氢对锆基块体非晶合金形变和开裂的影响

通过充氢和未充氢缺口拉伸试样和三点弯曲试样在SEM下的原位加载,研究了氢对Zr65Al7．5Ni10Cu17．5块体非晶合金形变和开裂过程的影响,结果表明,无论是否有氢,块体非晶的剪切带发展到临界尺寸,剪切裂纹就沿剪切带形核、扩展,它一旦张开就导致快速的断裂,断口边缘观察到的无特征区是剪切带,而不是剪切裂纹断口;剪切断口形貌和拉伸断口形貌没有本质区别,只有当长时间充氢才能形成氢鼓泡,如鼓泡很小或尚未形成,则氢对剪切带以及裂纹的形核、扩展没有明显影响;如存在较大的氢鼓泡,则当剪切带尚未充分发展时微裂纹就形核,导致低应力脆断。     

AZ31镁合金中绝热剪切带的组织演变规律

为了深入了解镁合金绝热剪切带与裂纹的关系,进而揭示镁合金在高速冲击载荷作用下局部变形绝热剪切的组织演变规律,采用分离式Hopkinson压杆对AZ31镁合金的帽状式样进行冲击压缩实验,而后利用光学显微镜,扫描电镜和维氏硬度计分别对冲击后的AZ31试样进行分析。结果表明,绝热剪切带形成于最大剪应力方向,随着冲击载荷的不断增加,沿着切应力方向上的微孔洞和微裂纹不断长大,直至彼此相互连接成裂纹,最终导致材料的断裂。经对剪切带及周围组织维氏硬度的测量发现,剪切带内细小晶粒区的硬度明显高于周围组织。     

Deformation behaviour of closed-cell aluminium foams in tension

The mechanical behaviour of commercially available ALPORAS aluminium foam with two different densities was studied under tension loading. In addition to the common stress–strain measurements, local deformation, notch-opening displacement and damage evolution were determined. The deformation characteristics deviated from those observed in aluminium foams under compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. Four regimes were evident in the stress–strain curves and deformation maps: the linear elastic regime, the plastic regime with no significant crack initiation and propagation, the regime of formation of a fracture process zone and, finally, the regime of fracture, where a main crack propagates through the specimen and leads to failure. The fracture strain was only a few per cent, with the higher-density foam showing a larger fracture strain, and the plastic Poisson's ratio was about 0.35. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state, caused by a non-vanishing Poisson's ratio, in front of the notch tip creates an increase of the fracture strength similar to the behaviour in ductile bulk metals.     

Torsional Fatigue Cracking and Fracture Behaviors of Cold-Drawn Copper:Effects of Microstructure and Axial Stress

The fatigue cracking and fracture behavior of cold-drawn copper subjected to cyclic torsional loading were investigated in this study.It was found that with increasing stress amplitude,the fracture mode of cold-drawn copper gradually changes from a shear fracture on transverse maximum shear stress plane to a mixed shear mode on both transverse and longitudinal shear planes and finally turns to the shear fracture on multiple longitudinal shear planes.Combining the cracking morphology and the relationship between torsional fatigue cracking and the grain boundaries,the fracture mechanism of cold-drawn copper under cyclic torsional loading was analyzed and proposed by considering the effects of the microstructure and axial stress caused by torsion.Because of the promotion of the grain boundary distribution on longitudinal crack propagation and the inhibition of axial stress on transverse crack grown,the tendency of crack propagation along the longitudinal direction increases with increasing stress levels.     

等通道转角挤压Al-Cu合金的应力和应变疲劳行为比较

通过恒应力和恒塑性应变控制疲劳实验,比较了等通道转角挤压四道次的Al—0．7％Cu(质量分数)合金的疲劳寿命、表面变形形貌、疲劳开裂和疲劳断口．结果表明：样品发生明显的循环软化,致使应变和应力疲劳寿命在高、低应力范围存在差别．在应变控制疲劳样品中,塑性变形既可由剪切带来承担,也可以由剪切带和形变带共同承担,进而疲劳裂纹分别沿剪切带或形变带萌生．而应力控制疲劳的塑性变形只集中在剪切带中,并导致剪切疲劳开裂．疲劳断口上存在典型的疲劳裂纹萌生区、缓慢扩展区、快速扩展区和最后瞬断区．     

SEM原位观察BT20钛合金激光焊接头的断裂行为

采用单边缺口试样用扫描电镜原位观察了BT20钛合金激光焊接头焊缝的裂纹萌生与扩展特征。焊后态试样裂纹一经萌生随即失稳断裂。焊后热处理试样经历了较长的裂纹亚稳扩展阶段，在切应力与正应力共同作用下，微裂纹沿滑移带萌生；裂纹扩展过程中遇到滑移带时发生偏转；新的微裂纹在裂纹尖端的滑移带相交处萌生，并随后与主裂纹相连。焊后热处理可显著提高激光焊接头的韧塑性。     

块状非晶态合金Zr65Al7.5Ni10Cu17.5的室温单轴压缩断裂行为

利用IUTM和SEM研究了Zr65Al7 5Ni10Cu17.5块状非晶合金的室温单轴压缩变形和断裂行为.该合金的室温压缩变形过程主要表现为弹性和塑性变形,并且塑性变形阶段没有加工硬化现象.宏观形貌观察发现样品形成与压缩方向呈约45°的变形带.塑性变形以剪切带粘性流层相对滑动的方式进行,在变形过程中形成脉络状断面形貌.裂纹于塑性变形积累到一定程度之后,在切变变形较大的区域表面形成.裂纹从萌生、扩展到断裂,时间极短.流变积累和应力集中综合作用的结果导致断裂.     

