Effect of grain size on high-cycle fatigue properties in alpha-type titanium alloy at cryogenic temperatures

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti-5%Al-2.5%Sn ELI alloy which was used for liquid hydrogen turbo-pumps of Japanese-built launch vehicles. Mean grain size of specimens was controlled to be about 30 or 80 μm. In the specimens with a grain size of 30 μm, fatigue strengths at 10⁶ cycles at 4 and 77 K are 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the specimen with a grain size of 80 μm, fatigue strengths at 10⁶ cycles at 4 and 77 K get lower to the same level as that at 293 K. Thus, it is concluded that refinement of α grains is one of important factors to obtain the good high-cycle fatigue properties for Ti-5%Al-2.5%Sn ELI alloy at cryogenic temperature.     

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 mm in the rolled material and 80 mm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 10⁶ cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 10⁶ cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.     

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 μm in the rolled material and 80 μm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 10⁶ cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 10⁶ cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.     

A comparison of maximum likelihood models for fatigue strength characterization in materials exhibiting a fatigue limit

In this study, various probabilistic models were considered to support fatigue strength design guidance in the ultra high-cycle regime (beyond 10⁸ cycles), with particular application to Ti-6Al-4V, a titanium alloy common to aerospace applications. The random fatigue limit model of Pascual and Meeker and two proposed simplified models (bilinear and hyperbolic) used maximum likelihood estimation techniques to fit probabilistic stress-life curves to experimental data. The bilinear and hyperbolic models provided a good fit to large-sample experimental data for dual-phase Ti-6Al-4V and were then applied to a small-sample data set for a beta annealed variant of this alloy, providing an initial probabilistic estimate of beta annealed Ti-6Al-4V fatigue strength in the gigacycle regime. The bilinear and hyperbolic models are recommended for use in estimating probabilistic fatigue strength parameters in support of very high-cycle design criteria for metals with clearly defined fatigue limits and fairly constant scatter in fatigue strength.     

Influence of mechanical surface treatments on the high cycle fatigue performance of TIMETAL 54M

TIMETAL 54M (in the following Ti-54M) is a newly developed (α + β) titanium alloy with nominal composition Ti-5Al-4V-0.6Mo-0.4Fe. The alloy can provide a cost benefit over Ti-6Al-4V due to improved machinability and formability. These attractive properties might be a driving force for replacing Ti-6Al-4V in many aircraft as well as biomedical applications. Since HCF performance is one of the most important requirements for these applications, it is essential to improve this property by microstructural optimization and by mechanical surface treatments such as shot peening or ball burnishing. The latter improvement is mainly the result of induced near-surface severe plastic deformation which results in work-hardening and the generation of compressive residual stresses that retard fatigue crack propagation. The main aim of the present study was to investigate the potential fatigue life improvements in Ti-54M due to shot peening and ball-burnishing. The process-induced residual stresses and stress-depth profiles were determined by energy-dispersive X-ray diffraction (ED) of synchrotron radiation with the beam energy of 10-80 keV. Results on Ti-54M and Ti-6Al-4V will be compared and correlated with the mean stress and environmental sensitivities of the fatigue strengths in the microstructures.     

Effect of combined plasma-carburizing and deep-rolling on notch fatigue property of Ti-6Al-4V alloy

This paper discusses the effects of a combination of plasma-carburizing and deep-rolling on notch fatigue properties of a Ti-6Al-4V alloy. Circumferentially V-notched cylindrical Ti-6Al-4V alloy specimens were plasma-carburized at a relatively low temperature for the improvement of wear resistance, and then, deep-rolled at the notch root for inducing compressive residual stress. Scanning electron microscopy, optical microscopy, laser scanning microscopy, surface roughness tester, and micro-hardness tester were used to characterize the modified surface layer at the notch root. Axial loading fatigue tests (R = 0.1) were performed using a servo-hydraulic testing machine in a laboratory atmosphere at an ambient temperature. The notch fatigue life of the specimen was reduced by plasma-carburizing due to the brittleness caused by the higher hardness in addition to the disappearance of compressive residual stress on the notched surface, but remarkably improved by the subsequent deep-rolling. The surface layer containing the compressive residual stress and the work hardening induced by deep-rolling effectively prevented and delayed the fatigue crack initiation and propagation of deep-rolled carburized specimen.     

