Al薄膜纳米压痕过程的多尺度模拟

采用多尺度准连续介质法分别模拟无缺陷和具有初始缺陷两种状态下,单晶Al薄膜纳米压痕初始塑性变形过程,得到载荷-位移响应曲线和应变能-位移变化曲线.研究了初始缺陷对纳米压痕过程中位错形核与发射、Peierls应力以及位错发射.临界载荷的影响.结果表明,在整个纳米压痕过程中出现了多次位错形核与发射现象,初始缺陷对第1和第3对位错的形核与发射影响较小,而对第2对位错的形核与发射具有明显的推迟作用,并伴随有裂纹扩展现象;由于初始缺陷引起薄膜材料内部严重的晶格畸变,导致系统应变能和位错运动的Peierls应力增加;裂纹扩展前,发射第2对位错需要的临界载荷增加,裂纹失稳后,位错发射需要的临界载荷下降.模拟获得的纳米硬度和Peierls应力与实验结果吻合.     

纳米压痕过程的多尺度模拟与Rice-Thomson位错模型分析

采用多尺度准连续介质法（简称QC方法）对单晶Ag薄膜纳米压痕过程进行模拟,研究压头宽度对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度的影响,并用Rice-Thomson位错模型（简称R-T位错模型）进行分析。结果表明,纳米压痕获得的载荷-位移曲线呈现出的不连续性与位错之间的协同作用密切相关;压头尺寸对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度具有明显的影响：随着压头宽度的增加,法向和切向接触应力以及纳米硬度值递减,呈现出明显的压头尺寸效应;而压头下方薄膜内位错形核临界载荷却递增,且与压头半宽度的平方根成正比。模拟结果与相应实验结果以及R-T位错模型计算结果吻合     

EB-PVD热障涂层的弹性模量和断裂韧性研究

用压痕法测量EB-PVD热障涂层的弹性模量及断裂韧性。发现热障涂层的Vickers显微硬度和弹性模量随施压载荷增大而减小,当载荷为2.94N时,显微硬度和弹性模量接近稳态值,分别为6.3GPa和172GPa;断裂韧性平均值约为1.81MPa·m1/2。压痕法测得陶瓷层断裂韧性数据波动较大,其主要原因是陶瓷层显微结构不均匀,使压痕裂纹在涂层不同局部区域所遇到的扩展阻力不同。     

TA15钛合金中纳米压痕附近残余应力-应变场及几何必须位错密度的分布

结合纳米压痕及高分辨电子背散射衍射技术(EBSD)测定了TA15钛合金中α及β相的弹性模量和纳米硬度,揭示了纳米压痕附近应力-应变场及几何必须位错(GND)密度的非均匀分布情况.利用高分辨EBSD测试过程中同步保存的背散射电子衍射花样,并基于cross-correlation的处理方法,计算得出了纳米压痕附近区域的残余弹性应力-应变场分布.结合应变梯度场理论,计算分析了纳米压痕附近区域的几何必须位错密度分布,进而对合金的微观塑性变形机制进行了讨论与分析.结果表明,α相的弹性模量及纳米硬度分别为129.05 GPa和6.44GPa,而β相的相应值为109.80 GPa和4.29 GPa.纳米压痕附近区域的残余Mises应力呈现明显的非均匀分布并受到相邻较软β相的显著影响.压痕附近的低残余应力区域伴随有显著较高的<α>形和柱面型几何必须位错密度分布.     

单晶硅压痕裂纹的氢致滞后扩展

利用压痕裂纹恒载荷试样，研究了单晶硅在空气中应力腐蚀以及动态充氢时氢致滞后开裂的可能性；利用卸载的压痕裂纹试样研究了残余应力引起氢致滞后开裂的可能性．结果表明，单晶硅压痕裂纹恒载荷试样当KI=KIC时在空气中并不发生应力腐蚀．在H2SO4溶液中动态充氢，则能发生氢致滞后开裂，止裂时归一化门槛应力强度因子为KIH／KIC≈0.9、卸载压痕裂纹的残余应力在充氢过程中也能引起氢致滞后开裂，归一化门槛应力强度因子为KIH／KIC≈0．9．     

