AZ31镁合金在高周疲劳过程中微观组织和孪晶演变特征

利用扫描电子显微镜（SEM）和电子背散射衍射（EBSD）技术研究了室温条件下AZ31镁合金在不同加载频率（3和30 Hz）和不同应力幅值（90,95,100,105,110 MPa）疲劳变形后的组织演变规律及断口形貌特征。结果表明：随着加载应力增加,基体内残余孪晶数量增加,残余孪晶主要以拉伸孪晶形式存在。随着应力幅值的增加晶粒逐渐细化,这是由于在循环过程中,拉伸孪晶演变诱导晶粒细化。随着应力幅值的增加,织构强度显著减弱,这与试样疲劳后的再结晶机制有关。通过对试样疲劳断口的分析,发现孪晶片层处容易引起裂纹萌生,随着应力的增加,试样中裂纹扩展区面积逐渐减小,在疲劳裂纹扩展区观察到明显的疲劳辉纹。最终断裂区表面粗糙,主要存在韧窝、撕裂脊以及二次裂纹等形貌。在最终断裂区可观察到韧窝,韧窝尺寸随着循环应力的增加,在较高加载频率下,韧窝的尺寸与数量均减小。     

小尺度材料的疲劳研究进展

小尺度材料广泛应用于微电子机械系统及大规模集成电路等微／纳米系统中．由于这些材料的几何尺度和微观结构尺度均在微米至纳米范围,它们对位错行为的约束作用以及表面和界面的影响导致了其疲劳行为与块体材料不同．本文就近年来国内外开展的有关小尺度材料(如薄膜材料)疲劳实验方法、循环形变行为、疲劳裂纹的萌生以及扩展行为进行了综述,对相关的疲劳尺寸效应及机理进行了探讨,并对今后这一领域的研究前景及方向进行了展望．     

30CrMnSiNi2A高强钢的疲劳小裂纹扩展特性及寿命预测

研究了３０ＣｒＭｎＳｉＮｉ２Ａ高强钢在恒幅载荷下的小裂纹起始特征和裂纹扩展特性。试验结果表明，小裂纹起始于缺口表面初始缺陷（如夹杂或孔洞）处。在应力比Ｒ＝０的恒幅载荷下，未显示小裂纹效应：在Ｒ＝－１的恒幅载荷下，则显示出小裂纹效应的存在。利用裂纹闭合模型和微观结构缺陷作为初始裂纹尺寸的方法预测了小裂纹扩展速率和疲劳全寿命，预测值与实验值符合良好。     

孪晶铜中孪晶尺寸对疲劳位错组态的影响

对孪晶铜进行塑性应变幅控制下的疲劳实验,研究了不同宽度的孪晶内疲劳位错组态及演化过程．结果表明,孪晶宽度不同时,孪晶内的疲劳位错组态不同．当孪晶宽度大于1μm时,孪晶内疲劳位错组态与单晶中情况类似;孪晶宽度介于1μm到200nm之间时,位错形成类PSBs结构;孪晶宽度介于200nm到20nm之间时,孪晶内只能形成一些位错碎片;孪晶宽度小于20nm时,孪晶内没有稳定的晶格位错存在．     

孪晶片层厚度对纳米孪晶Cu疲劳性能的影响

通过恒应力幅控制拉--拉疲劳实验, 比较了脉冲电解沉积制备的不同孪晶片层厚度纯Cu样品的疲劳寿命和疲劳耐久极限. 结果表明: 在应力疲劳下, 样品的疲劳寿命与疲劳耐久极限均随孪晶片层厚度的减小而提高. 疲劳样品的宏观表面变形形貌(SEM观察)和微观结构(TEM观察)表明:
当平均孪晶片层厚度为85 nm时, 材料的塑性形变由位错滑移和剪切带共同承担, 进而疲劳裂纹沿剪切带萌生; 而当平均孪晶片层厚度为32 nm时, 材料的塑性形变由位错--孪晶界交互作用主导, 从而导致疲劳裂纹沿孪晶界形成.     

