AFM evidence of surface relief formation and models of fatigue crack nucleation

Atomic force microscopy and high resolution scanning electron microscopy were used to find the true surface relief corresponding to persistent slip markings emerging from persistent slip bands on fatigued polycrystalline austenitic and ferritic stainless steels. The persistent slip markings are formed by extrusions and intrusions. The shape of extrusions and intrusions was documented and the kinetics of the extrusion growth was studied in both steels. The experimental data were discussed and compared with the predictions of the recent models of fatigue crack nucleation.     

驻留滑移带与晶界和孪晶界的交互作用

对含有退火孪晶的多晶铜进行了不同塑性应变幅下的应变疲劳实验,利用扫描电镜及其电子通道衬度技术（SEM-ECC）观察了表面滑移形貌、疲劳裂纹和位错组态,研究了驻留滑移带与晶界和孪晶界的交互作用．结果表明,在晶界附近和远离晶界处观察到位错组态分布的不均匀现象．这种不均匀性导致多晶铜中疲劳裂纹首先沿着普通大角晶界开裂,在孪晶界处由于应变相容性较好而难以产生疲劳裂纹．     

Experimental evidence and physical models of fatigue crack initiation

Results of the study of the early fatigue damage in a number of model and structural crystalline materials using modern experimental techniques are presented. The dislocation structure of the persistent slip bands and the evolution of the surface relief resulting in the formation of persistent slip markings during cyclic loading are documented. The dislocation mechanisms leading to production of point defects in cyclic loading are described and point defect production and annihilation rates are derived. The kinetics of point defect migration is characterized. The physically based models of the surface relief formation describing the formation of extrusions and intrusions are presented. The models are confronted with experimental evidence. It is concluded that intrusions representing sharp surface crack-like defects play the principal role in the initiation of fatigue cracks.     

Cyclic Saturation Dislocation Pattern Observation in the Vicinity of Grain Boundaries by Electron Channeling Contrast Technique

State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences,Shenyang, 110015, China)Abstract:The cyclic saturation dislocation patterns within grains and in the vicinity of low-angle grain boundaries in fatigued copper crystal were successfully observed by electron channeling contrast technique in SEM. The results show that the dislocation patterns within grains consisted of typical two-phase structure, i.e. persistent slip bands (PSB) and veins. With increasing plastic strain amplitude (γp1 ≥1.7×10-3), large amount of PSBs and regufar dislocation walls were observed.The dislocation walls and PSBs could cross through the low-angle grain boundaries continuously except that the dislocation-free zone (DFZs) appeared at some local regions. Combining with the cyclic stress-strain response and dislocation patterns, the effect of low-angle grain boundaries on cyclic deformation behavior was discussed.     

Half-cycle slip activity of persistent slip bands at different stages of fatigue life of polycrystalline nickel

The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.     

A finite element model of persistent slip band interaction with strengthened surface films during low cycle fatigue

Hardened surface layers such as those produced by ion implantation can inhibit the emergence of persistent slip bands (PSBs) at the free surface during low cycle fatigue, thereby extending crack initiation life. They have little effect, however, on bulk PSB formation. PSBs nucleated in the interior eventually impinge on the underside of the surface layer, producing localized cyclic shear strains in the film which lead to film rupture and rapid crack initiation. As a result, increases in fatigue life are modest. The present finite element model analysis shows that PSBs carrying plastic strains characteristic of fatigued f.c.c. metals produce stresses in an elastic surface layer which may exceed 1% of the surface film shear modulus. The detailed results are consistent with phenomenological data on surface damage accumulation during low cycle fatigue of ion-beam-modified nickel. Implications for the design of surface microalloys for inhibition of fatigue crack initiation are discussed.     

Growth of extrusions in localized cyclic plastic straining

Cyclic strain localization into persistent slip bands produces on the surface of cyclically deformed materials persistent slip markings in the form of extrusions and intrusions. The localized cyclic plastic straining in persistent slip bands leads to the production and annihilation of dislocations as well as point defects. Production, annihilation and migration of point defects, preferably vacancies, is quantitatively characterized. The effect of vacancy production and migration on the extrusion growth is treated under simplified conditions. At low temperatures when vacancies are immobile, a static extrusion is produced. At elevated temperature the diffusion equation is solved, a general expression for stabilized rate of extrusion growth is obtained and temperature dependence of the growth rate can be predicted. The predictions of the model are compared with experimental observations.     

