloys with different extrusion processes

The effect of Nd addition and the in?uence of extrusion processes on the microstructure and mechanical properties of Mg–6Zn–0.5Zr(ZK60) and Mg–6Zn–1.5Nd–0.5Zr(ZKNd602) alloys were investigated. Nd element can obviously re?ne the microstructure of both as-cast and asextruded Mg–Zn–Nd–Zr alloy. All of the extruded alloys exhibit a bimodal grain structure composed of equiaxed?ne recrystallized(DRXed) grains and elongated coarse un DRXed grains. It is necessary to achieve high strength,particularly the yield strength, for ZKNd602 alloy, when it is extruded with a lower extrusion temperature, a suitable extrusion ratio and a relatively lower extrusion ram speed. In this study, the ultimate tensile strength(UTS),yield strength(YS) and elongation(El) of the extruded ZKNd602 alloy were 421 MPa, 402 MPa and 6.7 %,respectively, with extrusion temperature of 290 °C, extrusion ratio of 18:1 and a ram speed of approximate0.4 mm·s~(-1). Meanwhile, the extrusion process has obvious effects on the room-temperature properties but weak effects on the high-temperature properties.     

The isothermal section at 400°C of the Nd–Cu–Sn ternary system

The Nd–Cu–Sn isothermal section at 400°C has been investigated by means of X-ray diffraction, optical and scanning electron microscopy and electron probe microanalysis. The following compounds have been identified or confirmed: NdCu₅Sn, Nd₇Cu₃₅Sn₁₁, NdCu₉Sn₄, NdCuSn, Nd₃Cu₄Sn₄, NdCu₂Sn₂, Nd₂Cu₃Sn₆ and NdCu_1−ySn₂, and their crystal structure have been determined or revised. Single crystal structure determination of the phases Nd₇Cu₃₅Sn₁₁ and Nd₃Cu₄Sn₄ and full profile powder refinement of the compound Nd₂Cu₃Sn₆, previously suggested as Nd₂Cu₄Sn₅, are also given. The subdivision of the composition triangle in tie triangles has been nearly completed. Of the predicted 36 three-phase equilibria, thirty have been defined. The general characteristics of the section have been discussed in comparison with those of other R–Cu–Sn systems formed by light rare earths (R= Ce, Pr).     

Hydrogen Storage Thermodynamics and Dynamics of Nd–Mg–NiBased Nd Mg_(12~-)Type Alloys Synthesized by Mechanical Milling

Nanocrystalline and amorphous Nd Mg_(12~-)type Nd Mg_(11)Ni+ x wt% Ni(x=100, 200) hydrogen storage alloys were synthesized by mechanical milling. The effects of Ni content and milling time on hydrogen storage thermodynamics and dynamics of the alloys were systematically investigated. The gaseous hydrogen absorption and desorption properties were investigated by Sieverts apparatus and differential scanning calorimeter connected with a H_2 detector. Results show that increasing Ni content significantly improves hydrogen absorption and desorption kinetics of the alloys. Furthermore,varying milling time has an obvious effect on the hydrogen storage properties of the alloys. Hydrogen absorption saturation ratio(R~a_(10); a ratio of the hydrogen absorption capacity in 10 min to the saturated hydrogen absorption capacity) of the alloys obtains the maximum value with varying milling time. Hydrogen desorption ratio(R~d_(20), a ratio of the hydrogen desorption capacity in 20 min to the saturated hydrogen absorption capacity) of the alloys always increases with extending milling time. The improved hydrogen desorption kinetics of the alloys are considered to be ascribed to the decreased hydrogen desorption activation energy caused by increasing Ni content and milling time.     

Microstructure and biocorrosion behaviors of solution treated and as-extruded Mg–2.2Nd–xSr–0.3Zr alloys

Mg–2.2Nd–xSr–0.3Zr alloys (x=0, 0.4 and 0.7, mass fraction, %) were prepared by gravity casting. Solution treatment was conducted on the as-cast alloys to homogenize microstructure, and hot extrusion was subsequently conducted. Microstructure was observed using an optical microscope and a scanning electron microscope. Biocorrosion behaviors of the alloy in simulated body fluid were analyzed by mass loss, hydrogen evolution and Tafel polarization experiments. The results show that the amount of residual eutectic phase of the solution treated alloys increases with increasing Sr addition, and the grains are significantly refined after hot extrusion. The corrosion resistance of the solution treated alloys deteriorates apparently with increasing Sr addition, while the corrosion resistance of the as-extruded alloys is improved with Sr addition. Nevertheless, the biocorrosion behavior of the as-extruded alloys obtained by Tafel polarization shows different trends from those obtained by the other two methods.     

