Thermal fatigue behavior of K465 superalloy

The thermal fatigue behavior of K465 superalloy was investigated at the peak temperature of 1050℃. By scanning electron microscopy （SEM） and optical microscopy, the main crack length was observed and measured. The initiation sites of the tested alloys are different in as-cast （named as K465） and solution heat treatment （named as SK465） conditions. In K465 alloy, most thermal fatigue cracks nucleate at （Nb,W,Ti）C carbides. In SK465 alloy, thermal fatigue cracks initiate in interdendritic regions, MC-type carbides and some interfaces. Thermal fatigue cracks propagate in transdendritic mode, and M6C-type carbides could retard thermal fatigue crack growth for SK465 superalloy.     

Corrosion and fatigue in micro-sized Ni cantilever beams

The Ni microcantilevers were fabricated by femtosecond laser. The corrosion behavior of the micro-sized Ni cantilever beams was studied by electrochemical noise and a newly developed fatigue testing method. The results show that the micro-sized specimens exhibit general corrosion behavior under the studied corrosion condition, whereas the ordinary-sized plates exhibit the localized corrosion behavior. The critical load amplitude of the micro-sized Ni specimens under corrosion fatigue status was determined to be 15 mN. The maximum bending loads, which were measured by fatigue tests, decrease gradually prior to final fracture. Corrosion fracture first occurs in the range of notch with a higher tensile bending stress, and exhibits clear evidence of trans-columnar fracture. The variation of maximum bending loads with time agrees with that creep deformation of the micro-sized Ni specimens can easily occur at room temperature, which implies that the micro-sized Ni specimens appear to have an improved resistance towards total crack as compared with the ordinary-sized Ni specimens.     

High-cycle fatigue performance of solution-treated metastable-β titanium alloys

The effect of grain boundary misorientation on the high-cycle fatigue performance of solution-treated, metastable-β titanium alloys was investigated. Initial damage during cyclic deformation was associated with the formation of coarse, planar slip bands, these often propagating through several grains without obstruction or redirection when intersecting with a grain boundary. This “continuous” slip through several grains was associated with the presence of a significant number of “low-angle” grain boundaries. Fatigue crack initiation was associated with crack initiation at intersecting planar slip bands at the free surface. The increase in operative slip length occasioned by the presence of low-angle grain boundaries lead to enhanced crack initiation and reduced lifetime. Fatigue crack propagation was characterized by step-like features formed through the interaction of the propagating crack and the coarse slip bands present in the plastic zone ahead of the crack tip. The direction of local fatigue crack propagation was also minimally affected when crossing low-angle grain boundaries.     

Improving fatigue property of Al-Li alloy by thermo-mechanical treatment

1　INTRODUCTIONAl Lialloyhasbeenusedwidelyasakindofnewstructuralspaceflightmaterialdependingonitshighstrengthandlowdensity[1,2 ] .ManyresearchworkshavebeendoneonAl Lialloy[36 ] ,butfewofthemareaboutfatiguefaliure .Becausemorethan 80 %ofstructuralmaterials failuresarecausedbythefatiguefailuresdirectlyorindirectly[7,8] ,itisnecessarytostudythefatiguepropertyofAl Lialloy[911] .ThereisobviousageingstrengtheningeffectwhenAl Lialloyisagedbelowthesolubilitycurveoftransientphaseδ′ .However,t…     

Effect of heat-treatment on fatigue property of Al-Li alloy

1　INTRODUCTIONMorethan 80 %ofstructuralmaterials failuresarecausedbyfatiguefailuredirectlyorindirectly [1,2 ] .Al Lialloyhasbeenusedasakindofnewstructuralspaceflightmaterialbecauseofitshighstrengthandlowdensity[3,4 ] .AlthoughalotofresearchesaboutAl Lialloy[58] havebeendoneinthepastfew years ,thede tailedstudyonthefatigueproperty[910 ] hasnotbeendonesomuch .Especially ,therewaslittleresearchonthefatiguecrackinitiatingandpropagatingbehavior.Be causethestrengthofAl Lialloycouldbeaffected…     

Low-cyclic fatigue properties of electrolytic low-titanium A356 alloys

The low-cycle fatigue （LCF） behavior of two kinds of A356 alloys produced by different titanium alloying methods was investigated and compared. The effect of titanium content and titanium alloying methods on LCF behavior is analyzed with plastic strain energy density. The results show that all alloys exhibit the cyclic hardening behavior. Raising Ti content can obviously increase the cyclic hardening ability. But the effect of Ti alloying method isn＇t distinct. Whether for the EA356 alloys or for MA356 alloys, the alloys with low titanium content have longer low-cycle fatigue life than that of the alloys With high titanium content. This is because that the alloys with low titanium content can consume higher cyclic plastic strain energy during cyclic deformation compared with alloys with high titanium content.     