Microstructural features of failure surfaces and low-temperature mechanical properties of Ti-6Al-4V ELI ultra-fine grained alloy

Microstructural regularities of failure surfaces and low-temperature mechanical characteristics in quasistatic uniaxial tension and compression have been studied for ultra-fine grained structural states of Ti-6Al-4V ELI alloy processed by equal channel angular pressing. Values of the yield stress and uniform strain at 300, 77, and 4.2 K have been compared for structural states of the Ti-6Al-4V ELI alloy that differ in the average grain size and the morphology of α and β phases. Statistical distributions of dimple sizes on the failure surfaces have been studied for different structural states and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 81–84, January–February, 2008.     

Effects of intermittent loading time and stress ratio on dwell fatigue behavior of titanium alloy Ti-6Al-4V ELI used in deep-sea submersibles

Different components of deep-sea submersibles,such as the pressure hull,are usually subjected to inter-mittent loading,dwell loading,and unloading during service.Therefore,for the design and reliability assessment of structural parts under dwell fatigue loading,understanding the effects of intermittent loading time on dwell fatigue behavior of the alloys is essential.In this study,the effects of the inter-mittent loading time and stress ratio on dwell fatigue behavior of the titanium alloy Ti-6Al-4V ELI were investigated.Results suggest that the dwell fatigue failure modes of Ti-6Al-4V ELI can be classified into three types,i.e.,fatigue failure mode,ductile failure mode,and mixed failure mode.The intermittent loading time does not affect the dwell fatigue behavior,whereas the stress ratio significantly affects the dwell fatigue life and dwell fatigue mechanism.The dwell fatigue life increases with an increase in the stress ratio for the same maximum stress,and specimens with a negative stress ratio tend to undergo ductile failure.The mechanism of dwell fatigue of titanium alloys is attribute to an increase in the plastic strain caused by the part of the dwell loading,thereby resulting in an increase in the actual stress of the specimens during the subsequent loading cycles and aiding the growth of the formed crack or damage,along with the local plastic strain or damage induced by the part of the fatigue load promoting the cumu-lative plastic strain during the dwell fatigue process.The interaction between dwell loading and fatigue loading accelerates specimen failure,in contrast to the case for individual creep or fatigue loading alone.The dwell fatigue life and cumulative maximum strain during the first loading cycle could be correlated by a linear relationship on the log-log scale.This relationship can be used to evaluate the dwell fatigue life of Ti alloys with the maximum stress dwell.     

The effect of initial notch shape of compact specimen on fatigue pre-cracking and fracture toughness testing

The circular notched compact specimens, along with standard specimens having straight or chevron notch are provided for fatigue and fracture toughness testings in order to study the crack observation capability during fatigue pre-cracking, skewness of the crack front, and the resulting fracture toughness K_Q. The test results indicated that circular notched specimens significantly facilitate the crack observation during fatigue testing as the cracks initiate on both surfaces of the specimen. No remarkable differences were observed on geometries of the fatigue crack front obtained and the resulting fracture toughness among these three types of specimen. The macroscopic observation of beach marks on the fracture surfaces revealed that, in the present material Ti-6Al-4V (ELI), the advance of only 1.3% of the whole crack length corresponded to the load level at which fracture toughness K_Q was evaluated.     

On the application of the Kitagawa-Takahashi diagram to foreign-object damage and high-cycle fatigue

