Gamma -> Epsilon Martensite Transformation and Twinning Deformation in fcc Cobalt During Surface Mechanical Attrition Treatment

The deformation microstructure of face-centered cubic cobalt subjected to surface mechanical attrition treatment was studied as a function of strain levels. Strain-induced gamma --> epsilon transformation and twinning deformation were evidenced by transmission electron microscopy and were found to progress continuously in ultrafine and nanocrystalline grains as the strain increased.

应变诱导晶粒细化与伸长率

针对表面机械研磨处理导致的纳米化过程,分析了晶体结构与层错能(SFE)对纳米/超细晶粒组织塑性变形及晶粒细化机制的影响.在低层错能、热力学亚稳态的纳米/超细晶粒组织中,存在应变诱导的马氏体相变、孪生与位错分解等塑性变形方式,拉伸变形时发生相变诱发塑性(TRIP)效应,指出TRIP效应可以是提高拉伸伸长率的机制.

Phase Evolution and its Effect on Magnetic Properties of Nd60Al10Fe20Co10 Bulk Metallic Glass

The thermal stability of nanocrystalline clusters, the phase evolution, and their effects on magnetic Propertieswere studied for as-cast Nd60Al10Fe20Co10 alloy using differential scanning calorimetry curves, x-ray diffraction patterns, scanning electron microscopy, and high-resolution transition electron microscopy. Thermomagnetic curves and hysteresis loops of the bulk metallic glass were measured during the annealing process. The high thermostability of the hardmagnetic properties of the samples observed is attributed to the stability of the nanocrystalline clusters upon annealing, while the slight enhancement in the magnetization is due to the precipitation of some Nd-rich metastable phases. The mechanism of thermostability of the nanocrystalline clusters and the formation of the metastable phases are discussed.

Strain-Induced Grain Refinement of Cobalt During Surface Mechanical Attrition Treatment

The microstructural evolution during surface mechanical attrition treatment of cobalt (a mixture of hexagonal close packed (hep) and face-centered cubic (fcc) phases) was investigated. In order to reveal the mechanism of grain refinement and strain accommodation. The microstructure was systematically characterized by both cross-sectional and planar-view transmission electron microscopy. In the hcp phase, the process of grain refinement. Accompanied by an increase in strain imposed in the surface layer. Involved: (1) the onset of 110 111 deformation twinning, (2) the operation of (1 120) 110 1 0} prismatic and (1 120) (000 1) basal slip, leading to the formation of low-angle dislocation boundaries, and (3) the successive subdivision of grains to a finer and finer scale. Ressulting in the formation of highly misoriented nanocrystalline grains. Moreover. The formation of nanocrystalliies at the grain boundary and triple junction was also observed to occur concurrently with straining. By contrast. The fec phase accommodated strain in a sequence as follows: (1) slip of dislocations by forming intersecting planar arrays of dislocations, (2) {1 1 1} deformation twinning, and (3) the gamma(fcc) --> epsilon(hcp) martensitic phase transformation. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization occurring in the hep phase and the gamma --> E: martensitic transformation in the fcc phase as well.

纳米孪晶铜力学性能和尺度效应的研究

采用基于机制的应变梯度塑性的传统理论(CMSG),对具有不同尺寸的铜纳米晶粒及孪晶的应力-应变关系进行了有限元模拟.在分析中提出了孪晶薄层强化带的概念并用粘聚力模型模拟晶界的滑移和分离现象,给出了在单向拉伸条件下不同厚度孪晶薄层和不同材料参数对孪晶铜总体应力-应变关系的影响,同时也给出了晶粒中孪晶薄层取向分布对孪晶铜应力-应变关系的影响.数值模拟结果显示:随着晶粒尺寸和孪晶薄层间距的减小,应变梯度效应逐渐增强,材料强化效果越明显;孪晶薄层的取向分布对材料整体的力学性能有较大影响,并且随着晶粒及孪晶薄层间距的减小,孪晶薄层取向的影响也越来越小.最后,有限元计算结果与实验数据进行了对比分析.

Simulations of Mechanical Behavior of Polycrystalline Copper with Nano-Twins

Mechanical behavior and microstructure evolution of polycrystalline copper with nano-twins were investigated in the present work by finite element simulations. The fracture of grain boundaries are described by a cohesive interface constitutive model based on the strain gradient plasticity theory. A systematic study of the strength and ductility for different grain sizes and twin lamellae distributions is performed. The results show that the material strength and ductility strongly depend on the grain size and the distribution of twin lamellae microstructures in the polycrystalline copper.

Microstructure and mechanical properties of Zr-Cu-Al bulk metallic glasses

Zr49Cu46Al5 and Zr48.5Cu46.5Al5 bulk metallic glasses(BMGs) with diameter of 5 mm were prepared through water-cooled copper mold casting. The phase structures of the two alloys were identified by X-ray diffractometry(XRD). The thermal stability was examined by differential scanning calorimetry(DSC). Zr49Cu46Al5 alloy shows a glass transition temperature, T, of about 689 K, an crystallization temperature, T-x, of about 736 K. The Zr48.5Cu46.5Al5 alloy shows no obvious exothermic peak. The microstructure of the as-cast alloys was analyzed by transmission electron microscopy(TEM). The aggregations of CuZr and CuZr2 nanocrystals with grain size of about 20 nm are observed in Zr49Cu46Al5 nanocrystalline composite, while the Zr48.5Cu46.5Al5 alloy containing many CuZr martensite plates is crystallized seriously. Mechanical properties of bulk Zr49Cu46Al5 nanocrystalline composite and Zr48.5Cu46.5Al5 alloy measured by compression tests at room temperature show that the work hardening ability of Zr48.5Cu46.5Al5 alloy is larger than that of Zr48.5Cu46.5Al5 alloy.

