Effects of grain size on the electrochemical corrosion behaviour of electrodeposited nanocrystalline Fe coatings in alkaline solution

Effect of grain size reduction on the electrochemical corrosion behaviour of nanocrystalline Fe was investigated using Tafel polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Nanocrystalline iron was fabricated by pulse electrodeposition using citric acid bath. The grain size of a nanocrystalline surface was analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The corrosion resistance of Fe in alkaline solution considerably increased as the grain size decreased from microcrystalline to nanocrystalline. The behaviour of passive film growth and corrosion was discussed in terms of excess of free energy caused by nanocrystalline surface.     

低温球磨和真空热压技术制备的纳米晶纯铝块体的强化机理

利用液氮球磨和真空热压技术制备了纳米晶纯铝块体材料。采用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)对材料的晶粒尺寸和微观组织进行研究,分析了影响热稳定性的因素和强化机理。研究表明,液氮球磨以后材料晶粒尺寸为37nm,经过真空热压和热挤压后晶粒有所长大,晶粒大小约300nm,纳米晶结构基本得到保持。分析晶粒热稳定性的原因在于球磨过程中生成的AlN等粒子的晶界钉扎以及引入的杂质溶质的拖拽作用。纳米晶纯铝块体的拉伸强度极限σb为173MPa,伸长率fε为17.5%。     

合金化元素对纳米晶铝合金热稳定性的影响

为研究加入Fe和Ti扩散系数有限的元素对纳米晶铝合金热稳定性的影响,制备Al?10%Fe(质量分数)和Al?10%Fe?5%Ti(质量分数)合金。将初始混合粉末在真空下球磨100 h,用高频感应加热烧结系统将球磨后的粉末制备成块体样品。采用X射线衍射仪、维氏显微硬度仪、场发射扫描电子显微镜和透射电子显微镜对球磨后的粉末和烧结后的块体样品进行表征。结果表明,Fe和Ti完全分散在基体中,与Al形成过饱和固溶体。另外,合金元素的加入使合金的硬度和屈服强度分别提高127%和152%。通过高温压缩试验评估合金的热稳定性,结果显示3种合金样品中Al?10%Fe?5%Ti合金的热稳定性最好。Fe和Ti的加入抑制合金的晶粒长大,阻碍第二相如Al13Fe4和Al13Ti的分解、长大,形成稳定的固溶体,从而提高合金的热稳定性。     

Substrate roughness and thickness effects on cold spray nanocrystalline Al−Mg coatings

Nanocrystalline Al−Mg coatings were produced using the cold gas dynamic-spraying technique. Unsieved Al−Mg powder of average nanocrystalline grain size in the range of 10 to 30 nm and with a particle size distribution from 10 to >100 μm was used as the feedstock powder. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy, as well as microhardness and nanoindentation measurements. Coating observations suggest that the wide particle size distribution of the feedstock powder has a detrimental effect on the coating quality but that it can be successfully mitigated by optimizing the spraying parameters. Nanohardness values close to 3.6 GPa were observed in both the feedstock powder and coatings, suggesting the absence of cold-working hardening effects during the process. The effects of the substrate surface roughness and thickness on coating quality were investigated. The deposited mass measurements performed on the coatings showed that the effect of using different grit sizes for the substrate preparation is limited to small changes in the deposition efficiency of only the first few layers of deposited material. The SEM observation showed that the substrate surface roughness has no significant effect on the macrostructures and microstructures of the coating. The ability to use the cold gas dynamic spraying process to produce coatings on thin parts without noticeable substrate damage and with the same quality as coatings produced on thicker substrates was demonstrated in this work. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and IIW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

烧结作用对CVD法超细铼粉性能的影响

研究了一种化学气相沉积(CVD)制备超细铼粉的新方法,即以NH4ReO7为原料,通过将其分解为Re2O7后气相输运至还原区,经氢气还原生成超细铼粉.对不同还原温度下制备的超细铼粉样品,采用XRD、SEM、激光粒度分析进行表征,实验揭示了烧结作用对晶粒尺寸、形貌、表面状态及粒度等粉末性能的影响规律.结果表明,随还原温度升高,烧结作用增强,制备的超细铼粉晶粒尺寸增大,具有更好的球形度,表面趋于光洁,平均粒径增大.     

