Ni-based speralloys have been widely used to make the blade parts of the advanced aeroengines for their high temperature tolerance and good mechanical property. During high temperature service, the materials endure the effects of temperature and alternating load, causing high-cycle fatigue deformation on the hot-end components. Meanwhile, the fatigue behaviors of the alloy are closely related to the deformation mechanisms and its microstructure characteristics, such as the size, distribution and morphology of γ' phase and carbides, and the fatigue fracture of the using materials possesses unpredictability. Therefore, investigating fatigue behaviors of the material is of significance in alloy design and life prediction. But the high-cycle fatigue behavior of K416B superalloy with high W content is still unclear up to now. For this reason, by means of high-cycle fatigue property measurement and microstructure observation, the high-cycle fatigue behavior of K416B Ni-based superalloy at 700 ℃ has been investigated. The results show that at 700 ℃ and stress ratio R=-1, the high-cycle fatigue life of K416B superalloy decreases with the stress increasing, and high-cycle fatigue strength of the alloy is 175 MPa. At the condition of low stress amplitude, the deformed dislocations may slip along different orientations in the matrix. With the stress amplitude increasing, the dislocations may shear into γ' phase and form the stacking fault. During tension and compression high-cycle fatigue, multiple slip systems are activated in the alloy, and the distortion occurs along various directions, resulting in stress concentration on the regions of γ +γ' eutectic and carbides. The crack sources may be initiated at the eutectic and blocky carbide near the surface of the alloy. As high-cycle fatigue goes on, the cracks propagate along the inter-dendrite in expansion region, and the typical cleavage fracture occurs in the final rupture region.
Empirical approaches to characterize the variability of high cycle fatigue have been widely used. However, little is understood about the intrinsic relationship of randomness of microstructure attributes on the overall variability in high cycle fatigue. The ability of quantifying the dispersivity of high cycle fatigue with physics based computational methods has great potential in design of minimum life and can aid in the improvement of fatigue resistance. To investigate the effects between microstructure attributes and high cycle fatigue dispersivity, the microstructure-sensitive extreme value probabilistic framework is introduced. First, the Voronoi algorithm is used to construct random polycrystalline microstructure representative volume elements. Different kinds of periodic boundary conditions are proposed to simulate the interior and surface constraints in polycrystalline microstructure representative volume elements. Then mechanical responses of both interior and surface microstructure representative volume elements under different strain amplitudes are simulated by internal state variable based crystal plasticity. The fatigue indicator parameter is introduced to characterize the driving force for fatigue crack formation dominated by maximum shear plastic strain amplitude. By computing a limited number of random polycrystalline microstructure representative volume elements, the distributions of fatigue indicator parameter under different strain amplitudes are obtained and analyzed with extreme value probability theory. The study with a kind of titanium alloy with material grade TC4 supports that the high cycle fatigue dispersivity increases with the decrease of the strain amplitude, especially under elastic limit. The extreme value of fatigue indicator parameter from random polycrystalline microstructure representative volume elements correlates well with the Gumbel extreme value distribution. Besides, the lower the average stress under different strain amplitudes, the fewer grains in polycrystalline microstructure representative volume element yield. Moreover, the grains on surface tend to have higher probability to initiate fatigue cracks and lower dispersivity in fatigue crack formation.
Ti-6Al-4V alloys are widely used in aero engine blades for their unique properties, such as high specific strength, high specific stiffness and high fatigue strength. Aero engine blades usually suffer a variety of cyclic loading during the period of services, which finally results in fatigue failure. Fatigue life of materials is known to highly depend on the surface quality. Consequently, more and more researches about the influence of machined surface roughness on the fatigue behavior of materials have been carried out in the last decades. However, there are less relevant results about the relationship between surface roughness and very high cycle fatigue (VHCF) properties of Ti-6Al-4V alloy. To investigate the effects of surface roughness on fatigue properties of Ti-6Al-4V alloy under very high cycle fatigue regimes, ultrasonic fatigue tests were conducted at the conditions of 20 kHz and stress ratio R1=-1 at room temperature in air. During ultrasonic fatigue testing, each specimen was water-cooled. The specimen surfaces were cut and grinded which gave different surface roughnesses. The surface roughness was characterized using profilometry. In order to explain the high dependence of stress-fatigue life curves on the surface roughness, an approach based on the finite element analysis of measured surface topography was proposed. The results show that the VHCF property of Ti-6Al-4V alloy was significantly affected by surface roughness. The critical flaw size was 0.49~1.10 μm when the ratio between spacing and height of circumferential grooves was between 2~10. When surface roughness was smaller than the critical flaw size, surface roughness exerted no influence on fatigue life. While surface roughness was greater than critical flaw size, fatigue life decreased with increasing surface roughness. Surface roughness played a more important role in long life regime than that in VHCF regime in which with the growth of surface roughness, the crack initiation site changed from single one to two or more ones, as well as changed from inside to subsurface. When the surface roughness was large enough, all cracks initiated from surface even in super long life regime.