基于纳米压痕测试的超细晶Si_2N_2O-Si_3N_4陶瓷常温弹塑性仿真

对分别在1 600℃、1 650℃、1 700℃条件下热压烧结制备的Si2N2O-Si3N4超细晶陶瓷进行纳米压痕试验测试,获得了材料的硬度值、弹性模量值和载荷-深度曲线。考虑试验中波士压针的磨损缺陷,通过理论和数值模拟相结合的方法,确定压针的尖端球面半径RBerk=500nm。以纳米压痕试验数据为依据,利用MSC.Marc有限元仿真软件模拟纳米压痕试验压针压入材料表面的过程,反推出所测试材料的应力应变关系曲线,其屈服应力随弹性模量的减小而降低,分别为47GPa、43GPa、35GPa。通过比较分析压痕区域的应变场和应力场,分析纳米压痕试验中材料的变形特征。     

梯度基体负偏压真空多弧离子镀沉积TiN涂层及其力学性能

目的通过梯度基体负偏压沉积工艺,获得综合性能优良的Ti N涂层。方法采用多弧离子镀工艺,在0~-180 V连续变化的梯度基体负偏压参数下沉积梯度Ti N涂层。通过X射线衍射仪和扫描电子显微镜对涂层的物相结构和形貌进行分析,通过纳米压痕和纳米划痕对涂层的力学性能进行系统研究。结果与无梯度沉积的涂层相比,梯度基体负偏压沉积Ti N涂层的(111)晶面衍射峰减小,厚度增加,表明涂层的沉积速率增大。经测试,梯度涂层的断裂临界载荷L_(c2)=215.21 m N,硬度值H=31.2GPa,弹性模量E=498 GPa,塑性变形临界载荷L_y=81.65 m N;无梯度沉积涂层的L_(c2)=248.63 m N,H=29.6 GPa,E=452 GPa,L_y=23.39 m N。二者相比之下,梯度涂层虽然断裂临界载荷有所减小,但硬度值和弹性模量均有所增大,并且塑性增大,塑性变形临界载荷大幅增加,综合力学性能提高。结论梯度基体负偏压沉积工艺改变了常规的单一参数设置,在沉积过程中,基体负偏压对涂层生长的影响不断改变,获得的涂层具有结构上的梯度变化,从而力学性能得到了改善。     

表面残余应力对玻璃压痕参数的影响

通过研究化学钢化前、钢化后钠钙玻璃的压痕形貌、几何尺寸和表面力学性能，分析和探讨了表面残余应力对玻璃压痕参数以及表面力学性能变化的影响机理。结果表明，普通玻璃的压痕与钢化玻璃的压痕有明显的差别，钢化玻璃的压痕周围呈圆形放射状光斑，压痕裂纹很小，其硬度值比相同载荷下钢化前玻璃硬度值增加了约15．8％，压痕断裂韧性有明显的提高，是钢化前玻璃断裂韧性的2．75倍，显示了钢化后表面残余压应力对玻璃的压痕参数和力学性能的影响。     

压痕法测定Mo_5SiB_2合金断裂韧性的经验方程确定

分析了载荷对Mo_5SiB_2合金压痕裂纹影响规律,观察了压痕裂纹形貌特征,表征了裂纹长度与载荷的关系,确定了压痕裂纹系统,选取不同的压痕方程计算了室温断裂韧性KIC值,探讨了压痕载荷以及压痕方程对KIC值的影响关系。结果表明,Mo_5SiB_2合金压痕裂纹系统属于中位裂纹,只要选择合适的压痕方程,载荷并不影响断裂韧性值。压痕法测定Mo_5SiB_2合金断裂韧性合适的经验方程为KIC=0.8193H~(0.5)E~(0.5)a~2c~(-1.5),测得的其KIC值为2.63 MPa·m~(1/2)。     

TiN附着膜的应力-应变关系

利用等离子体辅助化学气相沉积法(PACVD)在60Mn钢基片上沉积2．5μm厚的TiN膜．借助X射线应力分析技术和微拉伸设备，测量该附着膜纵向(加载方向)应力和横向应力及外载应变，进而求其等效应力-等效单轴应变关系，并由此算得它的条件屈服点σ0．1和σ0．2分别为4．2和4．4GPa，加工硬化指数为0．36．用纳米压痕仪测得其硬度为25GPa，弹性模量为420GPa．TiN膜在拉伸过程中发生了塑性变形．     

齿轮疲劳强度与裂纹萌生区域的预测研究

以经过表面强化后的国产某轿车变速箱齿轮为例,根据齿根附近沿深度的残余应力和硬度的分布对单齿弯曲疲劳强度和疲劳裂纹萌生区域的预测进行了分析和试验验证.通过强度和硬度之间的转换关系以及疲劳强度与残余应力的作用关系,得到了齿根附近沿深度的弯曲疲劳极限分布,齿根次表面下0.25～0.45mm之间的区域是疲劳危险区.齿轮表面强化的结果使得齿轮弯曲疲劳裂纹源从表面转入到次表面,并初步得到了微观验证.研究结果说明了基于残余应力和硬度可以进行疲劳强度和疲劳裂纹萌生区域的预测.     

