敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀的影响

为了研究敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀行为的影响,使用敏化处理的Z3CN20-09M不锈钢制成U弯试样,并置于250、290及320℃的高温水中进行应力腐蚀开裂实验,采用扫描电镜观察了高温水实验后试样的氧化膜厚度以及应力腐蚀裂纹的萌生及扩展行为.结果表明:敏化处理增加了氧化膜的厚度,降低了耐蚀能力,使SCC敏感性增大;温度较高时,敏化处理的影响较大;铁素体相容易被侵蚀,大多数点蚀坑产生于铁素体中;SCC裂纹优先在点蚀坑底部和奥氏体/铁素体相界位置处形成;相界面对SCC裂纹的影响取决于SCC裂纹相对于相界面的取向,SCC裂纹扩展方向平行于相界面时裂纹易沿着相界扩展,SCC裂纹扩展垂直于相界面方向时相界面对裂纹扩展起阻碍作用.     

原位研究M2高速钢微裂纹的萌生和扩展

使用扫描电子显微镜(SEM)内的原位加载台对M2高速钢进行了原位拉伸实验。结果表明：在M2高速钢的原位拉伸过程中,微裂纹主要在大尺寸共晶碳化物与基体的界面处萌生和扩展。与回火马氏体相比,裂纹更容易在残余奥氏体上萌生。碳化物的尺寸、形状和种类,对微裂纹的萌生和扩展也有重要的影响。减少块状残余奥氏体、一次共晶碳化物和MC碳化物的数量、减小碳化物的尺寸和改善碳化物形状,可减缓微裂纹的萌生和扩展。     

316L不锈钢扩散连接接头界面疲劳裂纹扩展行为

采用单边缺口试样对316L不锈钢扩散连接接头进行显微疲劳试验,实现微观尺度下界面裂纹扩展和微孔隙演化的原位观测.试验观察表明:动载荷下界面裂纹扩展时,裂纹尖端晶粒内产生局部的塑性变形,但几乎观测不到界面微孔隙的扩展,也未见微孔隙与微孔隙的相连;微孔隙对界面裂纹萌生和扩展的影响不大;扩散连接过程中所形成的谷脊状界面可以改变裂纹的扩展路径.     

核电主管道不锈钢在高温高压水环境下的疲劳裂纹萌生行为

利用腐蚀疲劳测试系统研究了高温高压水环境下两种压水堆核电站一回路主管道用不锈钢的腐蚀疲劳裂纹萌生行为。结果表明,316LN奥氏体不锈钢的裂纹主要在材料表面的驻留滑移带处萌生,少量裂纹在两簇驻留滑移带交界的亚晶界面处。含有少量铁素体的Z3CN20.09M奥氏体不锈钢的疲劳裂纹依次在试样表面的驻留滑移带处、相界处和点蚀坑处萌生,但主要是在驻留滑移带处。通过研究高温高压水环境下氧化膜的组成和腐蚀疲劳试样横截面的形貌,分析了疲劳裂纹在滑移带处萌生的机理。最后对比分析两种不锈钢裂纹萌生机制的异同,并讨论了铁素体对材料腐蚀疲劳性能的影响。     

SiCw/Al复合材料在TEM下的拉伸行为

在备有拉伸台的透射电镜上原位观察了SiCw/Al复合材料的变形、裂纹萌生及其扩展过程.结果发现,变形初期,位错主要从晶界及SiCw-Al界面处开动,在晶界与晶须周围产生大量位错.裂纹萌生源主要为:(1)材料制备过程中受损伤或破断的晶须;(2)晶须富集区.晶须与晶界对裂纹的扩展有阻碍作用.当裂纹取向与晶须大致垂直时,裂尖在晶须附近钝化然后绕过晶须扩展,当裂纹取向与晶须接近平行时,裂尖在晶须端部附近钝化后沿其一侧扩展.     

氢气环境下2205双相不锈钢的氢致开裂研究

为了研究氢气环境下双相不锈钢疲劳裂纹萌生和扩展的影响规律,建立氢气环境下双相不锈钢疲劳应变组织演化—氢致开裂之间的关联机制,在5 MPa氢气和5 MPa氮气2种环境中对2205双相不锈钢试样进行了慢应变速率拉伸和疲劳裂纹扩展速率试验。结果表明：在氢气环境下,2205双相不锈钢在慢应变速率拉伸过程中的氢脆敏感性不高,而在疲劳过程中氢脆现象显著,5 MPa氢气环境下2205双相不锈钢的疲劳裂纹扩展速率比氮气环境中的快18倍;氢气能够促进2205双向不锈钢疲劳裂纹尖端周围组织的局部塑性变形,并进一步导致氢致开裂。在氢气环境下2205双相不锈钢疲劳变形过程中,不同的相结构其氢致开裂机理也不同,铁素体相容易形成河流状花样断口形貌(解理断口),而奥氏体相断口形貌多呈现平行的滑移带特征,奥氏体相在铁素体相的解理开裂过程中对裂纹具有阻碍作用。     