球磨法制备Fe-1％C纳米晶及其热稳定性

微观应变的存在,获得的Fe-C纳米晶具有自发长大趋势.采用差热分析(DSC)手段研究不同等温条件下Fe-C纳米晶的热稳定性,结合晶粒长大热力学和动力学理论,求得晶界扩散激活能及稳定晶粒尺寸等参数,并分析讨论Fe-C纳米晶的晶粒稳定机制.     

Synthesis and grain growth kinetics of in-situ FeAl matrix nanocomposites（ Ⅰ ）： Mechanical alloying of Fe-A1-Ti-B composite powder

Three nanocrystalline alloys, FesoAlso, Fe42.5Al42.5Ti5B10 and Fe35Al35Ti10B20 （molar fraction, %）, were synthesized from elemental powders by high-energy ball milling. The structural evolutions and morphological changes of the milled powders were characterized by X-ray diffractometry（XRD）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. The effects of different Ti, B additions on the structure and phase transformation in these alloys were also discussed. It is observed that the diffusion of AI, Ti, B atoms into Fe lattice occurs during milling, leading to the formation of a BCC phase identified as Fe（Al） or Fe（Al, Ti, B） supersaturated solid solution. Fe-based solid solution with nanocrystalline structure is observed to be present as the only phase in all the alloy compositions after milling. Furthermore, the contents of Ti, B affect the formation of mechanical alloying products, changes in the lattice parameter as well as the grain size.     

Microstructure, permeability and rheological behavior of lost foam refractory coatings

The effect of microstructure on rheology and permeability of three commercial lost foam refractory coatings available on the market called samples I, II, and III, respectively was investigated in this study. Thermal gravimetric/differential thermal analysis (TG/DTA) method was used for detailed analysis of the organic components and to determine the thermal stability of the coatings. Particle shape and size and particle size distribution (PSD) were obtained by optical microscopy and morphological studies were carried out by scanning electron microscopy (SEM). Also the scanning electron microscopy energy dispersive X-ray analysis (SEM/EDXA) technique was used for elemental analysis of refractory particles. To determine the crystalline structure of the samples, X-ray diffraction (XRD) analysis was carried out. Permeability measurements were conducted with a modified apparatus originally used for determining the permeability of casting sand. Finally the rheological behavior of the samples was investigated using a rotating coaxial rheometer to provide the flow curve for coating suspensions. The studies revealed that coating I has the highest permeability, which can be due to its large mean particle size and wide particle size distribution (PSD). Furthermore it was found that the investigated refractory coatings behave as non-Newtonian fluids with shear thinning behavior. All coatings exhibited yield stresses, indicate that they behave as Bingham-type pseudoplastic fluids.     

Zr-4合金表面变形纳米结构TEM研究

利用透射电镜对Zr-4合金表面进行高能喷丸处理所致纳米结构的微观组织演变特征进行了研究.结果表明,经过喷丸处理变形后,Zr-4合金表面形成一层平均晶粒尺寸为几纳米至十几纳米的致密纳米层,随着离表面距离的增加,晶粒尺寸也不断增加.通过对Zr-4合金的变形行为以及微观结构组织演变的观察和分析,探讨了纳米晶的形成机制.     

镍钛形状记忆合金在局部包套压缩下的塑性屈服

作为一种崭新的尝试,局部包套压缩被应用于实现镍钛形状记忆合金在室温下的大塑性变形。基于主应力法和塑性屈服准则,分析了镍钛形状记忆合金局部包套的压缩塑性力学。采用透射电镜、高分辨透射电镜和扫描电镜研究镍钛形状记忆合金在局部包套压缩下的显微组织演变和变形行为。静水压力随着包套外径的增加而增加,有效地抑制了显微裂纹的萌生和扩展,有助于提高镍钛形状记忆合金的塑性,避免了脆性断裂的发生。在0．15～0．50的真实应变范围内,镍钛形状记忆合金在三向压应力状态下的塑性变形满足密席斯塑性屈服准则。在更大的塑性应变下,由于非晶相的出现,镍钛合金不能满足密席斯塑性屈服准则。     

