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Environmental fatigue crack propagation behavior of β-annealed Ti-6Al-4V alloy in NaCl solution under controlled potentials
International Journal of Fatigue 111 (2018) 186–195
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Environmental fatigue crack propagation behavior of β-annealed Ti-6Al-4V alloy in NaCl solution under controlled potentials Soojin Ahna, Daeho Jeonga, Yongnam Kwonb, Masahiro Gotoc, Hyokyung Sunga, Sangshik Kima,
T
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a
Dept. of Materials Engineering and Convergence Technology, ReCAPT, Gyeongsang National University, Jinju 52828, Republic of Korea Dept. of Materials Processing, Korea Institute of Materials Science, Changwon 51508, Republic of Korea c Dept. of Mechanical Engineering, Oita University, Oita 870-1192, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Fatigue crack propagation Ti-6Al-4V β -annealing Crack branching
The fatigue crack propagation (FCP) behavior of β-annealed Ti-6Al-4V (Ti64) alloy was examined in air and 0.6 M NaCl solution under anodic and cathodic applied potentials and at two different R ratios of 0.1 and 0.7. βannealed Ti64 alloy was sensitive to environmental FCP in NaCl solution under both anodic and cathodic applied potentials at an R ratio of 0.1, while the environmental effect was almost negligible at an R ratio of 0.7. The extent of crack branching in air and at an R ratio of 0.1 decreased substantially in NaCl solution and/or at an R ratio of 0.7. The EBSD (electron backscatter diffraction) and SEM (scanning electron microscope) fractographic analyses on the FCP tested specimens showed that microstructure-sensitive cracking, rather than crystallographic cleavage cracking, became encouraged in NaCl solution and/or high R ratio. It was suggested that the extent of crack branching played an important role in determining the environmental FCP behavior of β-annealed Ti64 alloy.
1. Introduction Ti-6Al-4V (Ti64) alloy is the most widely used α/β titanium (Ti) alloy used in a variety of industrial applications, such as energy, transportation and aerospace industries [1,2]. Excellent mechanical properties of Ti64 alloy, particularly in corrosive environment, have been reported in a number of literatures [3,4]. The mechanical properties of Ti64 alloy is greatly influenced by the heat treatment condition which is one of the most important variables in determining the microstructure [1,5–7]. The microstructure of α/β Ti64 alloy is classified as equiaxed α, acicular α and bimodal (equiaxed α + acicular α) structures. Among a number of heat treatment processes available for Ti64 alloy, β-annealing is known to provide an excellent resistance to fatigue crack propagation (FCP) [8–10]. β-annealed Ti64 alloy that has a microstructure of colonies of acicular α phases in large prior β grain boundaries is therefore considered as an excellent material for the structures requiring superior resistance to FCP [11,12]. The improved FCP resistance of β-annealed Ti64 alloy has often been attributed to crack branching [10,13,14]. Severe crack branching during fatigue loading in β-annealed Ti64 alloy can increase the resistance to FCP by reducing effective ΔK with a moderate sacrifice in strength and ductility [10,13,14]. It has been well established that Ti alloy has an excellent resistance
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to corrosion by forming a passive oxide film on the surface [15]. However, its pre-cracked specimen is not immune to environment-affected cracking (EAC) [16,17]. Hydrogen embrittlement (HE) is often attributed to the primary cause of environmental effect on the FCP behavior of Ti64 alloy in aqueous NaCl solution [18,19]. Hydrogen is soluble in solid solution in large quantity and can induce, for example, stress-assisted hydride formation at crack tip under fatigue loading [20,21]. Hydrogen diffused into a process zone at crack tip during fatigue loading may reduce the interatomic bond strength, causing an embrittlement [22,23]. Hydrogen may also encourage the emittance of dislocations at crack tip, which can further contribute to local embrittlement [22]. All these embrittlement mechanisms can increase the FCP rate of Ti64 alloy in hydrogen-generating environment, such as in aqueous NaCl solution under controlled potential [24,25]. The extent of hydrogen damage in α/β Ti64 alloy tends to vary with different microstructures [19]. For example, mill-annealed Ti64 alloy, the microstructure of which consists of equiaxed α phases with discontinuous network of β phase around α phases, has an excellent resistance to hydrogen degradation [19]. The structure of continuous network of β phase may suffer more hydrogen degradation than that of continuous α phase, since the diffusivity of hydrogen in Ti64 alloy is significantly greater in β phase than α phase [26]. β-annealed Ti64 alloy with the microstructure of acicular α platelets separated by continuous network
Corresponding author. E-mail address: [email protected] (S. Kim).