Gradual shear band cracking and apparent softening of metallic glass under low temperature compression

Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the shear band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal shear-band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major shear band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of shear band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to shear band cracking, rather than the change of shear band density.     

Experiments on formation of the adiabatic shear band in sheet metal

Formation of the adiabatic shear band in sheet metal is investigated with experiments for high strength steel sheets, 60 C and 60 TRIP. Since the adiabatic shear band is formed as a result of adiabatic shear failure with a narrow band of concentrated shear strain, the adiabatic shear band plays an important role in the analysis of high speed deformation phenomena. For shear band experiments with a tension split Hopkinson bar, specimens are designed to induced large shear strain. The experimental results show that the shear deformation modes of two sheet metals, 60 TRIP and 60 C, are quite different from each other in that the adiabatic shear band is observed only in 60 C. The shear deformation in 60 TRIP is restrained by the abrupt increase of strength due to the plastic strain, which interferes with propagation of the shear crack. Instead, a tensile crack developed at the corner where the shear crack should have been initiated. As a result, the load-displacement curves show that the tensile load of 60 TRIP specimens becomes higher than that of 60 C at the same displacement.     

Al—Li合金在慢拉伸条件下的形变与断裂行为

研究了Ａｌ－Ｌｉ合金在单向受力状态下的形变特征与断裂行为，测试了不同时效状态合金的慢拉伸裂纹扩展速率与应力强度因子的关系。结果表明，不同时效合金的范性形变机制、微观断裂模式和慢拉伸裂纹扩展阻力不同，它们视微观组织和晶界性态而异，应变局部化是低塑性的根源。     

Ⅲ型裂纹试样的氢致滞后断裂

利用复合型(Ⅰ+Ⅲ型)试样研究了应力状态对氢致表观屈服应力(它是产生局部宏观塑性变形所需的外应力)的影响,探讨了Ⅲ型裂纹试样产生氢脆(即氢致裂纹)的可能性。结果表明,对Ⅲ型裂纹试样,即使严重充氢也不会使表观扭转屈服应力下降。从而也不会沿原裂纹面产生氢致滞后裂纹。对复合型试样,只有当K_Ⅰ本身就能使原裂纹面产生滞后塑性变形时才能使表观扭转屈服应力随K_Ⅰ增大而下降。 对充氢的纯Ⅲ型裂纹试样,当扭矩大于临界值后保持一定时间就能在和原裂纹面成3/4π或-45°的面上产生氢致滞后裂纹,它导致典型的沿晶断口。如充氢试样直接扭断则得沿原裂纹面断裂的平剪切断口。当钢的强度和氢含量低于临界值就不会产生沿3/4π面的滞后裂纹。计算了Ⅲ型裂纹应力场和氢应变场的互作用能。结果表明,在和原裂纹面成3/4π的诸平面上互作用能有极小值,从而导致氢向该面浓集而形成氢致滞后裂纹。     

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.     

Stresses State and Mechanical Behaviors of the Green Body During Die Compaction and Ejection Process

Finite element simulations for metal powder compaction of a clutch plate were performed to examine the stresses during compaction, unloading, and ejection. To describe the mechanical behavior of compacted green body, a modified density-dependent Drucker-Prager Cap model was utilized to predict the stress and density distribution of the compacted clutch plate during loading and ejection stages. The results indicate that maximum tensile principal stress was a main driving force for the tensile crack initiation during ejection stage, and shear stress may be another driving force in both compaction and ejection stages for shear crack initiation. There were peak value of the stresses during ejection stage, and the stresses are in compressive state only during compaction stage. Therefore, the tensile crack initiation is not possible during compaction except shear crack. Hoop stress in the clutch plate is of less contribution to the crack initiation during compaction, unloading and ejection. Study of criteria of the crack initiation and fracture is necessary in order to obtain uniform density and crack-free components in the manufacturing of metal powder compaction.     

Shear band spacing under bending of Zr-based metallic glass plates

Metallic glasses often exhibit marked ductility when subjected to compressive or bending loads as a result of multiple shear band formation. This observed ductility depends upon sample geometry; thin plates show ductility in bending while thicker plates of the same composition fracture under similar loading. The thickness dependence of yielding and fracture of metallic glass plates subjected to bending is considered in terms of the shear band processes responsible for these properties. Experimental results show that shear band spacing and length scale with the thickness of the plate at a ratio of 1:10. Both shear band offset and shear band spacing increase with increasing curvature; shear band offset as the square of the plate thickness. As bending is increased beyond yield, shear band spacing continues to increase until the strain is accommodated by a few long shear bands. Continued bending leads to crack formation and failure.     

