Thermal expansion behavior and microstructure in bulk nanocrystalline selenium by thermomechanical analysis

Thermal expansion behavior of bulk nanocrystalline (NC) Se samples with a grain size range of 16–46 nm was studied by thermomechanical analysis (TMA) in the temperature range 290–373 K. Bulk NC Se samples were prepared by isothermally crystallizing the as-quenched bulk amorphous solid at 373–478 K. The glass transition and crystallization of the remaining amorphous Se in the partially crystallized samples were studied by TMA, and compared with the results of differential scanning calorimetry (DSC). The glass transition temperature, as determined from the thermal expansion behavior, was 308 K, 11 K lower than the value by DSC analysis. A structural densification phenomenon was observed in a grain growth process of an as-crystallized NC Se sample by TMA. It was found that the linear thermal expansion coefficient of the bulk NC Se sample increased with a reduction of grain size, from which the deduced thermal expansion coefficient of the interface decreased with the refinement of the grain size.     

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。     

Effect of Mo on microstructure and mechanical properties of bulk nanocrystalline Fe3Al materials prepared by aluminothermic reaction

Abstract

Bulk nanocrystalline Fe₃Al based materials with 5, 10 and 15 wt-%Mo were prepared by aluminothermic reaction. The microstructure and mechanical properties of the materials were investigated. It was shown that the materials consisted of a nanocrystalline matrix phase that was composed of Fe, Al and Mo and a little Al₂O₃ contamination phase. The nanocrystalline phase had a disordered bcc crystal structure. Average grain sizes of the nanocrystalline phase of the materials with 5, 10 and 15 wt-%Mo were 19, 31 and 24 nm respectively and that of the material with 5 wt-%Mo was the smallest. The materials with 10 and 15 wt-%Mo exhibited brittle behaviour in compression, whereas the material with 5 wt-%Mo had a large plastic deformation. The material with 5 wt-%Mo had the highest bending strength and the lowest compressive yield strength.     

Formation of fine nanocrystalline microstructure in bulk Fe-28Al-2Cr

Deformation induced grain refinement was examined in ≈100 nm Fe-28Al-2Cr intermetallic produced by ingot-metallurgical processing. Grain refinement to ≈7 nm (after room temperature compressive deformation) was observed in a small fraction of ingot-metallurgically processed 100 nm Fe-28Al-2Cr. As a first approach, it was demonstrated that fine nano crystalline microstructure can be produced in Fe-28Al-2Cr by a processing technique that does not require powder precursors.     

Consolidation,microstructure and mechanical properties of nanocrystalline metal powders

Nickel, iron and aluminum nanocrystalline powders prepared by the evaporation-condensation method were high pressure consolidated to densities close to the theoretical at room temperature and at 300°C. Microstructures of the materials obtained were characterized by employing scanning (SEM) and transmission (TEM) electron microscopy. Microhardness and yield stress of the materials fabricated from the elemental nanocrystalline powders were measured and compared to those of Ni, Fe and Al high pressure consolidated from the micron/submicron elemental powders, and of a nanocrystalline Ni-20 TiC composite processed via attrition milling of a nickel oxide (NiO)/TiC powder blend followed by the reduction of NiO and high pressure consolidation to full density.     

Mechanical properties of Gd123 bulk superconductors at room temperature

In order to investigate the mechanical properties of Gd123 single-grain bulk superconductors fabricated using a modified quench and melt growth method, tensile tests in the direction parallel and perpendicular to the c-axis have been carried out at 293 K by using the small specimens cut from bulk superconductors. As for the mechanical properties perpendicular to the c-axis, there was no significant difference between those in the crystal growth direction and those perpendicular to it. While the average value of the Young’s modulus of the bulk sample with 33.0 mol%-Gd211 secondary phase particles, 118 GPa, was higher than that of the bulk sample with 28.7 mol%-Gd211, 111 GPa, the average value of the tensile strength of the former, 36 MPa, was lower than that of the latter, 40 MPa. The tensile strength and the Young’s modulus in the c-axis, 10 MPa and 37 GPa, were quite low compared with those mentioned above. Poisson’s ratio based on the transverse strain in the c-axis, 0.15, was significantly smaller than that perpendicular to it, 0.30. In the specimens with higher length, however, the difference was decreased to some extent. With regard to the anisotropy of the Poisson’s ratio, the effect of a pre-existing micro-crack opening in the c-axis direction was discussed coupled with the constraints at the interfaces between the specimen and the sample holder.     

