微纳米尺度金属导电材料热疲劳研究进展

世界已逐渐进入以物联网和智能制造为主导的工业4.0时代,特别是人工智能和大数据处理的强烈需求,微纳米尺度器件的研发制造及广泛使用的日趋活跃使得小尺度材料得到广泛关注。由于这些材料的几何尺度和微观结构尺度的约束效应,其热疲劳损伤行为与块体材料不同。同时,材料尺度由微米向纳米量级的转变也会引起损伤机制的转变,使材料表现出不同的损伤形式,产生显著的尺寸效应。本文综述了近年来国内外开展的有关金属薄膜/线的热疲劳实验方法、热疲劳损伤行为及演化和热疲劳影响因素的研究进展,探讨了微纳米尺度金属材料热疲劳的微观机制和尺寸效应,并对这一领域的研究前景进行展望。     

西安交通大学学者发现金属孪晶变形的强烈晶体尺寸效应——孙军教授等完成的论文在《Nature》上发表

西安交通大学金属材料强度国家重点实验室微纳尺度材料行为研究中心研究生余倩,在导师孙军教授、肖林教授、马恩教授和单智伟教授的悉心指导下,与美国宾夕法尼亚大学李巨教授、丹麦瑞瑟国家实验室黄晓旭博士合作,对微小尺度金属单晶材料中的孪晶变形行为及其对材料力学性能的影响进行了深入的研究,发现了单晶体外观尺寸对其孪晶变形行为的强烈影响,以及相应材料力学性能的显著变化。     

亚微米厚铜薄膜的微观结构及疲劳损伤行为

在具有高弹性和力学稳定性的柔性基底上,用磁控溅射系统制备了亚微米厚铜薄膜,利用透射电镜（TEM）、扫描电镜（SEM）电子背散射成像及X射线衍射（XRD）对铜薄膜进行了微观结构表征．采用恒载荷幅控制研究了亚微米厚度铜薄膜的疲劳损伤行为．结果表明：退火后的铜薄膜呈现强烈的（111）织构,薄膜中存在大量的微米、纳米尺度孪晶．在恒载荷幅作用下,亚微米厚的薄膜不易产生疲劳挤出和微裂纹,疲劳裂纹容易在界面处萌生,孪晶附近的位错塞积及界面附近变形的不协调性导致了疲劳裂纹的产生．而亚微米厚铜薄膜疲劳强度的提高来源于薄膜厚度、晶粒尺寸和孪晶尺寸三个微尺度的约束．     

微米尺寸不锈钢的形变与疲劳行为的尺寸效应

采用聚焦离子束溅射蚀刻加工了微米尺寸304不锈钢悬臂梁试样．利用静态及动态弯曲加载研究了微米尺寸材料的形变与疲劳开裂行为．结果表明：随薄膜厚度的减小,材料的屈服强度升高,塑性下降．屈服强度随悬壁梁厚度的变化关系与Hall—Perch晶粒强化关系相似．微小悬壁梁屈服强度的升高来源于小尺度材料在非均匀变形下引起的应变梯度贡献的增加;而塑性下降则归因于较薄薄膜的晶粒内较少的可动位错．疲劳裂纹从尖缺口处萌生的门槛值接近块体材料．     

表面纳米化技术制备梯度纳米结构金属材料的研究进展

多数工程结构材料的失效都是从表面的薄弱环节开始发生或者传导,从而引起材料的性能下降,使用寿命缩短.受生物材料的梯度结构启发,近年来开发了多种表面纳米化技术,成功在工程材料表面制备了晶粒尺寸从表层纳米尺度连续变化到内部宏观尺度的梯度纳米结构,强化和保护了材料表面,有效地解决了上述问题.结合国内外表面纳米化的研究结果,综述了金属材料梯度纳米材料的研究进展.首先,介绍了梯度塑性变形、物理化学沉积等表面纳米化加工技术的最新进展.其次,对梯度等轴纳米晶、梯度纳米层片和梯度纳米孪晶等多种表面纳米化材料的微观结构进行了归纳,并对最新发展的梯度纳米结构材料表层晶粒的晶体学取向等微观信息表征方法进行了系统地阐述.随后,总结了梯度纳米结构对工程材料的表面强度、塑性、强-塑匹配、加工硬化、疲劳、耐磨、腐蚀和热稳定性等性能的影响.最后展望了表面纳米化技术制备梯度纳米结构金属材料的发展趋势及工程应用所面临的挑战.     