It is well known that the cyclic deformation behavior and dislocation structures of Cu single crystals with different orientations have been systematically investigated and understood. However, there is as yet no general and unequivocal knowledge of the thermal stability of fatigue-induced dislocation structures in Cu single crystals, which is particularly significant for the further improvement of low energy dislocation structure (LEDS) theory. In previous work, the thermal stability of fatigue dislocation structures in 18 41] single-slip and coplanar double-slip Cu single crystals have been reported. For deeply understanding the orientation-dependent thermal stability of fatigue dislocation structures, in the present work, conjugate and  critical double-slip-oriented Cu single crystals were cyclically deformed at different plastic strain amplitudes γpl up to saturation, and then annealed at different temperatures (300, 500 and 800 ℃) for 30 min, to examine the thermal stability of various fatigue-induced dislocation structures. It was found that an obvious recovery has occurred in various dislocation structures at 300 ℃. At the higher temperatures, e.g., 500 and 800 ℃, a remarkable recrystallization phenomenon takes place together with the formation of many annealing twins. The thermal stability of various dislocation structures produced in fatigued Cu single crystals with different orientations from high to low are on the order of vein structure, persistent slip band (PSB) structure, labyrinth structure and dislocation cells. The annealing twins formed in Cu single crystals with different orientations all develop strictly along the dislocation slip planes, which have been operated under fatigue deformation. The more serious the fatigue-induced slip deformation, the greater the amount of annealing twins would be. Furthermore, an over high annealing temperature, e.g. 800 ℃, would greatly speed up the migration of boundaries of recrystallized grains to restrain the formation of annealing twins, leading to, more or less, the decrease in the amount of twins.
Effects of welding heat input on the microstructure and dynamic fracture toughness (JId) of the CO2 shielded arc welded joints of Q460 high strength low alloy steel were investigated. The mechanism of effects on the dynamic facture behavior of the welded joint was also discussed. The results showed that there existed the allotriomorphic ferrite at the columnar interface in the fusion zone of welded joint under the condition of low heat input. The morphological characteristics of columnar crystal in the fusion zone gradually decreased and the allotriomorphic ferrite disappeared as the heat input increased. The fusion zone was mainly composed of acicular ferrite, and its average size increased with increasing heat input. The welded joint exhibited the optimal dynamic fracture toughness under the condition of medium heat input while it showed the lowest value under low heat input within the temperature range of -70 ℃ to room temperature. When the temperature decreased from room temperature to -70 ℃, the dynamic fracture mechanism of Q460 welded joint changed from ductile fracture to brittle cleavage fracture. Under the condition of low heat input, the allotriomorphic ferrite characterized by the planar growth at the columnar interface in the fusion zone of welded joint can lead to the rapid intergranular crack propagation at low temperature. The fine acicular ferrite in the fusion zone of the welding joint obtained at medium heat input which can hinder the crack propagation during the dynamic fracture at low temperature to the greatest extent is the reason why the welded joint exhibits high dynamic fracture toughness.
