纳米固体材料中微观结构的高分辨电镜研究

对纳米固体材料中晶界结构的特征一直存在争议,作者对两种不同方法(惰性气凝聚加原位加压法和非晶晶化法)所制备的纳米固体材料中的微观结构,特别是晶界结构进了较深入的研究,这两种方法都能提供具有清洁界面的纳米固体材料。 惰性气体凝聚加原位加压法:对纳米固体Pd样品的高分辨电镜研究表明,晶界是由序,局部无序区和纳米级空洞组成,并不具有“类气态”结构特征。一般来说,纳米固体Pd(平晶粒度为10nm)中大多数晶界结构特征类似于常规多晶材料中的晶界,但晶界较弯曲,而具有一定的畸变。此外,在Pd样品中,还存在大量的小角度晶界位错,晶粒内部也存在一些陷结构,如刃型位错,60°不全位错,五次孪晶及孪晶交割等,这些缺陷结构特征与一般多晶料中的缺陷十分相同。 非晶晶化法:对三种不同非晶合金(Ti基、Fe基等)进行晶化处理,制备出具有不同晶粒度的纳米合金样品,对其进行微观结构的研究表明,晶界上根本不存在“类气态”结构或无序的区域,样品中存在大量的大角度非对称晶界,相应的晶界具有高密度的位错。大多数晶界上有一定的应力存在,晶界附近的点阵发生轻微的畸变,这些晶界一般以半共格结合的形式存在。与惰性气体凝聚加原位加压法所制备的纳米固体Pd样品相比,除了晶界结构存在明显的差异外,非晶晶化法所制备     

纳米结构金属位错的研究进展

综述了国内外近年来对纳米结构金属位错的研究,包括位错的基本特征、研究方法以及定量分析.由于晶粒尺寸的减小,全位错的形成和运动变得困难甚至不可能,纳米结构金属更容易生成不全位错.在高分辨TEM图像观察实验中发现了大量孪晶或层错,也证实了不全位错的存在.着重讨论了晶界发射不全位错的形核、增殖以及在塑性变形过程中所起的作用.研究了纳米结构金属中的位错动力学,采用分子动力学模拟和高分辨透射电镜方法从不同层面上揭示了位错的形核、增殖、运动以及相互作用等过程.最后简单介绍了位错柏氏矢量以及密度的相关定量分析,其相关参数的表征对进一步弄清纳米结构金属的塑性变形机制具有重要意义.     

晶粒尺寸对Cu-30%Zn合金形变孪晶厚度的影响

对于面心立方结构的纳米金属,晶粒尺寸对孪生厚度(孪生核)的影响虽已有研究,但仍有待深入。本论文以Cu-30%Zn合金为模型材料,通过高压扭转变形技术、等径角挤压连同轧制技术变形得到晶粒尺寸在5~500nm的样品。透射电子显微镜观察发现:变形孪晶的片层厚度随晶粒尺寸的减小而减小,当晶粒尺寸小于20nm以后,孪晶厚度为(111)晶面间距(层错);另外,层错存在于各个不同尺寸范围的晶粒内,表明层错不受晶粒尺寸影响。研究结果表明在低层错能超细晶材料中,孪生变形是通过从晶界连续发射不全位错(层错)形成的。     

我国科学家发现纳米金属材料新特质

近期《科学》报道了中科院金属研究所沈阳材料科学国家（联合）实验室卢磊研究员领导的研究小组与卢柯研究员、丹麦Risφ国家实验室的黄晓旭博士合作研究的成果。他们利用共格孪晶界独特的稳定界面结构获得了具有超细特征尺寸的纳米结构金属,并发现减小孪晶片层厚度将增加材料的强度。     

中科院金属所研究金属材料获重大发现

据美国《科学》（Science）杂志报道,中科院金属研究所沈阳材料科学国家（联合）实验室研究员卢磊领导的研究小组与研究员卢柯、丹麦Risf国家实验室的黄晓旭博士合作,利用共格孪晶界独特的稳定界面结构获得了具有超细特征尺寸的纳米结构金属,并发现减小孪晶片层厚度将增加材料的强度。这一发现表明,当纯金属的特征尺寸降低至纳米量级时,     