https://doi.org/10.1016/j.ijfatigue.2018.02.013 Received 9 October 2017; Received in revised form 7 February 2018; Accepted 9 February 2018 Available online 10 February 2018 0142-1123/ © 2018 Elsevier Ltd. All rights reserved.
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acicular α phases, aligned along certain directions forming colonies, within large prior β grain boundaries. The FCP tests were conducted on CT specimens in air and 0.6 M NaCl on a servo-hydraulic testing machine (Instron model 8516) at a frequency of 10 Hz. For the fitting on NASGRO equation, positive R ratios of 0.1 and 0.7 were chosen for the tests [35]. The FCP tests in 0.6 M NaCl solution were performed while the CT specimens were polarized under an anodic (+0.05 V vs Ecorr) and a cathodic (−0.1 V vs Ecorr) applied potential, respectively, using a potentiostat (Wonatech model, WPG100E). The Ecorr value of β-annealed Ti64 alloy was measured in 0.6 M NaCl solution at a scan rate of 1 mV s−1 in three-electrode cell using a potentiostat (PAR model versa stat II). The open circuit potential of −0.51 VSCE was used as a reference value for polarizing the specimen. The length of fatigue crack in air and 0.6 M NaCl solution was measured by using a DCPD (direct current potential drop) technique, following an ASTM E647 standard [38]. Detailed set-up for an FCP test in aqueous environment is schematically described in Fig. 3. After the FCP tests, fracture surfaces and crack paths of tested specimens were examined by using an SEM and an optical microscope to identify any change in FCP mechanism with different environments and R ratios. Ultrasonic cleaning was utilized in distilled water for those specimens tested in NaCl environment to remove any calcareous deposits on the fracture surface.
Fig. 1. The schematic illustration of compact tension (CT) specimen used in this study. All the units are mm.
of β phase can therefore be more sensitive to hydrogen embrittlement [26]. The hydrogen diffused through the network of β phase may induce brittle fracture in β-annealed Ti64 alloy along α/β interfaces, either by the formation of hydrides at α/β interface or by the reduction in interatomic bond strength [20,21,26]. The environmental FCP behavior of β-annealed Ti64 alloy can also be enhanced by hydrogen-assisted separation along α/β interfaces [27–30]. Conversely, gaseous hydrogen in mill-annealed Ti64 alloy may induce brittle fracture in α phases, resulting in the crack deflection and the increase in FCP resistance [31]. It has been reported that β-annealing heat treatment enhances the FCP resistance of Ti64 alloy by inducing crack branching in the structure of acicular α phases [13,14]. Unfortunately, the research on the FCP behavior of β-annealed Ti64 alloy has mostly been done in the 70es and 80es [10,14,32,33]. Therefore, the advances in metal processing and metallurgical techniques have not been incorporated in the FCP characterization of β-annealed Ti64 alloy. Moreover the understanding on environmental FCP behavior has not been progressed since that time [10,14,32–34]. Despite the improvement in FCP prediction methodologies, such as NASGRO [35] and LAPS1 [36], factors affecting environmental FCP behavior of β-annealed Ti64 alloy have not been well established. In this study, the environmental FCP behavior of β-annealed Ti64 alloy was examined in an aqueous NaCl solution under controlled potentials. The FCP tests were conducted on β-annealed Ti64 alloy at two different stress (R) ratios of 0.1 and 0.7 in air and 0.6 M NaCl solution under anodic and cathodic applied potentials. The mechanisms associated with environmental FCP behavior of β-annealed Ti64 alloy was discussed based on the micrographic and fractographic observations.
