Effect of relative grain size on the flow stress of polycrystalline FCC sheets

A dynamic explicit finite element code and rate-dependent elastic-viscoplastic polycrystalline model were used to simulate the simple tension test of annealing FCC polycrystal and 6111-T4 aluminum alloy sheet metal. The variability of flow stress was investigated, which was induced by various grain boundary constraints when the ratio of thickness-to-grain size was changed. The simulated results show that, when the relative grain size increases, the constraint of grain boundary will increase accordingly, which results in the increase of the flow stress of polycrystal. The results agree with experiments.     

准D5晶粒各向异性集合的弹性本构研究

多晶体金属材料由微小晶粒集合而成,晶粒的物理性质和金属的微结构决定了该多晶体材料的宏观力学性质．常见金属多晶体材料的晶粒有面心立方、体心立方和密排六方结构．基于Voigt模型,Morris和Sayers给出了立方晶粒正交集合的弹性本构关系,Huang则给出了立方晶粒各向异性集合的弹性本构关系．假设一材料是由大量的准D5晶粒集合而成(称为准多D5晶体),利用准D5晶粒的对称性研究D5晶粒弹性本构关系,然后基于Voigt模型和Reuss模型分别推导出准D5晶粒各向异性集合的有效弹性刚度张量和有效弹性柔度张量．在本文中,品粒取向分布函数(ODF)w(R)用来描述多晶体中的晶粒取向为R的可能性密度,给出了弹性本构与织构系数之间的关系表达式．     

Synthesis and Characterization of Li-doped LaMnO3 CMR Materials

A series of bulk polycrystalline La1-xLixMnO3 samples with x ranging from 0.1 to 0.5 was prepared by sol-gel method,X-ray diffraction patterns show that the crystal structures are single rhombohedral perorskite for the x≤0.3 sample and the impurity appears when x〉0.3.Under the same synthesized conditions,the higher Li content samples display a higher content of liquid phase content and larger mean grain sizes,which leads to the increases of the effect of the grain boundaries.The experimental results show that the change of the ferromagnetic transition temperature and the resistivity can attribute to the effect of the grain boundary and the connectivity of the inter grains as well as the ratio of Mn^3＋ to Mn^4＋.     

纳米金属材料的强度与晶粒尺寸的关系

传统多晶金属材料的强度与晶粒尺寸的关系符合H-P关系,然而,对于纳米金属材料,随着晶粒尺寸的减小,强度与晶粒尺寸的关系不符合H-P关系.这与纳米材料晶界(包括三叉晶界、四方晶界或晶隅)所占的体积分数有很大关系.对此,通过几种修正模型对该问题进行了分析.     

Mechanical behaviors of polycrystalline NiTi SMAs of various grain sizes under impact loading

This work presents mechanical properties of the NiTi polycrystalline superelastic shape memory alloys(SMA) of 5 different grain sizes under high-speed impacts. The amorphous, nanocrystalline(40, 80, 120 nm) and coarse grain(20 μm) sheets are manufactured with cold rolling and suitable heat treatments. A Hopkinson tensile bar is used to perform tests up to 45 m/s. Highspeed camera system and digital image correlation method are used to get the strain field and particle velocity field at a sampling frequency of 2×10~6 frames/s with a resolution of 924×768 pixels. Nominal stress-strain curves are obtained for all the sheets with a strain rate of about 1000 s~(-1) and they have a similar evolution to the quasi-static case but with much higher stress levels. The rate sensitivity is increased with the grain size and the stress level can reach up to a 70% growth for a coarse grain sheet but be totally insensitive for the amorphous sheet in the strain rate from 10~(-4) to 10~3 s~(-1). A single transformation front can be found under high-speed impact(45 m/s) at the early loading stage. The speed of the transformation front is calculated from strain time histories and the highest front speed of 811 m/s is observed which is never observed before. It also reveals that the front speed depends also on the grain size. With the same loading speed, the bigger the grain size is, the slower the transformation front speed is.     

铈、磷晶界偏聚及其相互影响

本文通过改变10Mn2V钢中的铈、磷含量,研究了奥氏体状态下铈、磷在晶界的偏聚规律。研究发现:磷在奥氏体中的偏聚是一种平衡偏聚,随加热温度的升高,晶界的偏聚量下降,其偏聚自由能为-40～-44.5 kJ/mol;铈在奥氏体状态下也向晶界偏聚,但偏聚量随加热温度的升高而增加;当钢中铈、磷共存时,磷在晶界的偏聚量下降,而铈在晶界的偏聚量上升;钢中的铈、磷原子之间有较强的相互作用。     

Effects of Phosphorus Grain Boundary Segregation and Hardness on the Ductile-to-Brittle Transition for a 2.25CrlMo Steel

The combined effect of phosphorus grain boundary segregation and hardness on the ductile-to-brittle transition was examined for a P-doped 2.25CrlMo steel by using Auger electron spectroscopy in conjunction with hardness measurements, Charpy impact tests and scanning electron microscopy. With prolonging time at 540 ~C after water quenching from 980℃, the segregation of phosphorus increases and the hardness decreases. The DBTT （FATT） increases with increasing phosphorus segregation and decreases with decreasing hardness. The effect of phosphorus segregation is dominant until 100 h aging and after that the hardness effect becomes dominant. This effect makes the DBTT （FATT） decrease with further prolonging ageing time although the segregation of phosphorus still increases strongly.     