Some experimental studies of fatigue slip bands and persistent slip bands during fatigue process of low-carbon steel

Initiation and propagation behaviours of fatigue slip bands and persistent slip bands in lowcarbon steel have been investigated: fatigue slip bands were expressed quantitatively as a volume fraction which was products of the slip bands area (Σ A_s/A₀), the depth (h_s/h₀) and the number of slipped grains (G_s/G₀).

It was shown that distributions of fatigue slip bands and persistent slip bands showed the maximum at the angle division of 0° ˜ 10° (at and near the normal to the stress axis) and decreased gradually with increasing of the angle. And the normalized volume fraction of fatigue slip bands increased linearly in proportion to the number of cycles independently of the stress amplitude σ_a and the angle θ. The increment in volume fraction of fatigue slip bands meant the increment in fatigue damage and the fatigue life decreased in proportion to the increment in volume fraction of fatigue slip bands until its value reached a certain content. Then, the initiation rates of fatigue slip bands, persistent slip bands and stage I microcracks showed the increasing tendency with increasing of stress amplitude σ_a, and the propagating rates from fatigue slip bands to persistent slip bands and from fatigue slip bands to stage I micro-cracks were also similar increasing tendency with stress amplitude.     

Observation of fatigue damage in structural steel by scanning atomic force microscopy

Fatigue slip bands and plastic deformation around fatigue microcracks were observed by scanning atomic force microscopy. In fatigue slip bands, extrusions were observed but intrusions were not detected. Large extrusions were found in slip bands whose traces at specimen surface were almost perpendicular to the loading axis. Microcracks propagated under mixed mode condition of Mode I, Mode II, and Mode III.     

NUCLEATION AND SHORT CRACK GROWTH IN FATIGUED POLYCRYSTALLINE COPPER

Surface evolution in polycrystalline copper specimens with a shallow notch has been studied in interrupted constant strain amplitude cyclic loading. The inhomogeneous strain distribution close to stress amplitude saturation leads to the formation of extrusions and intrusions along persistent slip bands within the grain and also in suitably oriented grain boundaries. Numerous primary cracks within a grain or at a grain boundary are nucleated. Some cracks can grow further either by linking with existing cracks or by nucleation of new elementary cracks ahead of the crack tip. Crack growth rates of individual cracks fluctuate considerably but for each strain amplitude, which results in a saturated plastic strain amplitude, a crack growth rate of an equivalent crack can be established. This crack growth rate was found to depend strongly on the plastic strain amplitude in agreement with the Manson-Coffin law.     

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。     

Initiation and short crack growth in austenitic–ferritic duplex steel‐effect of positive mean stress

The effect of the mean stress on the crack initiation and short crack growth of austenitic–ferritic duplex steel has been studied. High mean stresses and stress amplitudes result in appreciable mean strain relaxation and long‐term hardening. Mean stress produces unidirectional slip bands and slip steps that serve as nuclei for persistent slip bands and persistent slip markings. It leads to the acceleration of the crack initiation and production of a high density of cracks. Crack linkage contributes to the growth of short cracks. The concept of equivalent crack was used to describe the crack growth. The kinetics of short crack growth with positive mean stress is similar to that in symmetric loading, that is, exponential growth is observed. Positive mean stress results in earlier crack initiation and in the acceleration of the crack growth rate. Both factors contribute to the decrease of the fatigue life.     

Ultrasonic back reflection evaluation of crack growth from PSBs in low-cycle fatigue of stainless steel under constant load amplitude

The objective of this study is to develop a method for evaluating crack growth from persistent slip bands (PSBs) in low-cycle fatigue of stainless steel, using an ultrasonic back reflection wave during the early stages of its fatigue life. Changes in the back reflection intensity from surface of the material under cyclic loading are measured. Back reflection intensity decreased due to the evolution of PSBs before the start of fatigue crack growth from the crack initiated along PSBs with increase in the number of cyclic loads. The average dislocation density in a grain including PSBs corresponds to the attenuation change measured during the fatigue test, from the initial state to the nucleation and growth of the fatigue crack. The attenuation is caused by the movement of dislocation due to ultrasonic waves, whose mechanism was considered quantitatively. In this study, micromechanical modeling was conducted as a prediction method for remaining fatigue life to start crack growth from PSBs based on the changes in ultrasonic back reflection intensity.     