3. Results and discussion 3.1. FCP behavior of β-annealed Ti64 in NaCl at an R ratio of 0.1
To examine the environmental FCP behavior of β-annealed Ti64 alloy, 12.7 mm thick compact tension (CT) specimens with L (longitudinal)-T (transverse) orientation were prepared from the central portion of 105 mm thick plate. Fig. 1 shows the schematic illustration of CT specimen used in this study. The β-annealing treatment was conducted by heating at 1050 °C for 1 h and furnace cooling. The specimens were then heated again at 730 °C for 2 h and furnace cooled for the purpose of stabilization [9]. For a micrographic observation, β-annealed Ti64 alloy was polished and etched by using a Kroll’s reagent (85 mL H2O + 3 mL HNO3 + 5 mL HF) [37]. Fig. 2 shows the microstructure of β-annealed Ti64 alloy examined by using (a) an optical microscope and (b) an SEM (scanning electron microscope), showing
Fig. 4 shows the da/dN vs ΔK (stress intensity factor range) curves of β-annealed Ti64 alloy at an R ratio of 0.1 in air and 0.6 M NaCl solution under an anodic applied potential of +0.05 V vs Ecorr and a cathodic applied potential of −0.1 V vs Ecorr, respectively. Since a metallic structure is designed to spend the most of its fatigue life at low ΔK regime, the near-threshold ΔK (ΔKth) value, below which fatigue crack does not grow, provides an excellent indication of FCP resistance of metal [39–44]. The ΔKth value of β-annealed Ti64 alloy was 8.8 MPa√m in air which was substantially greater than the reported value of 3.9 MPa√m for the mill-annealed counterpart [8]. The improved FCP resistance of Ti64 alloy with β-annealing has been attributed to crack branching, as a crack tends to grow along α/β interfaces within colonies of acicular α phases [8,13,14,45]. Yoder and co-workers have proposed that crystallographic crack branching can occur in β-annealed Ti64 alloy when the size of cyclic plastic zone2 is smaller than the size of relevant structural barriers (i.e., widmanstatten packet size, colony boundaries, α phase grain size and prior β grain size) [10,32]. Crack branching is then expected to decrease the FCP rates by reducing effective ΔK as a result of dispersing strain field energy of crack among multiple crack tips [8,9,32]. It has been suggested that fatigue crack in β-annealed Ti64 alloy grows basically in crystallographic manner, trespassing a bundle of acicular α platelets [8,10,32]. When a growing crack meets the structural barriers, it may also follow the boundaries of these barriers [8,10,32,45]. The α/ β interfaces can play a similar role as that of colony boundaries and β grain boundaries, when acicular α platelets are positioned to be roughly perpendicular to the direction of crack growth [8,10,32,45]. These two favorable crack growth mechanisms competing in β-annealed Ti64 alloy, i.e., crystallographic cleavage and structure-sensitive fracture, are believed to induce crack branching [8,10,13,14]. It was also noted in Fig. 4 that the FCP rates of β-annealed Ti64 alloy was lower in air than those in NaCl solution below the ΔK value of approximately 25 MPa√m, while the difference in FCP rates between anodic and cathodic applied potential was not significant. The ΔKth
1 The simplified fatigue life equation formulated from low cycle fatigue data of smooth specimen and the Rice-Kujawski-Ellyin asymptotic field.
2 Yoder et al. utilized the following equation to calculate cyclic plastic zone size, ryc = 0.132(ΔK/2σcy)2.
2. Experimental procedures
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Fig. 2. The microstructure of β-annealed Ti64 alloy examined by using (a) an optical microscope and (b) an SEM.