YAl 2P / Mg -Li -Al复合材料动态拉伸断裂过程原位观察

通过 SEM中静载动态拉伸 , 原位观察和研究了搅拌铸造法制备的 YAl _2P/ Mg-Li-Al 基复合材料裂纹的萌生及扩展机制。结果表明 , 微裂纹萌生位置主要为复合材料的铸造缺陷处、 复合材料的基体中以及颗粒/基体界面处 , 并以在复合材料基体中萌生为主。微裂纹的扩展主要在基体和 YAl ₂颗粒/基体界面处进行。主裂纹的长大方向具有选择性 , 裂纹主要沿颗粒贫化区与颗粒富集区的交界处开裂 , 主裂纹扩展到一定程度后 , 试样全面失稳而迅速断裂。基体的断裂失效在 YAl 2P/ Mg2Li2Al复合材料拉伸断裂过程中起很大作用。     

X80管线钢焊接接头微观断裂行为的TEM原位观察

为了从微纳米尺度研究管线钢的断裂方式,通过透射电镜原位拉伸方法,从焊缝区和热影响区直接取样,直观测试了X80管线钢在晶粒尺度范围的裂纹生长、扩展等断裂过程和机理.研究表明:在原位拉伸过程中,晶内发射的螺型位错与刃型位错速率之比约为4∶1;晶界裂纹为不连续扩展,而裂纹在晶内沿其DFZ的方向萌生扩展,其扩展是连续的.在加载过程中,裂纹会越过晶界扩展,当裂纹越过大角度晶界时,裂纹扩展方向改变约为30°,扩展方式也会有所变化;当裂纹越过小角度晶界时,裂纹扩展方向不变,扩展方式也不变.     

7A04铝合金应力腐蚀过程中裂纹萌生和扩展规律研究

为了研究不同拉伸速率对应力腐蚀敏感性及应力腐蚀过程(SCC)中裂纹萌生与扩展规律的影响,本文选用7A04铝合金在3.5%wt NaCl溶液中进行慢应变速率拉伸实验(SSRT),采用扫描电子显微镜(SEM)、体式显微镜、电化学方法、高分辨透射电子显微镜(HRTEM)等方法进行研究;提出一种原位预测裂纹萌生和扩张的新方法即相移与电化学阻抗谱相结合的方法来系统研究应力腐蚀过程(SCC)中裂纹萌生和扩展规律.结果表明:当拉伸速率为3.0 um/min时,在3 h附近裂纹开始萌生,在8 h附近裂纹发生明显扩展;通过高分辨透射电子显微镜(HRTEM)对裂纹萌生和扩展机理进行研究,验证了新方法的可行性,推测裂纹的萌生和扩展机理可能是由晶界出产生的析出物引起的.     

ODS铁素体钢中微裂纹形核过程的TEM原位观察

采用透射电子显微镜原位位伸范膜试样法研究了ＯＤＳ铁素体钢中位错发射及微裂纹形核，钝化和扩展的过程。结果表明，ＯＤＳ铁素体钢在原位拉伸时，裂尖首先发出大量位错并形成无位错区，保持恒位移，纳米级微裂纹在裂纹顶端连贯形核或在ＤＦＺ中不连续形核，这个微裂纹并不钝化成孔洞。     

Characteristics of microcrack initiation in CuNiAl shape memory alloy

The mechanism of microcrack initiation in CuNiAl shape memory alloy was investigated through in situ tensile tests under optical microscopy, scanning electron microscopy and transmission electron microscopy. The results indicated that various martensite phases appeared first around a notch tip upon loading, and then microcracks initiated along the martensite/parent interface. The small martensite regions could transform back to the parent phase not only upon unloading but also upon loading or during crack propagation. There was no dislocation emission and motion before martensite formation. The slip bands formed after the crack had propagated a definite distance, and the microcracks nucleated and the cracks propagated either along the slip bands or the martensite/parent interface.     

