纯稀土纳米晶Gd块体材料的结构与磁性

采用放电等离子烧结技术（简称SPS技术）及热处理制备了全致密纯稀土纳米晶Gd块体材料，研究了材料的微结构和磁性。X衍射结果表明材料在烧结过程中形成了一定程度的c轴织构。TEM观察显示，烧结态纳米晶Gd的平均晶粒尺度在10nm左右；热处理后，平均晶粒尺寸达到100nm。PPMS测试发现与粗晶Gd相比，纳米晶Gd的居里温度以及饱和磁化强度均有所下降。说明Gd的纳米化对其磁性具有重要影响。     

高能球磨制备纳米晶Al－Cu合金

利用高能球磨工艺制备了纳米晶Ａｌ－Ｃｕ合金。发现按Ａｌ_（５０）Ｃｕ_（５０）配比纯元素粉末高能球磨时，形成了纳米晶的Ｃｕ_９Ａｌ_４金属间化合物。初步探讨了高能球磨纳米晶金属间化合物的形成过程。     

一维CuO纳米针材料的直径可控制备研究

采用脉冲电沉积技术在金属Cu基板表面电沉积纳米晶Cu薄膜，在空气中采用热氧化法制备出一维金属CuO纳米针材料。实验研究了脉冲电沉积参数（输出脉冲频率f和占空比r）对电沉积纳米晶的微结构和晶粒度的影响，发现一维CuO纳米针的根部直径与电沉积纳米晶Cu的晶粒度完全一致。通过调节脉冲电沉积参数，能控制电沉积纳米晶Cu的晶粒度，进而实现一维CuO纳米针材料的直径可控分布。     

高能球磨制备纳米晶Al—Cu合金

利用高能球磨工艺制备了纳米晶Ａｌ－Ｃｕ合金，发现按Ａｌ５０Ｃｕ５０配比纯元素粉同能球地，形成了纳米晶的Ｃｕ９Ａｌ４金属间化合物，初步探讨了高能纳米晶金属间化合物的形成过程。     

纳米Mg_2Si块体的制备及其热稳定性的研究

采用机械球磨和真空热压相结合的方法,在1.5GPa压力下于400℃热压1h制得纳米晶Mg2Si金属间化合物块体.纳米晶Mg2Si块体晶粒度为30nm,致密度为91.45%.采用原位高温XRD对纳米Mg2Si块体的热稳定性进行了研究.纳米晶Mg2Si块体在700℃、800℃和900℃时晶粒的生长指数分别为6、5和4,说明其具有良好的热稳定性.     

金属铝纳米晶块体材料的制备与性能表征

在惰性气体保护的全封闭系统内,采用直流氢电弧蒸发-冷凝与放电等离子烧结相结合的技术制备出高致密度、晶粒尺寸细小且分布均匀的纯铝纳米晶块体材料.对制备材料的结构分析表明,铝纳米晶块体具有很高的纯度,纳米晶界面洁净无杂相.对制备的铝纳米晶块体的力学性能测试表明,其显微硬度为2.12GPa,比铝粗晶材料提高了约6倍;而弹性模量与粗晶块体相比变化不大.     

纳米晶金属块体材料制备技术与力学性能研究进展

主要总结了纳米晶金属块体材料的制备技术与力学性能的研究进展.讨论了各种制备技术的特点与现存的问题,以及纳米晶金属块体材料的强度、塑性、应变速率敏感性、超塑性、变形机制与断裂机制等力学性能问题.     

纳米晶材料在外场作用下的结构稳定性研究

综述了近年来纳米晶材料在外场作用下的结构稳定性的研究进展,着重介绍了纳米晶材料在温度场和应力场、电场、磁场联合作用下的晶粒长大行为。对纳米晶材料而言,外加应力场促进其晶粒长大;外加电场抑制其晶粒长大;而外加磁场的影响则有待进一步研究。     

溶质对单相纳米晶材料晶粒长大行为的影响

从热力学角度来看,溶质在晶界偏聚,通过降低晶界能来降低晶粒长大的驱动力,从而抑制晶粒长大。从动力学角度来看,溶质与晶界交互作用,钉扎晶界,使晶界迁移速率降低,从而抑制晶粒长大。本文从热力学和动力学两方面综述了溶质对单相纳米晶材料晶粒长大行为的影响,并展望了其发展方向。     