Fig. 3. The fatigue crack propagation testing system in aqueous environment under controlled potential.
value decreased from 8.8 MPa√m in air to 6.5 MPa√m @ +0.05 V vs Ecorr and 5.7 MPa√m @ −0.1 V vs Ecorr, representing the reduction of 26% and 35%, respectively. For the case of mill-annealed Ti64 alloy, no notable reduction in ΔKth value has been reported in NaCl solution under both anodic and cathodic applied potentials [31,46]. It is therefore clearly suggested that β-annealed Ti64 alloy with the microstructure of acicular α phases is more sensitive to environmental FCP in NaCl solution than mill-annealed counterpart with the microstructure of equiaxed α phases [20,21,26]. At present, the reason for the increased sensitivity with the microstructure of acicular α phases has not been clearly understood [32,47–49]. The ease of hydrogen-transport may vary with different microstructural arrays of α and β phases, since each phase has different hydrogen diffusivity [27,31,48]. Continuous network of β phases between acicular α platelets in β-annealed Ti64 alloy can provide a path for easy hydrogen-transport [18,26,50]. Discontinuous nature of β phases in mill-annealed Ti64 alloy, on the other hand, is not a favorable microstructure for hydrogen-transport. It has been well established that hydrogen influences the FCP behavior of metals [24–30]. Even in NaCl solution under an anodic applied potential, the chemical dissociation at crack tip enables hydrogen generation and absorption causing an intrinsic hydrogen-damage [25,50]. To understand the environmental FCP behavior of β-annealed Ti64 alloy in NaCl solution, the SEM fractographic analysis was
Fig. 4. The da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.1 in air and 0.6 M NaCl solution under controlled potentials.
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Fig. 5. Low- and high-magnification SEM fractographs of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in near-threshold ΔK regime.
solution under both applied potentials. A number of literatures have suggested that the secondary cracks on the out of plane are the result of crack branching, which tend to decrease the FCP rates by reducing effective ΔK at crack tip [8,9,13,14]. As compared to that in air, the fracture surface appeared to be smoothened in NaCl solution under both applied potentials. The change in crystallographic cleavage plane from low-index to high-index plane in aggressive environment has often been attributed to “smoothen” the cleavage facets [51–53]. To identify any change in crystallographic cleavage plane with exposure to NaCl solution, the EBSD technique was utilized.
conducted. Fig. 5 shows low- and high-magnification SEM fractographs of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the fractographs were documented in near-threshold ΔK regime. Several things were notable on the fracture mode of β-annealed Ti64 alloy. First of all, a number of secondary cracks were observed on the specimen tested in air, while not in NaCl solution. Fig. 6 shows high-magnification SEM fractographs matching those represented in Fig. 5. It was clearly demonstrated that the tendency for secondary cracking was substantially reduced in NaCl
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Fig. 6. High-magnification SEM fractographs showing secondary cracks on the fracture surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were documented in near-threshold ΔK regime.
Fig. 7. The SEM fractographs and the EBSD orientation image maps of metallographically polished surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
3.2. EBSD analysis on fracture surface of β-annealed Ti64
orientation image maps of each specimen indicated the change in facet orientation at grain boundaries. For clear interpretation, facet crystallographic analysis and pole figure analysis were conducted. Fig. 8 shows the pole figures, inverse pole figures and facet crystallography results plotted on an irreducible triangle projection of β-annealed Ti64 specimens tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. The pole figure analysis indicated that texture was moderately strong in between rolling direction (RD) and transverse direction (TD), while the intensity of texture varied for each specimen.