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未发生明显改变. 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.     

高性能铜系层状金属材料设计:纳米尺度下强化能力与韧化能力思考

对近年来国内外在纳米层状金属材料的强化能力及其尺度与界面效应、塑性形变行为及稳定性等基本规律的研究进展进行了系统的总结,并对纳米层状材料的强化能力与韧化能力进行了深入的探讨.最后,对纳米层状金属材料的强化与韧化能力中的关键科学问题及未来研究进行了展望.     

纳米金属材料的界面力学行为研究

将常规多晶材料的粗晶粒尺寸缩小到纳米尺度时,这些纳米晶体材料会呈现出与其对应的粗晶材料迥异的物理现象.与材料力学行为最相关的是强度及塑形变形机理这两个方面.考虑到晶界的变形与破坏可能是纳米晶体材料低塑性的根源,克服纳米晶体材料中强度与韧性之间存在的"熊掌和鱼不可兼得"的问题,也通常称为晶界工程.在众多的晶界中,孪晶界面被发现可同时保持材料的强度和韧性.本文主要就纳米金属材料中界面的力学行为做一个简要综述,包含晶界的强化力学机理以及新型孪晶界面的力学行为与揭示内在尺度效应的模型研究.     

纳米压痕技术在材料力学测试中的应用

近年来,材料纳米级力学测试日益引起广大研究者的重视。纳米压痕仪凭借极高的载荷和位移分辨率,广泛应用于材料表面的微纳米级力学性能的测试,包括硬度、弹性模量、塑性应变、薄膜界面结合强度以及材料疲劳特性等。综述了几种纳米压痕和纳米冲击技术测试材料力学性能的方法和原理,介绍了纳米压痕技术在材料力学性能测试方面的若干先进应用实例及其测试机理,以及原子力显微镜和扫描探针显微镜在力学测试方面的原理和应用。最后,提出了纳米压痕仪存在的若干问题,并对纳米压痕技术的发展进行了展望,认为纳米压痕技术结合有限元模拟建立材料疲劳断裂模型,是纳米压痕在力学测试方面发展的必然趋势。     

45碳钢低周疲劳与应力循环棘轮失效的实验研究

对调质处理的45碳素结构钢进行了应变循环低周疲劳实验以及应力控制棘轮失效实验．对于前者进行了带平均应变和不带平均应变的实验,以研究平均应变对低周疲劳特性的影响;对于后者研究了平均应力和应力幅值对棘轮失效的影响．应变循环实验表明：平均应变对循环饱和行为以及低周疲劳循环失效圈数并没有明显的影响,但对材料的循环初期循环塑性行为有影响．棘轮失效实验结果表明：当应力幅值较大而平均应力较小时,材料的棘轮失效主要归于较大塑性应变幅值引起的低周疲劳破坏;当平均应力较大而应力幅值相对较小时,材料的棘轮失效主要归于较大的棘轮应变引起的材料韧性破坏,其失效准则可以用最大单调极限应变来表征．     

Small fatigue crack growth in metallic materials: A model and its application to engineering alloys

Characterization of the growth behavior of small fatigue cracks is important for materials used in structurally demanding applications such as aircraft turbine discs and some automotive engine components. Here, we present a general, dislocation-based fracture mechanics approach to predict the growth rate of small fatigue cracks in metallic materials. The applicability of the model to the small fatigue crack growth behavior of four engineering alloys was examined. Small fatigue cracks were initiated and propagated, in a controlled manner, from micronotches fabricated by femtosecond pulsed laser micromachining. The results suggest that a methodology consisting of crack-tip damage accumulation and fracture provides a common framework to estimate the fatigue crack propagation lifetime of structural materials.     

