非晶态金属的力学性能

一、引言对于非晶态金属的研究最初是集中在它的结构和物理性能上。关于非晶态金属的力学性能的研究是在1970年,当用离心铸造法成功地制造了尺寸均匀的带状试样以后,对Pd80 Si20合金的力学性能和变形、断裂过程进行了研究。相继,由Chen等又发表了用轧辊淬火法制造的同系合金的力学性能的研究。以后,作为非晶态金属的一个重要性能——力学性能引起了人们的注意,特别是数年来的进展是相当惊人的。非晶态金属从其结构上来说,原子排列是混乱的(同晶态金属相比),更确切地说,原子排列不具有长程序,只在局部区域保持短程序。因此,非晶态金属的结构没有各向异性,没有特定的滑移面,更没有在晶态金属中见到的晶界、孪晶、堆垛层错等局部不均匀组织。为此,非晶态金属的力学性能同晶态金属是颇不相同的。下面,我们仅就在文献中见到的有关非晶态金属的力学性能方面的问题作一简要概括。     

失配位错对Cu/Ni多层膜力学性能的影响

通过对Cu／Ni多层膜纳米压痕过程的二维分子动力学模拟,研究了失配位错对多层膜力学性能的影响。结果表明,失配位错网对滑移位错的阻碍作用使Cu／Ni多层膜得以强化,并且这种强化作用依赖于多层膜的调制波长(相邻两膜层的厚度之和)。当调制波长大于临界值λc时,失配位错的应力场随膜层厚度变化不大;当调制波长小于临界值λc时,失配位错的运动导致了界面处的应力集中,从而使多层膜的力学性能下降。为了优化多层膜的力学性能,临界调制波长要大于在单膜层内产生位错的深度。     

片层状TiAl-Nb合金中γ/γ界面体系拉伸行为的原子模拟

全片层组织结构的TiAl基合金在发生塑性变形时，因具有多个可阻碍位错迁移的界面，增加了位错迁移所需要的应变能，从而使变形能力和强度强烈依赖于这种显微组织中的层状界面。本工作采用分子动力学方法研究了单轴拉伸载荷下具有γ/γ界面的TiAl-Nb合金的变形行为。从原子尺度上讨论了真孪晶(True-twin, TT)、旋转界面(Rotational boundary, RB)、伪孪晶(Pseudo-twin, PT)三种不同界面下，片层状TiAl-Nb合金的力学响应、位错演化和断裂机制；阐述了材料力学响应与微观缺陷演化之间的关系，表明含不同界面的TiAl-Nb合金力学性能具有显著的层状边界效应。通过观察位错与界面的交互作用发现位错与界面相遇后，三个界面及附近都会产生无序原子区；而RB/PT试样中无序原子区作为位错源会向另一片层发射位错，TT试样中的无序原子区不会作为位错源向另一片层发射位错。     

CdZnTe晶体缺陷的透射电子显微分析

采用透射电子显微镜对CdZnTe晶体材料的缺陷特性进行了分析。在（111）面的透射电镜明场像中，观察到了棱柱位错环、位错墙、沉淀相、层错及倾斜的孪晶界面。应力是位错形成的主要原因，棱柱位错环的产生是由于沉淀相粒子在基体上产生错配应力；而位错网络与位错墙是两种热应力联合作用于晶体边缘的结果。晶体生长过程中，液固界面生长形态从平界面向胞状界面发展产生的沉淀相衬度不同于由于Te原子溶解度的回退产生的沉淀相衬度。CdZnTe晶体中的堆跺层错和孪晶与固液界面的稳定性相关。     

热处理温度对双连续β-氮化硅增强铝基复合材料性能的影响

通过常压烧结法制备了气孔率为53.43%的多孔氮化硅预制体,并利用挤压铸造法制备双连续β-氮化硅增强铝基复合材料,并研究了热处理温度对复合材料显微组织和力学性能的影响。利用位错增殖强化理论和界面反应理论分析复合材料的显微组织和力学性能,结果表明,随着热处理温度的升高,铝合金基体发生位错增殖,β-氮化硅增强体和铝合金基体的界面反应程度增强,复合材料的显微硬度增高,断裂韧性降低,弯曲强度先增加后降低,复合材料断裂模式从沿晶韧性断裂转变为沿晶脆性断裂。当热处理温度为850℃,复合材料的界面层厚度约为20~50 nm,其综合力学性能达到最佳。     

