Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.     

Environmental fatigue crack propagation in medium strength titanium sheet alloys

Results are reported for an investigation of environmental fatigue crack propagation resistance in four commercial titanium alloys of medium strength. The materials were IMI 130 (commercially pure titanium with low oxygen content), Ti-70 (commercially pure titanium with high oxygen content), IMI 230 (Ti-2.5 Cu) and Ti-5Al-2.5Sn. The environments were dry argon, normal air, distilled water and 3.5% aqueous NaCl. The conclusions were (1) the ranking of the materials in terms of conventional mechanical properties does not permit a ranking in terms of crack propagation resistance, (2) the material with the highest elastic moduli, Ti-5Al-2.5Sn, also had the best crack propagation resistance in the absence of stress corrosion, (3) there is a correspondence between the degree of isotropy of the static yield strength and the orientation dependence of crack propagation resistance, (4) for all the materials there was a trend of higher crack growth rates at similar ΔK values in the order; dry argon, air, distilled water, 3.5% aqueous NaCl, (5) in the aqueous environments only Ti-5Al-2.5Sn gave evidence of stress corrosion cracking.     

PASSIVATION EFFECTS ON CRACK CLOSURE IN THE FATIGUE CRACK PROPAGATION OF A PEAK-AGED Al-Li-Zr ALLOY IN NaCl AND Na2SO4 SOLUTIONS

Abstract Fatigue cracking of a peak-aged Al-Li-Zr alloy was investigated by measuring crack closure as a function of applied anodic potential in 0.6 M NaCl and 0.5 M Na₂SO₄ solutions with an unloading elastic compliance technique, and by comparison with crack closure in dry air. The present work involves complementary anodic behaviour of the Al-Li-Zr alloy in both solutions by potentiodynamic polarization and potentiostatic current transient experiments. From the repassivation rates in the passivation potential range in both solutions, it is indicated that a more stable passive film is formed at lower applied anodic potential than at higher applied anodic potential. The intrinsic fatigue crack propagation (FCP) rates under unstable passivation potential in both solutions were significantly larger than those obtained in dry air. Under stable passivation potential in both solutions, however, the intrinsic FCP rates in the low ΔK_eff range were slightly lower than those obtained in dry air. The crack closure in the low ΔK_eff range increased under stable passivation potential, in dry air and under unstable passivation potential. The high crack closures appearing in the low ΔK_eff range were characterized by a tortuous fracture surface in dry air, and the occurrence of various crack paths such as rolling plane delamination under unstable passivation potential. The difference between environmental crack closures under stable and unstable passivation conditions is discussed in terms of environment-assisted crack-tip damage processes.     

FATIGUE AT NOTCHES SUBJECTED TO REVERSED TORSION AND STATIC AXIAL LOADS

Reversed torsion with and without a superimposed end load has been applied to 1% Cr-Mo-V steel specimens containing sharp notches. Crack propagation was monitored by a sensitive d.c. potential drop system that measured crack depths between 25 μm and 0.6 mm from the root of the notch. Stress intensity factors do not satisfactorily correlate all the crack growth data but a strain intensity factor which is a function of material properties and notch plastic zone size shows a significant improvement and provides a single upper bound solution for both ambient and elevated temperatures. This solution permits designers to make safe lifetime assessments. At room temperature cracks initially propagate by mode II along the surface, and mode III radially but at low stresses crack growth is continued by mode I propagation. At higher stresses a transition to mode I cracking is avoided. Elevated temperature causes a brittle layer to form and in this case cracks initially propagate by mode I which then translates to mode III cracking at high stresses. Mode III thresholds are significantly higher than mode I thresholds but a constrained shear strain zone, as found at the root of notches subjected to torsion, permits the initiation and generation of a mode III crack. The application of an axial load enhances the mode III crack propagation rate since this increases the effective crack tip intensification factor.     

敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀的影响

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

Environmental assisted fracture for 4340 steel in water and air of various humidities

This paper reports on measurements of crack growth by environmental assisted fracture (EAF) for 4340 steel in water and in air at various relative humidities. Of most interest is the observation of slow crack propagation in dry air. Fractographic analysis leads to the strong suggestion that this slow crack propagation is due to hydrogen cracking caused by internal hydrogen in solid solution inside the sample material. This revised version was published online in November 2006 with corrections to the Cover Date.     

Correlation between Porosity and Fracture Mechanism in High Pressure Die Casting of AM60B Alloy

X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation,propagation and subsequent fracture of specimen. Results showed that four types of porosities,including gas-shrinkage pore,gas pore,net-shrinkage and island-shrinkage,could be identified according to the formation mechanism and morphology characterization. During tensile deformation,it was shown that the gas-shrinkage pore and net-shrinkage,rather than gas pore or island-shrinkage,were the main sources for crack initiation. In addition,the crack propagated by interconnecting the porosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals(ESCs) were present,the crack would propagate along the ESC boundaries in an intergranular mode,while at these locations without ESCs,the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.     

铈离子对高强铝合金应力腐蚀开裂的缓蚀作用

基于慢应变速率拉伸实验(SSRT),采用恒电流极化、电化学噪声(ECN)与电化学阻抗(EIS)等方法,研究7A04铝合金在3.5%(质量分数)NaCl水溶液中的应力腐蚀开裂(SCC)行为以及Ce~(3+)对其SCC的缓蚀作用,探讨Ce~(3+)对裂纹孕育与发展过程的抑制机理。结果表明:无论是阳极还是阴极极化,均会促进7A04的SCC倾向,前者增加了裂尖的阳极溶解,后者则加速了裂尖的氢脆效应。Ce~(3+)的加入能延缓7A04的SCC断裂时间,但其有效性仅限于裂纹的萌生阶段。由于Ce~(3+)能够抑制铝合金表面的亚稳态点蚀发育和长大,因而使裂纹的孕育时间显著延长,降低了SCC的敏感性。不过一旦裂纹进入扩展阶段或者试样表面有预裂纹,则由于Ce~(3+)很难迁移到裂纹尖端或在裂尖区难以成膜,不能对裂纹的生长起到有效抑制作用,因而无法降低7A04的SCC发展速率。SEM分析表明7A04铝合金光滑试样SCC主要源于亚稳态或稳态点蚀的诱导作用。     

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air,dry air,and vacuum

Environmental influences on near‐threshold fatigue crack growth in wrought magnesium alloy AZ61 were investigated. Fatigue tests were performed in ambient (humid) air, dry air, vacuum, and dry nitrogen gas at 19 kHz cycling frequency and load ratio R = ?1. Threshold stress intensity factor amplitudes, K_th, determined for limiting growth rates below 5 × 10^?13 m/cycle were 1.1 MPam^1/2 in ambient air and 1.2 MPam^1/2 in dry air. A much higher K_th of 1.9 MPam^1/2 was measured in vacuum and dry nitrogen gas. This suggests oxygen to be the most detrimental constituent of ambient air that increases near‐threshold crack propagation rates and decreases K_th. The deleterious effect of humidity is comparatively small. Corrosive influences are effective at ultrasonic cycling frequency for growth rates below approximately 3 × 10^?9 m/cycle. The crack propagation curves in ambient and dry air show a plateau‐like regime where the fracture mode changes from purely ductile to a mixed ductile and brittle mode. In vacuum and dry nitrogen gas, a ductile crack path is found for all investigated crack growth rates.     