Comparison of the Fatigue Performance of Commercially Produced Nitinol Samples versus Sputter-Deposited Nitinol

Self-expanding vascular implants are typically manufactured from Nitinol tubing, using laser cutting, shape setting, and electropolishing processes. The mechanical and fatigue behavior of those devices are affected by the raw material and its processing such as the melting process and subsequent warm and cold forming processes. Current trends focus on the use of raw material with fewer inclusions to improve the fatigue performance. Further device miniaturization and higher fatigue life requirements will drive the need toward smaller inclusions and new manufacturing methods. As published previously, the high-cycle fatigue region of medical devices from standard processed Nitinol is usually about 0.4-0.5% half-alternating strain. However, these results highly depend on the ingot and semi-finished materials, the applied manufacturing processes, the final dimensions of test samples, and applied test methods. Fabrication by sputter deposition is favorable, because it allows the manufacturing of micro-patterned Nitinol thin-film devices without small burrs, heat-affected zones, microcracks, or any contamination with carbides, as well as the fabrication of complex components e.g., 3D geometries. Today, however, there is limited data available on the fatigue behavior for real stent devices based on such sputter-deposited Nitinol. A detailed study (e.g., using metallographic methods, corrosion, tensile, and fatigue testing) was conducted for the first time in order to characterize the micro-patterned Nitinol thin-film material.     

合金材料超高周疲劳行为的基本特征和影响因素

合金材料在超高周疲劳下具有与低周和高周疲劳不同的裂纹萌生和扩展行为以及不同的 S--N曲线特征. 材料的强度、循环加载的频率、所处的环境等都显著影响超高周疲劳的特性. 本文综述了合金材料超高周疲劳行为的基本特征和影响因素的研究进展.     

Effects of continuous cast section size on torsion deformation and fatigue of induction hardened 1050 steel shafts

This study investigates the influence of continuous cast section size on the mechanical performance of induction hardened parts produced from steel bars. SAE 1050 steel from commercially produced Jumbo Blooms, Blooms, Rotary Round, and Billet were hot rolled into round bars with diameters of 37-44 mm. These bars were then normalized, machined into test specimens, the gauge sections were polished, and the specimens were case-hardened by induction hardening. Torsional monotonic and fully reversed cyclic fatigue tests were conducted to study the effect of the initial continuous cast section size on deformation and fatigue behaviors. Reduction ratios in this study ranged from a low of 20.4:1 for the Billet, up to a high of 142:1 for the Jumbo Bloom. Test results indicate that the continuous cast section size has only small effects on the torsion monotonic and cyclic deformation properties and negligible effect on the torsion fatigue performance. Small differences observed in deformation and fatigue properties between the four processes are attributed mostly to the variation in case and core hardness levels caused by small differences in chemistry, particularly carbon content. Variations in sulfur content also influence ductility and fatigue behavior. At high strains, the cracks initiated in shear as longitudinal cracks for all four materials. At low strains, the cracks initiated at the surface in tension as spiral cracks due to normal tensile stresses.     

Fatigue cracking at twin boundaries: Effects of crystallographic orientation and stacking fault energy

The combined effects of crystallographic orientation and stacking fault energy (SFE) on the cracking behaviors of twin boundaries (TB) under low-cycle fatigue (LCF) tests were studied in pure Cu, Cu–Al and Cu–Zn alloys. A new approach, called the slipping morphology method, based on the crystallographic characteristics of Σ3 TB in face-centered cubic materials, was developed to determine the grain orientations by studying the twin-slip morphology characteristics on the sample surfaces after LCF tests. Through analyzing the dislocation–TB interaction and the damage this causes to TBs, a new parameter, defined as the difference of Schmid factors (DSF), was proposed to describe the effects of crystallographic orientation on the LCF cracking behaviors of TBs. A semi-quantitative relationship was established among DSF, SFE, dislocation slip mode and the critical conditions of TB cracking by systematically studying more than a hundred post-fatigue surface morphologies of pure Cu, Cu–Al and Cu–Zn alloys. It is interesting to find that the TB cracking relies strongly on the cooperation of both DSF and SFE. Furthermore, taking into account the interactions between slip dislocations and different boundaries, the fatigue cracking possibilities of several typical interfaces were compared and discussed. The results demonstrate that low-angle grain boundaries (GBs) are the strongest in resisting fatigue cracking, high-angle GBs are the weakest, and TBs are in between, which contributes the most to the final fatigue performance of materials. This new finding will help understanding of the interfacial properties under cyclic loading and may be beneficial to the design of high-performance materials with optimal fatigue properties in the future.     