TWIP钢的低周疲劳断裂机制

通过采用扫描电镜及透射电镜等手段,观察并研究了TWIP钢在低周单轴循环对称拉压载荷下的疲劳断裂后的显微组织。结果表明:TWIP钢矩形试样的疲劳裂纹一般萌生于角部,从表面萌生时可能表现为多个疲劳源。在低周疲劳变形过程中,TWIP钢不但产生了形变孪晶,还产生了大量的微条带,其实质为细微孪晶片层和驻留滑移带。疲劳裂纹主要萌生于微条带对晶界和孪晶界的撞击引发的孔洞。孔洞串连接起来成为裂纹,夹杂物促进了裂纹扩展。随着裂纹的扩展,试样的承载面积不断减小,最终发生快速的韧性断裂。     

亚微米晶铜中孪晶对位错储存能力的影响

利用电解沉积技术制备出系列亚微米晶纯铜样品,并在样品中引入不同密度的纳米孪晶．室温轧制具有不同孪晶密度的纯铜样品,使样品中储存大量位错．结果表明,具有高密度纳米孪晶结构的纯铜样品,40％轧制变形可使其屈服强度从690MPa增加到850MPa;而无孪晶的亚微米铜样品的屈服强度在同样轧制变形条件下只从230MPa增加到330MPa．这表明具有高密度纳米孪晶结构的纯铜样品具有很高的位错储存能力．     

表面机械研磨诱导AISl 304不锈钢表层纳米化I.组织与性能

采用表面机械研磨处理(SMAT)在AISl 304不锈钢上制备出纳米结构表层，研究纳米化行为及其对硬度的影响．结果表明：经过SMAT后，样品表面形成了厚度约为30μm的纳米晶层，显微组织由平均晶粒尺寸约为10nm的单一马氏体相演变为尺寸稍大的双相组织，在距表面30—300μm的范围内，显微组织由以亚微米级的奥氏体多系孪晶为主逐渐演变为单系孪晶．表面纳米化是晶粒碎化与纳米尺度新相形成共同作用的结果．与心部相比，表面硬度显著提高．     

PST双相TiAl类单晶中的微孪晶变形

利用透射电子显微镜研究了ＰＳＴ双相ＴｉＡｌ类单晶中的微孪晶变形行为，研究结果表明，孪晶面为（１１１），是由（α／６）Ｓｈｏｃｋｌｅｙ不全位错在（１１１）面上运动产生的，当该位错遇到到互在孪晶关系的两片ＴｉＡｌ界面时，将在另一片ＴｉＡｌ相中诱发出新的微孪晶，其孪晶系统不变，这说明互成孪晶关系的ＴｉＡｌ相界面对微孪晶变形阻碍作用不大，本文讨论了微孪晶变形对材料变形的影响。     

孪晶对多晶铜疲劳行为的影响

对含有高密度孪晶的多晶铜进行了塑性应变幅控制下的疲劳实验.结果表明,塑性应变幅小于8.14×10-4时,孪晶对 疲劳行为的影响不大;塑性应变幅大于8.14×10-4时,孪晶的约束作用、孪晶界与位错的反应及孪晶中位错的特殊组态,使多晶 铜的循环饱和应力提高,硬化曲线中应力饱和平台区延长.     

应力幅对LY12CZ铝合金腐蚀疲劳应变电流响应的影响

研究了ＬＹ１２ＣＺ铝合金在应力控制下于３．５％ＮａＣｌ溶液中恒定电位下,应力幅对腐蚀疲劳（ＣＦ）应变电流行为的影响以及应变电流与腐蚀疲劳损伤相对应的关系,结果表明：应变电流能反映ＣＦ不同阶段的损伤行为,应变电流的波型随ＣＦ过程所发生的变化主要在应变电流的波幅、波峰数及其与应力波的相位差上,不同应力幅下的应变电流行为的差异主要表现在：高应力幅下,在一个循环周次内,瞬变电流在整个腐蚀疲劳过程中均出现两     

Improving the fatigue endurance of lead zirconate titanate thin films through PbO interfacial modification

The effects of the modification of electrode/ceramic interfaces through a chemical solution deposition-derived PbO buffer layer on the fatigue endurance of lead zirconate titanate(PZT) thin films were investigated.The grain size and the surface roughness of the PZT films increased through PbO interfacial modification.Moreover,the PZT films with PbO interfacial modification had a better crystallographic structure and no evident secondary phases were observed.While the remanent polarization and dielectric constant were reduced,the fatigue endurance was improved.Based on the results,the mechanism for the fatigue endurance improvement was discussed.     