Mechanical properties in tension of mechanically attrited nanocrystalline iron by the use of the miniaturized disk bend test

The mechanical properties of warm compacted nanocrystalline (nc) iron powder compacts of near theoretical density in the grain size range between 8 and 33 nm were investigated. The elastic and plastic behavior were characterized by miniaturized disk bend tests and hardness measurements. Light and scanning electron microscopy (SEM) were used to document the deformation and fracture morphologies. The Young's modulus of the nc Fe was essentially the same as that of coarse grained Fe. All samples failed in a macroscopically brittle manner. Local plasticity in shear bands was observed in the samples with the larger grain sizes (>20 nm). An increasing failure stress with increasing grain size is probably due to a processing effect on the flaw controlled failure of the samples. The results are discussed in the context of the deformation and fracture behavior of micrometer grain size metals and alloys.     

Microstructure and properties of hydrogenated TB8 alloy

Thermohydrogen processing can enhance workability, decrease flow stress and deforming temperature of titanium alloys. In this study, thermohydrogen processing was carried out for metastable b-type TB8 alloy. The microstructures of hydrogenated TB8 alloy were investigated based on scanning electron microscopy(SEM), transmission electronic microscopy(TEM) as well as X-ray diffraction(XRD) analysis. The results reveal that d hydride phase forms in the hydrogenated TB8 alloy, but the amount of b phase increases with hydrogen content increasing. Single b phase appears when the hydrogen content reaches 0.7 wt%. The alloying elements redistribute in the hydrogenated TB8 alloy, and hydrogen leads to the reduction of the alloying elements in b phase. The room-temperature compression tests were performed on a MTS809 machine. It is found that the room-temperature yield strength of hydrogenated TB8 alloy decreases. And minimum yield strength is obtained at a hydrogen content of 0.5 wt%. The ductility does not decrease within 0.7 wt%hydrogen content. These results provide theoretical basis for improving the formability and promoting the applications of TB8 alloy.     

Influence of Temperature on the Inverse Hall-Petch Effect in Nanocrystalline Materials:Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

柠檬酸络合盐燃烧法合成DyFeO3纳米粉体

利用溶胶凝胶自蔓延燃烧合成工艺制备得到了DyFeO3纳米粉体,并通过热综合分析(DTA/TG),X射线衍射(XRD)和扫描电子显微镜(SEM)分析探讨了溶液pH值对单相DyFeO3的形成和粉末晶粒尺寸的影响。结果表明,干凝胶能够在400℃以下燃烧基本完全,当pH值为3时,几乎没有杂相生成,将制得的DyFeO3纳米粉体置于750℃煅烧温度下并保温3h,微晶的尺寸在55～90nm范围内。     

高能球磨制备Al-Pb-Si-Sn-Cu纳米晶粉末的特性

通过机械合金化制备了Al-15％Pb-4％Si-1％Sn-1．5％Cu(质量分数)纳米晶粉末。采用X射线衍射(XRD)，扫描电镜(SEM)和透射电镜(TEM)对不同球磨时间的混合粉末的组织结构、晶粒大小、微观形貌以及颗粒中化学成分分布情况进行了研究。结果表明混合粉末经过球磨后形成了纳米晶，其组织非常均匀。球磨对Pb的作用效果明显大于对Al的作用效果，经过40h球磨后Pb粒子达到40nm，而Al在球磨60h后晶粒为65nm；经球磨后，Cu和Si固溶于Al的晶格中，而Sn则固溶于Pb晶格中，并且Al和Pb发生了互溶，形成了Pb(Al）超饱和固溶体；在球磨过程中硬度高的脆性粒子Si难于完全实现合金化。     

晶粒尺寸对纳米晶钴摩擦磨损性能的影响

采用脉冲电沉积法制备了不同晶粒尺寸的纳米晶钴镀层,考察了晶粒尺寸对纳米晶钴硬度和摩擦磨损性能的影响.结果表明:随着晶粒尺寸的减小,钴镀层的硬度显著升高,符合经典的Hall-Petch效应;而纳米晶钴的抗磨性并没有随着晶粒的减小而显著增加,同时其摩擦系数随着晶粒的减小而略有增加.     