Fatigue and fracture toughness of epoxy nanocomposites

The fatigue and failure mechanisms of epoxy composites have been researched extensively because of their commercial importance in fields demanding materials with high specific strength. Particulate, sheets, short and long fibers with dimensions in the micrometer and nanometer range are the major fillers which have been studied for enhancing the fatigue resistance of epoxies. The nano and micro scale dimensions of the fillers give rise to unexpected and fascinating mechanical properties, often superior to the matrix including fracture toughness and fatigue crack propagation resistance. Such properties are dependent on each other (e.g., the fatigue properties of the polymer composites have been found to be strongly influenced by its toughness). This article is a review of the various developments in this field and the underlying mechanisms which are responsible for performance improvements in such composites.     

Effect of backbone polymer on properties of 316L stainless steel MIM compact

This study compares the effects of three backbone polymers, LDPE, HDPE and LDPE/HDPE, on the dimensions and mechanical properties of 316L stainless steel MIM compact. MIM parts of optimal quality can be produced using properly formulated binders. A spiral flow test is performed firstly to elucidate the flow behaviors of MIM feedstocks. Secondly, the injection molding of tensile bars is tested to examine the dimensional stability and the mechanical strength of sintered parts against variation in the binder formula. Among the three backbone polymers considered herein, HDPE performs best in terms of both the stability of flow and the MIM compact quality; LDPE performs the worst. HDPE has significantly better length, width, density, and hardness by up to 24%, 27%, 30%, and 64%, respectively. In summary, this work has demonstrated that a backbone polymer strongly affects the dimensions and the mechanical properties of the sintered part. The proper selection of a backbone polymer, such as HDPE, is required to increase the dimensional accuracy and quality of 316L sintered parts.     

Evaluation of Tensile and Fatigue Properties of Metals Using Small Specimens

Small specimens are increasingly being used in getting mechanical properties directly when there are limited materials to facilitate standard specimens, which play a great role in the rapid measurement of mechanical properties and residual life assessment of in-service reactor components. Although tensile and fatigue properties of the small specimens are investigated extensively, theoretical models for describing the mechanical properties of small specimens need to be established. Here, we conduct a systematic investigation of tensile and fatigue properties of pure Cu specimens with thicknesses ranging from 3 to 0.2 mm. The results show that the decrease in uniform elongation of the 0.2 mm-thick specimens is mainly due to the effects of grain boundary and free surface on the strain hardening rate. A modified theoretical model correlated with the ratio of the surface grain layer thickness to the grain size is proposed to predict variation in yield strength of the small specimens more accurately. Furthermore, the mechanism for the difference in fatigue life between the 0.2 mm-thick specimen and other thicker specimens is elucidated. The Basquin equation-based model is presented as a potential way to evaluate the fatigue life of metals using small specimens.     

Effects of γ-Ray Irradiation on the Fatigue Strength,Thermal Conductivities and Thermal Stabilities of the Glass Fibres/Epoxy Resins Composites

Glass fibres/epoxy resins composites have been performed as ideal materials to make support instruments for high-energy and nuclear physics experiments. The effects of the γ-ray irradiation on the fatigue strength, thermal conductivities and thermal stabilities of the glass fibres/epoxy resins composites were investigated. And a two-parameter fatigue life model was established to predict the fatigue life of the composites. Results revealed that the γ-ray irradiation could probably result in the degradation of epoxy resins, but hardly damage to the glass fibres. And the γ-ray irradiation treatment could significantly affect the fatigue strength of the composites at a low-cycle fatigue stage, but seldom influence at a high-cycle fatigue stage. Furthermore, the fabricated glass fibres/epoxy resins composites after the γ-ray irradiation still presented excellent fatigue strength, ideal thermal conductivities, remarkable dimensional and thermal stabilities, which can meet the actual requirements of normal operation for supporting instruments under high-energy and nuclear physics experiments.