镍基单晶高温合金热机疲劳断裂特征

为了进一步提高镍基单晶高温合金的热机疲劳性能,通过微观结构解析研究了合金热机疲劳断裂特征.通过金相和扫描电子显微镜研究了热机疲劳断裂的断口特征和微观结构.研究表明:裂纹起源于形变孪晶与试样外表面的交截处,过程中的氧化有助于裂纹的长大;裂纹尖端的应力场诱发出大量形变孪晶,而形变孪晶的存在为裂纹进一步沿着孪晶界扩展提供了便利条件;镍基单晶高温合金的疲劳断裂主要是由于形变孪晶的形成以及裂纹沿孪晶界的扩展造成的.形变孪晶与高温合金疲劳断裂密切相关.     

硅纳米晶微观结构缺陷的高分辨电子显微学研究

利用离子注入结合后续高温退火的方法成功地制备出包埋在二氧化硅(SiO_2)基质中的硅纳米晶.利用透射电子显微学对所制备的硅纳米晶(离子注入浓度为3×10~(17)cm~(-2))的微观结构缺陷进行了详细的研究.通过高分辨像分析发现:较大的纳米晶(直径＞6nm)中存在很多面缺陷,主要为孪晶与层错.孪晶包括一次孪晶、二重孪晶、三重孪晶及五重孪晶.层错分为内禀和外禀两种类型,并讨论了内禀层错占多数的原因.除了面缺陷以外,还有一部分纳米晶中存在位错.     

LaAlO_3基体上CaBaCo_2O_(5+δ)多晶薄膜的微观结构以及生长方式

采用脉冲激光沉积方法,在(001)LaAlO3上沉积CaBaCo2O5+δ多晶薄膜,用X射线衍射(XRD)和透射电子显微镜(TEM)对基体和薄膜的微观结构进行观察,并探讨了薄膜的生长机制以及改善薄膜质量的可能工艺。结果表明,(1)LaAlO3基体中存在位错、孪晶等缺陷,且孪晶类型以(100)楔形孪晶为主;(2)CaBaCo2O5+δ薄膜为多晶薄膜,薄膜底部保留了薄膜生长过程中的初始晶粒结构,晶粒大小约5~8 nm,厚度约为5~7 nm。薄膜顶层的晶粒较大,约为11~16 nm。     

全片层γ-TiAl基合金屈服应力的微结构尺度效应

本文基于多位错塞积模型,研究了γ-TiAl基全片层组织合金的屈服应力与微观结构多尺度的关系,给出了全片层γ-TiAl基合金屈服应力的解析计算公式.重点分析了合金中PST颗粒片层界面强度τλc、晶界强度τdc、片层厚度λ及PST颗粒大小尺度d对合金屈服应力大小的影响.     

选区激光熔化316L不锈钢高应变率压缩下的塑性变形行为

对选区激光熔化316L不锈钢(SLM-316L)的高应变率(1000、2000、3000 s^-1)压缩力学性能进行测试，用扫描电镜和背散射衍射(EBSD)等手段表征冲击加载前后试样的微观结构，并分析晶体结构的差异以及位错滑移、孪生行为等微观变形机制。结果表明：SLM-316L不锈钢在高应变率载荷作用下有显著的应变率强化效应，其微观组织由截面呈不规则多边形的柱状胞晶密排结构组成，高应变率加载使晶体取向的择优性降低、小角度晶界和孪晶界数量增加，且孪晶界在小角度晶界的交叉缠绕区分布密集，试样的塑性变形过程伴随着位错滑移及孪生行为。     

Formation of lamellar M23C6 on and near twin boundaries in austenitic stainless steels

Thin foil electron microscopy studies were made on the precipitation of lamellar M₂₃C₆ during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation on incoherent twin boundaries M₂₃C₆ formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature. The lamellar morphology of M₂₃C₆ is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching.     