纳米TiC颗粒弥散增强超细晶钨基复合材料的组织结构与力学性能

采用高能球磨和热压烧结的方法成功制备了纳米TiC颗粒弥散增强超细晶W基复合材料,并对其组织结构、室温力学性能进行了研究.研究结果表明,当纳米TiC颗粒含量较小时,高能球磨可以使TiC颗粒均匀分散到W基体中,烧结后,TiC颗粒尺寸约100nm,当纳米TiC颗粒含量较高时,局部出现团聚现象;纳米TiC的加入强烈的阻碍了W晶粒的长大并使复合材料的断裂模式由沿晶断裂为主向穿晶断裂为主转变,提高了材料的力学性能;在TiC含量为1%(质量分数,下同)时,材料的致密度、维氏显微硬度、弹性模量、抗弯强度分别达到98.4%、4.33、396GPa、1065MPa.纳米TiC颗粒对复合材料的强化机制主要是细晶强化和晶界强化.     

Thermodynamic Properties of Nanograin Boundary and Thermal Stability of Nanograin Structure

The thermal features of the nanograin boundary were described by a developed thermodynamic model.Using the nanocrystalline Cu as an example,the pressure,the bulk modulus,and the volume thermal expansion coefficient were calculated to characterize the thermodynamic properties of the grain boundaries on the nanoscale.Based on the parabolatype relationship between the excess free energy and the excess volume of the nanograin boundary,the thermal stability,as well as its evolution characteristics,was analyzed.T...     

Incontinuous grain growth in cobalt nanocrystalline powders prepared by high-energy mechanical milling

Cobalt nanocrystalline powders with the average grain size of about 17 nm were prepared by high-energy mechanical milling. Grain growth in highly pure and particle-containing nanocrystalline Co powders were investigated respectively by a series of annealing experiments at different temperatures. The characteristics of incontinuous grain growth were found in both the pure and the particle-containing nanocrystalline powders. It is proposed by the authors that the sharp increase in nanograin size in the transition between the low and high temperature regions is a result of enhanced grain growth promoted by the stored energy as a supplied driving force, based on which rapid grain growth occurs through a particular dominant mechanism of nanograin rotations in the pure nanocrystalline powders, and that through off-pinning of grain boundaries in the particle-containing nanocrystalline powders.     

Microstructure and fundamental properties of nanostructured gadolinium (Gd)

The nanoparticles and nanocrystalline bulk of pure gadolinium (Gd) were prepared by a novel route. The nanostructures of the single particle and the bulk of Gd were investigated, and the crystal structure was characterized. The fundamental properties, namely the physical, thermal, and mechanical characteristics, were studied for the prepared Gd bulk with an ultrafine nanograin structure. As compared with the conventional polycrystalline metal, the ultrafine nanocrystalline Gd has greatly enhanced functional and structural properties. The physical background for the changes of the fundamental properties with the reduction of the grain size to the nanoscale was analyzed.     

Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

Grain growth and shrinkage are essential to the thermal and mechanical stability of nanocrystalline metals,which are assumed to be governed by the coordinated deformation between neighboring grain boundaries(GBs)in the nanosized grains.However,the dynamics of such coordination has rarely been reported,especially in experiments.In this work,we systematically investigate the atomistic mechanism of coordinated GB deformation during grain shrinkage in an Au nanocrystal film through combined state-of-the-art in situ shear testing and atomistic simulations.We demonstrate that an embedded nanograin experiences shrinkage and eventually annihilation during a typical shear loading cycle.The continu-ous grain shrinkage is accommodated by the coordinated evolution of the surrounding GB network via dislocation-mediated migration,while the final grain annihilation proceeds through the sequen-tial dislocation-annihilation-induced grain rotation and merging of opposite GBs.Both experiments and simulations show that stress distribution and GB structure play important roles in the coordinated defor-mation of different GBs and control the grain shrinkage/annihilation under shear loading.Our findings establish a mechanistic relation between coordinated GB deformation and grain shrinkage,which reveals a general deformation phenomenon in nanocrystalline metals and enriches our understanding on the atomistic origin of structural stability in nanocrystalline metals under mechanical loading.     

Deformation Behavior of Bulk Ultrafine Grained Copper Prepared by Sub-Zero Rolling and Controlled Recrystallization

Present study aims to investigate the microstructure evolution and mechanical properties of bulk nanocrystalline/ultrafine grained pure Cu obtained after controlled recrystallization of samples heavily cold-deformed at cryogenic temperature. Microstructural characterization has been carried out by X-ray diffraction analysis and hardness measurements. Mechanical behavior has been investigated by tensile testing. Significant dependence of mechanical property on grain size has been recorded. Present study indicates that conventional rolling at subzero temperature followed by controlled recrystallization may be utilized as an effective method for development of bulk Cu with two- to three-fold improvement in yield strength in comparison to its coarse grained counterpart with moderate ductility and toughness.     

