[en] This paper reports the synthesis and characterization of nanostructured coatings. To improve the mechanical properties and oxidation resistance of the materials, two new types of nanostructured coatings including CoNiCrAlY-MoSi/sub 2/ and Ni60-TiB/sub 2/ were designed. The nanocrystalline feedstock powders were prepared by high energy ball milling (HEBM). The particle size, morphology and grain size of the feed stocks were investigated. The preparation, microstructure, mechanical properties, and anti-oxidation behavior of the nanostructured CoNiCrAlY-MoSi/sub 2/ and Ni60-TiB/sub 2/ coatings are presented. With a lamellar and compact structure, the optimized nano-composite CoNiCrAlY-MoSi/sub 2/ coatings is metallurgically bonded with the substrate. It exhibits low porosity, high fracture toughness and excellent thermal shock resistance. The nanostructured Ni60-TiB/sub 2/ composite coatings also exhibited better mechanical properties and wear-corrosion resistance than those of its conventional counterpart. This work is expected to play an important role in the preparation and application of high performance nanostructured coatings. (author)

[en] Nanocrystalline NdFeB magnets were prepared by spark plasma sintering (SPS) and SPS followed by HD using melt spun ribbons as the starting materials. The microstructure of SPSed and HDed magnets were analyzed. The effects of process including temperature and compression ratio on the microstructure and properties were investigated. High magnetic properties were obtained in anisotropic HDed magnets. The combination of Zn and Dy additions was successfully employed to improve the coercivity and thermal stability of the SPSed magnets. Open recoil loops were found in these magnets with Nd-rich composition and without soft magnetic phase for the first time. The relationship between the recoil loops and microstructure for SPS and HD NdFeB magnets were investigated. The investigations showed that the magnetic properties of SPS+HDed magnets are related to the extent of the aggregation of Nd-rich phase, which was formed during HD due to existence of porosity in SPSed precursor. Large local demagnetization fields induced by the Nd-rich phase aggregation leads to the open loops and significantly reduced the coercivity. By reducing the recoil loop openness, the magnetic properties of HDed NdFeB magnets were successfully improved. (author)

[en] This paper reports the synthesis and characterization of nanostructured coatings. To improve the mechanical properties and oxidation resistance of the materials, two new types of nanostructured coatings including CoNiCrAlY-MoSi_2 and Ni60-TiB_2 were designed. The nanocrystalline feedstock powders were prepared by high energy ball milling (HEBM). The particle size, morphology and grain size of the feed stocks were investigated. The preparation, microstructure, mechanical properties, and anti-oxidation behavior of the nanostructured CoNiCrAlY-MoSi_2 and Ni60-TiB_2 coatings are presented. With a lamellar and compact structure, the optimized nano-composite CoNiCrAlY-MoSi_2 coatings is metallurgically bonded with the substrate. It exhibits low porosity, high fracture toughness and excellent thermal shock resistance. The nanostructured Ni60-TiB2 composite coatings also exhibited better mechanical properties and wear-corrosion resistance than those of its conventional counterpart. This work is expected to play an important role in the preparation and application of high performance nanostructured coatings

[en] In situ multiphase structure Gd₆₅Mn₂₅Si₁₀ alloys were fabricated by melt spinning and subsequent crystallization treatment. In the process of crystallization, the α -Gd, GdMn₂ and Gd₅Si₃ phases precipitate in the amorphous matrix in turn. The Curie temperature ( T _C) values for the α -Gd crystallization phase and amorphous matrix can be tailored by tuning the crystallization treatment time. All three multiphase alloys obtained by crystallization treatment at 637 K for 20, 30 and 40 min, respectively, undergo multiple successive magnetic phase transitions. A table-like magnetic entropy change over a wide temperature range (∼90–120 K) and a large full width at half maximum (Δ T _FWHM) magnetic entropy change (∼230 K) were achieved in the above-mentioned crystallized alloys, resulting in large refrigerant capacities (RCs). The enhanced RCs of the three crystallized alloys for a magnetic field change of 0–5 T are in the range of 541–614 J kg⁻¹. Large Δ T _FWHM and RC values and a table-like (−Δ S _M)^max feature obtained in in situ multiphase Gd₆₅Mn₂₅Si₁₀ crystallized alloys make them suitable for potential application in efficient Ericsson-cycle magnetic refrigeration working in a temperature range from 74 to 310 K. (paper)

[en] To understand the structure and phase precipitation behavior of the ternary La–Fe–B alloy, the effects of wheel speed and heat treatment on the melt spun La₂Fe₁₄B alloy were investigated. The results show that the hard magnetic La₂Fe₁₄B (2:14:1) phase is difficult to obtain in the ternary alloy. The α -Fe and La phases mainly precipitated as spun ribbons prepared at relatively low wheel speeds, and a more amorphous phase formed at higher wheel speeds. With the annealing temperature increasing from 400 °C to 500 °C and 800 °C, the α -Fe and La phases also gradually precipitated from the amorphous matrix. A liquid La-rich phase and α -Fe phase could coexist in the alloy at high temperature. However, the substitution of Nd for La can greatly enhance the formation of a hard 2:14:1 phase. The effects of Nd substitution on the structure and phase precipitation during annealing for the (La_1−x Nd_x )₂Fe₁₄B ( x   =  0.1. 0.2, 0.3 and 0.4) alloys have been also investigated. The present results are important for exploring high-performance low-cost La-based Nd–Fe–B magnets. (paper)