Diffusive healing of internal fatigue micro-cracks in pure titanium

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｉｔａｎｉｕｍａｎｄｉｔｓａｌｌｏｙｓｗｉｔｈｐａｒｔｉｃｕｌａｒｐｒｏｐｅｒｔｉｅｓａｒｅｗｉｄｅｌｙｕｓｅｄｉｎｍａｎｙｉｎｄｕｓｔｒｙｆｉｅｌｄｓ .Ｔｈｅｙｕｎ ａｖｏｉｄａｂｌｙｕｎｄｅｒｇｏｖａｒｉｏｕｓｄａｍａｇｅｓｉｎｓｅｒｖｉｃｅ .Ｔｈｅｎｕｃｌｅａｔｉｏｎａｎｄｇｒｏｗｔｈｏｆｉｎｔｅｒｎａｌｆａｔｉｇｕｅｍｉｃｒｏ ｃｒａｃｋｓｈａｖｅｄｅｌｅｔｅｒｉｏｕｓｅｆｆｅｃｔｓｏｎｔｈｅｓｔｒｅｎｇｔｈａｎｄｔｏｕｇｈｎｅｓｓｏｆｔｈｅａｌｌｏｙｓ .Ｈｉ…     

Cyclic deformation and fatigue behaviour of 7Mo–0.5N superaustenitic stainless steel—stress–strain relations and fatigue life

The cyclic deformation characteristics and fatigue behaviour of a superaustenitic stainless steel with composition Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%) have been investigated. Detailed studies were performed on cyclic hardening/softening behaviour, hysteresis loops, waveform, fatigue lifetime, and internal as well as effective stresses during cyclic straining in the total strain amplitude range 2.7·10⁻³–1.0·10⁻². Special attention is paid to the role of nitrogen and the interaction between nitrogen and molybdenum. Immediate cyclic softening takes place at small strain amplitudes, whereas hardening occurs during the first few cycles at large strain amplitudes followed by softening. For all strain amplitudes a virtually stationary state develops after about 10% of the lifetime with only a weak decrease of the peak stresses. In the cyclic stress–strain curve the material hardens linearly during multi step testing, whereas single step testing leads to excessive hardening at the largest strain amplitudes. During strain cycling the internal stresses develop like the total stresses, while the effective stresses decrease with increasing number of cycles for all strain amplitudes and also diminish with increased strain amplitude. This behaviour is discussed in terms of developing dislocation structures, studied in an accompanying paper. A double slope behaviour in Coffin–Manson diagrams is observed. The fatigue lifetime resembles that of AISI 316 with 0.29 wt% nitrogen at high strain amplitudes but is shorter at lower strain amplitudes. However, in stress controlled situations the superaustenitic material is superior.     

Forecasting method of fatigue life test data for metal materials

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｆａｔｉｇｕｅｌｉｆｅｏｆｔｈｅｍｅｔａｌｍａｔｅｒｉａｌｓｉｓｔｈｅｂａｓｉｃｄａｔａｆｏｒｃａｌｃｕｌａｔｉｎｇｔｈｅｉｎｔｅｎｓｉｔｙｏｆｔｈｅｐａｒｔｓｏｆｔｈｅｍｅｃｈａｎｉｃａｌｐｒｏｄｕｃｔｓ .Ｉｎｇｅｎｅｒａｌ,ｔｈｅｆａｔｉｇｕｅｌｉｆｅｔｅｓｔｄａｔａａｒｅｏｂｔａｉｎｅｄｔｈｒｏｕｇｈｔｈｅｒｕｉｎｏｕｓｔｅｓｔｏｎｔｈｅｍａ ｔｅｒｉａｌｓｐｅｃｉｍｅｎ .Ｂｕｔｔｈｅｔｉｍｅｏｆｔｈｅｆａｔｉｇｕｅｌｉｆｅｔｅｓｔｉｓｌｏｎｇａｎｄｔｈｅｃ…     

Low-cycle fatigue behavior of DZ125 superalloy under prior exposure conditions

Low-cycle fatigue(LCF) behavior of the directionally solidified(DS) nickel-based DZ125 superalloy was studied at elevated temperature(980 ℃).Specimens were,respectively,exposed for 0,2,25,50,and 100 h in air.The fatigue life of pre-exposed specimens is lower than that of unexposed specimens.The result is closely associated with fatigue crack initiation and propagation due to oxygen embrittlement and cycle loading.Detailed fractographic evaluations indicate the fatigue life is closely related to ...     

Nickel multilayers and their effects on bend fatigue strength of leads

Nickel is commonly coated on the outer leads for T8 metal package. The leads electrodeposited by conventional dull or bright nickel over 5 μm at direct current from Watt bath are hard to pass the bend fatigue test for three times. Nickel electrodeposited at multi waveform current including direct current, single and double pulse from sulfamate bath can improve the bend fatigue strength of leads. Such nickel plating has a multilayer structure, its morphology of sublayers can be clearly seen in its cross section with SEM. The electrochemical study shows that these sublayers have different corrosion potentials. The bend fatigue test of leads with such plating for T8 metal package shows that the number of bend increases with the decrease of average current density of multi waveform, which can be attributed to the reduction of every sublayer thickness and the increase of layer numbers under the same condition of total thickness.     