放电等离子烧结制备块状纳米晶SmCo5烧结磁体

采用放电等离子烧结（SPS）技术制备了致密纳米晶SmCo5烧结磁体，研究了磁体的结构和磁性能。X衍射结果表明，烧结磁体具有CaCu5结构，说明SPS过程可以获得稳定的1：5相。TEM观察显示，磁体由平均晶粒尺寸约为30nm的1：5相构成。室温时磁体的矫顽力高达2208kA/m，而剩磁比高达0，7，说明在纳米晶之间存在强烈的晶间交换耦合作用。烧结磁体具有良好的高温性能，773K时的矫顽力为456kA/m，矫顽力温度系数β为-0．212％/K。     

P92钢表层纳米组织的热稳定性

本文通过表面纳米化处理(SMAT)在P92钢表层中形成纳米组织结构,研究了回火温度对表层纳米组织演化行为和析出行为的影响.结果表明:淬火态和回火态P92钢组织经过SMAT处理后沿深度方向依次是纳米层、剧烈变形层、最终过渡到正常组织.随后分别研究了淬火和回火态SMAT试样经不同温度回火后微观组织的再结晶及长大行为.经SMAT处理的铁素体纳米晶粒在550℃时仍能保持较好的纳米结构,甚至高达650℃时表层晶粒仍为纳米晶,当温度超过760℃时表层组织发生显著的再结晶和晶粒长大现象.纳米晶界能抑制淬火态P92钢在较低温度回火时M23C6碳化物的析出.纳米组织提高了高温回火过程中合金元素在铁素体中的扩散速率,加速了M23C6碳化物的长大过程.不同温度回火过程中SMAT纳米层中析出行为的变化将在本文中详细描述.     

纳米晶软磁材料的磁性能

介绍了纳米晶软磁材料所具有的独特结构和优异的磁性能。从纳米晶软磁材料的微观组织结构和宏观磁特性紧密相关的角度,探讨了铁基纳米晶合金的结构与磁性之间的依赖关系。     

SmCo6.6Nb0．4块状纳米晶烧结磁体的结构和磁性能

采用SPS技术制备了纳米晶SmCo6．6Nb0.4烧结磁体，研究了磁体的结构和磁性能．X衍射结果表．4明烧结磁体具有TbCu7结构，说明SPS过程可以获得稳定的1：7相．TEM观察显示，磁体由平均晶粒尺寸约为30nm的1：7相构成．室温时磁体的矫顽力高达2．8T，而剩磁比高达0．74，说明在纳米晶之间存在强烈的晶间交换耦合作用．烧结磁体具有良好的高温性能，矫顽力温度系数β为-0.169％/K。     

Thermal properties of ball-milled nanocrystalline Fe,Co and Cr powders

Ball-milled Fe, Co and Cr powders have been investigated by differential scanning calorimetry to study the stored enthalpy and the origin of the enhanced specific heat in these materials. The samples were prepared of high purity powder of 40–50 μm initial particle size and milling times up to 1 month have been applied. The results of calorimetric measurements have been correlated with structural parameters (average grain size and dislocation density) obtained by high resolution X-ray diffractometry.     

SmCo7块状纳米晶烧结磁体的制备和性能

采用放电等离子烧结技术制备了致密纳米晶SmCo6.6Nb0.4烧结磁体,研究了磁体的结构和磁性能.结果表明,烧结磁体具有TbCu7结构,说明通过SPS过程可以获得稳定的1:7相;磁体由平均晶粒尺寸约为30 nm的1:7相构成,磁体的室温矫顽力高达2.8 T,而剩磁比高达0.74,说明在纳米晶之间存在强烈的晶间交换耦合作用.烧结磁体具有良好的高温性能,773K时的矫顽力为0.48 T,矫顽力温度系数β为-0.169%/K.     

Gd–Ni–Al bulk glasses with great glass-forming ability and better mechanical properties

A series of Gd–Ni–Al ternary glassy alloys with the maximum diameter of 4 mm were obtained by common copper mold casting. The maximum values of the reduce glass transformation temperature (T _g/T _m) and the distance of supercooling region ΔT _x of these alloys in this study were 0.648 and 50 K, respectively. The compressive fracture strength (σ _f) and Young’s modulus (E) of Gd–Ni–Al glassy alloys were 1,240–1,330 MPa and 63–67 GPa, respectively. The magnetic properties of these BMGs were investigated. The Gd–Ni–Al bulk glassy alloys with great glass forming ability and good mechanical properties are promising for the future development as a new type of function materials.     

稀土元素Gd对Al-Si-Mg铸造合金微观组织和力学性能的影响

为了系统地研究稀土Gd对铸造Al-Si-Mg(A357)合金组织和性能的影响,采用OM,SEM,EPMA,XRD,DSC,TEM及拉伸实验等方法对不同Gd含量A357合金进行研究。结果表明:Gd的添加可以细化A357合金的晶粒并减小二次枝晶间距。此外,Gd可以有效地细化合金中的共晶硅,但是对片状共晶硅的形貌影响不大。晶粒和共晶硅的细化及二次枝晶间距的减小使添加Gd后的A357合金的力学性能有了显著的提高。其中,A357-0.5Gd(质量分数/%)合金热处理态抗拉强度为355MPa,相对于未添加Gd元素的A357合金提高了37MPa。当Gd质量分数为1.0%时,尽管组织得到进一步细化,但是大量粗大Al 2Si 2Gd第二相的形成导致了合金力学性能的下降。同时对Gd的细化机制进行探究,结合TEM分析结果可以推断,Gd变质处理后共晶硅上的孪晶密度并不足以引起共晶硅形貌的转变,使得Gd变质效果较弱。而Gd对共晶硅的细化作用可能与Gd增加成分过冷以及形成纳米相阻碍共晶硅生长有关。     