[en] Nanocrystalline NdFeB magnets were prepared by spark plasma sintering (SPS) and SPS followed by HD using melt spun ribbons as the starting materials. The microstructure of SPSed and HDed magnets were analyzed. The effects of process including temperature and compression ratio on the microstructure and properties were investigated. High magnetic properties were obtained in anisotropic HDed magnets. The combination of Zn and Dy additions was successfully employed to improve the coercivity and thermal stability of the SPSed magnets. Open recoil loops were found in these magnets with Nd-rich composition and without soft magnetic phase for the first time. The relationship between the recoil loops and microstructure for SPS and HD NdFeB magnets were investigated. The investigations showed that the magnetic properties of SPS+HDed magnets are related to the extent of the aggregation of Nd-rich phase, which was formed during HD due to existence of porosity in SPSed precursor. Large local demagnetization fields induced by the Nd-rich phase aggregation leads to the open loops and significantly reduced the coercivity. By reducing the recoil loop openness, the magnetic properties of HDed NdFeB magnets were successfully improved

[en] Partially inverse spinel CoFe₂O₄, which may be prepared through various heat treatments, differs remarkably from the ideal inverse spinel in many properties. The structure of partially inverse spinel CoFe₂O₄ as well as its electronic and magnetic properties through a systemic theoretical calculation of (Co_1-xFe_x)_Tet(Co_xFe_2-x)_OctO₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) have been investigated by the generalized gradient approximation (GGA) + U approach. It is found that the Co and Fe ions prefer their high spin configurations with higher spin moments at octahedral sites in all the studied cases, in line with experimental observations. The Co ions at the octahedral sites favour being far away from each other in the partial inverse spinels, which also show half metallicity at certain inversion degrees.

[en] Isotropic and anisotropic NdFeB magnets were synthesized by spark plasma sintering (SPS) and SPS+HD (hot deformation), respectively, using melt-spun ribbons as the starting materials. Spark plasma sintered magnets sintered at low temperatures (<700 ⁰C) almost maintained the uniform fine grain structure inherited from rapid quenching. At higher temperatures, due to the local high-temperature field caused by the spark plasma discharge, the grain growth occurred at the initial particle surfaces and the coarse grain zones formed in the vicinity of the particle boundaries. Since the interior of the particles maintained the fine grain structure, a distinct two-zone structure was formed in the spark plasma sintered magnets. The SPS temperature and pressure have important effects on the widths of coarse and fine grain zones, as well as the grain sizes in two zones. The changes in grain structure led to variations in the magnetic properties. By employing low SPS temperature and high pressure, high-density magnets with negligible coarse grain zone and an excellent combination of magnetic properties can be obtained. An anisotropic magnet with a maximum energy product of ∼30 MG Oe was produced by the SPS+HD process. HD at 750 ⁰C did not lead to obvious grain growth and the two-zone structure still existed in the hot deformed magnets. Intergranular exchange coupling was demonstrated in the spark plasma sintered magnets and was enhanced by the HD process, which reduced the coercivity. Good temperature stability was manifested by low temperature coefficients of remanence and coercivity. The results indicated that nanocrystalline NdFeB magnets without significant grain growth and with excellent properties could be obtained by SPS and HD processes.

[en] The structure and magnetic properties of LaFe_11.6Si_1.4/Ce₄₀Co₆₀ bulk composites prepared by hot pressing sintering (HPS), followed by high-temperature diffusion annealing, were investigated. During the HPS process, the La(Fe,Si)₁₃ alloy powder particles combined with Ce₄₀Co₆₀ binder and a porous structure with high mechanical strength were obtained. The addition of Ce₄₀Co₆₀ can also promote the peritectic reaction. Annealing has important effects on the magnetocaloric properties due to the diffusion of Ce and Co into the La(Fe,Si)₁₃ phase during annealing. HPS samples were annealed at 1373 K for 24 h, and a table-like magnetocaloric effect, exhibiting a constant high magnetic entropy change (− ∆S_M) of ~ 4.0 J/(kg K) in a commercially useful temperature region of 28 K (236–264 K), was obtained for a field change of 2 T. The values of full width at half maximum of (− ∆S_M)–T plots (ΔT_FWHM) and enhanced refrigeration capacity for the composites are 55 K and 173 J/kg, respectively, for a field change of 2 T. The composites exhibit high compressive strength of up to 312 MPa. The present results indicate that LaFe_11.6Si_1.4/Ce₄₀Co₆₀ bulk composites can meet the requirement of near room temperature magnetic refrigeration based on the Ericsson cycle.