金属层状复合材料的力学行为及微观变形机理

金属层状复合材料作为一种典型的非均质材料,通过调控其内部的多尺度微结构特性,可以实现金属结构材料强度-韧性的协同提升,在高端先进制造领域具有潜在的应用前景。金属层状复合材料的宏观力学性能显著依赖于各组元层的性能、厚度和异质界面的结构特性。变形过程中材料内部的微观应力/应变在异质界面处的协调特性对组元金属的形变微观机制产生重要影响,进而影响复合材料整体的性能。因此,探索金属层状复合材料的“微观结构-力学行为-变形机制-宏观力学性能”的内在关联并揭示其对应的微观形变机理,对设计具有优异综合力学性能的金属层状复合材料有重要的理论指导意义和实际应用价值。聚焦于晶态金属层状复合材料的微观力学行为及变形机理,介绍了其力学行为的尺寸与界面效应,着重讨论了室温下材料的微观形变物理过程,阐明了非均匀金属层状复合体强韧化的机理。最后,对金属层状复合材料力学行为的研究进行了简要展望。     

化学镀镍－硼合金层组织结构的研究

化学镀Ｎｉ－Ｂ合金层的结构与镀层的硼含量有关。硼含量低的镀层为晶态；硼含量高的镀层为非晶态。在加热时，非晶态合金向晶态转变，晶态和非晶态的Ｎｉ－Ｂ合金都会析出Ｎｉ＿３Ｂ相。     

化学镀镍—硼合金层组织结构的研究

化学镀Ｎｉ－Ｂ合金层的结构与镀层的硼含量有关。硼含量低的镀层为晶态；硼含量高的镀层为非晶态。在加热时，非晶态合金向晶态转变，晶态和非晶态的Ｎｉ－Ｂ合金都会析出Ｎｉ３Ｂ相。     

高压扭转对SiC_P/Al复合材料微观组织和力学性能的影响

为研究大塑性变形对金属基复合材料微观组织和力学性能的影响,利用高压扭转工艺(HPT)在200℃下将纯Al粉末和经氧化处理的SiC粉末混合固结成10wt%SiC_P/Al复合材料。采用TEM观察HPT变形后不同圈数试样的SiC-Al界面及Al基体微观组织,采用EDS能谱仪分析界面处原子扩散现象,采用万能拉伸试验机测试研究不同扭转圈数试样的力学性能。结果表明:不同圈数试样Al基体内出现大量位错、非平衡晶界等晶格缺陷;组织内存在两种SiC-Al界面,含SiO_2层的原始界面和因颗粒破碎而新生成的界面。两种界面结合良好,界面处元素相互扩散;随着扭转圈数的增加,10wt%SiC_P/Al复合材料抗拉强度增加,延伸率得到较大提高。分析发现高压扭转后不同圈数组织内产生的大量晶格缺陷和细小晶粒,促进界面处元素的相互扩散,使界面结合良好,同时大量晶格缺陷和细小晶粒的产生以及结合良好的SiC-Al界面是SiC_P/Al复合材料力学性能大幅提升的主要原因。     

镀镍石墨烯/钛复合材料界面力学行为的分子动力学研究

目的 研究拉伸载荷下镀镍石墨烯/钛复合材料界面的应力-应变行为。方法 采用分子动力学模拟方法,建立了镀镍石墨烯/钛复合材料界面的单晶模型,研究了不同方向拉伸载荷下材料的力学行为。结果 镀镍石墨烯复合材料的力学性能随着镀层厚度的增大而提高,镀层镍可以作为位错源使复合材料的位错密度提高,镀层镍塑性变形的滞后使镀镍石墨烯/钛复合材料具有更高的塑性变形能力。结论 材料的力学性能更多依赖于镀镍层数,随着镀层厚度的增大,镀镍石墨烯/钛复合材料表现出了更高的抗拉强度,但界面处的裂纹和空洞数量也有所增加,材料的延伸率有所下降。     

Recent advances in modeling of interfaces and mechanical behavior of multilayer metallic/ceramic composites