In situ study of martensitic transformation and nucleation and propagation of cracks in Cu-Ni-Al shape memory alloy

Abstract

The stress induced martensitic transformation and the relationship between it and the nucleation and propagation of cracks in the Cu-Ni-Al shape memory alloy were investigated through in situ tensile tests by SEM and TEM. The results indicated that the stress concentration ahead of the crack tip could induce formation of stacking faults and different types of martensites. Transmission electron microscope observations showed that the martensites could transform from one type to another type and even reversely to parent during loading. The microcracks nucleated along the martensite/parent interface and intersections between two martensites. When the crack propagated a certain distance, the stress concentration ahead of the crack tip was large enough to result in formation of slip bands, in this condition the microcrack nucleated along slip bands more easily.     

In-situ observation of microcrack evolution in a dual-phase steel during tensile straining

ABSTRACT

In this paper, the microcrack evolution in DP590 dual-phase steel was observed by in-situ straining in straining transmission electron microscopy. It was found that a void initiated ahead of a main crack. After the load was applied, a thinned area was nucleated ahead of the void tip, and it grew gradually into shallow nanovoid, penetrating nanovoid, and then new void. Meanwhile, the old void connected with the main crack. The repetitions of the procedures resulted in the continuous propagation of a crack. Interaction between microcracks was observed. The propagation direction of one microcrack may change, affected by other microcracks. Voids were observed between microcracks and have a significant effect on the coalescence of microcracks.     

HIGH TEMPERATURE SHORT FATIGUE CRACK BEHAVIOUR IN A STAINLESS STEEL

Abstract— Continuous low cycle fatigue (LCF) tests with-and without-hold time in push-pull and torsion loading modes and sequential LCF tests in push-pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and creep-fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.
Crack density was higher on the surface of a CF tested specimen than that of a PF tested specimen. Mainly short cracks oriented at 45° to the specimen axis were observed on a torsion fatigue tested specimen surface. For push-pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45° to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis.
Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent CF loading and this produced a total damage summation significantly larger than one.     

The sensitivity of surface crack initiation to surface roughness in low-cycle fatigue at high temperature

Low-cycle fatigue tests have been carried out on AISI 304L stainless steel and Cr---Mo---V steel specimens with two different modes of surface roughness at 823 K. In the case of Cr---Mo---V steel, grain boundary cavities were not formed during the test. Transgranular cracks were formed and then propagated. The number of cycles required for the crack initiation was observed to be a very large fraction of the toral fatigue life. In the case of AISI 304L stainless steel, grain boundary cavities formed and intergranular crack initiation and propagation was also observed to occur. The number of cycles required for crack initiation was negligible in comparison with the total low-cycle fatigue life.     

FATIGUE CRACK GROWTH UNDER MODE II LOADING

Abstract— The behavior of fatigue crack growth for low and medium carbon steels, an austenitic stainless steel and an aluminum alloy under pure Mode II loading was investigated experimentally, using cruciform specimens. The results show that under pure Mode II loading, fatigue crack propagation has three possibilities, namely, bifurcation into two branches, propagation along the original Mode II direction, and the mixture of these two situations, depending on the material. The growth rate da/dN vs. ΔK_II relation for Mode II propagation is similar to a Pans type law for Mode I growth. Fractographic observations by optical microscopy and SEM were made also on all specimens tested. When a crack branched, striations parallel to the crack front which were often associated with Mode I fatigue crack growth were observed and long marks parallel to the crack propagation direction were also found for slanted fracture surfaces. When a crack propagated along the original Mode II direction, many frictional marks parallel to the crack propagation direction were observed.     

Growth of short cracks during low and high cycle fatigue in a duplex stainless steel

Damage evolution during low- and high-cycle fatigue in an embrittled duplex stainless steel is characterized in this paper. Moreover, scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) were employed in order to analyze microcracks formation and propagation. During low-cycle fatigue, microcracks initiate the ferrite phase either along slip planes with the highest Schmid factor (SF) inside the grains or at the α/α grain boundary. Then, microcracks propagation take place in ferrite or austenite grains with the highest SF. An analysis of the dislocation structure in the near-surface and in ferritic grains in the bulk of the specimen has shown that dislocation microbands are associated with microcrack initiation.In the high-cycle fatigue regime, damage generally initiates in the austenite by slip band formation followed by crack initiation either at an α–α boundary or at an α–γ boundary in the intersection of slip bands in the austenite. The microstructure in the austenite consists of a low density of dislocation pile-ups while the ferrite is practically inactive or develops only micro-yielding at boundaries.Despite the differences in both fatigue regimes, phase boundaries are an effective barrier against crack propagation because they delay the advance of the crack tip.