Tensile properties of Cu with deformation twins induced by SMAT

High density nano-scale deformation twins were introduced in the surface layer of Cu sample by means of surface mechanical attrition treatment （SMAT） at room temperature. The Cu sample with deformation twins shows a yield strength of about 470 MPa in tension tests. The significant strengthening may be attributed to the effective inhibition of slip dislocations by abundant twin boundaries.     

Evolution of a martensitic structure in a Cu–Al alloy during processing by high-pressure torsion

A Cu-11.8 wt% Al alloy was quenched in iced water from a high temperature (850 °C) to introduce a martensitic phase and then the alloy was processed using quasi-constrained high-pressure torsion (HPT). The micro-hardness and the microstructures of the unprocessed and severely deformed materials were investigated using a wide range of experimental techniques (X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and high- resolution TEM). During HPT, a stress-induced martensite–martensite transformation occurs and an $ \alpha^{\prime}_{1} $ martensite phase is formed. In the deformed material, there are nanoscale deformation bands having high densities of defects and twins in the $ \alpha^{\prime}_{1} $ martensite. It was observed that a high density of dislocations became pinned and accumulated in the vicinity of twin boundaries, thereby demonstrating a strong interaction between twin boundaries and dislocations during the HPT process.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

In Situ Observation of Twin Boundary Sliding in Single Crystalline Cu Nanowires

Using a homemade, novel, in situ transmission electron microscopy (TEM) double tilt tensile device, plastic behavior of single crystalline Cu nanowires of around 150 nm are studied. Deformation twins occur during the tests as predesigned before the experiments. In situ observation of twin boundary sliding (TBS) caused by full dislocation (extended dislocation) is first revealed at the atomic scale which is confirmed by molecular dynamics (MD) simulation results. Combined with twin boundary migration and multiple dislocations nucleated from surface, TBS causes a superlarge fracture strain which is over 166% and a severe necking which is over 93%, far beyond the typical values for most nanomaterials without twins.     

Effect of Ultrasonic Nanocrytalline Surface Modification on the Microstructural Evolution of Inconel 690 Alloy

Specimens of Inconel 690 were investigated after ultrasonic nanocrystalline surface modification (UNSM) using microhardness tests, electron backscattered diffraction, and transmission electron microscopy (TEM). After UNSM treatment, a 60% increase of hardness up to ～310 µm in depth was observed. Layer-by-layer TEM analysis showed well-refined grains and twins in addition to the high dislocations density. The mechanism of the microstructure refinement was attributed to the development of nano-grains, twin structures, and dislocations.     

Microstructural investigations of reduced magnesium titanate spinels which have shown anomalous resistance behaviour

Samples from the reduced magnesium titanate spinel system (Mg_{1 + y}Ti_{2 − y}^x+O₄) have been examined using selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) imaging. Unusual microstructural features are observed in samples in the region of the system (i.e., where x, the average Ti valency, lies between 3.26 < x < 3.33) where transitions to zero electrical resistance on cooling below 50 K are reported. These features do not occur in samples from outside this compositional range. Lamellar features with wavelengths up to 750 nm are observed, and associated electron diffraction patterns demonstrate streaking of diffraction spots around the direct beam. The presence of streaks, rather than divergent spots, indicates fanning of the structure across the lamellar boundaries and HREM images further indicate that interlamellar boundaries are coherent. Linear streaking parallel to 111 observed in some crystals may represent the same feature viewed perpendicular to a lamellar boundary. These observations are consistent with exsolution of a single high temperature phase to two co-existing spinels of slightly different compositions. The observation of coherent lamellar boundaries and streaking of electron diffraction spots are consistent with spinodal decomposition as the mechanism of exsolution.Fine-scale lamellar structures parallel to “111” are also observed, which are texturally distinct from those described above. Streaking of electron diffraction spots parallel to 110 may be associated with these features. These lamellae probably represent intimate spinel-spinelloid intergrowths, rather than ordering of cation vacancies or another process.

