AZ61镁合金的动态力学性能与显微分析

利用SHPB(分离式霍布金森压杆)技术对AZ61镁合金进行了动态压缩实验,并利用金相和扫描电镜对冲击后的试样进行了显微分析.讨论了该合金在常温下的动态断裂和塑性变形机制.结果表明:AZ61镁合会的动态压缩应力-应变曲线表现出较强的应变硬化特性,其屈服强度随应变速率增大而升高,断口呈现大量解理台阶和少量韧窝并存的混合形貌,塑性变形方式为滑移和孪生共存.     

轧制AZ31镁合金板材(4mm)动态压缩性能与失效行为

为了研究轧制AZ31镁合金板材(4mm)在高应变速率下的动态力学性能和失效行为,采用分离式霍普金森压杆装置(SHPB)在室温下应变速率为500～2600s-1范围内对其进行了动态压缩实验,并利用金相显微镜(OM)和扫描电镜(SM)对冲击后的试样进行了显微分析.探讨了轧制AZ31镁合金板材沿轧制方向(RD)、横向(TD)和法向(ND)的动态压缩性能和失效行为.结果表明:轧制AZ31镁合金4mm板材动态压缩性能存在各向异性.沿RD和TD方向压缩的动态性能相同,沿ND方向压缩的动态断裂强度最大.AZ31镁合金4mm板材的动态压缩断裂机制为解理断裂.变形机制为沿RD和TD方向高速压缩时,{101-2}<112-0>拉伸孪晶参与变形;沿ND方向高速压缩时,{101-1}<112-0>压缩孪晶参与变形.     

ZK60稀土镁合金高温塑性变形行为研究

为了研究镁合金高温塑性变形行为,采用Gleeble-1500型热/力压缩模拟机对ZK60-RE稀土镁合金在423~673 K及0.002~0.1 s-1应变速率进行不同变形程度的高温压缩模拟试验,分析了实验合金在高温压缩变形时流变应力、应变速率以及变形温度之间的关系,推导并计算了不同应变速率和不同温度下的变形激活能,并观察了不同变形程度的显微组织.结果表明:试验合金在一定变形速度下,较低的温度压缩时以加工硬化为主,较高的温度下以动态再结晶为主.峰值应力随变形速度的降低和温度的升高而下降.合金的变形激活能在523~623 K内迅速上升.     

镁合金动态力学性能的研究现状及发展方向

概述了镁合金高应变加载条件下的塑性变形微观机制和损伤形式.综述了国内外镁合金动态力学性能的研究现状.针对镁合金动态力学性能的不足之处,指出未来镁合金动态力学性能研究的发展方向为:系统研究现有牌号镁合金的动态力学性能;探讨合金元素、加工工艺和热处理对镁合金动态力学性能的影响;构建镁合金动态本构方程.     

高应变率下AZ31镁合金焊接接头动态力学性能

采用分离式Hopkinson压杆在应变速率为900~2500s-1范围内对轧制态AZ31镁合金氩弧焊(TIG)和搅拌摩擦焊(FSW)焊接接头进行了高速冲击压缩实验,利用金相显微镜和扫描电子显微镜对压缩后的接头组织和断口进行了观察。结果表明:随着应变速率的增大,合金的真应力-应变曲线变化不大,说明AZ31镁合金两种焊接接头对应变速率的敏感性较小;在高应变速率下FSW焊接接头的强度及塑性均优于TIG焊接接头;两种接头在高应变速率下的断裂方式均为解理断裂,但相对于TIG焊接接头,FSW焊接接头更加平整光滑;两种接头的显微组织对应变率均不敏感,并且在高应变率压缩下的变形方式相同,主要为滑移。     

电子束熔炼钒合金动态压缩实验及细观分析

本文利用霍普金森压杆(SHPB)进行了动态压缩实验,获得了电子束熔炼钒合金V-5Cr-5Ti的动态压缩应力应变曲线。采用应变冻结的方法,对材料进行不同变形量的动态加载,并对加载后的试件进行观察,研究不同变形范围下材料的细观变形机制,考察变形量对变形机制的影响,并与电弧熔炼钒合金的细观变形机制进行了比较。结果表明:当动态压缩应变率为3600s-1时,电子束熔炼钒合金的流变应力约为700MPa;该材料在常温下的塑性变形机制为位错滑移。     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。     