Fig. 7 shows the SEM fractographs and the EBSD orientation image maps of metallographically polished surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in near-threshold ΔK regime. Each specimen was sectioned normal to the direction of crack growth, and grain orientation was measured from the surface adjoining to the fracture surface by using an EBSD technique. The EBSD studies on
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Fig. 8. Pole figures, inverse pole figures and facet crystallography results plotted on an irreducible triangle projection on β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
From the analysis of inverse pole figure, preferred facet orientations were determined. The maximum intensity of texture was close to {2 1 1 0 } pole for the specimen tested in air along with a weak texture close to {0 0 0 1} pole, indicating low-index planes. In NaCl solution under anodic and cathodic applied potentials, on the other hand, the maximum intensity of texture was measured in between {0 0 0 1} and {2 1 1 0 } pole, indicating high-index planes. To identify the change in cleavage facet plane from low- to high-index planes with the exposure to NaCl solution, the facet orientations, located exactly adjacent to fatigue cracked plane, are marked by blue circles on an irreducible triangle projection. The HCP (hexagonal close packed) crystal lattices for some selected facets are also demonstrated in Fig. 8. It is shown that the orientations of facets exactly match to the maximum texture intensity regime, confirming the change in tendency of crack growth from low- to high-index planes with the presence of aggressive environment.
boundaries), it tends to bifurcate [8,9,13,14]. The crack growing in NaCl solution, on the other hand, has a tendency of following α/β interfaces, as shown in Fig. 9(b) and (c). The change in preferred crack path in NaCl solution is possibly associated with hydrogen-enhanced cracking along α/β interfaces. As mentioned previously, hydrogen diffusivity and solubility are greater in β phase than α phase [26]. The ribs of β phase in α/β interlamellar structure can therefore provide easy crack path in hydrogen-generating environment [19,26,27]. The fracture surface of crack growing through the interface of α/β interlamellar structure may be confused with the facets of crystallographic cleavage crack. High-magnification SEM fractographic analysis was conducted on these facets to identify any difference in facet morphology, as shown in Fig. 10. Featureless surface morphology was observed on the facets in air, representing the crystallographic cleavage faceting (Fig. 10(a)). On the other hand, fine steps were observed on the facets in NaCl solution as a result of separating acicular α phase and interfacial β phase (Fig. 10(b) and (c)). The crack growing mechanism would change from crystallographic cleavage cracking in air to microstructure-sensitive cracking in hydrogen-generating environment, restraining crack branching. This supports the proposition of Irving and Beevers that crack branching-related transition phenomenon in da/dN vs ΔK curves of Ti alloys only occurs in relatively inert environment, but not in aggressive environment [33]. The present study strongly suggests that environmental effect on the FCP behavior of β-annealed Ti64 alloy may be related to crack branching, the extent of which is influenced by different environment.
3.3. Crack branching phenomenon in β-annealed Ti64 Fig. 9 shows low- and high-magnification crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in low ΔK regime. It was notable that crack branching was significant in air, while the extent decreased considerably in NaCl solution. In air, the crack tends to cut through the bundle of acicular α platelets, due to strong tendency for crystallographic cleavage cracking. When the crack meets the structural barriers (i.e., α/β interface, colony boundaries, prior β
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Fig. 9. Low- and high-magnification crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were documented in low ΔK regime.