Electrochemical corrosion kinetics of spark plasma sintered Nd–Fe–B permanent magnets in NaCl electrolyte

Abstract

Static state immersion experiments, polarisation curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in NaCl electrolyte. Effect of microstructure modification on their chemical stability of the magnets was discussed. The electrochemical reactions of both magnets are controlled by the step of active substances adsorption process at the open circuit potential and the anodic potential, which turn to diffusion process at the cathodal potential. Although both magnets are susceptible to corrosion in saline electrolytes, SPS magnets are more corrosion resistant than conventional sintered magnets due to their special microstructure that is different from those of conventional sintered magnets. In SPS magnets, the grain size of the Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd rich phase form along the grain boundaries of Nd₂Fe₁₄B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure effectively restrains the aggressive intergranular corrosion along Nd rich phases. As a result, the SPS magnet possesses excellent corrosion resistance in NaCl electrolyte.     

Preparation and properties of hot-deformed magnets processed from nanocrystalline/amorphous Nd–Fe–B powders

The hot-deformed magnets processed from nanocrystalline/amorphous Nd–Fe–B powders were preparedunderdifferenthot-pressingtemperatures(600–750 °C, at intervals of 25 °C) by the self-made hotpressing equipment. The microstructure and magnetic properties of hot-deformed magnets prepared at different temperatures were also investigated. When the temperature is above 650 °C, the density of magnet reaches 7.5 g·cm~(-3).The optimum magnetic properties of magnetic induction intensity of Br= 1.3 T, optimum energy product of(BH)max= 282.5 kJ·m~(-3),intrinsiccoercivityof Hcj= 1130.0 kA·m~(-1) of hot-deformed magnets are obtained at hot-pressing temperature of 650 °C. X-ray diffractometer pattern shows that the(00 L) texture has been obtained. For the microstructural characteristic, on the one hand, the good magnetic performance is attributed to the fine platelet-like grains with an average length of 410–440 nm at the hot-pressing temperature range from625 to 675 °C. On the other hand, the unaligned coarse grains are observed in all the samples. And the areal fraction of those is gradually increasing with the rise of the hot-pressing temperature, which tends to deteriorate the magnetic properties. The composition map shows the accumulation of Nd/Pr-rich phase in the coarse grains' region.     

Effects of Microstructure and Rare-Earth Constituent on the Oxidation Behavior of Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd Titanium Alloy

The isothermal oxidation behavior in air of high-temperature titanium alloy Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd with bimodal and lamellar microstructures was investigated at 600–800 °C. The results revealed that the alloy with lamellar microstructure has better oxidation resistance than that with bimodal microstructure. The porous oxide scales that form mainly contain TiO₂. A noticeable observation concerns the preferential attack around rare-earth particles, associated rapid oxygen diffusion along the incoherent rare-earth precipitate/matrix interface and cracks formed during oxidation. The resulting internal attack caused fragmentation of rare-earth particles and further oxidation of substrate to form TiO₂ scale with some fine dispersoids of Al₂O₃. Tensile tests showed that the ultimate strength and ductility of the specimens with removed surface were higher than that with a surface scale.     

Intermediate hydrogenation phase in the hydrogenation–disproportionation–desorption–recombination process of Nd2Fe14B-based anisotropic magnets

To understand the disproportionation and anisotropy-inducing mechanism of the hydrogenation–disproportionation–desorption–recombination process of Nd₂Fe₁₄B-based anisotropic magnets, the disproportionated structure of Nd_13.0Fe_67.9Co_11.0B_7.0Ga_1.0Zr_0.1 alloys has been investigated via transmission electron microscopy. An intermediate hydrogenation phase of the tetragonal Fe₃B (t-Fe₃B) has been revealed to appear in the early stage of disproportionation. The t-Fe₃B possesses lattice coherency and an associated “one-to-one” orientation relationship with the parent Nd₂Fe₁₄B phase. Within the t-Fe₃B grain, fine Nd₂Fe₁₄B particles exist along with NdH₂ particles. The diameter of the Nd₂Fe₁₄B particles is around 50 nm, and they have close orientation to that of original Nd₂Fe₁₄B. An anisotropy-mediating disproportionation scheme with the intermediate hydrogenation phase is proposed.