Effects of Phosphorus Grain Boundary Segregation and Hardness on the Ductile-to-Brittle Transition for a 2.25CrMo Steel

The combined effect of phosphorus grain boundary segregation and hardness on the ductile- to-brittle transition was examined for a P-doped 2.25Cr1Mo steel by using Auger electron spectroscopy in conjunction with hardness measurements, Charpy impact tests and scanning electron microscopy. With prolonging time at 540 oC after water quenching from 980 oC, the segregation of phosphorus increases and the hardness decreases. The DBTT (FATT) increases with increasing phosphorus segregation and decreases with decreasing hardness. The effect of phosphorus segregation is dominant until 100 h aging and after that the hardness effect becomes dominant. This effect makes the DBTT (FATT) decrease with further prolonging ageing time although the segregation of phosphorus still increases strongly.     

2.25Cr1Mo钢韧脆转变温度影响因素分析

采用不同的时效工艺,运用冲击试验、俄歇能谱、扫描电镜等分析方法,研究2.25Cr1Mo钢的韧脆转变温度变化规律及其影响因素。结果表明,2.25Cr1Mo试验钢650℃时效2h后的韧脆转变温度为-55℃,560℃时效100h后的韧脆转变温度为-25℃,480℃时效1 200h后的韧脆转变温度升高至-10℃;P在晶界处的偏聚行为是导致试验钢韧脆转变温度变化的主要因素。     

FCC晶体结构多晶铜的动态本构方程参数确定

以带有同步组装技术的分离式Hopkinson压杆作为热模拟装置，采用基于MTS(Mechanical Threshold Stress)模型建立的FCC晶体结构的动态本构方程，确定了多晶铜动态本构方程参数，研究了连铸多晶铜的动态冲击特性，获得的理论应力一应变曲线与实验结果相当吻合，但不能描述485～585K间回复与再结晶温度区间的变形。     

高矫顽力烧结钕铁硼永磁研究进展

烧结钕铁硼稀土永磁具有超高的磁性能,但是居里温度较低,使其在高温领域的应用受到一定的限制.新能源汽车、风力发电等新兴产业的快速发展,大幅拉升了对耐高温烧结钕铁硼的市场需求.因此,为了提高钕铁硼材料的使用温度,通过开发新技术来提高烧结钕铁硼矫顽力;同时,为了降低产品成本和对重稀土金属的消耗,开发低Dy或无Dy的高矫顽力烧结钕铁硼磁体成为目前的研究热点.研究发现,采用细化晶粒技术、晶界扩散技术以及晶界掺杂技术均能不同程度地提高钕铁硼磁体的矫顽力.综述了这3种新技术的最新研究进展.     

细晶工艺制备高性能烧结钕铁硼的研究

烧结钕铁硼稀土永磁具有超高的磁性能,但是居里温度较低,使其在高温领域的应用受到一定的限制.新能源汽车、风力发电等新兴产业的快速发展,大幅拉升了对耐高温烧结钕铁硼的市场需求.因此,为了提高钕铁硼材料的使用温度,通过开发新技术来提高烧结钕铁硼矫顽力;同时,为了降低产品成本和对重稀土金属的消耗,开发低Dy或无Dy的高矫顽力烧结钕铁硼磁体成为目前的研究热点.研究发现,采用细化晶粒技术、晶界扩散技术以及晶界掺杂技术均能不同程度地提高钕铁硼磁体的矫顽力.综述了这3种新技术的最新研究进展.

     

Refinement mechanism of in-situ Al4C3 particles on AZ91 magnesium alloys

The effects of SrCO3 on the microstructure of AZ91 magnesium alloy were investigated by OM, SEM, EDS, XRD and DTA analyses. The results show that AZ91 magnesium alloy with 1.0% SrCO3 addition has the best refining effect at 730 ℃ for 20 min, the average size of the α-Mg grain in AZ91 matrix alloy is reduced from about 91 μm to 58 μm, with reduction of about 36%. Based on the analysis results of EDS, binding energy, and Gibbs free energy, it is evident that the grain is refined because of the generation of A14 C3 particles which can be used as the crystal nucleus of the magnesium alloy when SrCO 3 is added. Owing to the generation of A14C3 in AZ91 magnesium alloy, the grain boundary is pined and the grain growth is inhibited. In the accession to the Al4C3, small subcooling temperature difference leads to the formation of fine grain microstructure in alloy melt. Fine grain microstructure in AZ91 alloy melt with the addition of SrCO3 can be obtained on the condition of lower undercooling temperature according to the DTA analysis results.     