3.4. FCP behavior of β-annealed Ti64 in NaCl at an R ratio of 0.7
ratio of 0.7. No notable difference in fracture morphology was observed between different R ratios of 0.7 and 0.1, suggesting that the effect of environment was similar for both high and low R ratios. It has been previously demonstrated that crack branching in β-annealed Ti64 alloy is restrained in air above the ΔK value of approximately 25 MPa√m at an R ratio of 0.1 and/or at high R ratio of 0.7 [9]. This study also shows that crack branching is largely suppressed at an R ratio of 0.7 in NaCl solution. It is therefore suggested that the extent of crack branching with different environments (Fig. 9) and different R ratios (Fig. 12) is an important parameter to determine the FCP resistance of β-annealed Ti64 alloy. It is hypothesized that two competing crack growth mechanisms, including crystallographic cleavage cracking and microstructure-sensitive cracking, are responsible for the crack branching in β-annealed Ti64 alloy. The crack branching is suppressed, as the tendency increases for microstructure-sensitive cracking, such as cracking along α/β interfaces, colony boundaries and β grain boundaries. The present fractographic analysis suggests that microstructuresensitive cracking in β-annealed Ti64 alloy is encouraged in NaCl solution and at high R ratios. Resultantly, crack branching is restrained in NaCl solution and/or high R ratio, increasing the FCP rates of β-
The FCP tests of β-annealed Ti64 alloy in hydrogen-generating environment were conducted at an R ratio of 0.7 to understand how the environmental sensitivity varied with different R ratios. It is because hydrogen intake through adsorption at crack tip can be encouraged with greater CTOD (crack tip opening displacement) at high R ratios [54,55]. Hydrogen transport by means of moving dislocations at the crack tip can also be enhanced with increasing mean stress [16,54,55]. Fig. 11 shows the da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.7 in air and NaCl solution under a cathodic applied potential of −0.1 V vs Ecorr. Unlike those at an R ratio of 0.1, environmental effect was negligible at high R ratio of 0.7. To identify the reason for the apparently reduced environmental effect at an R ratio of 0.7, the fractographic and micrographic analysis were conducted. Fig. 12 shows the SEM fractographs and the matching crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.7 in (a) air and (b) 0.6 M NaCl solution under an applied potential of −0.1 V vs Ecorr, documented in the near-threshold ΔK regime. As for the case of R ratio of 0.1, relatively smoothened fracture surface was also observed in NaCl solution at an R
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Fig. 10. The High-magnification examination on the facets of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
4. Conclusions The environment-enhanced FCP behavior of β-annealed Ti64 alloy was investigated at R ratios of 0.1 and 0.7 in air and 0.6 M NaCl solution under anodic and cathodic applied potentials, and the following conclusions were drawn. (1) β-annealed Ti64 alloy was sensitive to environmental FCP in NaCl solution at an R ratio of 0.1. The EBSD analysis of fracture surface and the SEM observation of crack path suggested that crystallographic cleavage crack growth was suppressed in NaCl solution, while microstructure-sensitive crack growth along α/β interfaces was encouraged. (2) Significant extent of crack branching in β-annealed Ti64 alloy, as known to increase the resistance to FCP by reducing effective ΔK, was observed in air at an R ratio of 0.1. On the other hand, crack branching was largely suppressed in NaCl solution and/or at an R ratio of 0.7, reducing the resistance to FCP. (3) Two competing crack growth mechanisms, including crystallographic cleavage cracking and microstructure-sensitive cracking, were responsible for the crack branching in β-annealed Ti64 alloy. The presence of NaCl solution and/or high R ratios in β-annealed Ti64 alloy tended to encourage the tendency for microstructuresensitive cracking, restraining crack branching during fatigue loading. (4) Environmental effect on the FCP of β-annealed Ti64 alloy was almost negligible at an R ratio of 0.7. It was therefore suggested that the extent of crack branching, rather than hydrogen-damage, played an important role in determining the environmental FCP behavior of β-annealed Ti64 alloy at high R ratio.
Fig. 11. The da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.7 in air and 0.6 M NaCl solution under a cathodic applied potential of −0.1 V vs Ecorr.
annealed Ti64 alloy. The trends in Figs. 11 and 12 suggested that the extent of crack branching, rather than hydrogen-damage, played an important role in determining the environmental FCP behavior of βannealed Ti64 alloy.
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Fig. 12. The SEM fractographs and the matching crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.7 in (a) air and (b) NaCl solution under an applied potential of −0.1 V vs Ecorr. All the pictures were documented in near-threshold ΔK regime.