TC4 titanium alloy is usually used to manufacture engine blades, blings or blisks and fatigue is the main failure of these components due to its high strength, good corrosion resistance and light weight. In engineering applications, three typical surface modification processes such as shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) were employed to improve fatigue performance. In this work, SP, LSP and LSB were taken to enhance surface layer of TC4 titanium alloy. The surface integrity of specimens including surface roughness, microhardness, residual stresses and microstructure was investigated to obtain the effects of modification on surface layer by different methods. The rotating-bending fatigue performance was tested at room temperature and fatigue fracture surfaces were analyzed by SEM. Fatigue life was compared at the same stress 760 MPa with the reference machinced specimen. Fatigue strength was determined by stair method for 1×107 cyc. The results show that both the rotating-bending fatigue life and fatigue strength of TC4 titanium alloy are increased by these surface modification processes. The fatigue life prolonging factor (FLPF) for SPed specimens is 20.4, and FLPF for LSPed specimens and LPBed specimens is 89.6 and 99, respectively. Meanwhile, fatigue strength improvement percentage (FSIP) for SPed, LSPed and LPBed specimens is 36.3%, 37.8% and 38.8%, respectively. Moreover, the fatigue cracks initiate beneath surface enhanced layer for surface-modified specimens, while they are located at surfaces for un-surface-enhanced ones. Based on dislocation theory, the subsurface cracks initiation resistance and fatigue strength for surface-enhanced specimens were analysied. Finally, surface modification mechanisms were discussed and some quantitative analysis methods on surface modification effects were proposed. For surface-enhanced smooth specimens, the FSIP limit is 40% based on proposed analysis model and it is verified in this work by different surface layer enhancement processes (36.3% for SPed specimens, 37.8% for LSPed and 38.8% for LPBed specimens are near to 40%). Fatigue total life including initiation and propagation is a complex problem, and therefore it is difficult to give accurate life prediction and analysis, especially for small crack growth, although some invesitigations on total fatigue life can be roughly estimated based on Basquin relation for stress fatigue life or Coffin and Marson eqution for strain fatigue life which have not any physical meaning or any mechanism.
Titanium alloys have been widely used as superior engineering materials because of their high specific strength, high temperature resistance and high corrosion resistance. In their engineering applications such as used in aircraft engines, titanium alloys may experience even 1010 fatigue cycles. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes of titanium alloys, which resulted in a sharp decrease in fatigue strength. Therefore, the HCF and VHCF of titanium alloys have both scientific significance and engineering requirement. In this work, the effects of microstructure and stress ratio (R) on HCF and VHCF of a Ti-6Al-4V alloy have been investigated. Fatigue tests were conducted on a rotating-bending fatigue machine and an ultrasonic fatigue machine. All the fatigue fracture surfaces were observed by SEM. The results show that the HCF and VHCF behaviors of the fully-equiaxed and the bimodal Ti-6Al-4V alloy are similar. The observations of fracture surface indicate that two crack initiation mechanisms prevail, i.e. slip mechanism and cleavage mechanism. With the increase of stress ratio, the crack initiation mechanism switches from slip to cleavage. The S-N curves present the single-line type or the bilinear type. For the cases of rotating-bending and ultrasonic axial cycling with R= -1.0, -0.5 and 0.5, the S-N curves are single-line type corresponding to the slip mechanism or cleavage mechanism. For the cases of R= -0.1 and 0.1, the S-N curves are bilinear type corresponding to both slip and cleavage mechanisms. A model based on fatigue life and fatigue limit is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results.