Molecular dynamics simulation of plastic deformation of nanotwinned copper

The plastic deformation of polycrystalline Cu with ultrathin lamella twins has been studied using molecular dynamics simulations. The results of uniaxial tensile deformation simulation show that the abundance of twin boundaries provides obstacles to dislocation motion, which in consequence leads to a high strain hardening rate in the nanotwinned Cu. We also show that the twin lamellar spacing plays a vital role in controlling the strengthening effects, i.e., the thinner the thickness of the twin lamella, the harder the material. Additionally, twin boundaries can act as dislocation nucleation sites as they gradually lose coherency at large strain. These results indicate that controlled introduction of nanosized twins into metals can be an effective way of improving strength without suppression tensile ductility. (C) 2007 American Institute of Physics.

Vacancy clusters in ultrafine grained Al by severe plastic deformation

Bulk nanostructured metals are often formed via severe plastic deformation (SPD). The dislocations generated during SPD evolve into boundaries to decompose the grains. Vacancies are also produced in large numbers during SPD, but have received much less attention. Using transmission electron microscopy, here we demonstrate a high density of unusually large vacancy Frank loops in SPD-processed Al. They are shown to impede moving dislocations and should be a contributor to strength. (C) 2007 American Institute of Physics.

变形过程中纳米金属镍的微结构演变

利用透射电镜对冷轧变形所致纳米结构金属镍的微结构组织演变特征进行了研究。结果表明，经过轧制变形后，纳米晶的平均晶粒尺寸为50-70mm，有少量位错结构，但没有发现位错堆积缠结；在晶界处及附近有台阶界面结构，以及与台阶形状相对应的应力场衬度。在变形后期，可以靠晶界发射不全位错从而促使层错生成，并依赖层错尺寸长大（即层错界面位错）的运动进行。而当层错尺寸长大时，其前沿局部变形应力逐渐变大，直至该应力大到一定值时，位错停止运动，层错尺寸也不再长大，并留下了台阶结构。

微纳米晶金属的应变率敏感性及应变硬化行为分析

基于亚微米、纳米晶粒组织塑性变形过程中多种变形机制(位错机制、扩散机制及晶界滑动机制)共存,建立了理论模型,用于定量研究亚微米、纳米晶粒组织的塑性变形行为.以铜为模型材料,计算分析了晶粒尺度、应变率以及温度对亚微米、纳米晶粒组织塑性变形行为的影响.结果表明:相比粗晶铜,亚微米晶铜表现出明显的应变率敏感性,并且应变率敏感系数随晶粒尺度及变形速率的减小而增大;同时,增大变形速率或降低变形温度都能提高材料的应变硬化能力,延缓颈缩发生,进而提高材料的延性.计算分析结果与实验报道吻合.

纳米晶体微观结构与力学性质的分子动力学模拟

纳米晶体材料是由尺度在1-100nm的微小颗粒组成的体系.该文应用分子动力学结合Finnis-Sinclair多体势函数模拟了纳米晶铜的微观结构和单向拉伸变形,以及纳米铜晶粒的结构与扩散性质,并采用了局部晶序分析、晶向分布函数等多种手段,对它们的结构进行了分析.研究了晶粒尺寸的变化对它们的影响.对纳米晶铜的微观结构的模拟表明：随着晶粒尺寸的减小,纳米晶体的晶界结构变化并不明显,而晶粒内部的晶格畸变加剧,导致其结构的无序度明显增加,并且晶粒内部结构和晶界结构的差别也越来越小;晶粒内部的原子的平均能量明显升高,但界面原子的平均能量变化很小.由于受晶格畸变和晶界比例增加的影响,纳米晶体的密度小于单晶的密度.对纳米晶粒的结构与扩散性质进行了分子动力学模拟的结果显示：随着晶粒尺寸的减小,晶粒表面层包含的原子比例迅速增加,表面层的原子平均能量上升,而晶粒内部的保持不变,但不仍然要高于相应单晶体的值,而表面层的厚度基本为一常数.纳米晶粒的扩散系数随着它的尺寸的增加而迅速减小,虽现指数衰减关系.这种减小主要是由于晶粒的表面层原子比例的减小和这些原子的能量降低有关.表面原子的扩散在晶粒的扩散中占主导的地位.

Curie Transition Of Nc Nickel By Mechanical Spectroscopy And Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline ( NC) nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the Curie temperature of NC nickel is lower than that of coarse-grained nickel is explained by an analytical model based on the weakening of cohesive energy.

纳米晶Ni疲劳行为的实验研究

系统研究了纳米晶Ni与粗晶Ni的疲劳行为。通过疲劳实验获得了这2种材料的疲劳应力--寿命曲线，并采用AFM对纳米晶Ni样品表面进行观察以研究其裂纹萌生的微观机制，利用纳米压痕仪对疲劳实验前后样品的力学性能和显微组织变化进行了研究。结果表明，纳米晶Ni具有比粗晶Ni更高的疲劳极限。AFM观察表明纳米晶疲劳后样品表面出现平均尺寸为73 nm的胞状起伏，疲劳后样品的晶粒尺寸没有发生明显改变。压痕硬度结果表明疲劳过程材料的力学性能也未发生明显变化。