共查询到20条相似文献,搜索用时 31 毫秒
1.
Shigeaki Kobayashi Toshiyuki Inomata Hiroyuki Kobayashi Sadahiro Tsurekawa Tadao Watanabe 《Journal of Materials Science》2008,43(11):3792-3799
The effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation were studied in coarse-grained
polycrystalline aluminum specimens whose grain boundary microstructures were analyzed by SEM-EBSD/OIM technique. Fatigue crack
nucleation occurred mainly along grain boundaries and depended strongly on both the grain boundary character and grain boundary
configuration with respect to the persistent slip bands. However, it was little dependent on the geometrical arrangements
between the grain boundary plane and the stress axis. Particularly, random boundaries become preferential sites for fatigue
crack nucleation. The fatigue cracks were also observed at CSL boundaries when the grain-boundary trace on the specimen surface
was parallel to persistent slip bands. On the other hand, no intergranular fatigue cracks were observed at low-angle boundaries.
The fatigue cracks were observed at triple junctions as well as grain boundaries. Their nucleation considerably occurred at
triple junctions where random boundaries were interconnected. The grain boundary engineering for improvement in fatigue property
was discussed on the basis of the results of the structure-dependent intergranular and triple junction fatigue crack nucleation. 相似文献
2.
探索了奥氏体晶粒尺寸对珠光体等温转变组织特征以及对韧性性能的影响规律.研究表明,在相同等温转变温度下,珠光体片层间距无明显变化,随奥氏体晶粒尺寸的增加,先共析铁素体量减少而珠光体团尺寸增加.珠光体断裂韧性受控于裂纹前沿塑性影响区尺寸(1~2)δc,其中δc为临界裂纹张开位移,当原奥氏体晶粒大于(1~2)δc时,裂纹扩展阻力主要来自穿越珠光体片层α、θ相的颈缩、破断.当原奥氏体晶粒尺寸接近或小于(1~2)δc时,裂纹主要沿晶界、珠光体团界、α+θ片层界面扩展,通过扩展路径发生多次弯折消耗能量,随原奥氏体晶粒尺寸增加,准静态断裂韧度J变化幅度较小.而冲击韧性缺口前沿塑性影响区远大于原奥氏体晶粒,大角度晶界将促使裂纹的转折而提高扩展阻力,提高裂纹前沿塑性区大角度晶界密度有利于提高冲击功,冲击韧性A随晶粒尺寸的增加显著下降. 相似文献
3.
Shigeaki Kobayashi Manabu Nakamura Sadahiro Tsurekawa Tadao Watanabe 《Journal of Materials Science》2011,46(12):4254-4260
The effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel was investigated in
order to control fatigue crack propagation. The fraction of low-Σ coincidence boundaries in specimens was controlled by thermomechanical
processing. The specimen with the higher fraction of low-Σ boundaries (73%) showed the lower propagation rate of fatigue crack
than the specimen with the lower fraction of low-Σ boundaries (53%). The ratio of intergranular fracture segments to the total
crack length was lower for the specimen with the higher fraction of low-Σ boundaries. Moreover, the roles of grain boundaries
in the fatigue crack propagation were investigated in connection with grain boundary microstructure, i.e., the character distribution
and geometrical configuration of grain boundaries. It is evidenced that the approach to grain boundary engineering is applicable
to controlling fatigue crack propagation in austenitic stainless steel. 相似文献
4.
《Materials Science & Technology》2013,29(11):1247-1254
AbstractPolycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s?1 under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement. 相似文献
5.
Tingguang Liu Xiangkun Ru Shuang Xia Xiangyu Zhong Yonghao Lu 《Materials Science & Technology》2019,35(4):477-487
Grain boundary engineering (GBE) via low strain tension and annealing was used to enhance the resistance to stress corrosion cracking of a 304 stainless steel. Electron backscattered diffraction (EBSD) analysis exhibited that the GBE steel had a higher fraction of low-∑ coincidence site lattice (CSL) boundaries, larger grain-clusters, longer twin boundary chains, and fewer paths of connected non-twin boundaries with a more zigzag shape. Slow strain rate tests in high-temperature water showed that the GBE steel performed better plasticity, higher tensile strength, and similar yield strength compared to conventional steel. The low fraction of random boundaries in GBE steel resulted in a lower frequency of intergranular crack initiation, and the zigzag paths of non-twin boundaries made the intergranular crack propagation more difficult. 相似文献
6.