After decades of development, mechanical properties of pipeline steels have a good combination of strength and toughness. But after welding, in the heat affected zone (HAZ), microstructure of the base plate was erased by the welding thermal cycle. Several subzones with different microstructures were formed in the HAZ due to different thermal histories they went through. Toughness of the HAZ varies due to the heterogeneous microstructure. In this work, toughness of the HAZ of X100 pipeline steel was examined with two notch locations. Low toughness of 51 J was obtained when the notch encountered intercritically reheated coarsen-grained (ICCG) HAZ and high toughness of 183 J when the notch did not contain ICCGHAZ. Meanwhile, different sub-zones in the HAZ were simulated using Gleeble thermal simulation machine. Simulated coarsen-grained (CG) HAZ, fine-grained (FG) HAZ and intercritically reheated (IC) HAZ with uniform microstructure had good toughness of 244, 164 and 196 J, respectively. In contrast, toughness of simulated ICCGHAZ was only 32 J. Therefore, ICCGHAZ consisting of coarse granular/upper bainite and necklace-type martensite-austenite (M-A) constituent along grain boundaries was proved to be the primary reason for low toughness. Instrumented Charpy impact test results showed that ICCGHAZ could notably embrittle the sample and lower the crack initiation energy. Characterization on the fracture surfaces of the as-fractured Charpy impact specimens showed that ICCGHAZ was found to be the crack initiation site of the whole fracture, and M-A constituent in the ICCGHAZ was characterized as cleavage facet initiation. Fracture mechanisms in the CGHAZ and ICCGHAZ were separately investigated using EBSD. The results showed that necklace-type M-A constituent in the ICCGHAZ notably increased the frequency of cleavage microcracks nucleation. Fracture mechanism changed from nucleation controlled in the CGHAZ to propagation controlled in the ICCGHAZ due to the existence of necklace-type M-A constituent. Therefore, the formation of necklace-type M-A constituent in the ICCGHAZ could not only cause notable drop of toughness in the HAZ, but also change the fracture behavior/mechanism. Hence, research on how to control the distribution status of M-A constituent in the ICCGHAZ is the key to improve the toughness of a weld joint.
Recently, low-cost advanced high strength steels (AHSS) with high toughness and fatigue limit have been developed in order to ensure the safety and lightweight of the engineering components. As promising candidates for next generation of AHSS, the bainite/martensite multiphase high strength steels exhibit excellent combination of strength and toughness due to the refined multiphase microstructure and retained austenite films located between bainitic ferrite laths. The previous works showed that the mechanical properties of bainite/martensite multiphase steels can be further improved through quenching-partitioning-tempering (Q-P-T) process. In the present work, the effect of Q-P-T process on the microstructure and fatigue behaviors of steels was investigated, and the relationship between the microstructure and the fatigue crack propagation was discussed in detail. Here, a 20Mn2SiCrNiMo bainite/martensite multiphase steel was treated by Q-P-T processes: (1) quenching to 200 ℃, partitioning at 280 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT200 sample); (2) quenching to 320 ℃, partitioning at 360 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT320 sample). Microstructure observations showed that the QPT200 sample consisted of leaf-shaped bainite, martensite and filmy retained austenite (RA), while some blocky martensite/austenite (M/A) islands were observed in QPT320 sample. The volume fractions of retained austenite in QPT200 and QPT320 samples are 4.5% and 9.8%, respectively. The fatigue crack propagation rate da/dN and threshold value of fatigue cracking ΔKth were measured using compact-tensile specimens. The results showed that the Q-P-T process parameters had a significant influence on the microstructures and fatigue properties of the bainite/martensite multiphase steels. The bainite/martensite multiphase steel after appropriate Q-P-T treatment (QPT 200 sample in the present work) has higher ΔKth and lower da/dN, which originates from the resistance on fatigue crack propagation due to the presence of leaf-shaped bainite and nanometer-sized retained austenite films. Furthermore, although the volume fraction of retained austenite in QPT320 sample is higher than that in QPT200 sample, the ΔKth of QPT 320 sample is lower than that of QPT200 sample. It is suggested that the effect of retained austenite on the fatigue behaviors depends on its volume fraction, size and morphology.
Owing to the unique amorphous structure, metallic glasses (MGs) exhibit quite distinctive deformation and fracture behaviors from the conventional crystalline materials. The high strength, brittleness and macroscopic homogenous and isotropic structural features make MGs ideal model materials for the investigations of the strength theory of high-strength materials. Hence the fracture behavior and strength theory of MGs have attracted very extensive interests of researchers from the fields of materials, mechanics and physics. This paper is based on the research works of the authors on the fracture and strength of MGs in the past decade, and concentrates on discussing the current knowledge and recent advances on the fracture behavior and strength theory of ductile and brittle MGs. Firstly, the fracture behaviors of ductile and brittle MGs including tension-compression strength asymmetry, fracture mechanism and ductile-to-brittle transition will be briefly elaborated. Then the strength theories of MGs will be discussed, with our emphasis on the foundation, validation, further development and application of the ellipse criterion. At last, some unsolved issues associated with the fracture and strength of MGs are proposed.