Ti/Al/Mg/Al/Ti层合板变形行为及断裂形式的原位研究

为阐明Ti/Al/Mg/Al/Ti层合板的变形行为及断裂形式,对轧制态和退火态的层合板进行原位弯曲和拉伸试验,研究组元板和金属间化合物的裂纹萌生和扩展情况.结果表明:无论Al/Mg界面有无金属间化合物,Al/Mg界面分层都是最先出现的断裂形式,因此Al/Mg界面结合强度决定层合板的力学性能.在退火态层合板中,金属间化合...     

Quantitative calculation of local shear deformation in adiabatic shear band for Ti-6Al-4V

JOHNSON-COOK（J-C） model was used to calculate flow shear stress-shear strain curve for Ti-6Al-4V in dynamic torsion test. The predicted curve was compared with experimental result. Gradient-dependent plasticity（GDP） was introduced into J-C model and GDP was involved in the measured flow shear stress-shear strain curve, respectively, to calculate the distribution of local total shear deformation（LTSD） in adiabatic shear band（ASB）. The predicted LTSDs at different flow shear stresses were compared with experimental measurements. J-C model can well predict the flow shear stress-shear strain curve in strain-hardening stage and in strain-softening stage where flow shear stress slowly decreases. Beyond the occurrence of ASB, with a decrease of flow shear stress, the increase of local plastic shear deformation in ASB is faster than the decrease of elastic shear deformation, leading to more and more apparent shear localization. According to the measured flow shear stress-shear strain curve and GDP, the calculated LTSDs in ASB are lower than experimental results. At earlier stage of ASB, though J-C model overestimates the flow shear stress at the same shear strain, the model can reasonably assess the LTSDs in ASB. According to the measured flow shear stress-shear strain curve and GDP, the calculated local plastic shear strains in ASB agree with experimental results except for the vicinity of shear fracture surface. In the strain-softening stage where flow shear stress sharply decreases, J-C model cannot be used. When flow shear stress decreases to a certain value, shear fracture takes place so that GDP cannot be used.     

Development of a Non-collinear Nonlinear Ultrasonic-Based Technique for the Assessment of Crack Tip Deformation

Non-collinear nonlinear ultrasonic (NCNLU) wave mixing technique has been established to study the localized plastic deformation at the crack tip during fatigue. A pair of ultrasonic shear wave was mixed non-collinearly to obtain a longitudinal wave of frequency equal to the sum of the two shear wave frequencies under a resonant condition. Experiments were carried out on notched 9Cr-1Mo 3-point bend specimen during high-cycle fatigue. The variation of the NCNLU parameter with the stress accumulation at the crack tip during the fatigue crack initiation and propagation and mapping of the deformation zone around the crack tip are described in this paper.     

TEM OBSERVATION OF LOCAL DEFORMATION BAND AT CRACK TIP

The thin foil specimen of a ferrite-austenite duplex stainless steel was tensiled under transmis-sion electron microscope(TEM).It was found that both in ferrite and austenite the localdeformation band at crack tip was formed near to the crack propagating direction.Its forma-tion was related with the crack tip Schmid factor,dislocation shielding,latent hardening andhardening coefficient.When the crack tip emitted dislocations to a slip system by the action ofpure mode Ⅱ stress resolute,and the decreasing rate of hardening coefficient was suitable,lo-cal deformation band was easy to form.     

Microstructure and tensile properties of high-strength high-ductility Ti-based amorphous matrix composites containing ductile dendrites

In the present study, two Ti-based amorphous matrix composites containing ductile dendrites dispersed in an amorphous matrix were fabricated by a vacuum arc melting method, and deformation mechanisms related to the improvement of strength and ductility were investigated by focusing on how ductile dendrites affected the initiation and propagation of deformation bands, shear bands or twins. Ti-based amorphous matrix composites contained 70–73 vol.% coarse dendrites of size 90–180 μm, and had excellent tensile properties of the yield strength (1.2–1.3 GPa) and elongation (8–9%). The Ta-containing composite showed strain hardening after yielding, and reached fracture without showing necking, whereas necking occurred straight after yielding without strain hardening in the Nb-containing composite. The improved tensile elongation and strain hardening behavior was explained by the homogeneous distribution of dendrites large enough to form deformation bands or twins, the role of β phases surrounding α phases to prevent the formation of twins, and deformation mechanisms such as strain-induced β to α transformation.     

组合加载下碳素工具钢中疲劳小裂纹的门槛区行为特征

通过考察碳素工具钢的表面滑移、疲劳破坏及表面裂纹的扩展条件，以统一的局部应力讨论了光滑试样和裂纹试样的疲劳特征。分析和实验结果表明，在裂纹试样的名义应力小于光滑试样表面滑移门槛应力的范围内，线弹性断裂力学是有效的，对应裂纹扩展门槛值的裂尖性区为一常数。