Since the introduction of the term “nanolaminate” in the mid-1990s, considerable research activities on metallic/ceramic nanolaminates (MCN) have been conducted. Incorporating ceramics with high hardness and high melting point together with high ductile metals can improve their thermomechanical behavior in corrosive environments. A great number of researchers have reported that MCNs exhibit outstanding thermomechanical properties compared with the constituent layers and bulk material, which is attributed to the atomic structure and high density of the interfaces. This article provides a review of recent advances in modeling of the mechanical behavior of MCN composites, with focus on Nb/NbC and Ti/TiN multilayer composites. The main strengthening mechanisms of MCNs, based on the layer thickness, the interface structure, and the interaction of threading dislocations with the interface as well as dislocations nucleation from the interface, are reviewed, and recently, obtained results from molecular dynamics simulations, along with these findings, are presented. Moreover, MD-based flow surfaces for use in large-scale continuum models are reviewed in connection with results from MD of MCNs under various mechanical loading conditions, including uniaxial and biaxial loadings.     

Symbiotic crystal-glass alloys via dynamic chemical partitioning

The design of high performance structural materials is always pursuing combinations of excellent yet often mutually exclusive properties such as mechanical strength, ductility and thermal stability. Although crystal-glass composite alloys provide better ductility compared to fully amorphous alloys, their thermal stability is poor, due to heterogeneous nucleation at the crystal-glass interface. Here we present a new strategy to develop thermally stable, ultrastrong and deformable crystal-glass nanocomposites through a thermodynamically guided alloy design approach, which mimics the mutual stabilization principle known from symbiotic ecosystems. We realized this in form of a model Cr-Co-Ni (crystalline)/Ti-Zr-Nb-Hf-Cr-Co-Ni (amorphous) laminate composite alloy. The symbiotic alloy has an ultrahigh compressive yield strength of 3.6 GPa and large homogeneous deformation of ∼15% strain at ambient temperature, values which surpass those of conventional metallic glasses and nanolaminate alloys. Furthermore, the alloy exhibits ∼200 K higher crystallization temperature (T_X > 973 K) compared to that of the original TiZrNbHf-based amorphous phase. The elemental partitioning among adjacent amorphous and crystalline phases leads to their mutual thermodynamic and mechanical stabilization, opening up a new symbiotic approach for stable, strong and ductile materials.     

含界面相三维全五向编织复合材料拉伸行为模拟及失效机制研究

基于三维全五向（Q5D）编织复合材料的细观结构模型,通过引入界面相单元,建立了含界面相Q5D编织复合材料单轴拉伸损伤失效分析模型。应用Python语言实现对ABAQUS的二次开发,将Linde等提出的失效准则和Von-Mises应力准则分别用于纱线和基体的渐进损伤判断,并确定材料的整体失效模式;对于界面相,采用Quads准则进行损伤判断。利用周期性位移边界条件,对含界面相Q5D编织复合材料的纵向拉伸应力-应变行为进行了渐进损伤数值模拟,详细讨论了在纵向拉伸载荷作用下材料的细观损伤起始、扩展和最终失效的演化过程,分析了材料的细观损伤失效机制,预测了材料的极限破坏强度,并研究了界面相性能对材料整体力学行为的影响规律。研究结果表明,数值模拟结果与实验值吻合较好,验证了渐进损伤模型的有效性,为该类材料的力学分析和优化设计奠定了基础。     

Micro-scale modeling of interface-dominated mechanical behavior

A micro-scale interface dislocation dynamics approach to model the mechanical behavior of crystalline nanolaminates is presented. To circumvent the exhaustive atomistic modeling of interfaces and dislocations in nanolaminates, an atomistically informed dislocation dynamics model was developed in which interfaces are categorized using a geometrical interface classification scheme and the interface-dominated mechanical response is related to nucleation, glide, and reactions of lattice and interface dislocations at/within/across interfaces. We show that such a scheme is effective in mapping the structure–property relations of various types of interfaces.     

