Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn

The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately by cleavage ofα grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in terms of the effective stress intensity,K _eff. From Arrhenius plots of crack growth rates for variousK _eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through theβ phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem.     

Subcritical crack-growth under sustained load in Ti-6AI-6V-2Sn

The subcritical sustained-load cracking (SLC) behavior of beta annealed, recrystallization annealed, and solution-treat-and-aged Ti-6Al-6V-2Sn was studied in dry argon and moist air. The effects of microstructure, internal hydrogen concentration, specimen orientation, and specimen thickness on threshold stress-intensity and crack-growth rate were determined under increasing stress-intensity conditions using wedge-opening load (WOL) specimens, and the fracture morphologies were studied by scanning electron microscopy. The SLC threshold stress intensities are lower and crack growth rates are higher in solution-treatand-aged condition than in beta-annealed and recrystallization conditions. The plane strain conditions increase the susceptibility to SLC. The texture effects on SLC are intensified when the cracking plane is close to the basal plane. For low interstitial-hydrogen concentrations ≈10 ppm, sustained-load crack growth is controlled by creep at the crack tip. Increased hydrogen concentration results in enhanced cleavage, lower threshold stress-intensity, and accelerated crack-growth. Of the possible mechanisms for hydrogen-assisted SLC in Ti-6Al-6V-2Sn, the most likely is that involving preferential segregation of hydrogen to beta phase, leading to fracture by α-β interface separation. Formerly with McDonnell Douglas Research Laboratories.     

Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions

Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions.     

The Effect of Stress State on Internal Hydrogen-Induced Crack Growth in Ti-6AI-6V-2Sn

The effect of stress state on threshold stress intensity and crack growth rates was studied in Ti-6A1-6V-2Sn containing 38 ppm hydrogen. Threshold stress intensities approached a lower limiting value near 32 MPa-m1/2 as total plane strain conditions were approached. Under increasing plane stress conditions, thresholds increased significantly toward a value indicating immunity from hydrogen effects. Crack growth rates depended on both stress intensity and stress state. The thinner samples required much greater stress intensities to achieve crack growth rates comparable to the thick samples. This was due to plane stress plastic zones at the sample surfaces acting as a constraint on crack opening. Under plane strain conditions, fracture was characterized by extensive cleavage of thea grains. Fracture occurred by a ductile mode in the plane stress regions. A threshold model based on hydride formation and fracture was modified to account for stress state effects. The predicted stress intensities were in good agreement with the observed values. N. R. MOODY, formerly a Graduate Student at the University of Minnesota     

Characteristics of sustained-load cracking and hydrogen effects in Ti-6AI-4V

Sustained-load cracking (SLC) characteristics of Ti-6A1-4V are significantly influenced by 1) exposure temperature, 2) hydrogen content, and 3) basal plane crystallographic texture. The stress intensity applied to precracked specimens did not play a major role in affecting crack initiation or crack growth rate except that there would appear to be a required minimum level. Increasing hydrogen content raises the temperature at which SLC occurs and increases crack growth rate. A model for SLC has been proposed based on hydride precipitation at the crack tip and subsequent crack propagation by creep, cleavage, andJor interfacial separation at the hydrideJmatrix interface. Formerly with the Boeing Commercial Airplane Co., is presently an Engineering Consultant.     

In situ scanning auger analysis of hydrogen-induced fracture in Ti-6Al-6V-2Sn

Metallurgical and Materials Transactions A -     

Fabrication and properties of heavy section extrusions of Ti-6Al-6V-2Sn and Ti-8Mo-8V-2Fe-3AI alloys

This study is concerned with the fabrication in heavy section of the titanium alloys Ti-8Mo-8V-2Fe-3Al (Ti-8823) and Ti-6Al-6V-2Sn (Ti-662). The technique utilized to achieve 84 pct reduction during the extrusion of heavy section cylindrical hollows is given. The response to subsequent aging of both alloys is described in terms of the effect on important mechanical properties such as yield and ultimate tensile strength, ductility and fracture toughness. For the Ti-8823 alloy extruded in heavy section, it is shown that the optimum heat treatment consists of aging directly after hot working rather than the more common solution treatment and age cycle. With the former heat treatment, uniform through the thickness mechanical properties are obtained. Uniform mechanical properties are also obtained through the section of the Ti-662 extrusion with a solutionizing and overaging heat treatment.     