Observations of slip patterns on the surface of fatigued gold using the scanning tunnelling microscope

Observations of fatigue-induced slip patterns on the surface of polycrystalline gold (+99.9%) using the scanning tunnelling microscope (STM) are presented. The samples were cycled in four-point bending between zero and 0.0025 strain for 250 and 2500 cycles. STM observations of the sample fatigued for 250 cycles revealed broad slip bands 0.38–0.69 m wide containing narrow slip bands 0.015–0.123 m wide and fine slip lines 0.006 m wide. The depth of these features are 20–57 nm for broad slip bands, 3–7 nm for narrow slip bands, and approximately 2–4 nm for fine slip lines. The sample fatigued for 2500 cycles showed similar values for the width of the slip bands and slip lines except that their depth was increased by a factor of between 2 and 4 times for the narrow slip bands and the fine slip lines. Apparent persistent slip bands (PSBs) spaced 2.4 m, 300–500 nm deep were also observed. These results demonstrate that STM is a significant new tool for observing and distinguishing various types of fatigue-induced surface slip patterns on suitably prepared samples. The vertical resolution obtained with STM is vastly superior to current SEM and TEM methods.     

FATIGUE LIFE BEHAVIOR OF COPPER SINGLE CRYSTALS. PART I: OBSERVATIONS OF CRACK NUCLEATION

In low strain fatigue, cracks often form in persistent slip bands (PSB) in a wide variety of materials. Observations of crack nucleation involving PSB have been made in copper single crystals by careful interferometric studies. The width, length, step height and thus strain localization in the PSBs remain very stable during saturation, even up to three-quarters of the life. For a given applied strain amplitude, the specimen will form PSBs having a certain distribution of slip offsets. The fatal crack is found to nucleate in the group of micro-PSBs having the largest offsets, and the largest strain localization. The PSB volume fraction for a given strain amplitude depends only very slightly on crystal orientation, and the slip offsets not at all. Thus the kinetics of crack nucleation are not affected by orientation. The mechanism of nucleation is concluded to be one of random walk in the slip offsets of PSBs.     

On the formation and stability of dislocation patterns—III: Three-dimensional considerations

A model is proposed to describe the occurrence of the vein structures and the nucleation of persistent slip bands in fatigued metals as a cooperative phenomenon of motion and interaction of dislocation populations. It is based on the competition between diffusive-like mobilities for random or stress induced dislocation motion and cubic-like non-linearities for the pinning of stress driven dislocations by immobile dipoles. It describes the coexistence of rod-like vein structures and ladder-like persistent slip bands in three dimensions, integrating the qualitative picture of pattern formation in one and two dimensions given earlier in Parts I and II.     

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

Abstract

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.     

Evaluating Schmid criterion for predicting preferential locations of persistent slip markings obtained after very high cycle fatigue for polycrystalline pure copper

Very high cycle fatigue carried out on pure copper polycrystals promotes early slip markings, labelled as slip markings of types II and III, localized close to grain or twin boundaries. In this work, we focus on whether Schmid criterion can predict the preferential sites of slip markings of types II and III and identify the active slip systems. Combining observations of slip markings and polycrystalline modeling, it is shown that considering pure cubic elastic behavior, maximum resolved shear stress as a criterion for type II slip markings preferential sites is 70% reliable criterion. Concerning slip markings of type III, the reliability falls to 30%. The role of cross slip is highlighted and a scenario rationalizing the stress amplitude conditions and sites to observe early slip markings of type II or III for copper polycrystals is proposed.     

Dislocation patterning in fatigued metals: Labyrinth structures and rotational effects

The nucleation of persistent slip bands (PSBs) in stressed metals may be interpreted as a cooperative phenomenon for dislocation populations. The competition between their mobility and nonlinear interaction (creation, annihilation, pinning) induces an instability of uniform dislocation distributions vs inhomogeneous ones and leads to the formation and persistence of spatial patterns. In particular, we have shown in preceding publications that the nucleation and coexistence of the layered structure of PSBs and the rodlike structure of the surrounding matrix can be described by a system of coupled partial differential equations of the reaction-diffusion type for immobile and mobile dislocations. Here we consider explicitly the motion and interaction of positive and negative mobile dislocations and derive an expression for their effective diffusivity. Moreover we show how these effects together with secondary slip activity may trigger the formation of walls in different directions in analogy with observations of labyrinth structures.