Change in electrical resistivity of commercial purity aluminium severely plastic deformed

Commercial purity aluminium sheets were severely plastic deformed by accumulative roll bonding (ARB). Changes in electrical resistivity at 77 K and microstructure during the ARB process were traced up to 12 cycles, which corresponded to an equivalent strain of 10. The resistivity at 77 K increased with increasing number of ARB cycles, then saturated after about the sixth ARB cycle with a maximum increment of resistivity from starting material of about 1.1 nΩ m. Since lattice defects affect the resistivity of metals, the internal dislocation density and the density of grain boundaries were evaluated from scanning transmission electron microscopy images using Ham's method and grain boundary maps obtained from electron back-scattering diffraction, respectively. The relationship between the change in resistivity and the lattice defects is discussed.     

The effect of hardening by annealing in ultrafine-grained Al–0.4Zr alloy: influence of Zr microadditives

Influence of annealing on the microstructure and mechanical properties has been studied for Al–Zr (0.4 wt.%) alloy with the ultrafine-grained (UFG) structure formed by high-pressure torsion (HPT) at room temperature. A drastic hardening effect by short-term annealing in the temperature range of 90–280°С was observed for the HPT-processed Al–Zr alloy. The effect of hardening by annealing for the HPT-processed Al–Zr alloy is compared with that for the HPT-processed commercial purity (CP) Al. It was shown that addition of 0.4 wt.% Zr in Al does not cause a significant impact on the magnitude of hardening by annealing up to 150°С, however it leads to a shift of its maximum to higher annealing temperatures and expansion of the thermal stability range of strength up to 280°С. The kinetics of hardening by annealing for CP Al and Al–Zr alloys in the UFG state has been studied for the first time. It was shown that in both materials the strength first increases linearly with the duration of annealing and then reaches saturation. The temperature dependence of the rate of hardening by annealing was analysed through an Arrhenius law, and apparent activation energy was extracted for both systems. The addition of Zr results in the reduction of the activation energy of annealing-induced hardening by ～2 times. Possible physical mechanisms controlling the kinetics of hardening by annealing are discussed for the ultrafine-grained CP Al and Al–Zr alloy.     

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given te...     

高温塑性变形引起的P非平衡晶界偏聚

阐述了高温塑性变形引起的非平衡晶界偏聚的准热力学和动力学,并使用该模型预测了低合金结构钢中高温塑性变形导致的P在奥氏体晶界的非平衡偏聚.研究发现：当变形为20%,应变速率为1×10^-3 s^-1时,在800 ℃左右会出现一个P的晶界偏聚浓度峰值;在1000 ℃变形为20％时,晶界偏聚浓度随着应变速率的增加而增加.预测结果与现有的实验结果基本一致. 关键词： 非平衡偏聚 晶界 塑性变形 磷     

Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel. The simulation results suggest that the sliding of the {001}/{110} type and {110}/{111} type grain boundary can improve the impact toughness. Then, the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding. Finally, the sliding of the grain boundary is analyzed from the standpoint of the energy. We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.     

Grain boundary misorientation changes during grain growth in pure aluminium

A new method is described for data-logging large amounts of grain boundary misorientation information from channelling patterns in the scanning electron microscope (SEM). The method relies on producing specimens where the grain size is larger than the specimen thickness and where the grain boundary planes are perpendicular to the specimen plane (the so-called columnar structure). Results for grain growth in pure aluminium at 460 and 500°C are presented. There is an increase in the proportion of low angle boundaries at the expense of high angle boundaries during growth times of up to a few hours. The reasons are thought to be partly connected with lower low angle boundary mobility compared with high angle boundaries. However, the growth kinetics appear to be normal over the entire growth time range.     

高压下锐钛矿TiO2的电阻率及能带结构的研究

应用在位电阻率测量方法研究高压下锐钛矿TiO2的电学性质。通过研究电阻率随压力变化的异常变化点,观察到了TiO2从锐钛矿－柯铁矿－斜锆石的相变。卸压后,电阻率数值和初始值相差2个数量级,说明该相变为不可逆相变。结合第一性原理计算结果表明,柯铁矿结构更小的带隙是导致TiO2电阻率减小的根本原因。     

高压下锐钛矿TiO2的电阻率及能带结构的研究

应用在位电阻率测量方法研究高压下锐钛矿TiO2的电学性质. 通过研究电阻率随压力变化的异常变化点, 观察到了TiO2从锐钛矿-柯铁矿-斜锆石的相变. 卸压后,电阻率和初始值相差2个数量级, 说明该相变为不可逆相变. 结合第一性原理计算结果表明, 柯铁矿结构更小的带隙是导致TiO2电阻率减小的根本原因.     