Spannungsrißkorrosion in hochreinem Wasser von 3–3 ½% NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen. Teil I

SCC in High Parity Water In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S or Nacl, which initiate SCC, could be found. To clarify the SCC-behaviour experiments on turbine disc steels with different chemical compositions an yield strength were performed in high purity water. The results show, that chemical composition has no effect on the crack initiation. Under high purity water conditions no crack initiation due to stress corrosion cracking is observed on the steel with a yield strength of 850 N/mm². On the steel with a yield strength of 1250 N/mm² which is not used in service, crack initiation occurs in pure water. But if sharp cracks already exist, crack propagation occurs in both cases. The investigations showed, that stress corrosion cracking of turbine discs can be prevented by a good water chemistry with a cation conductivity less than 0.2 μS/cm (μmho/cm).     

Investigation on cracking mechanism of austenite stainless steel during in situ tension in transmission electron microscope

Twin-roll strip casting technology is a new one to produce austenite stainless steel strip directly. However, during this process, the cracking occurs usually on the surface of the steel strip. The technique of in situ tension in transmission electron microscope was used to observe and analyze the crack initiation and propagation in austenite stainless steel produced by twin-roll strip casting technology in this work. The results show that the crack initiates in dislocation free-zone firstly and then propagates along the direction vertical to the tensile force. The crack may propagate in continuous propagation way and discontinuous one respectively. When the shear stress on the close-packing face and the normal stress on the secondary close-packing face are at the same side of the tensile force, the cracks propagate continuously. However, when the shear stress on the close-packing face and the normal stress on the secondary close-packing face are in two sides, the cracks propagate discontinuously.     

A shear band decohesion model for small fatigue carck growth in an ultra-fine grain aluminium alloy

A study of small fatigue crack growth behaviour of an ultra-fine grain size aluminum alloy IN 9052 has been carried out. Specimens were tested in tension–tension at R0.1 in laboratory air and in the vacuum chamber of a field emission gun scanning electron microscope. Loading and unloading experiments were carried out within the SEM to study the displacements of the crack faces, the shape change at the crack tip, the shear bands around the crack tip and the fatigue crack growth mechanism. The in-situ SEM observations revealed that shear bands formed in the crack tip region were directly associated with the growth mechanism of the crack. The shear bands localized the decohesion in the crack tip region and the cracking along the shear bands was observed to occur during the loading part of the load cycle. The overall behaviour of the crack tip region subjected to cyclic loading is summarized by a qualitative model for small crack growth in an ultra-fine grained material.     

Improving resistance to dynamic embrittlement and intergranular oxidation of nickel based superalloys by grain boundary engineering type processing

Abstract

Polycrystalline 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.     

Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments

Fatigue crack propagation (FCP) was studied on wrought magnesium alloys, AZ31 and AZ61, in laboratory air, dry air and distilled water. In laboratory air, the FCP rate versus stress intensity factor plots consisted of two sections with different slopes, which was clearly recognized after allowing for crack closure. This was attributed to the transition in fracture mechanisms operated. In distilled water, FCP rates were nearly the same as in laboratory air, while in dry air, an order of magnitude slower than in laboratory air and distilled water. After allowing for crack closure, the environmental effects still existed and FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractography revealed that the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively.     

Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu

Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.     

Fatigue crack growth resistance of ECAPed ultrafine-grained copper

This work presents the experimental results of fatigue crack growth resistance of ultrafine-grained (UFG) copper. The UFG copper has a commercial purity level (99.90%) and an average grain size of 300 nm obtained by a 8-passes route Bc ECAP process. The fatigue propagation tests are conducted in air, at load ratios R = K_min/K_max varying from 0.1 to 0.7, on small Disk Shaped CT specimens. Both stage I and stage II regime of growth rate are explored. Results are partially in contrast with the few experimental data available in the technical literature, that are by the way about high purity UFG copper. In fact, the present material shows a relatively high fatigue crack resistance with respect to the unprocessed coarse-grained alloy, especially at high values of applied stress intensity factor ΔK. At higher R-ratio a smaller threshold intensity factor is found, together with a lower stage II fatigue crack growth rate. The explanation of such crack growth retardation is based on a diffuse branching mechanism observed especially at higher average ΔK.     