A previous four-dimensional in situ study of a short crack in a magnesium alloy King et al. (2011), Elektron 21, used synchrotron X-ray computed micro-tomography to follow its three-dimensional development with progressive fatigue cycling through the microstructure, which had been mapped by diffraction contrast tomography to measure grain shapes and crystal orientations in three dimensions. In the present work, very high-resolution post-test examination of the same sample by Serial Block Face Scanning Electron Microscopy (SBFSEM) provided three-dimensional fractographs to investigate the influence of microstructural features on the measured crack propagation rates. Digital volume correlation, applied to the X-ray computed micro-tomography datasets, measured the three-dimensional crack opening displacements and hence the crack opening modes. The short fatigue crack in magnesium propagated with mixed mode opening. Basal plane fracture is a dominant mechanism; hence, boundaries that disrupt the continuity of the basal plane are proposed to influence the crack propagation rate. 相似文献
7.
K. Kurihara H. Kokawa S. Sato Y. S. Sato H. T. Fujii M. Kawai 《Journal of Materials Science》2011,46(12):4270-4275
Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along
grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in
materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized
304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless
steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small
pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized
321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization
subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one.
The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region
and remarkable suppression of intergranular corrosion during GBE. 相似文献
8.
The fracture behaviour of Cu-14 wt% Al alloys has been studied as a function of nickel content varying from 0 to 10 wt%. It was found that the presence of a brittle phase
2 at the grain boundaries is responsible for intergranular fracture in low nickel alloys. Severe intergranular embrittlement exhibited by high nickel alloys in not associated with any precipitate at the grain boundaries. In fact when high nickel alloys are cooled slowly, a ductile phase () forms along the grain boundaries that resists the propagation of crack through grain boundaries and the fracture is transgranular. 相似文献
9.
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. 相似文献
10.
W. LuJ. Chen X. KongS.S. Chakravarthula Y. Qiao 《Mechanics of materials : an international journal》2011,43(10):567-573
The nonuniform characteristics of cleavage cracking across high-angle grain boundaries are analyzed in considerable detail. To break through a grain boundary, a cleavage front would first penetrate across the boundary at its central part, with the side sections being locally arrested. Such a front behavior causes a strong crack trapping effect and a large increase in required crack growth driving force. Eventually, as the persistent grain boundary areas are separated apart, the crack front bypasses the grain boundary. The critical condition of the unstable crack propagation is determined by both the local fracture resistance and its increase rate with respect to the expansion of the break-through window. The grain boundary toughness is dominated by the effective grain boundary ductility. 相似文献
11.
The competition between intergranular (IG) and transgranular (TG) fracture in fcc polycrystalline aggregates with physically representative GB misorientation distributions comprised of random low-angle, random high-angle, and coincident site lattice (CSL) GBs has been investigated. Physically-based critical conditions for IG fracture, due to the formation of dislocation pileups, and TG fracture, due to the propagation of cracks on cleavage planes, were coupled to a dislocation-density-based crystal plasticity formulation and a computational fracture scheme for crack branching to investigate how dislocation–GB interactions influence dislocation transmission, pileup formation, and local failure modes. The predictions indicate that aggregates with a large fraction of random and CSL high-angle GBs are dominated by IG fracture, as low GB transmission leads to extensive dislocation-density pileup formation and localized stress accumulations that induce IG fracture. Aggregates with a majority of low-angle GBs are dominated by TG failure, which is consistent with experimental observations. This investigation provides a fundamental understanding of the physical mechanisms governing IG and TG fracture in polycrystalline aggregates. 相似文献
12.