Dislocations in single hemp fibres—investigations into the relationship of structural distortions and tensile properties at the cell wall level

The relationship between dislocations and mechanical properties of single hemp fibres (Cannabis sativa L. var. Felina) was studied using a microtensile testing setup in a 2-fold approach. In a first investigation the percentage of dislocations was quantified using polarized light microscopy (PLM) prior to microtensile testing of the fibres. In a second approach PLM was used to monitor the dislocations while straining single fibres. The first part of the study comprised 53 hemp fibres with up to 20% of their cell wall consisting of dislocations. For this data set the percentage of dislocations did not affect the mechanical properties. In the second part of the study it was found that dislocations disappeared during tensile testing, and that they did not reappear until several weeks after failure. A strain stiffening effect due to the straightening of the dislocations was not observed. It is possible that the former positions of the dislocations functioned as locations for crack initiation. However, the crack does not propagate transversely all the way trough the dislocation but results in a shear failure between the microfibrils. In rheological studies fibres were strained at constant stress levels, and dislocations that had disappeared did not reappear during that period.     

The scratch behaviors of copper bi-layers by a diamond tip: a molecular statics study

The scratch deformation behaviors of two bicrystal coppers (Cu(100)/Cu(110) and Cu(110)/Cu(100)) during the nanoscratching process were explored and compared with their single crystal ingredients by the molecular statics simulations. The effects of lattice configuration and scratch depth were investigated in this study. The results showed that the motion of dislocations was blocked in the bicrystal interface until the dislocations accumulated enough energy to move. From the study, it was found that the bicrystal interfaces can provide resistance to the motion of dislocations, and can strengthen the mechanical properties of copper materials.     

Microstructure and properties of zirconia-alumina nanolaminate sol-gel coatings

Zirconia-alumina multilayer nanolaminate coatings were applied on stainless steel 316 substrates by a sol-gel dipping method. The coatings were characterised using X-ray diffraction, optical and scanning electron microscopy. The hardness and elastic modulus, the wear resistance and the oxidation resistance of the coatings were measured and assessed. It was observed that the coatings possessed fine grains, fine pores and high retention of tetragonal zirconia phase. The coatings exhibited high hardness and elastic modulus as well as good resistance to oxidation at high temperatures. However, these properties may be influenced by the interactions at the coating/substrate interface.     

RSMA增容PA6/PP共混物的形态结构与增容机理

采用ＲＳＭＡ为增容剂制备了ＰＡ６／ＰＰ共混物,研究了ＲＳＭＡ增容ＰＡ６／ＰＰ共混物的形态结构和热行为以及晶态结构,并探讨ＲＳＭＡ增容ＰＡ６／ＰＰ共混物的增容机理结果表明,ＰＡ６／ＰＰ共混物为热力不相容的海岛型两相结构,ＲＳＭＡ的加入改善ＢＰＡ６与ＰＰ相间的相容性,使两相分均匀,分散度提高。ＲＳＭＡ对ＰＡ／ＰＰ共混物的增容机理可用界面－分散相复合模型描述。     

Crack formation in sapphire/niobium/sapphire joints under compression

Compression tests were carried out on UHV diffusion bonded single crystalline sapphire/Nb/sapphire joints to investigate their mechanical properties, the mechanisms that lead to the failure of the joints and the dislocation-interface interaction. The tests were performed for different orientation relationships (OR) at the interface to study the influence of different bond strength on the mechanical behavior. Additionally, the metal layer thickness was varied for each OR to alter the influence of the interface. The experimental results showed, that with decreasing metal layer thickness the stress needed to form a crack increases drastically, whereas for the Nb/sapphire system the bond strength at the interface seems to have no significant influence on the mechanical behavior of the joint. A theoretical model will be presented that explains the experimentally observed relationship between metal layer thickness and crack stress.     

Tailoring the crystal structure of individual silicon nanowires by polarized laser annealing

We study the effect of polarized laser annealing on the crystalline structure of individual crystalline-amorphous core-shell silicon nanowires (NWs) using Raman spectroscopy. The crystalline fraction of the annealed spot increases dramatically from 0 to 0.93 with increasing incident laser power. We observe Raman lineshape narrowing and frequency hardening upon laser annealing due to the growth of the crystalline core, which is confirmed by high resolution transmission electron microscopy (HRTEM). The anti-Stokes:Stokes Raman intensity ratio is used to determine the local heating temperature caused by the intense focused laser, which exhibits a strong polarization dependence in Si NWs. The most efficient annealing occurs when the laser polarization is aligned along the axis of the NWs, which results in an amorphous-crystalline interface less than 0.5 μm in length. This paper demonstrates a new approach to control the crystal structure of NWs on the sub-micron length scale.