Cr12 MoV导辊中贯穿裂纹的形成机理研究

目的分析导辊贯穿裂纹萌发和扩展的机理。方法采用扫描电镜和能谱等手段分析贯穿裂纹的裂纹源和扩展区域的显微图像和化学成分,结合X射线衍射仪和维氏硬度仪研究裂纹导辊组织中碳化物的种类及硬度,运用数值模拟分析裂纹扩展过程中的应力情况,分析Cr12Mo V导辊中贯穿裂纹的萌发原因和扩展机理。结果元素偏析是Cr12Mo V导辊贯穿裂纹萌发的主要原因,近表面氧、铬两种元素明显高于裂纹扩展区域,导辊在内应力的作用下分离,在近表面处产生裂缝,形成裂纹源。高硬度共晶碳化物是Cr12Mo V导辊裂纹扩展的主要原因,共晶碳化物主要是(Fe,Cr)7C3型块状颗粒,硬度为1327HV,其周围产生大量微裂纹,并相互连接形成网状而分割机体,在外力的作用下扩展,形成贯穿裂纹。采用扩展有限元法(XFEM)对贯穿裂纹进行模拟仿真,对裂纹扩展应力进行分析,裂纹尖端应力最大,约为裂纹扩展应力的1.5倍;沿裂纹扩展方向,应力先急剧减小,然后呈抛物线型,在导辊径向厚度达最大时存在一个极大值,在导辊上下表面应力最小。结论导辊中合金元素和共晶碳化物对其裂纹的萌发和扩展具有较大影响,裂纹扩展应力场与初始裂纹位置和导辊尺寸直接相关。     

十字轴断裂原因分析

通过扫描电镜观察、金相检验、化学分析以及硬度检测等手段对十字轴的断裂原因进行了分析,分析结果表明：该轴的硫含量超标;表面裂纹源处存在夹杂物富集现象,在使用时引起应力集中,从而导致其疲劳断裂.     

The effects of film formation and mechanical factors on the initiation of stress corrosion cracking of unalloyed steels in carbonate solutions

Investigations into the intergranular stress corrosion cracking (SCC) of unalloyed steel St 36 in carbonate-bicarbonate solutions have shown that SCC susceptibility is limited to the potential range of magnetite formation for both constant load and constant strain rate tests. Experiments to determine the creep behaviour of the steel in the corrosive solution show that creep rates corresponding to the critical strain rates in the constant strain rate test occur during the crack initiation state after loading of the tensile specimen with a constant crack initiating load. These experimental findings establish, therefore, that critical crack initiating strain rates of the unalloyed steel are always present during the initial stage for SCC regardless of the test method used. The cause of crack initiation at grain boundaries can be attributed to the fact that the magnetite layers feature faults and grooves in the oxide along the grain boundaries in the entire critical potential range. These oxide grooves, which are also observed on unstressed steel, cause a notch-like localized corrosion attack. The depth and crack tip radius of the resultant grain boundary notches appear to be sufficient to initiate propagating cracks under the single action of predominately pure stresses without macroscopic deformation of the steel. This does not necessarily mean, however, that a purely stress induced corrosion process is involved, for it is not out of question that the concentration of stress in the base of grain boundary notches is sufficient to cause localized deformation processes required for crack initiation.     

Inhibition effect of metal cations to intergranular stress corrosion cracking of sensitized Type 304 stainless steel

Inhibition effect of metal cation to intergranular stress corrosion cracking (IGSCC) of sensitized Type 304 stainless steel has been investigated by slow strain rate technique (SSRT) with a dynamic crack observation system. The SCC tests were conducted in fully-deaerated aqueous solutions containing various metal sulfates of 10⁻⁵ kmol/m³ at 95 °C. Metal cations of Na⁺, Ca²⁺, Mn²⁺ and Zn²⁺ were selected in this study, and were characterized by a hardness based on the hard and soft acids and bases (HSAB) concept. As a result, it was found that a hard metal cation in the test solution increased crack initiation time and decreased mean crack initiation frequency. In other words, metal cations with larger hardness have an ability of suppressing initiation process of the IGSCC. On the other hand, apparent mean crack velocity was independent of the hardness of metal cation. The finding that the metal cations with large hardness inhibit initiation of the IGSCC was able to be rationally explained on the basis of the passive film model combined with the HSAB concept.     