Preparation of nanocrystalline CuTi by quenching the melt under high pressure

A new method of preparing a bulk nanocrystalline alloy by means of quenching a melt under high pressure has been developed. Using this method, a bulk CuTi alloy with 10–20 nm crystallites was synthesized. The structures and grain sizes of the alloy were examined by means of X-ray diffraction and TEM. We know of no precedent for using this method to directly prepare nanocrystalline alloys. The interfaces within the bulk alloys are very clean, and there is no porosity. The mechanism for nanometer-sized crystallite formation by this method is discussed.     

On the nanocrystalline to glass transition during ball milling

Mechanical alloying performed by ball milling metallic powders leads to a nanocrystalline state and metastable phases such as supersaturated solid solutions and amorphous phases. The nanocrystalline state may act as a transition state for the crystal to glass transition. Assuming polymorphic (or partitionless) melting of a nanocrystalline supersaturated solid solution, it is found that a critical nanograin size for amorphization may be defined. This critical size depends on the concentration of the supersaturated solid solution.Application to the Zr based hexagonal solid solution Zr-Ni allows a quantitative evaluation of this effect. It is shown that for nanocrystalline size the classical T₀ curve is significantly lowered in temperature, yielding a polymorphous crystal to glass transition for smaller nickel concentration than for conventional crystalline sizes. Therefore, both supersaturation and grain refining to nanocrystalline dimensions work towards an easier amorphization by ball milling.     

Crystallization and microstructure of metastable water quenched nanostructured 8 mol% yttria-stabilized zirconia using the solution precursor plasma spray method

A t′ tetragonally structured metastable 8 mol% yttria-stabilized zirconia (8 mol% YSZ) nanomaterial was synthesized by means of solution thermal plasma spray with water quenching of reacted species. Synthesis of the 8 mol% YSZ powder involved vaporization of a liquid precursor injected into a plasma jet where individual droplets, depending on their trajectory within the plasma, experienced varied thermal histories. Thus, not all the material produced underwent a complete gel → glass → nanocrystalline transformation sequence. Consequently, the collected powder contained a proportion of gel and glass (amorphous) state material. Additionally, the powder contained nano-scale and small micron-scale rapidly solidified 8 mol% YSZ particles. Following thermal treatment, the gel and the amorphous content transformed to produce (i) densely packed nanograin and (ii) chain-like nanograin aggregates. The nanograin aggregates are suggestive of a strong, yet short-range intergranular attraction, as predicted in computer simulation studies presented in the literature. Interestingly, this mixed morphology powder, after compaction and heat treatment at 1400 °C for 2 h, transformed into 98 % dense material with a homogeneous 200–500 nm grain size. For generating 8 mol% YSZ, the solution precursor plasma spray method offers a high synthesis rate using a low-cost precursor to produce powder that can be consolidated into morphologically homogeneous bulk nanomaterial.     

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.     

Preparation and characterization of dysprosium (Dy) ultrafine nanocrystalline structures

By combining the inert-gas condensation with the SPS technology in an entirely closed system with the oxygen concentration below 0.5 ppm, the pure Dy bulk with the ultrafine nanocrystalline structure has been prepared. Thus a novel and efficient route of preparing nano rare-earth metals, as well as metallic nanomaterials that are highly reactive in the air, is proposed. The thermal, physical and mechanical properties of the prepared ultrafine nanocrystalline Dy bulk have been characterized and compared with those of the conventional raw polycrystalline bulk. It is found that in the nanocrystalline Dy bulk, the starting temperature of phase transformation from hexagonal to rhombohedral crystal structure is reduced by 70 degrees C in comparison with that of the raw polycrystalline bulk. The electrical resistivity of the ultrafine nanocrystalline bulk is slightly increased by a few percent as compared with that of the polycrystalline bulk, while the thermal conductivity is reduced by 28-35%. The microhardness and the elastic modulus of the ultrafine nanocrystalline Dy bulk are found to be remarkably improved as compared with those of the raw polycrystalline bulk, e.g., the microhardness and the elastic modulus are approximately 2.4 and 2.0 times as high as those of the raw polycrystalline bulk, respectively.