Structural and Thermal Behavior of Fe-Cr-Mo-P-B-C-Si Amorphous and Nanocrystalline HVOF Coatings

The microstructure, thermal behavior, and mechanical properties of amorphous/nanocrystalline 70Fe-15Cr-4Mo-5P-4B-1C-1Si (wt.%) coatings produced by high velocity oxy fuel (HVOF) spraying of mechanically alloyed powders were investigated by x-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermal stability of samples was investigated using differential scanning calorimetry (DSC). The results show that by adjusting the HVOF parameters especially fuel/oxygen ratio and proper selection of powder composition, the desired microstructure with different amount of amorphous and nanocrystalline phases and therefore with different mechanical properties could be obtained.     

硫脲对喷射电铸纳米晶铜组织与性能的影响

采用喷射电铸法制备了不同硫脲浓度下的纳米晶铜铸层,用SEM、XRD方法分析了硫脲对纳米晶铜铸层微观结构和晶粒大小的影响,并对纳米晶铜铸层的力学性能进行了测试.结果表明,硫脲能细化晶粒,致密铸层组织,显著提高铸层的抗拉强度和显微硬度;电铸液中硫脲浓度为10 mg/L时,可获得晶粒尺寸为29 nm的致密铜铸层,其显微硬度和抗拉强度分别为3.1 GPa和645 MPa.电铸液中硫脲浓度从5 mg/L增至10 mg/L时,纳米晶铜铸层的择优取向由(220)晶面转变为(111)晶面.     

Friction and wear behaviors of nanocrystalline surface layer of chrome-silicon alloy steel

A nanocrystalline surface layer of about 25 μm thickness was fabricated on a quenched and tempered chrome-silicon alloy steel using Supersonic Fine Particles Bombardment (SFPB). The microstructural features in the treated surface layer were characterized using scanning electron microscopy and transmission electron microscopy observations. The grain size is about 16 nm in the top surface layer. Nanoindentation tests indicate the hardness of the top nanocrystalline layer is about 2 times of that of the matrix. The tribological behavior of the nanocrystalline surface layer was investigated under dry conditions. Experimental results show that the friction coefficients and wear volume loss of the surface nanocrystallized samples are lower than those of the untreated samples, and the wear resistance is remarkably improved. After surface nanocrystallization, there occurs a transition of dominant wear mechanics from the combined action of abrasive wear and adhesive wear to the abrasive wear. The advantages realized in the friction and wear properties of the SFPB treated sample may be attributed to the duo enhancement of the hardness and the surface activity caused by the grain refinement, which, in turn, result in the improvements in forming oxide layer and resistance to plastic removal.     

Nanocrystalline spherical iron–nickel (Fe–Ni) alloy particles prepared by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR)

FeCl₂ and NiCl₂ were used for synthesis of nanocrystalline spherical Fe–Ni alloy particles by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR). Spherical ultrafine Fe–Ni particles were obtained by USP of aqueous solutions of iron–nickel chloride followed by thermal decomposition of generated aerosols in hydrogen atmosphere. Particle sizes of the produced Fe–Ni particles can be controlled by the change of the concentration of an initial solution. The effect of the precursor solution in the range of 0.05, 0.1, 0.2 and 0.4 M on the morphology and crystallite size of the Fe–Ni alloy particles are investigated under the conditions of 1.5 h running time, 900 °C reduction temperature, and 1.0 L/min H₂ volumetric flow rate. X-ray diffraction (XRD) studies and Scherrer crystallite size calculations show that the crystalline size was nearly 28 nm. Energy dispersive spectroscopy (EDS) was performed to determine the chemical composition of the particles. Transmission electron microscope (TEM) was used to confirm the crystalline size, that was determined using XRD results. Scanning electron microscopy (SEM) observations reveal that the precursor solution strongly influences the particle size of the synthesized Fe–Ni alloy particles. Spherical nanocrystalline Fe–Ni alloy particles in the range of 80 and 878 nm were obtained at 900 °C.