A model of heating a flux-cored wire in arc metallizing and analysis of the structure of the coating

Summary

This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the softer base metal side before the critical stress conditions for brittle fracture initiation occur in the CGHAZ. This asymmetrical plastic deformation promotes deviation of ductile crack growth from the crack tip CGHAZ. In this case, the critical CTOD value does not always reflect the CGHAZ toughness itself.

A notch location nearer the weld metal sometimes causes fracture initiation in the weld metal if the fatigue crack tip samples the CGHAZ. Such experimental data do not reflect the real CGHAZ toughness.

The significance of the critical CTOD value obtained in the tests must be determined in the fracture toughness evaluation of the weld CGHAZ. This paper presents a procedure for evaluation of CTOD test results obtained for HAZ‐notched welds that considers the strength mismatch effect.     

慢加载条件下氢对裂纹萌生与扩展的作用

在扫描电镜原位拉伸条件下，研究了氢原子对正火态的Ｃｒ－１Ｍｏ钢裂纹萌生与扩展的作用结果发现：氢原子使缺口根部韧性裂纹萌生时的临界塑变降低，并使裂纹的扩展方式从沿最大切应力方向的连续扩展改变成沿最大主应力面的跳跃扩展．试验未发现明显的氢致塑变特征．文中对氢原子的影响机理进行了分析．     

Zr基块状非晶的氢损伤与氢致滞后断裂

研究了Zr41.2Ti13.8Ni10Cu12.5Be22.5块状非晶在充氢过程中氢致滞后断裂规律以及氢鼓泡的形核、长大及破裂过程.结果表明,当充氢电流i<20 mA/cm2时,不出现氢损伤(鼓泡及微裂纹),但在恒载荷条件下能发生氢致滞后断裂,其归一化门槛应力强度因子为KIH/KIC=0.63.当i>20 mA/cm2后,无载荷下充氢会产生氢损伤,恒载荷下发生滞后断裂时KIH/KIC从0.63降为0.26.氢鼓泡(直径约为30 nm)形核时的内压pi≈3.6 GPa.随氢的进入,鼓泡不断长大;内压增大至pC≈3.9 GPa时,鼓泡就会解理扩展变成裂纹;但扩展20-30 μm后,内压下降从而止裂.当一定量原子氢进入氢鼓泡后,它又能解理扩展,从而在鼓泡边缘局部解理断口上可以观察到止裂线.     

Fracture toughness of structural aluminium alloy thick plate joints by friction stir welding

Abstract

The fracture toughness in a friction stir welded joint of thick plates of structural aluminium alloy type A5083-O is investigated. A joint between two 25 mm thick plates is fabricated by one sided, one pass friction stir welding. The Charpy impact energy and critical crack tip opening displacement (CTOD) in the friction stir weld are much higher than those in the base metal or heat affected zone, whereas mechanical properties such as stress–strain curve and Vickers hardness are not conspicuously different. The effects of the microstructure on crack initiation and propagation are studied in order to clarify the difference in fracture toughness between the stir zone and base metal. The analyses of the fracture resistance curves and the diameters of dimples in the fracture surface after both tensile and bending tests show that the fine grained microstructure in the stir zone helps to increase ductile crack initiation and propagation resistance. It is found that the high fracture toughness value in the stir zone is affected by the fine grained microstructure in friction stir welds.     

TP304不锈钢三通失效分析

电厂脱硝设施中所使用的TP304不锈钢三通出现裂纹缺陷,通过成分分析、断口宏观和微观分析、微观组织观察、硬度检测,并结合生产工艺对三通裂纹的产生原因进行分析。研究结果表明:材料中Ni含量偏低使材料的奥氏体稳定化程度降低;基体内含有的2.5~3.0级球状非金属夹杂物对基体造成撕裂作用,促进疲劳裂纹的萌生;同时,三通在加工完成后也缺少固溶处理步骤,使得材料内应力过大。这些原因综合导致材料奥氏体含量降低,硬度过高,同时造成材料塑性变形部位内应力过高,在支管颈部产生较大的应力集中现象。三通内表面同时存在大量加工损伤,强化了应力集中现象,在疲劳载荷的作用下,促进了三通裂纹的萌生。