The prospect of joining titanium and aluminum components into structures is desirable for a wide range of aerospace and automobile industry applications. One of the problems related with the joining processes for dissimilar metals such as Ti and Al is the formation of residual stress in the bonded joint, which has significant effect on the joint mechanical properties. In this work, joining of a titanium alloy to an aluminum alloy by ultrasonic assisted brazing using a Zn-Al filler metal was investigated. The microstructures of the titanium/aluminum brazed joints were determined by OM, SEM and TEM. The local tensile deformation characteristics of the brazed joints were also examined using the digital image correlation (DIC) methodology by mapping the local strain distribution during in situ tensile tests. The results showed that the Ti7Al5Si12 phase and the TiAl3 phase were formed at the titanium/brazing seam interface. The brazing seam was primarily composed of a Zn-rich phase and a Zn-24.14%Al (mass fraction) eutectoid structure. At the aluminum/brazing seam interface, no interfacial reaction layer was observed and the primary phase Zn-Al dendrites nucleated at the aluminum base metal and grew into the inside of the bonding region. A diffusion layer was formed in the aluminum base metal. It was found that the tensile deformation of the brazed joints was highly heterogeneous, which led to the deflection of the crack during propagating in the joint. The fracture initiated at the Zn-rich phases, where contained the highest stress concentration due to their low elastic modulus, and propagated in the Zn-rich phases or through the interface between Zn-rich phase and Zn-Al eutectoid structure.
Cu-Cr-Zr alloy usually applys to the complex environment at high temperature. The mechanical behaviors of alloy are different from the condition of normal temperature. At high temperature, grains and precipitates of ultra-fine grain Cu-Cr-Zr alloy become coarse and it would affect the hot deformation behavior of alloy. To solve the thermal stability of the ultra-fine grain materials, the grain growth mechanism and the driving force of ultra-fine grain materials must be studied, as well as trace elements on the thermal stability mechanism. Tensile properties, microstructure of fracture and fracture mechanism of ultra-fine grain (UFG) Cu-Cr-Zr alloy made by two different treatment methods were studied at the temperature range of room temperature to 600 ℃. The results show that the ultimate tensile strength (UTS) of alloys decreases with increasing temperature. The UTS and elongation of No.1 alloys are about 577.17 MPa and 14.6% at room temperature, respectively. And No.1 alloy start to occur dynamic recrystallization and UTS decreases fast at 300 ℃. The UTS of No.1 alloy are only 59.12 MPa at 600 ℃. The UTS and elongation of No.2 alloy are about 636.71 MPa and 12.1% at room temperature, respectively. The pinning effect by precipitation on grain boundary in the No.2 alloy begins to weaken at 400 ℃. The UTS of No.2 alloy decreases fast and are only 65.20 MPa at 600 ℃. Compared to No.1 alloy, No.2 alloy have better room temperature property and thermal stability. The elongation of all alloys increases with increasing temperature and show superplasticity on elevated temperature. The high temperature tensile fracture morphologies are an intense and deep dimple pattern. The high temperature fracture mechanism is ductile fracture by gathered microporous.
Wheel is one of the key components of a train to transmit power and affect the security operation. With the rapidly development of high-speed railway, rolling contact fatigue of railway wheels has become an important issue with respect to failure. With the increasing of train speeds and axle loads, the wheel-rail dynamic stress and contact stress were increased, resulting in wheel out of round with off-axis wear and potential for derailment. SAE 1050 steel as a typical wheel steel is widely used in high-speed wheel and wagon wheel. Consequently, the wheel rolling contact fatigue performance under service process and the fatigue performance of wheel steel materials have been studied. However, there are less relevant results about the anisotropy of rolling and off-axis loading of wheel steel materials. To investigate the effect of anisotropy and off-axis loading on fatigue property of 1050 wheel steel, uniaxial fatigue tests were conducted at the conditions of 120 Hz and stress ratio R=0.1, and off-axis fatigue tests were conducted at the conditions of 55 Hz and R=0.1 at room temperature in air. All fatigue specimens were cut from bar round with the angles (0°, 30° and 45°) to rolling direction. The fatigue limit of specimens under two kinds of special loading conditions was obtained. Fracture surface of the specimen was observed by SEM. The finite element (FEM) analysis software (Ansys 14.0) was used to analyze static mechanics of specimens under three different off-axis loading angles (0°, 30° and 45°). The results showed that the fatigue limit decreased with increasing angle to rolling direction and the percentage of decline was 9%. The fatigue limit decreased with increasing off-axis loading angle and the percentage of decline was 85%. The shear stress and Von Mises stress were larger and increased with increasing off-axis loading angle when the specimen was subjected to off-axis loading.