ZK60和ZK60-1.0Er镁合金热压缩变形和加工图

采用Gleeble-1500D热模拟试验机对ZK60和ZK60-1.0Er镁合金进行了热压缩实验,分析了合金在温度为160~420℃,应变速率为0.0001~1.0s-1条件下的流变应力变化特征。结果表明:两种镁合金在热压缩过程中的流变应力随变形温度的降低和应变速率的升高而增加,在流变应力达到峰值后随即进入稳态流变;稀土Er的加入使得平均变形激活能珚Q值由183kJ/mol降到153kJ/mol,应力指数n值由6提高到8;发生动态再结晶的临界应力σc值随变形温度升高和应变速率降低而降低,在420℃/1.0s-1高温高应变速率时,稀土Er的加入使得ZK60镁合金发生动态再结晶的临界应力值σc由76MPa降到50MPa。通过动态模型构建热加工图并结合金相组织观察可知:稀土Er的加入缩小了ZK60镁合金的热加工失稳区,增加了热加工安全区的功率耗散效率峰值η_(max),由35%增大到45%,促进了动态再结晶晶粒的形核,但抑制了再结晶晶粒的长大。     

镁合金塑性变形力学行为与微观组织研究进展

介绍了镁合金在单轴压缩、单轴拉伸、轧制和挤压条件下塑性变形的力学行为及微观组织结构演变规律。简述了镁合金中二次拉伸孪生现象以及各种变形条件下孪生与孪生变体类型的选择规律。基于对镁合金位错滑移、机械孪生及动态回复与再结晶行为的认识,对镁合金力学行为的各向异性、轧制与挤压成型能力的影响规律进行了探讨,强调了初始织构对变形机制、动态再结晶及成型能力的重要影响。最后讨论了析出强化镁合金塑性变形与强韧化机理。     

镁合金的塑性变形机制和孪生变形研究

概述了镁合金的塑性变形机制,介绍了镁合金的主要孪生系及其表征技术,详细分析了变形温度、变形速率、受力方向和晶粒尺寸等对镁合金孪生变形的影响,讨论了孪生变形对镁合金塑性变形、动态再结晶、力学性能与断裂的影响。孪生通常发生在粗大晶粒中,晶粒细化可以激活镁合金中的非基面滑移,抑制孪生变形和降低镁合金的各向异性,指出细晶镁合金的研制和工业化生产是变形镁合金发展的重要方向。     

Cleavage step deformation in brittle solids

The nature and origin of sub-surface deformation associated with steps on cleavage surfaces of brittle solids is investigated. The geometrical configuration of step formation during cleavage is first discussed in terms of an interaction between crack segments propagating on adjacent planes. Observations of cleavage surfaces, using optical microscopy, scanning electron microscopy, and X-ray topography, are then described. It is concluded that the cleavage steps form primarily according to a mechanism in which adjacent crack segments first overlap to produce a connecting cleavage sliver, and subsequently join at one end (sometimes both ends) of the sliver to complete the separation of the cleavage halves. The slivers close in imperfect registry with the underlying parent material, thereby leaving a residual gap or dislocation network beneath the undercut steps. This mechanism is in accord with previous observations of step-associated deformation, hitherto taken as evidence of local plasticity.     

Gefügeuntersuchungen an Proben aus IN 939 nach Wechselverformung bei hohen Temperaturen

The Influence of Cyclic Deformation at High Temperatures on the Microstructure of IN 939 The influence of cyclic deformation at high temperatures (700°C, 850°C) on the microstructure of the cast nickelbase superalloy IN 939 was investigated. As methods light microscopy, scanning microscopy, transmission microscopy and X-ray deflection were used. As fracture origin casting pores could be detected in all cases. This observation and the observed small deformation along the sample lead to the assumption that crack propagation from existing casting defects is the essential fracture mechanism for fatigue of smooth specimens.     

Dynamic studies of peel failure in metallized polyester films

Peel failure in PET-aluminium-PET laminates has been studied by dynamic scanning electron microscopy (SEM). The mechanisms of failure are identified, and dynamic observations correlated with conventional SEM fractography. Plastic deformation is highly localized in a region several microns thick. In well-bonded regions the “peel crack” propagates by an advance microcracking mechanism, allowing changes of fracture plane which are accompanied by the formation, extension and breakage of ligaments drawn out from the surface layers of PET. The origin of localized deformation and possible factors affecting the locus of failure are dicussed.     

Some aspects of the fracture behaviour of Mg65Cu25Y10 bulk metallic glass during room-temperature bending

The fracture behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) during room-temperature three-point bending was investigated. The BMG was initially produced by casting into a wedge-shaped mold which generated an amorphous structure below the ∼4 mm thickness zone of the wedge. Three-point bend testing was then carried out on the BMG with the fracture angles and salient features of the fracture surfaces examined by scanning electron microscopy. Observations indicate that this type of deformation mode results in fracture via crack propagation from both surfaces of the samples where the tensile and compressive stresses are greatest. The direction of crack propagation was also found to deviate considerably from 45° to the length direction of sample. A scanning electron microscopy (SEM) study of the fracture surfaces indicated that deformation banding was a feature of crack propagation within compressive zone whereas the tensile zone generated a featureless surface characteristic of brittle failure. The mechanism of failure of the present alloy is discussed on the basis of the observed features on the fracture surfaces and the direction of propagation of cracks during failure and compared with the failure mechanism of samples fractured under both simple tension and compression.     