Phase transformations in Ti- 3AI- 8V- 6Cr- 4Zr- 4Mo

The time-temperature-transformation behavior of a metastable β-titanium alloy, Ti-3A1-8V-6Cr-4Zr-4Mo (RMI 38644), has been examined using hardness, X-ray measurements, and optical and transmission electron microscopy. This examination has shown that a variety of metastable and stable phases may form in this alloy system. These include metastable zones, ω, both Burgers (Type 1) α and non-Burgers (Type 2) α, TiCr₂, and a Ti-Zr-silicide. A composite TTT diagram is presented which describes the influence of aging time and temperature on the stability of these phases. Finally, the appearance of these transformation products is correlated with the observed hardness changes when RMI 38644 is aged at elevated temperatures.     

The influence of laser glazing on fatigue crack growth in Ti-24AI-11Nb

The influence of laser glazing on fatigue crack growth (FCG) in a titanium aluminide (Ti₃Al) alloy Ti-24A1-11Nb has been studied. Glazing leads to retention of beta phase in the melt zone and is accompanied by a significant increase in hardness as compared to that of the base microstructure. Laser glazing strongly influences the FCG behavior. Crack growth rates (CGRs) at a given stress intensity are reduced by up to three orders of magnitude. Maximum retardation in the CGR is observed when the crack front is around 4.0 mm ahead of the laser track. The observed slowing of CGR can be understood on the basis of the superposition principle that accounts for the influence of residual stresses on FCG. The equilibrating compressive stresses resulting from the tensile thermal residual stresses reduce the local effective stress intensity and, thereby, lead to reduced CGRs. In the absence of residual stresses, when subjected to a thermal treatment (650 °C/100 h), no observable variation in CGR is noticed over the entire crack length that encompassed two laser tracks. Leave from Defence Metallurgical Research Laboratory, Hyderabad, India,. U.S. government work not protected by u.s. copyright     

Basal and near- basal hydrides in Ti- 5AI- 2.5Sn

Cleavage cracking of titanium alloys has been reported to occur under conditions of stress corrosion cracking and sustained load cracking and, in such cases, the cleavage plane has often been reported as being near-basal, typically 14 to 16 deg from (000I)α However, the indices of this cleavage plane have not yet been definitely established, nor has the reason for its occurrence. In the present work the Ti-5Al-2.5Sn alloy has been hydrogenated to various levels and three hydride phases are found to occur. The first to precipitate is the y-hydride, occurring at 00123300 ppm H₂. At 00123500 ppm H₂ a near-basal hydride of unknown crystal structure is formed. There is no unique habit plane. At concentrations in excess of 600 ppm, hydrogen-stabilized stacking faults occur on the basal plane. The significance of these hydride phases is discussed with reference to stress corrosion cracking and sustained load cracking.     

Environmentally assisted,sustained-load crack growth in powder metallurgy nickel-based superalloys

To examine the influence of niobium (Nb) on sustained-load crack growth (SLCG) in oxygen, three powder metallurgy (P/M) nickel-based superalloys, with nominal compositions similar to IN100, but with 0, 2.5, and 5 wt pct of Nb, are used. These alloys are gamma-prime (γ’) strengthened and have comparable volume fractions (53 vol pct) of γ’ precipitates. The SLCG experiments are conducted in high-purity oxygen and argon at 873, 923, and 973 K. The environmental cracking sensitivity (ECS) for the alloys with 2.5 and 5 wt pct of Nb is consistent with that of INCONEL 718 and supports the previously identified role of Nb-rich carbides in enhancing crack growth. The susceptibility of the Nb-free alloy to oxygen, however, is much greater than expected. The apparent activation energy for crack growth in oxygen was found to depend on stress-intensity-factor (K) levels for the Nb-containing alloys and ranged from about 320 to 260 kJ/mol for K levels of 35 to 60 MPa√m. It was nearly independent of K at about 250 kJ/mol for the Nb-free alloy. The results are discussed in terms of the rate-controlling process and of the mechanism for crack-growth enhancement.     