High cycle fatigue and fracture behavior of 2124-T851 aluminum alloy

The high cycle fatigue properties and fracture behavior of 2124-T851 aluminum alloy were investigated roundly, including the fatigue crack growth rate, fracture toughness and fatigue S--N curve. Furthermore, the fatigue crack growth rate was analyzed by fitting the curves. And the microstructure of the alloy was studied using by optical microscopy, transmission electron microscopy and X-ray diffractometry, scanning electron microscop1/2 The results show that the fatigue strength and the fracture toughness of 2124-T851 thick plate are 243 MPa and 29.6 MPa.m at room temperature and R=0. 1, respectively. At high cycle fatigue condition, the characteristics of fatigue facture were observed obviously. And the higher the stress amplitude, the wider the space between the fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area and the greater the ratio between the intermittent fracture area and the whole fracture area.     

Microporosity effects on cyclic plasticity and fatigue of LENS™-processed steel

Special microstructures of the newly developed Laser Engineered Net Shaping (LENS?)-processed steel induce a new variability in fatigue damage formation and evolution mechanisms. The microporosity and mechanism of fatigue damage formation and growth were invested using X-ray computed tomography and scanning electron microscopy. Systematic observations were made of the variations in the fracture surfaces according to three fatigue damage evolution stages: fatigue crack formation (incubation), microstructurally/physically small cracks, and long cracks. The fatigue crack was formed almost exclusively at the relatively large pores located at or near the specimen surface, with rare cases at incompletely melted power particles on the surface. Distributed cracks from large interior pores coalesced with each other in the microstructurally small crack regime to form the major critical crack that eventually fractured the specimen. This coalescence accelerated the fatigue crack growth, which in turn decreased the fatigue life but not significantly. In the long-crack regime, the fracture surface was rougher, but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I type fracture. Cyclic strain-softening, with reduced strain-hardening, was also observed. The multistage fatigue model of McDowell et al. was used to capture the microstructure effects in the three fatigue damage evolution regimes, and the upper and lower bounds for the strain–life are predicted.     

Twin boundary: Stronger or weaker interface to resist fatigue cracking?

The fatigue cracking mechanisms at twin boundaries (TBs) are investigated by considering the stacking fault energy, dislocation slip mode and the difference in the Schmid factors (DSF) between matrix and twin. In Cu and its alloys, the occurrence of TB cracking becomes much more frequent with either increasing the alloying component or the DSF. Considering the interactions between dislocations and TBs, a semiquantitative relationship based on the statistical experimental results is established to describe the TB cracking mechanism.     

Application of the local approach to the fatigue assessment for welded joints

The fatigue behavior of welded structures is currently determined by means of recommendations defined in terms of S-N curve corresponding to the detail classes of welded joints without taking account of the actual geometry of the weld. A new fatigue strength assessment method based on Dang Van multiaxial fatigue limit criterion was introduced, which is named the local approach and presented by lnstitut de Soudure recently. The local approach has advantages in taking welding residual stresses and the geometry of the weld toe and weld root into consideration. The application of the local approach to the fatigue strength assessment of low carbon steel Q235B welded joints was studied. The fatigue tests and finite element analysis results show that the local approach parameters recommended by lnstitut de Soudure were incorrectly for low carbon steel Q235B welded joints. With aluminum alloy welded joints being used widely, the parameters of the local approach used for aluminum alloy welded joints were obtained and verified on bases of the fatigue tests and finite element analysis.     

Study of fatigue properties for weld HAZ of EH36MOD steel

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Metallography of fatigue crack initiation in coarse-grained Astroloy at 20°C

Fatigue crack initiation in coarse-grained Astroloy was studied by initiating cracks under bending loads. The specimen was not replicated in order to preserve any debris or extrusions associated with the cracks. Cracks were found to have initiated at pores and along slip lines. Debris and tonguelike extrusions were found coming from some cracks, but not all of them. Illustrations are given of the five different crack extrusion-related phenomena that were observed. It is concluded that the extrusion of material in the form of tonguelike or other debris may accompany the initiation of fatigue cracks, but that it is not a requisite part of the processes causing the crack.     

Subsection method of fatigue design for welded joints treated by ultrasonic peening

Concerning the notable difference between the S-N curve slope of welded joints treated by ultrasonic peening treatment （UPT） and that of as-welded joints, the subsection method is put forward for fatigue design of welded joints treated by UPT, using the design method of nominal S-N curves. Results show that, in medium life zone, strength grade of the fatigue design curves for UPT welded joints is two grades higher than that for as-welded joints. Furthermore, in medium life zone, strength grade of the fatigue design curves for UPT welded joints is three grades lower than that in long life zone. Conclusion of the comparison is that as for different joint types in different life zones, fatigue design should be processed according to different S-N curves respectively.