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.     

冷变形多晶Cu的疲劳软化行为及其门槛

对预变形多晶Cu,在不同载荷下进行拉压对称的恒应力循环试验,比较了不同预变形对其饱和行为及位错组态的影响。结果表明,低应力幅下循环饱和行为仍与历史相关,疲劳后的位错组态具有“继承”性。计算不同载荷下的软化量并外推出疲劳软化门槛,约为流变应力的0.43—0.45左右,其值与低碳钢的几乎相同。本文还描述了循环过程的三个阶段,并对其机理进行了探讨。     

Tribological properties of nanoporous anodic aluminum oxide film

Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28 nm, 45 nm, 95 nm, and 200 nm diameter and 60-100 μm thick) were synthesized by anodization. Reciprocating wear tests using 1 mm diameter steel balls as counterpart were carried out for a wide range of load (from 1 mN to 1 N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1 N and 1 N) whereas it was negligible at low loads (1 mN and 10 mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size.     

金属材料疲劳损伤的界面效应

总结了不同金属材料在低周疲劳过程中典型的晶界、孪晶界、相界和微电子互连界面的损伤开裂行为. 纯Cu中疲劳裂纹萌生的难易顺序为: 小角度晶界、驻留滑移带和大角度晶界. 对于纯Cu与铜合金中退火孪晶界, 是否萌生疲劳裂纹与合金成分有关, 随合金元素的加入降低了层错能, 退火孪晶界相对容易萌生疲劳裂纹. 对于Cu--Ag二元合金, 由于存在不同的晶界和相界面, 是否萌生疲劳裂纹取决于界面两侧晶体的取向差, 通常两侧取向差大的界面容易萌生疲劳裂纹. 在微电子互连界面中, 疲劳裂纹萌生位置与焊料成分和时效时间有关,对于Sn--Ag/Cu互连界面, 疲劳裂纹通常沿焊料与界面化合物结合处萌生; 对于Sn--Bi/Cu互连界面, 随时效时间增加会出现明显的由于Bi元素偏聚造成的界面脆性.     

FATIGUE SOFTENING BEHAVIOR AND ITS THRESHOLD OF COLD STRAINED POLYCRYSTALLINE Cu

The symmetrical push-pull fatigue tests in load control were carried out for prestrained polycrystalline Cu.The change in mechanical behaviour during cycling was recorded in detail and the effect of various degree of prestraining on saturated behavior and dislocation structure was compared.The results show that the cyclic saturation state is still history-dependent at low stress amplitude and the dislocation structure after fatigue exhibits an “inheritance” be- havior.The level of softening was calculated at various load amplitudes and a softening threshold of about 0.43 to 0.45 of the flow stress was obtained.This is the same as that of low carbon steel.Three stages of cyclic processes and their mechanism have been described.     

Twin stability in highly nanotwinned Cu under compression,torsion and tension

Twin stability under four distinct mechanical loading states has been investigated for highly nanotwinned Cu containing parallel nanotwins ～40 nm thick. Observed deformation-induced microstructural changes under tension, compression, tension–tension fatigue and torsion are qualitatively compared in order to assess twin stability as a function of the loading direction and stress. It is observed that the twins are very stable although small microstructural changes vary with deformation mode. Shear bands, deformation-induced grain growth and detwinning are also discussed.     

Fatigue strength of tungsten inert gas-repaired weld joints in airplane critical structures

In this work the effect of Gas Tungsten Arc Welding (GTAW) repairs on the axial fatigue strength of an AISI 4130 steel welded joint used in airframe critical to the flight-safety was investigated. Fatigue tests were performed at room temperature on 0.89 mm thick hot-rolled plates with constant amplitude and load ratio of R = 0.1, at 20 Hz frequency. Monotonic tensile tests, optical metallography and microhardness, residual stress and weld geometric factors measurements were also performed. The fatigue strength decreased with the number of GTAW repairs, and was related to microstructural and microhardness changes, as well as residual stress field and weld profile geometry factors, which gave origin to high stress concentration at the weld toe.