The role of foreign-object damage (FOD) and its effect on high-cycle fatigue (HGF) failures in a turbine engine Ti-6Al-4V alloy is examined in the context of the use of the Kitagawa-Takahashi diagram to describe the limiting conditions for such failures. Experimentally, FOD is simulated by firing 1 and 3.2 mm diameter steel spheres onto the flat specimen surface of tensile fatigue specimens at velocities of 200 and 300 m/s. Such damage was found to markedly reduce the fatigue strength of the alloy, primarily due to four factors: stress concentration, microcrack formation, impact-induced plasticity and tensile residual stresses associated with the impact damage. Two groups of fatigue failures could be identified. The first group initiated directly at the impact site, and can be readily described through the use of a fatigue-crack growth threshold concept. Specifically, a Kitagawa-Takahashi approach is presented where the limiting threshold conditions are defined by the stress-concentration corrected smooth-bar fatigue limit (at microstructurally small crack sizes) and a “worst-case” fatigue-crack growth threshold (at larger crack sizes). The second group of failures was caused by fatigue cracks that initiated at locations far from the impact site in regions of high tensile residual stresses, the magnitude of which was computed numerically and measured experimentally using synchrotron X-ray diffraction. Specifically, these failures could be rationalized due to the superposition of the residual stresses on the far-field applied mean stress, leading to a locally elevated load ratio (ratio of minimum to maximum loads). The effects of residual stress, stress concentration, and microstructurally small cracks are combined in a modified Kitagawa-Takahashi approach to provide a mechanistic basis for evaluating the detrimental effect of FOD on HCF failures in Ti-6Al-4V blade alloys.     

Ti-12Mo-6Zr-2Fe钛合金的纳米压入特征

利用纳米压入测量仪测试了Ti-12Mo-6Zr-2Fe钛合金的硬度和模量.测试表明,Ti-12Mo-6Zr-2Fe钛合金的硬度比Ti-6Al-4V ELI高,而模量比后者低,与常规方法得到的规律一致,表明该方法能很好地表征钛合金的力学特性.对测试过程的分析表明这种方法得到的结果不能与传统方法得到的结果互换,表面状态对测量结果有一定影响.     

Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, titanium alloy (Ti-10V-2Fe-3Al) and titanium alloy (Ti-5Al-2.5Sn) were used to investigate the toughening mechanisms with new approaches. The results show that Ti-5Al-2.5Sn alloy possesses good combination of strength and ductility as well as satisfied low-cycle fatigue life both at 293 K and 77 K. As for Ti-10V-2Fe-3Al alloy, the microstructure with metastable phase shows lower strength and ductility but higher threshold stress intensity factor (K _th) than solution treated and aged microstructure composed of and phases. The microstructures also show that twinning occurs in deformation of Ti-5Al-2.5Sn alloy at 77 K. Twinning seems to be helpful for improving the low-cycle fatigue life to a great extent at cryogenic temperature. It's also found that owing to stress-assisted martensite transformation in metastable Ti-10V-2Fe-3Al alloy, the fatigue crack propagation path shows a very tortuous way, which decrease the effective stress intensity factor (K _eff) at crack tip, and increase threshold stress intensity factor (K _th).     

High-cycle fatigue properties in Ti–5% Al–2.5% Sn ELI alloy with large grain size at cryogenic temperatures

High‐cycle fatigue properties were investigated for Ti–5% Al–2.5% Sn ELI alloy with a mean α grain size of 80 μm, which had been used for liquid hydrogen turbo‐pumps of Japanese‐built launch vehicles. At cryogenic temperatures, the fatigue strength in high‐cycle region did not increase in proportion to increments of the ultimate tensile strength and the fatigue strengths at around 10⁶ cycles were about 300 MPa independent of test temperatures. Fatigue cracks initiated in the specimen interior independent of the test temperatures of 4 K, 77 K and 293 K. At 4 K and 77 K, several crystallographic facet‐like structures were formed at crack initiation sites. On the other hand, there were no facet‐like structures that could be clearly identified at the crack initiation sites at 293 K. Low fatigue strengths in longer‐life region at cryogenic temperatures could be attributable to the formation of large sub‐surface crack initiation sites, where large facet‐like structure are formed.     

增材制造Ti-6Al-4V钛合金低周疲劳性能研究进展

金属增材制造技术可用于大型、复杂高性能钛合金结构件的制备,在航空航天等领域具有显著的优势和巨大的发展潜力。虽然增材制造Ti-6Al-4V合金构件的强度已经能够超过锻件,但它仍存在内部孔隙、熔合不良、粗大的柱状晶及残余拉应力等问题,使其在疲劳性能上与锻件具有一定的差距。本文在介绍直接能量沉积、选区激光熔化和电子束选区熔化3种代表性增材制造技术的原理及特点的基础上,简述了3种工艺制备Ti-6Al-4V合金构件的微观组织、静态力学性能及低周疲劳性能的研究进展,重点讨论了打印方向、缺陷、显微组织和表面处理对低周疲劳性能的影响。分析了增材制造Ti-6Al-4V合金构件低周疲劳性能、拉伸性能与微观组织之间的内在关系,并对提高构件低周疲劳性能的方法和推动其广泛应用的发展方向进行展望。     

Effect of dwell on the growth of fatigue cracks in Ti-6Al-4V alloy bar

The growth of cracks in Ti-6Al-4V alloy was studied in fatigue tests in which the maximum load each cycle was held constant for a chosen dwell-time, and the results were compared with those obtained using sinusoidal stressing.