Fracture behavior dependence on load-bearing capacity of filler in nano- and microcomposites of polypropylene containing calcium carbonate

The fracture toughness and deformation mechanism of PP/CaCO₃ (15 wt.%) composites were studied and related to load-bearing capacity of the particles. To alter the load-bearing capacity of the particles, different particle sizes (0.07–7 μm) with or without stearic acid coating were incorporated. The fracture toughness of the composites was determined using J-Integral method and the deformation mechanism was studied by transmission optical microscopy of the crack tip damage zone. It was observed that the load-bearing capacity of the particles decreased by reduction of particle size and application of coating. A linear relationship between normalized fracture toughness and inverse of load-bearing capacity of particles was found. The crack tip damage zone in composites, which consists in massive crazing, further grows by reduction in load-bearing capacity.     

Micromechanisms of fatigue crack growth in a forged Inconel 718 nickel-based superalloy

The micromechanisms of fatigue crack propagation in a forged, polycrystalline IN 718 nickel-based superalloy are evaluated. Fracture modes under cyclic loading were established by scanning electron microscopy analysis. The results of the fractographic analysis are presented on a fracture mechanism map that shows the dependence of fracture modes on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Plastic deformation associated with fatigue crack growth was studied using transmission electron microscopy. The effects of ΔK and K_max on the mechanisms of fatigue crack growth in this alloy are discussed within the context of a two-parameter crack growth law. Possible extensions to the Paris law are also proposed for crack growth in the near-threshold and high ΔK regimes.     

Damage mechanisms of fatigue fracture in a metastable beta Ti-15-3 alloy

The mechanism of crack tip deformation in metastable beta Ti-15-3 alloy under fatigue loading has been examined. In spite of the small thickness of the test specimens (1 mm), the plastic zone revealed plane strain conditions which was transformed to a plane stress zone when its size became 0.25 of the crack length. Slip processes whose density increased with crack length were the dominant microscopic feature of crack tip plasticity. Microcracks emanating from the main crack appeared as a result of extensive slip damage. Transmission electron microscopy (TEM) and X-ray evidence indicate the absence of twinning or phase transformation and that dislocation processes constitute the microstructural origin of crack propagation resistance in the alloy. Energy calculations show that the specific energy of slip, 20 MJ m⁻³, exceeds that of microcracking by three orders of magnitude.     

Microfailure behaviour of randomly dispersed short fibre reinforced thermoplastic composites obtained by direct SEM observation

The microfailure behaviour of thermoplastic polyamide 6,6 composites reinforced with randomly dispersed short glass fibres was studied. Scanning electron microscopy was carried out on the surface of the composites under load to observe directly the behaviour. The microfailure proceeds following the steps (1) interfacial microfailure occurs at the fibre tips, (2) the microfailure propagates along the fibre sides, (3) plastic deformation bands of the matrix occurs from the interfacial one, (4) crack opening occurs in the band and the crack grows slowly through the band, (5) finally a catastrophic crack propagation occurs through the matrix with pulling-out fibres from the matrix. A model for the microfailure mechanism of the composites is proposed and some methods to improve the mechanical properties of the composites are discussed on the basis of the mechanism.     

有限变形理论能量释放率增率形式

基于有限变形理论中的能量原理和变分原理,考虑以有裂纹的瞬时位形为参考,建立增量变形引起的裂纹扩展方程能够更真实的描述裂纹尖端的扩展机制,在含有裂纹物体的瞬时变形的基础上,推导了裂纹体的能量释放率和增率的形式,提出了裂纹扩展判据.该判据反映了单位时间内裂纹扩展单位面积可以提供的能量与单位时间内裂纹扩展单位面积所需要的能量...     

ELEVATED TEMPERATURE FATIGUE CRACK GROWTH IN ALLOY 718—PART II: EFFECTS OF ENVIRONMENTAL AND MATERIAL VARIABLES

Observations concerning the effects of the environment and material variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of deformation characteristics in the crack tip region. The review of the role of material variables has focused on the effects of chemical composition and microstructure parameters including precipitate size and morphology as well as grain size and morphology. These analyses have suggested that the governing mechanism at the crack tip is the degree of homogeneity of plastic deformation and associated slip density. For conditions promoting homogeneous plastic deformation, with a high degree of slip density, the environmental damage contribution is shown to be limited, thus permitting the dominance of cyclic damage effects which are characterized by a transgranular crack growth mode and a lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and related intergranular fracture mode. Considering that the crack growth damage mechanism in alloy 718 ranges from fully cycle dependent to fully environment dependent, conflicting experimental observations under different operating conditions are examined and a sensitizing approach is suggested to increase the alloy resistance to environmental effects.