C对Gd5SiGe3磁致冷材料组织结构和磁相变的影响

主要研究C对Gd5SiGe3磁致冷合金组织结构与居里温度的影响.使用商业蒸馏Gd为原料,采用非自耗电弧炉熔炼了Gd5SiGe3Cx(x=0、0.1、0.3、1.0)系列合金.粉末XRD结果表明,少量C(x=0.1、0.3)加入合金后,样品主相具有单斜的Gd5Si2Ge2型结构,当x=1.0时,合金中出现了CSi相和GdGe相.采用光学显微镜观察了了合金纵截面的微观形貌,纵截面的柱状晶和晶粒内的线条组织是Gd5SiGe3Cx合金显微组织的两大显著特征.用样品振动磁强计测定了样品在低场下(7960A/m)的M-T曲线,结果表明,Gd5SiGe3Cx系列合金的居里温度随C含量的增加而逐渐降低,x=1.0时居里温度达到最低值108K.     

半固态处理对Ti基非晶复合材料的组织和力学性能影响

利用"形变诱导相变机制"对非晶基体同时增强增韧,采用悬浮熔炼-铜模吸铸法制备了Ti基非晶复合材料。在此基础上,通过半固态处理工艺对凝固过程动力学控制优化组织,研究了半固态处理对试样的组织和力学行为的影响。结果表明,铸态试样组织为过冷奥氏体相、热致马氏体相和非晶复合结构,应力加载形变过程中通过形状记忆晶相TRIP效应对非晶基体增韧,表现为加工硬化行为。铸态试样的心部组织为较粗大的树枝晶,且生长不均匀;经半固态处理后,先析出相按照尖角溶解平面析出长大形式近球化,形成等轴晶,并在磁悬浮搅拌作用下均匀化,获得奥氏体相晶粒细小、圆整度高、组织致密,复合材料组织得到有效优化,(Ti0.5Ni0.48Co0.02)80Cu20合金断裂强度和塑性变形量由2 582 MPa和15%分别提高至2 745MPa和21.5%。     

Cu纳米晶块体的制备及纳米晶粒长大动力学研究

采用高能球磨的方法制取金属纳米晶粉末，然后利用放电等离子烧结（SPS）技术制备出金属纳米晶块体材料。设计系列实验研究金属纳米晶材料的晶粒长大行为，获得了纳米晶粒长大的动力学规律。根据已有的工作基础和对实验结果的深入分析，确定动力学参数对稳定相晶粒长大行为的影响。实验发现了高能球磨配合SPS技术制备的Cu纳米晶块体发生快速晶粒长大的临界温度，并结合纳米晶界过剩体积与过剩自由能的关系，分析了纳米晶粒组织的能量因素对晶粒长大行为及动力学的影响。     

Bias effect on microstructure and mechanical properties of magnetron sputtered nanocrystalline titanium carbide thin films

Nanocrystalline titanium carbide (TiC) thin films were prepared by magnetron sputtering deposition at 473 K. The effect of substrate bias on microstructure and mechanical properties was studied in details using X-ray photoelectron spectroscopy, X-ray diffraction, field emission scanning electron microscopy, indentation and scanning microscratch. The TiC films exhibit a (111) preferential orientation. Substrate bias decreases grain size and deposition rate of the TiC films. The TiC films have columnar structure which becomes finer at high substrate bias. Nanoindentation hardness, Young's modulus, and toughness of the films are increased as the substrate bias goes up. However, the adhesion peaks at substrate bias of − 100 V and drops when bias is increased further.     

烧结温度对WZV材料力学性能和显微结构的影响

为了开发一种新型刀具材料,以WC、ZrO2和VC为原料,利用热压烧结工艺,分别在1500、1550、1600℃和1650℃烧结温度下制备了4种相同成分的WC/ZrO2/VC(WZV)复合材料.分析了烧结温度与刀具材料相对密度、硬度、抗弯强度和断裂韧性之间的关系,研究了烧结温度对刀具材料力学性能和显微结构的影响,确定了该材料合理的烧结温度为1550℃.试验结果表明,ZrO2质量分数为10%的WZV复合粉末经过48 h的高能球磨,在1550℃、30 MPa的热压烧结条件下,可获得相对密度为99.2%,维氏硬度为17.6 GPa,抗弯强度为786 MPa,断裂韧性为11.51 MPa.m1/2的优异性能.此外,通过对材料显微结构和断裂方式的分析,发现烧结温度对材料的断裂方式具有重要影响.