These microstructural features appear to relate to the critical resistivity transition, as they have only been observed in samples close to the compositions which have displayed such electrical behaviour. Similar studies on samples away from the region of interest fail to show these textures. A relationship between the solvus and the zero electrical resistance behaviour is inferred. In the simplest case, the zero electrical resistance material may represent a metastable homogeneous high temperature spinel from above the solvus; alternatively, the electrical behaviour may derive from the strained interlamellar boundaries. Fine-scale linear features parallel to “111”, often associated with the interlamellar boundaries, are interpreted as fine spinelloid domains; the electrical behaviour may also relate in some way to these features.

It should be noted that the presence of no single microstructural feature correlates directly with the occurrence of this behaviour, nor does the loss of any feature correlate with degradation; however, it is also clear that the microstructure is continually developing as the sample ages at room temperature. Different degradation rates in air and vacuum are explained in terms of the continuing exsolution being slowed by the development of spontaneous strain at interlamellar boundaries. Chemical attack by air releases interlamellar strain and allows exsolution to progress at full speed.     

Twin crystal structured Al-10 wt.％ Mg alloy over broad velocity conditions achieved by high thermal gradient directional solidification

Twin crystal structured Al-10 wt.％ Mg alloys that were grown over a broad solidification velocity range were prepared and studied for the first time.The high thermal gradient (G) and growth velocity (V) of directional solidification resulted in the dominant solidification of twins: the twinned dendrite trunks at constant high Vs curved in the G direction with large angles in 7 mm diameter crucibles and invaded regular columnar grains because of a distinct kinetics growth advantage.Transitive deceleration experiments were designed to produce twin crystals that evolved with lower values of V (100,10,and 0.5 μm/s) and had a structural coarsening trend.Twin cell growth in the absence of arms occurred at a growth velocity of 10 μm/s.A coherency loss was observed at a growth velocity of 0.5 μm/s with straight coherent twin boundaries turning into curved incoherent boundaries.Linear theoretical analyses were performed to understand the structural evolution of the twins.These results demonstrate the possibility of producing dense and controlled twin crystals in the Al-Mg system under most industrial production conditions;thus,this approach can be a new structural choice for designing Al-Mg-based alloys that have widespread commercial applications.     

Planar defects in GeSe and GeS crystals

Planar defects in layered GeSe and GeS crystals grown by vapour transport are studied by transmission electron microscopy. The principal defects are (110) twins, which appear as narrow parallel bands, and (001) low-angle grain boundaries, containing networks of boundary dislocations. In order to propose an atomistic model, the structure of the materials is described in terms of hexagonal rings and of trigonal pyramids projected along the c-axis. These units are used for the construction of the model of the twin structure which is free of dangling bonds. The relationship of the defects with the transport properties of the materials is discussed.     

Effect of Laser Shock Peening on the Microstructures and Properties of Oxide‐Dispersion‐Strengthened Austenitic Steels

Oxide‐dispersion‐strengthened (ODS) austenitic steels are promising materials for next‐generation fossil and nuclear energy systems. In this study, laser shock peening (LSP) has been applied to ODS 304 austenitic steels, during which a high density of dislocations, stacking faults, and deformation twins are generated in the near surface of the material due to the interaction of laser‐driven shock waves and the austenitic steel matrix. The dispersion particles impede the propagation of dislocations. The compressive residual stress generated by LSP increases with successive LSP scans and decreases along the depth, with a maximum value of ?369 MPa. The hardness on the surface can be improved by 12% using LSP. In situ transmission electron microscopy (TEM) irradiation studies reveal that dislocations and incoherent twin boundaries induced by LSP serve as effective sinks to annihilate irradiation defects. These findings suggest that LSP can improve the mechanical properties and irradiation resistance of ODS austenitic steels in nuclear reactor environments.