Effect of optimized aging processing on properties of the sintered Dy-doped Nd–Fe–B permanent magnet

We investigate the effect of the optimized aging processing on magnetism and mechanical property of the sintered Dydoped Nd–Fe–B permanent magnet. The experimental results show that the magnetism, especially intrinsic coercivity, of the optimized aged Dy-doped Nd–Fe–B magnet is more excellent than that of the sintered one, but the former＇s strength and hardness are lower than that of the latter. It was observed that the optimized aged Dy-doped Nd–Fe–B magnet have more uniform grain size, thinner（Nd, Dy）-rich boundary phase. By means of the EBSD technology, the number of larger angle grain boundaries in the optimized aged Dy-doped Nd–Fe–B magnet is more than that of the sintered one. The reasons for the increased intrinsic coercivity and decreased mechanical properties of the optimized aged Dy-doped Nd–Fe–B magnet are also discussed.

固溶时效处理Mg-Zn-Al-Sn-Mn合金的微观组织演变及腐蚀行为研究

通过电子背散射衍射、扫描电子显微术、透射电子显微术、电化学试验和浸泡试验,研究了Mg-Zn-Al-Sn-Mn合金热挤压、固溶和人工时效过程中的微观组织演变和腐蚀行为。研究发现,挤压型材表现出由变形晶粒和再结晶晶粒组成的多峰结构特点。固溶处理后,大部分第二相颗粒在固溶过程中溶解进镁基体中,合金中形成了完全再结晶的晶粒结构,且晶粒尺寸显著增大。上述两方面使得固溶处理后的合金表现出最佳的耐腐蚀性能。时效处理24 h后,合金硬度达到峰值水平(HV 84),其强化效果主要来源于密集分布的τ-Mg_(32)(Al,Zn)49析出相。随着时效时间的延长,过时效状态下析出相尺寸出现典型的双峰分布。然而,时效处理过程中析出的大量τ-Mg_(32)(Al,Z)49颗粒导致了合金耐腐蚀性能的降低。此外,腐蚀机制也由点蚀和丝状腐蚀转变为晶间腐蚀。

Effect of optimal aging treatment on magnetic performance and mechanical properties of sintered Nd-Fe-B permanent magnets

The magnetic performance and mechanical properties including hardness, brittleness, fracture toughness and strength characteristics of the as-sintered and the optimal aged Nd-Fe-B magnets were examined in this work. A new method of Vickers hardness indentation combined with acoustic emission was used to test the brittleness of the magnets.The results show that the magnetic properties of the magnets could be improved through aging treatment, especially the intrinsic coercive force. But it is accompanied by a decrease of strength and fracture toughness. Theoretical calculation confirms that acoustic emission energy accumulated count value could be used to characterize the material brittleness. The bending fracture morphologies of the as-sintered and the optimal aged Nd Fe B magnets were investigated with the emphasis on the relationship between mechanical properties and microstructure using a field emission scanning electron microscopy(FE-SEM). The research results indicate that the intergranular fracture is the primary fracture mechanism for both as-sintered and optimal aged Nd Fe B magnets. Aging treatment changes the morphology and distribution of the Nd-rich phases, reducing the sliding resistance between Nd_2Fe_(14)B main crystal grains and lowers the grain boundary strength, which is the main reason for the strength and fracture toughness decrease of the aged Nd-Fe-B magnets.

Effects of CeO_2 coating on oxidation behavior of T91 steel in water vapor

Oxidation behaviors of blank and CeO2 coated T91 steel were investigated at 600 °C in water vapor for up to 150 h.Gold marker was used to define the mass transport direction.The oxide scales were studied with X-ray diffractometry (XRD),scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA).The oxidation resistance of the steel is improved by CeO2 coating,though the improvement is not remarkable.Ce-rich oxide band is located at the interface of the inner equiaxed layer and the outer columnar layer after oxidation,which is not consistent with the original surface.The results show that outward iron transport is blocked by the Ce-rich band.A new oxide nucleating and growing site (reaction front) is induced at the inner surface of the Ce rich band.

Surface resistivity of carbonaceous fiber/PTFE antistatic coatings

PAN(Polyacrylonitrile)-based carbonaceous fibers were prepared at the heat treatment temperature(HTT)range of 650 to 900 oC.The relationships among HTT,carbon content and volume resistivity of the carbonaceous fibers were investigated.The carbonaceous fibers/PTFE(Polytetrafluoroethylene)antistatic coatings were prepared by the spraying technology and the effects of carbonaceous fibers and pigments on surface resistivity of the coatings were systematically discussed.Micrographs provide insight into the antistatic mechanism of the coating.The results show that carbon content of the carbonaceous fibers increases from 68.8% to 74.8%(mass fraction)and the volume resistivity decreases drastically from 1.94×103 to 8.27×10-2 Ω·cm.The surface resistivity of the antistatic coating is adjustable between 105 and 108 Ω to fit the different antistatic materials.Static is dissipated by a conductive network of short fibers and the tunneling effect between the neighboring fibers and conductive pigments.Conductive pigments make the conductive network more perfect and improve the antistatic ability,but insulating pigments acting as barriers for the formation of conductive channel increases the surface resistivity of the coatings.The influence of pigments on the surface resistivity drops gradually with the decrease of the carbonaceous fibers volume resistivity.