Duplex stainless steels are widely used in nuclear industry for their excellent mechanical behavior, good weldability and superior ressistance to corrosion, the fracture toughness of which will be deteriorated with ageing time, as they are exposed to a certain temperature (204~538 ℃). In the present work, hot forging will be employed to induce the change of ferrite grain orientation and refinement of austenite grains; it is expected to improve the impact toughness after long-term thermal ageing. The microstructure and impact surface morphology of Z3CN20-09M duplex stainless steel were investigated by using SEM and EBSD. The micro-mechanical properties and impact properties of Z3CN20-09M duplex stainless steel at different thermal ageing time were tested by a nano-indenter and an instrumented impact tester. The results show that the crystal orientation of ferrite changes obviously and the austenite is changed from the original coarse columnar grains to the fine equiaxed grains after hot working. The im pact toughness of cast materials and forged materials decreases greatly with ageing time. The charpy impact energy of both aged and unaged forged-materials is higher than that of cast material. Cast material and forged material exhibit microvoid coalescence fracture in the early of thermal ageing; after 3000 h thermal ageing, the impact fracture features changes from ductile dimples to brittle cleavages in ferrites and tearings or dimples in austenites. However, cleavage features in forged material are significantly less than those in cast material due to the difference in ferrite crystal orientation.
Annealing treatment is an effective method for improving structural stability of ultrafine-grained (UFG) or nanostructured (NS) materials produced by severe plastic deformation (SPD). This work focuses on the effect of annealing temperature on the tensile fracture behavior of UFG Cu produced by accumulative roll bonding (ARB). Annealing treatment was performed for 10 min at temperatures of 100, 150, 200 and 250 ℃. The microstructure of annealed and ARBed UFG Cu was observed by TEM. The uniaxial static tensile test was performed by utilizing fatigue testing machine (IBTC-5000) with an initial strain rate of 10-2 s-1. Fracture morphology was observed by SEM. The results suggested that yield strength and tensile strength decreased after annealing treatment compared with initial sample. However, yield strength and tensile strength of ARB-Cu increased with increasing annealing temperature below recrystallization temperature. When annealing temperature is higher than recrystallization temperature, the strength decreased rapidly. With increasing the annealing temperature, the grain size of ARB-Cu increases and gradually tends to bimodal distribution, and the fracture morphology shows a trend of increasing plasticity gradually. The annealing treatment is helpful to bonding efficiency E. The relationship between the theoretical bonding efficiency E and the ARB passes n can be expressed in E=(1-0.5n)×100%.
Titanium alloys have been widely used in bearing force components in aeronautical structures, such as blades and beams to withstand the high frequency dynamic loads, which requires an outstanding fatigue resistance performance in very high cycle regime during their service life. In this work, very high cycle fatigue failure property of TC17 alloy used as aircraft engine blade material was studied by ultrasonic fatigue test and electromagnetic resonance fatigue test under 110 Hz and 20 kHz sinusoidal load, and crack initiation mechanism of different failure mode was analyzed. The results showed that, fatigue failure modes of TC17 alloy could be divided into surface induced failure and interior induced failure. Surface induced failure was caused by the machine defect and surface slide trace that triggered by the asymmetric loading. Interior induced failure was caused by slid fracture of primary α phase under asymmetric loading. Fatigue resistance of TC17 alloy was influenced by the fatigue crack initiation mechanism but concerned little about the loading frequency. The variation of the fatigue failure mechanism resulted in the S-N curves presenting bilinear. A fatigue strength predicted model is established based on the parameter of the weak crystal orientation area, which is in good agreement with the fatigue test result.
Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually happens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This paper describes a new framework for the micromechanism of cleavage fracture of high strength low alloy (HSLA) steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The toughness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle-sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The "critical event" of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weakest microstructural component and its critical size which specifies the local cleavage fracture stress σf for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the combined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analyses. A case study, using the new framework of micromechanism for analyzing toughness of 8%Ni steel welding metals is presented to analyze the experimental results.