Al-6Mg-1Zn合金超塑变形机理的研究

本文在测定Al-6Mg-1Zn合金超塑拉伸性能的基础上,利用金相显微镜对一系应变速率下超塑变形试样进行了晶粒度、等轴比的测定。结果表明,在最佳应变速率条件下,等轴比最小,说明最佳应变速率条件下变形,晶界滑动在变形中所占比重最大,且晶内位错运动在变形中起有一定的作用。本文所研究的这种新型合金超塑性能较好,但晶粒稳定性不很好,这是国内外文献中少见的,其变形机理的研究为这种材料进一步研究和将来的使用奠定了基础,并为开发其他超塑合金提供了一些资料。     

周期性边界条件下MC模拟晶粒数的统计

本文应用Monte Carlo(MC)法模拟二维条件下晶粒长大的原理，晶粒长大结晶界处的格点迁移为主要特征，并用它模拟了在周期性的边界条件下的晶粒长大行为，提出了一种二维条件下精确的统计晶粒尺寸的方法，可以对各种形态的晶粒进行统计，并且考虑了模拟过程中的周期性边界条件对晶粒统计的影响，通过模拟，揭示了晶粒长大过程中材料组织演变时拓扑学和动力学特征。     

Rate dependent rheological stress-strain behavior of porous nanocrystalline materials

To completely understand the rate-dependent stress-strain behavior of the porous nanocrystalline materials,it is necessary to formulate a constitutive model that can reflect the complicated experimentally observed stress-strain relations of nanocrystalline materials.The nanocrystalline materials consisting grain interior and grain boundary are considered as viscoplastic and porous materials for the reasons that their mechanical deformation is commonly governed by both dislocation glide and diffusion,and pores commonly exist in the nanocrystalline materials.A constitutive law of the unified theory reflecting the stress-strain relations was established and verified by experimental data of bulk nanocrystalline Ni prepared by hydrogen direct current arc plasma evaporation method and hot compression.The effect of the evolution of porosity on stress-strain relations was taken into account to make that the predicted results can keep good agreements with the corresponding experimental results.     

Spontaneous Magnetic Transitions and Corresponding Magnetoelastic Properties of Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Ge<Subscript>2</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Gd,Tb and Dy)

The spontaneous magnetic transitions and corresponding magnetoelastic properties of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb and Dy) were investigated by using the X-ray diffraction method and magnetic measurement. The results showed that the compounds experience two magnetic transitions, namely the second-order paramagnetic to antiferromagnetic transition at temperature T_N (T_N=368, 423 and 443 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) and the first-order antiferromagnetic - ferrimagnetic transition at temperature T_t (T_t=96, 80 and 40 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) as the temperature decreases. The temperature dependence of the lattice constant a(T) displays a negative magnetoelastic anomaly at the second-order transition point T_N and, at the first-order transition T_t, a increases abruptly for GdMn₂Ge₂ and TbMn₂Ge₂, Δa/a about 10^-3. Nevertheless, the lattice constant c almost does not change at these transition points indicating that such magnetoelastic anomalies are mainly contributed by the Mn-sublattice. The transitions of the magnetoelastic properties are also evidenced on the temperature dependence of magnetic susceptibility χ. The first-order transition behavior at T_t is explained by the Kittel mode of exchange inversion.     

Effect of grain boundary on electric performance of ZnO nanowire transistor with wrap-around gate

A novel grain boundary(GB) model characterized with different angles and positions in the nanowire was set up.By means of device simulator,the effects of grain boundary angle and location on the electrical performance of ZnO nanowire FET(Nanowire Field-Effect Transistor) with a wrap-around gate configuration,were explored.With the increase of the grain boundary angle,the electrical performance degrades gradually.When a grain boundary with a smaller angle,such as 5° GB,is located close to the source or drain...     

Oxidation kinetics and high temperature in-situ observation of the oxidation behaviour of NbSi2 at 1023 K

By using high temperature optical microscopy, oxidation behaviors of poly-and single crystalline NbSi₂ were observed in-situ at 1023 K. The effects of micro-cracks, porosity and grain boundary on the oxidation behavior of NbSi₂ have been studied. The results indicate that new cracks initiate and extend from the pre-existing cracking areas in the arc-melted poly-crystalline specimen, leading to fragmentation after 220 min at 1023 K. However, although pores and grain boundary are the preferential oxidation site, they do not directly lead to fragmentation during oxidation, indicating that the pre-existing cracks in specimen are the key reason to pesting. The oxidation kinetics of the studied NbSi₂ specimens corresponds well with the in-situ observation.     

内应力假说和脆性材料特性参数的尺寸效应(Ⅲ)

内应力假说建立了一组描述脆性材料强度指标和弹模指标尺寸效应的方程 ;该假说认为内应力场或者说内应力能会随试件尺寸加大而加大 ,在引起材料强度指标随试件尺寸加大而减小的同时 ,还会引起材料初始切线弹模随试件尺寸加大而加大的弹模尺寸效应 .文中利用尺寸效应的方程对 3组不同尺寸的混凝土试件强度和弹模试验数据以及多组不同冰晶粒径的多晶冰试件强度和弹模试验数据进行了计算 ,结果表明理论公式在描述材料强度尺寸效应 (或粒径效应 )和弹模指标尺寸效应时具有良好的精度 ,证明了内应力理论是正确的 .对内应力理论在解释脆性材料破坏形态、尺寸效应等多种力学特性方面的应用进行了总结 .