Microstructural influences on fatigue crack propagation in Ti-10V-2Fe-3Al

Tensile properties and Fatigue Crack Propagation (FCP) rates have been measured in laboratory air for a wide variety of different Ti-10-2-3 microstructures, all having nominal yield strengths of 1240 MPa. These microstructural variations included both recrystallized and unrecrystallized conditions, as well as varying amounts of primary α, both α- and Ω-aged conditions, and also several hot forming histories. With one exception, microstructure was found to have very little or no influence on FCP. Not only was the rate of FCP unaffected, but moreover, the macroscopic fracture surface roughness was largely unaltered. The one exception was a notable one: the Ω-aged condition. Strengthening through the precipitation of Ω instead of α was found to retard FCP significantly; no significant change in macroscopic fracture surface roughness was found, however. The retardation was attributed to changes in slip character and reversibility. Formerly with Carnegie-Mellon University Formerly with BrownBoveri in Switzerland     

Elevated temperature tensile behavior of Ti-25AI-10Nb-3V-1Mo

The effects of microstructure and temperature on tensile and fracture behavior were explored for the titanium aluminide alloy Ti-25Al-10Nb-3V-lMo (atomic percent). Three microstructures were selected for study in an attempt to determine the role of the individual microstructural constituents in this α₂ + B2 alloy. Tensile testing of both round and flat specimens in vacuum indicated a change in deformation behavior from 25 °C to 450 °C. Observations suggested that this change in deformation behavior occurred within the α₂ phase. Failure initiation at 450 °C and above was by a ductile process and was associated with the B2 phase. Above 600 °C and at high strains, plastic deformation occurred predominantly in the B2 phase. Strain localization was observed above 600 °C and found to be due to the lower work-hardening rate of the B2 phase. Strain localization at slip band intersections with prior β grain boundaries resulted in rapid strain accumulation in the B2 phase. Alignment of secondary α₂ laths with the tensile axis at high deformation levels appeared to inhibit shear band localization between voids due to a lack of participation of the α₂ phase in deformation. Formerly Materials Scientist, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433, is Program Manager, Air Force Office of Scientific Research, Boiling AFB, Washington, DC 20332. Formerly Professor, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA 15213, is Scientist, Lawrence Berkeley Laboratory, Berkeley, CA 94720.     

Void formation,void growth and tensile fracture in Ti-6AI-4V

Metallurgical and Materials Transactions A - The influence of microstructure on void formation, void growth and tensile fracture was investigated for the Ti-6A1-4V alloy, aged to yield strengths of...     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part II. crack path and microstructure