A 57 mm thick forged and rolled bar of Ti-6Al-4V alloy was used. It had been annealed at 700°C for 2 hr and the microstructure was mainly hexagonal -phase with a strong preferred orientation (TD texture). Test pieces were cut from the bar to select crystal orientations of fracture planes and directions. When the stressing direction was parallel to the basal plane, the crack growth rate of dwell-cycling at room temperature was similar to that for sinusoidal stressing at 0.3 and 25 Hz. But when the stress was normal to the basal plane the rate of growth was increased considerably, depending on the dwell-time and the stress intensity factor range (ΔK).

The dependence of the dwell-effect on microstructure and temperature was investigated and the preferred plane of dwell-fracture determined. The failure mechanism is discussed, together with the implications of the results for aircraft components subjected to in-service cyclic loads such that each flight is broadly simulated by a stress pattern having a dwell time at maximum load.     

Fatigue life prediction of small notched Ti-6Al-4V specimens using critical distance

This study investigated the method of estimating the fatigue strength of small notched Ti-6Al-4V specimen using the theory of critical distance that employs the stress distribution in the vicinity of the notch root. Circumferential-notched round-bar fatigue tests were conducted to quantify the effects of notch radius and notch depth on fatigue strength. The fatigue tests show that the larger notch radius increases the fatigue strength and the greater notch depth decreases the fatigue strength. The theory of critical distance assumes that fatigue damage can be correctly estimated only if the entire stress field damaging the fatigue fracture process zone is taken into account. Critical distance stress is defined as the average stress within the critical distance from notch root. The region from the notch root to the critical distance corresponds to the fatigue fracture process zone for crack initiation. It has been found that a good correlation exists between the critical distance stress and crack initiation life of small notched specimens if the critical distance is calibrated by the two notched fatigue failure curves of different notch root radii. The calibrated critical distances did not vary clearly over a wide range of fatigue failure cycles from medium-cycle low-cycle fatigue regime to high-cycle fatigue regime and have an almost constant value. This critical distance corresponds to the size of crystallographic facet at the fatigue crack initiation site for the wide range of fatigue cycles.     

Microtwin formation in the α phase of duplex titanium alloys affected by strain rate

The effect of tensile strain rate on deformation microstructure was investigated in Ti-6-4 (Ti-6Al-4V) and SP700 (Ti-4.5Al-3V-2Mo-2Fe) of the duplex titanium alloys. Below a strain rate of 10⁻² s⁻¹, Ti-6-4 alloy had a higher ultimate tensile strength than SP700 alloy. However, the yield strength of SP700 was consistently greater than Ti-6-4 at different strain rates. The ductility of SP700 alloy associated with twin formation (especially at the slow strain rate of 10⁻⁴ s⁻¹), always exceeded that of Ti-6-4 alloy at different strain rates. It is caused by a large quantity of deformation twins took place in the α phase of SP700 due to the lower stacking fault energy by the β stabilizer of molybdenum alloying. In addition, the local deformation more was imposed on the α grains from the surrounding β-rich grains by redistributing strain as the strain rate decreased in SP700 duplex alloy.     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

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.     

A mathematical equation relating low cycle fatigue data to fatigue crack propagation rates

A mathematical equation is derived to predict fatigue crack growth rates on the basis of a J integral analysis from the fatigue fracture behaviour of low cycle fatigue samples. According to this equation, the fatigue crack propagation curves can be predicted if low cycle fatigue data and an initial microcrack size are available. The results obtained from this study show that the predicted fatigue crack propagation rates for Ti-24V, Ti-6Al-4V and Al-6Zn-2Mg alloys are very close to experimental values.