Spatial heterogeneity in liquid–liquid phase transition

Molecular dynamics simulations are performed to investigate the liquid-liquid phase transition （LLPT） and the spatial heterogeneity in A1-Pb monotectic alloys. The results reveal that homogeneous liquid AI-Pb alloy undergoes an LLPT, separating into Al-rich and Pb-rich domains, which is quite different from the isocompositional liquid water with a transition between low-density liquid （LDL） and high-density liquid （HDL）. With spatial heterogeneity becoming large, LLPT takes place correspondingly. The relationship between the cooling rate, relaxation temperature and percentage of A1 and the spatial heterogeneity is also reported. This study may throw light on the relationship between the structure heterogeneity and LLPT, which provides novel strategies to control the microstructures in the fabrication of the material with high performance.

Effects of Finish Rolling Temperature on Microstructure and Mechanical Properties of Ferritic-Rolled P-Added High Strength Interstitial-Free Steel Sheets

The steels were rolled at 3 different finishing temperatures. The mechanical properties were tested by tensile tests. The results show that as finish rolling temperature decreases from 620 to 560 ℃ in ferrite region,the deep drawability of ferritic-rolled P-added high strength IF steels is improved,and r value rises from 1.14 to 1.37. Finish rolling temperature （FT） has less influence on other mechanical properties,such as yield strength,tensile strength and elongation. Microstructures of hot rolled and annealed steel sheets and precipitates of annealed steel sheets were also analyzed.

Influence of B,Al and Nb addition on the glass forming ability and stability of mechanically alloyed Zr-Ni amorphous alloys

We investigated the influence of minor additions of B,Al and Nb that have representative atomic sizes on the glass forming ability (GFA) and stability of Zr-Ni amorphous alloys during mechanical alloying. The results show that the minor addition of B,Al or Nb does not shorten the initial time of the full amorphization reaction or improve the glass forming ability of the Zr-Ni alloys at a low rotation speed. However,B addition can effectively improve the mechanical stability of the amorphous phase against mechanically induced crystallization. Furthermore,the amorphous phase gradually transforms into a metastable fcc-phase with increasing milling time. The addition of Al and Nb that have similar atomic sizes has a similar effect on the GFA and the mechani-cal stability of the Zr-Ni amorphous phase. Moreover,Al and Nb addition can alter the crystallization behavior and improve the thermal stability of the Zr-Ni amorphous phase.

Catalytic oxidation of phenol in wastewater-A new application of the amorphous Fe_(78)Si_9B_(13) alloy

The amorphous Fe78Si9B13 alloy was used as a heterogeneous Fenton catalyst in the process of phenol degradation.The influences of main operating parameters such as reaction temperature,catalyst amount,hydrogen peroxide dosage and initial pH of solution on phenol degradation rate were investigated.The maximum mineralization of phenol was achieved at 60°C,6 g/L Fe78Si9B13, 0.31 mol/L hydrogen peroxide,with an initial pH of 2.5.More than 99%of phenol was completely removed under the optimum conditions within 10 min for a solution containing 1000 mg/L of phenol.Batch experiments for solutions containing phenol con- centrations ranging from 50 to 2000 mg/L were investigated under the above conditions and the same excellent degradation rate was obtained.The Fe78Si9B13 showed better catalytic activity than iron powder and Fe 2+ .Addition of n-butannol(hydroxyl radical scavenger)decreased the degradation rate of phenol,which demonstrates that hydroxyl radicals were mainly responsible for the removal of phenol.We demonstrated that phenol may be degraded by hydroxyl radicals decomposed by hydrogen peroxide on the surface of Fe78Si9B13 and illustrated the reaction mechanism for this process.This amorphous alloy exhibited high stability in recycling experiments and showed excellent reuse performance even after continuous operations of 8 cycles.

Thermodynamic determination of fragility in La-based glass-forming liquid

Differences in the thermodynamic functions between the liquid and crystalline states of La-based bulk metallic glasses alloys were calculated with the specific heat capacity Cp and the fusion heat ΔHf,which we measured. Fragility indexes having different thermodynamic definitions were calculated from the temperature dependence of excess entropy ΔSliq-cry. It is ambiguous for La-based glass-forming liquid to evaluate fragility from the intercepts of ΔSliq-cry-temperature curves. We found that the thermodynamic fragility MΔS,denoted ΔS,decreases at the onset melting temperatures Tm rather than at glass transition temperatures Tg,and relates linearly with the kinetic fragility m. The correlations between thermodynamic fragility and kinetic fragility are discussed within the frameworks of the Adam-Gibbs relationship and potential energy landscape theory.

Antibacterial effect of metallic glasses

The antibacterial effect of Fe-based,Ni-based and Cu-based metallic glasses has been studied in this research.All test metallic glasses were found to inhibit growth of Esherichia coli.The optical density value of E.coli flocculation exposed to metallic glasses,from which the number of bacteria was determined,was 65% lower than that of the match group,and 57% lower than that of the ferrite group.Moreover,the antibacterial effect was not significantly different(σ=0.05) between Cu-based,Ni-based and Fe-based metallic glasses.These results extend our understanding of the antibacterial effect of metallic antibacterial materials,and suggest new applications for metallic glasses.