Internal fatigue cracks are growing in vacuum

Cylindrical specimens of 2024 and 7075 Al alloy material were heat treated with a cold water quench to obtain high residual tensile stresses at the interior. Fatigue tests showed internal cracks growing in the shear mode. By drilling a hole along the centre line internal cracks were given access to air, which then produced tensile mode cracks. Prestraining of specimens eliminated residual stresses thus producing crack initiation at the outer surface with crack growth in the tensile mode. Cracking in the tensile mode was sensitive to mean stress, whereas cracking in the shear mode was not. The shear mode crack on a micro level appeared to be slip band cracking.     

FATIGUE MACROCRACK GROWTH IN TEMPERED HY80, HY130, AND 4140 STEELS:THRESHOLD AND MID-ΔK RANGE

Abstract— –The rate of propagation of macrofatigue cracks down to near threshold was measured in air in three tempered martensitic steels; HY80, HY130 and 4140 (650°C temper). The value of Δ K _th was determined by the load-shedding technique in center notched panel specimens. Of the three steels, 4140 tempered at 650°C had the lowest Δ K _th, 3–5 MN/m^3/2, while HY80 had the highest, 4.2 MN/m^3/2. The 4140 (650°C temper) is intermediate in strength between HY80 and HY130. The results are discussed in terms of a recent theory of one of the authors.
The fatigue crack propagation rates in the mid-Δ Krange in HY80 and HY130 in argon were also studied by measuring, with foil strain gages, the cyclic plastic work to propagate a fatigue crack by a unit area, U.HY 80 has a lower crack propagation rate and correspondingly higher U .This was attributed in part to the higher yield strength of HY130 but the dislocation structure and carbide composition and morphology also play roles. Microstructural changes due to cyclic plastic deformation inside the plastic zone in HY80 and HY130 were observed by TEM of thin foils. SEM studies of the fracture surfaces at Δ K = 20 MN/m^3/2 indicate a more ductile fracture mode for HY80 than for HY130. The fatigue crack propagation rate of HY130 is substantially higher in laboratory air (47% relative humidity) than in dry argon. This is not the case for HY80.     

Fatigue crack growth behaviour in aluminium alloy 7475 under different aging conditions

Abstract

The effects of aging temperature and aging time on fatigue crack growth resistance have been studied for a 7475 Al-Zn-Mg based aluminium alloy. The alloy was tested in the underaged, peak aged, and overaged conditions after aging at 120 and 160C. Fatigue crack propagation tests were conducted in laboratory air using compact tension specimens in L-S orientation, under constant amplitude sinusoidal loading with an R ratio of zero. Results are discussed on the basis of resultant microstructures, fatigue crack growth rate diagrams and fractographic analysis. At 120C, a considerable effect of aging time on crack velocities at high stress intensities was seen. However, at 160C no significant dependency of crack growth rate on aging time was observed. The fatigue performance of overaged specimens was better for both aging temperatures. Also, lower aging temperature resulted in a more resistant structure against fatigue crack growth. Fractographic inspection showed that intermetallic particles play an important role in the crack growth behaviour of the Al-Zn-Mg alloy.     

Welding solidification cracking susceptibility and behavior of a Ni-28W-6Cr alloy

Welding solidification cracking of alloys is associated with the range of solidification temperature that can be greatly affected by the amount of refractory metals and other additives. In this work, solidification cracking of Ni-28W-6Cr alloy with high W content was studied by gas tungsten arc welding, showing that the welding current, alloying elements and precipitates all affect the cracking susceptibility. The lengths of cracks increase linearly with the welding current in the range from 150 to 250 A. The relatively high cracking susceptibility is mainly attributed to the high content of Si, which tends to segregate with other elements including W, Cr, Mn as films or components with low melting point in the last solidification stage and weaken the binding force of grain boundaries. Moreover, the existence of precipitated continuous eutectic M_6C carbides in the grain boundaries also acts as nucleation sites of crack initiation, and the cracks often propagate along solidification grain boundary.