《Materials Science & Technology》2013,29(6):683-687
AbstractMonte Carlo simulations were used to investigate the effect of grain growth on the coincidence site lattice (CSL) boundary content of randomly textured polycrystals. Each grain was assigned an orientation, and grain boundary properties were dependent on both the boundary misorientation and the CSL character. While low misorientation angle boundaries (LABs) increase during growth, the fraction of CSL boundaries does not change with time. Decreasing CSL boundary energy and mobility did not alter these results. In contrast with LABs, which are characterised by a scalar misorientation angle, a particular combination of three independent rotation variables is required to create a low energy CSL boundary; thus, these boundaries are unlikely to form or to persist in a random polycrystal. While texture influences boundary formation, a texture that can enhance CSL boundaries is not apparent. Boundary plane effects should not increase CSL fraction during grain growth. 相似文献
13.
This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding. 相似文献
14.
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. 相似文献
15.
We present a phase-field model to simulate intergranular and transgranular crack propagation in ferroelectric polycrystals.
The proposed model couples three phase-fields describing (1) the polycrystalline structure, (2) the location of the cracks,
and (3) the ferroelectric domain microstructure. Different polycrystalline microstructures are obtained from computer simulations
of grain growth. Then, a phase-field model for fracture in ferroelectric single-crystals is extended to polycrystals by incorporating
the differential fracture toughness of the bulk and the grain boundaries, and the different crystal orientations of the grains.
Our simulation results show intergranular crack propagation in fine-grain microstructures, while transgranular crack propagation
is observed in coarse grains. Crack deflection is shown as the main toughening mechanism in the fine-grain structure. Due
to the ferroelectric domain switching mechanism, noticeable fracture toughness enhancement is also obtained for transgranular
crack propagation. These observations agree with experiment. 相似文献
16.
17.
L. Tan L. Rakotojaona T.R. AllenR.K. Nanstad J.T. Busby 《Materials Science and Engineering: A》2011,528(6):2755-2761
The microstructural evolution, specifically of grain boundaries, precipitates, and dislocations in thermomechanically processed (TMP) Alloy 800H samples was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The TMP not only significantly increased the fraction of low-Σ coincidence site lattice boundaries, but also introduced nanoscale precipitates in the matrix and altered the distribution of dislocations. Statistical analysis indicates that the morphology and distribution of grain boundary precipitates were dependent on grain boundary types. The microstructure optimization played a synergistic effect on the significantly increased strength with comparable ductility and enhanced intergranular corrosion resistance and creep-fatigue life compared to the as-received samples. 相似文献
18.
19.
The slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip band blocked by the grain boundary, a slip band propagated into an adjacent grain, and a slip band propagated through one and then blocked by a second grain boundary. The theory for continuously distributed dislocations is used to calculate the crack-tip sliding or opening displacement and the microscopic stress intensity factor under tensile and shear loading. Assuming that the range of the tip displacement directly determines the propagation rate of both Stage I and II cracks, prediction of the propagation behavior of a small crack is made as a function of the distance between the crack tip and the grain boundary, and of the difficulty to propagate slip into adjacent grains, as well as a function of crack length and stress level. The directions for further development of modelling are discussed. 相似文献
20.
Tanguy Lacondemine Clément Roux-Langlois Tanguy Rouxel 《International Journal of Fracture》2017,205(1):73-82
The hydrogen-related fracture propagation process in martensitic steel was investigated through crystallographic orientation and fracture surface topography analyses. The hydrogen-related fracture surface consisted of three typical surfaces, namely smooth surfaces, surfaces with serrated markings, and surfaces with dimples. Crystallographic orientation analysis suggested that the smooth surface was generated by intergranular fracture at prior austenite grain boundaries, and the surface with serrated markings originated from quasi-cleavage fracture propagated along \(\{011\}\) planes. According to the reconstructed fracture propagation process by fracture surface topography analysis, the intergranular fracture at prior austenite grain boundaries initiated and propagated suddenly at the early stages of fracture. The quasi-cleavage fracture along \(\{011\}\) planes then gradually propagated within the prior austenite grains. At the final stages of fracture, ductile fracture accompanied by dimples occurred around the edge of the specimen. The results clearly indicated that the fracture propagation path changed with the proceeding fracture from the prior austenite grain boundaries to along \(\{011\}\) planes within the prior austenite grains. 相似文献