The low cycle fatigue (LCF) experiments of nickel-based turbine disc alloy GH4738 have been carried out at different temperatures in air to investigate the influence of temperature on fatigue crack growth (FCG) behavior of GH4738 alloy. The FCG curves (da/dN-ΔK and a-N) and their regularity have been obtained. The results show that there is a sensitive range of temperature in which the fatigue life for GH4738 decreases sharply. The microstructures and fracture surface morphologies of the GH4738 samples tested at different temperatures were observed by FE-SEM, and changes of the mechanical properties of GH4738 at high temperature were also taken into account through modifying the stress intensity factor amplitude, ΔK. The interruption experiments were carried out at 700 ℃ and room temperature, respectively, to investigate the crack growth mode and oxidation degree at the crack tip and grain boundary of the samples. And the essential reason of temperature influence on FCG behavior of GH4738 was discussed. The result showed that as the temperature increases, the fatigue crack growth rate (FCGR) of GH4738 accelerates, the fracture surface tends to coarse, and the failure mode converts from a mixed transgranular and intergranular fracture to totally intergranular fracture. The fatigue crack growth lifetime decreases remarkably at 650~700 ℃, existing a temperature-sensitive region under ΔK=40 MPam1/2 and 30~40 μm grain size conditions, which is mainly caused by the oxidation at elevated temperature, independent of the microstructure and mechanical property. Oxygen diffuses into the grain boundary through crack tip and slip band, or penetrates directly into the grain boundary, reacts with active elements (Co, Ti, Al) and generates brittle oxides. These brittle oxides result in weakening of grain boundary and significant decrease of fatigue property of GH4738.
The world has gradually entered the industrial 4.0 Era, which is dominated by the Internet of Things (IOT) and intelligent manufacturing. Especially, strong requirement for artificial intelligence and big data processing, the development and preparation of micro/nano electronic devices is becoming increasingly active, and much more concerns have been attracted to small-scale materials. Because of the constraint effect of geometric and microstructural dimensions of these materials, the thermal fatigue damage behavior is different from that of the bulk counterparts. At the same time, the change of the material scale from microns to nanometers also results in the transformation of the deformation mechanism, so that the materials exhibit different damage behaviors and significant size effects. In this paper, thermal fatigue testing methods, thermal fatigue damage and evolution, and the factors influencing thermal fatigue properties of metal film/line are reviewed, the corresponding mechanism of thermal fatigue and the size effect of the micro/nano-scale metals are discussed. The prospective research of this field in the future is addressed.
With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.
Since structural components in the nuclear power plant are unable to be disassembled during their in service process, it is an urgent and key problem how to quickly and non-destructively evaluate fatigue reliability of these key structural components by using miniature specimens. Fatigue properties of miniature specimens of CA6NM martensite stainless steel for impellers in the nuclear pump were obtained by using symmetrically bending fatigue loading and uniaxial tension-tension fatigue loading, respectively. A comparison of fatigue properties between the miniature specimens and bulk specimens was conducted to examine feasibility for the evaluation of fatigue reliability of the CA6NM steel using miniature specimens. The results show that tensile strength of the 40 μm-thick CA6NM specimens is slightly higher than that of the bulk specimens, but elongation of the 40 μm-thick specimens is lower than that of the bulk counterparts. In low cycle fatigue regime, fatigue strength of the 40 μm-thick specimens subjected to uniaxial tension-tension fatigue loading is lower than that of the standard bulk counterparts. With decreasing the applied stress amplitude, the difference in fatigue properties gradually decreases, and the fatigue limit of the miniature specimen is close to that of the bulk counterparts. Fatigue strength of the 40 μm-thick specimens subjected to bending fatigue loading is much higher than that subjected to uniaxial tension-tension fatigue loading, and also higher than that of the bulk counterparts. Fatigue strength of the miniature specimens is related to the loading mode. The difference in the fatigue mechanism between the miniature specimens and the bulk counterparts is discussed, and the feasibility to evaluate fatigue reliability of the steel using miniature specimens is addressed.