In Part I of this article, the influence of various testing parameters and environments on the fatigue-crack growth rates in samples of beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials) ELI was reported.^[1] A design-of-experiments (DOE) approach was used to survey different combinations of variables, all expected to be important for dynamically loaded offshore oil and gas production risers, and to identify significant effects on the fatigue-crack propagation rate at a stress-intensity range of ΔK=17 MPa . The da/dN vs ΔK curves also were examined for the DOE and supplementary tests, and the results of the two approaches were compared. In this part of the study, the microstructural basis for the robust fatigue-crack growth resistance of beta-annealed Ti-6Al-4V-0.1Ru (ELI) samples was investigated with optical metallography and scanning electron microscopy (SEM). A gradual transition from structure-sensitive (microfacet formation) to structure-insensitive (striation formation) crack propagation centered at ΔK _trans ≅24 MPa , regardless of the combination of testing/environmental conditions examined; the absence of a sharp transition in the slope of the da/dN vs ΔK curves was, therefore, entirely consistent with the fracture-surface morphology. The size of the reversed cyclic plastic zone at the transition ΔK value correlated with the size of the lamella packets, but the number of cycles required to generate a striation ranged between one and ten, suggesting that the crack actively grew over only a portion of its front at any one instant. It is interesting to note that ΔK _trans was the same as the stress range where the da/dN curves at 0.2, 2, and 20 Hz (in seawater) converged to a single curve (refer to Part I of this article): at lower stress ranges, crack growth rates at 20 Hz were significantly higher than those at 0.2 and 2 Hz. The quantitative data showed that fatigue cracks propagated parallel to lamellae interfaces when the long axes of the lamellae made a relatively small angle (<30 deg) to the nominal crack-propagation direction. The crack cut directly across lamellae (i.e., perpendicular to their surfaces) when the long axes of the lamellae were nearly perpendicular to the nominal crack-propagation direction. If the lamellae long axes lay 45 deg to the crack-propagation direction, the crack deflected to run parallel or perpendicular to the lamellae. This behavior occurred regardless of the environment and loading conditions investigated. There was considerable variation in the amount that the cracks deviated from their nominal plane (i.e., the plane normal to the load axis and through the notch tip), with much greater deflections in the cold-rolled than in the parent material, but the angle of the macroscopic crack plane did not exceed 11 deg. Crack branching was observed both at the center and outer surfaces of the samples, regardless of ΔK or other parameters. The relationship between micro- and macrobranches was examined, and branching was more prominent below ΔK _trans, which separated the structure-sensitive and continuum-mode crack-propagation regimes. The relative amounts of micro- and microbranches are reported, and this branching may explain the large scatter in the measurements of the fatigue-crack growth rate often encountered in Ti-6Al-4V and its variants and points to the need for thorough characterization of crack paths, both midplane and surface, as part of the interpretation of da/dN vs ΔK data.     

Fatigue crack propagation resistance of beta-annealed Ti-6AI-4V alloys of differing interstitial oxygen contents

Fatigue crack growth rates have been determined for beta-annealed Ti-6Al-4V alloys with respective oxygen contents of 0.06, 0.11, 0.18 and 0.20 wt pct. For each of these alloys, transitional crack growth behavior has been observed which appears to correlate with a critical value of the reversed plastic zone size,viz the Widmanstätten packet size. Moreover, growth rates below transitional levels order in terms of packet size, with lower growth rates associated with larger packets. The present results suggest that intersti-tial oxygen content and prior beta grain size significantly affect fatigue crack growth rates through control of the Widmanstätten packet size.     

稀土Gd对Ti-46.5Al-2.5V-1.0Cr合金组织的影响

 通过光学金相和扫描电镜观察分析了Ti-46.5Al-2.5V-1.0Cr（at.%）合金中添加微量稀土Gd对合金铸态和变形组织的影响。发现,含稀土Gd的TiAl合金铸造组织晶粒尺寸细小;变形宏观组织中均匀变形区较大,变形组织动态再结晶的体积分数高,层片分解较完全。     

Room temperature creep of Ti-6AI-4V

Recent investigations have shown both that Ti-6A1-4V does and does not exhibit room temperature creep at low stress. In the present investigation three different microstructures (α β anneal, recrystallization anneal and β anneal) of Ti-6A1-4V were examined under dead weight torsional loading. The loading sequence was forward, reverse and second forward loading. It was concluded that even at a stress level well below the yield stress the alloy exhibits creep in forward loading and increased creep strain in reverse loading and second forward loading. Furthermore the rate of creep at constant stress was different in different microstructures of the alloy; the maximum creep rate occurred with the recrystallization anneal and the minimum creep rate occurred with the β anneal. At the maximum stress in forward loading there appeared to be a change in creep mechanism.