Effect of iron contamination on grain boundary states at a direct silicon bonded (110)/(100) interface

The effect of iron contamination on the electrical property of a (110)/(100) interfacial grain boundary (GB) in a p‐type direct‐silicon‐bonded wafer has been investigated by current–voltage and capacitance–voltage characteristics. It is found that iron contamination can change the charge property of this “model” GB. Based on proper physical modeling, it is found that compared to a “clean” grain boundary, iron contamination can significantly increase the density of GB states, and the corresponding GB neutral level has also been modified. These results show us a clear physical picture of Fe contamination influencing the GB states. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamics and structural aspects of grain boundary segregation

Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summarized. Useful methods of predicting grain boundary segregation are presented. Finally, segregation behavior of solutes at grain boundaries is compared with that at free surfaces, and examples of chemical composition of intexphase boundaries are given.     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

电磁感应式液固态金属电阻率定性测量装置及应用

介绍了一种自主研制的非接触式电阻率测量装置,该装置基于电磁感应原理,可实现降温过程中金属从液态到固态各阶段电阻率的定性测量,为实时表征金属材料的结构变化提供了一种有效的测量工具.详细介绍了该装置的设计原理及其结构,同时利用该装置,测量了金属Zn,Sb以及过共晶Zn-70 wt.% Sb合金降温过程中电阻率随温度的变化情况,验证了该装置的可靠性. 关键词： 电阻率 电磁感应 液态金属 结构转变     

Sr偏析Al晶界结构的第一性原理计算

采用基于密度泛函理论和局域密度近似的第一性原理赝势方法,计算了纯Al晶界和杂质Sr偏析Al晶界的原子结构和电子结构.结果表明Sr偏析引起了晶界膨胀和晶界处电子密度的大幅度降低,从而导致晶界结合力的减弱.这应为Sr杂质偏析引起的Al晶界脆化的主要根源所在. 关键词： Al晶界 Sr 杂质偏析 第一性原理计算     

Atomic scale investigation of grain boundary structure role on intergranular deformation in aluminium

The role that grain boundary (GB) structure plays on the directional asymmetry of an intergranular crack (i.e. cleavage behaviour is favoured along one direction, while ductile behaviour along the other direction of the interface) was investigated using atomistic simulations for aluminium 〈1 1 0〉 symmetric tilt GBs. Middle-tension (M(T)) and Mode-I crack propagation specimens were used to evaluate the predictive capability of the Rice criterion. The stress–strain response of the GBs for the M(T) specimens highlighted the importance of the GB structure. The observed crack tip behaviour for certain GBs (Σ9 (2 2 1), Σ11 (3 3 2) and Σ33 (4 4 1)) with the M(T) specimen displayed an absence of directional asymmetry which is in disagreement with the Rice criterion. Moreover, in these GBs with the M(T) specimen, the dislocation emission from a GB source at a finite distance ahead of the crack tip was observed rather than from the crack tip, as suggested by the Rice criterion. In an attempt to understand discrepancy between the theoretical predictions and atomistic observations, the effect of boundary conditions (M(T), Mode-I and the edge crack) on the crack tip events was examined and it was concluded that the incipient plastic events observed were strongly influenced by the boundary conditions (i.e. activation of dislocation sources along the GB, in contrast to dislocation nucleation directly from the crack tip). In summary, these findings provide new insights into crack growth behaviour along GB interfaces and provide a physical basis for examining the role of the GB character on incipient event ahead of a crack tip and interface properties, as an input to higher scale models.     

Influence of Fe doping on electrical properties of LCMO

The polycrystalline samples La_0.67Ca_0.33Mn_(1?x)Fe _x O₃ (x?=?0.00,?0.01,?0.03, and 0.1) have been grown in single phase by solid state route. The analysis of the reaction has been done by thermogravimetry and differential thermal analysis measurements. DC electrical resistivity measurements have been carried out down to 15?K. The samples with x?=?0.00, 0.01, and 0.03 exhibit metal–insulator (MI) transition at temperatures 221.5?K, 217?K, and 215?K respectively, whereas the sample with x?=?0.1 is insulating in nature for entire temperature range. Interestingly, the electric transport properties of these samples are not consistent with their magnetic phase transitions and the samples show MI transition at a temperature, T _MI, which is significantly lower than the paramagnetic to ferromagnetic transition temperature (T _c). The resistivity data below T _MI has been analyzed using the empirical relation ρ?=?ρ₀?+?ρ₁ T ⁿ and the data above this temperature has been analyzed using two existing models, Mott's variable range hopping model and spin polaronic conduction model.     

ZA27合金晶界处铁、稀土元素的有序化与交互作用

为从理论上揭示铁、稀土元素在锌铝合金晶界处的行为本质，建立了ZA27合金中α相大角度重位点阵晶界模型，利用递归法(Recursion)计算了晶界的电子结构(状态密度、费米能级、结构能).用晶界结构能定义合金的团簇能(有序能)，并计算了偏聚铁及稀土晶界的团簇能.计算结果表明：铁、稀土元素在锌铝合金晶界处团簇能为正值，不能形成团簇，具有有序化倾向，趋于形成稳定的金属间化合物.铁与稀土元素在晶界形成负电中心，降低晶界的费米能级. 关键词： 稀土 晶界 电子结构 有序化     

ZA27合金中稀土及铁的晶界偏聚与交互作用

根据分子动力学理论建立了液态ZA27合金的原子集团，结合计算机编程构造出ZA27合金α相与液相共存时的原子构形及α相大角度重位点阵晶界模型.利用递归法计算了铁、稀土元素 固溶 于晶粒内，游离于固液相界面及其在α相晶界处的环境敏感镶嵌能，计算了铁、稀土元素与 Al 的键级积分.由此得出：铁、稀土处于固液相界区比在晶内更稳定，解释了铁、稀土在α相内溶解度很小，结晶时富集于固液相界前沿液体中，从而导致凝固结束后铁、稀土元素偏聚于晶界，并形成成分复杂的稀土化合物的事实. 关键词： 稀土 晶界 递归方法 电子结构     

Different effects of grain boundary scattering on chargeand heat transport in polycrystalline platinum and goldnanofilms

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann--Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann--Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.