Acknowledgments
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Environmental fatigue crack propagation behavior of β-annealed Ti-6Al-4V alloy in NaCl solution under controlled potentials Soojin Ahna, Daeho Jeonga, Yongnam Kwonb, Masahiro Gotoc, Hyokyung Sunga, Sangshik Kima,
T
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a
Dept. of Materials Engineering and Convergence Technology, ReCAPT, Gyeongsang National University, Jinju 52828, Republic of Korea Dept. of Materials Processing, Korea Institute of Materials Science, Changwon 51508, Republic of Korea c Dept. of Mechanical Engineering, Oita University, Oita 870-1192, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Fatigue crack propagation Ti-6Al-4V β -annealing Crack branching
The fatigue crack propagation (FCP) behavior of β-annealed Ti-6Al-4V (Ti64) alloy was examined in air and 0.6 M NaCl solution under anodic and cathodic applied potentials and at two different R ratios of 0.1 and 0.7. βannealed Ti64 alloy was sensitive to environmental FCP in NaCl solution under both anodic and cathodic applied potentials at an R ratio of 0.1, while the environmental effect was almost negligible at an R ratio of 0.7. The extent of crack branching in air and at an R ratio of 0.1 decreased substantially in NaCl solution and/or at an R ratio of 0.7. The EBSD (electron backscatter diffraction) and SEM (scanning electron microscope) fractographic analyses on the FCP tested specimens showed that microstructure-sensitive cracking, rather than crystallographic cleavage cracking, became encouraged in NaCl solution and/or high R ratio. It was suggested that the extent of crack branching played an important role in determining the environmental FCP behavior of β-annealed Ti64 alloy.
1. Introduction Ti-6Al-4V (Ti64) alloy is the most widely used α/β titanium (Ti) alloy used in a variety of industrial applications, such as energy, transportation and aerospace industries [1,2]. Excellent mechanical properties of Ti64 alloy, particularly in corrosive environment, have been reported in a number of literatures [3,4]. The mechanical properties of Ti64 alloy is greatly influenced by the heat treatment condition which is one of the most important variables in determining the microstructure [1,5–7]. The microstructure of α/β Ti64 alloy is classified as equiaxed α, acicular α and bimodal (equiaxed α + acicular α) structures. Among a number of heat treatment processes available for Ti64 alloy, β-annealing is known to provide an excellent resistance to fatigue crack propagation (FCP) [8–10]. β-annealed Ti64 alloy that has a microstructure of colonies of acicular α phases in large prior β grain boundaries is therefore considered as an excellent material for the structures requiring superior resistance to FCP [11,12]. The improved FCP resistance of β-annealed Ti64 alloy has often been attributed to crack branching [10,13,14]. Severe crack branching during fatigue loading in β-annealed Ti64 alloy can increase the resistance to FCP by reducing effective ΔK with a moderate sacrifice in strength and ductility [10,13,14]. It has been well established that Ti alloy has an excellent resistance
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to corrosion by forming a passive oxide film on the surface [15]. However, its pre-cracked specimen is not immune to environment-affected cracking (EAC) [16,17]. Hydrogen embrittlement (HE) is often attributed to the primary cause of environmental effect on the FCP behavior of Ti64 alloy in aqueous NaCl solution [18,19]. Hydrogen is soluble in solid solution in large quantity and can induce, for example, stress-assisted hydride formation at crack tip under fatigue loading [20,21]. Hydrogen diffused into a process zone at crack tip during fatigue loading may reduce the interatomic bond strength, causing an embrittlement [22,23]. Hydrogen may also encourage the emittance of dislocations at crack tip, which can further contribute to local embrittlement [22]. All these embrittlement mechanisms can increase the FCP rate of Ti64 alloy in hydrogen-generating environment, such as in aqueous NaCl solution under controlled potential [24,25]. The extent of hydrogen damage in α/β Ti64 alloy tends to vary with different microstructures [19]. For example, mill-annealed Ti64 alloy, the microstructure of which consists of equiaxed α phases with discontinuous network of β phase around α phases, has an excellent resistance to hydrogen degradation [19]. The structure of continuous network of β phase may suffer more hydrogen degradation than that of continuous α phase, since the diffusivity of hydrogen in Ti64 alloy is significantly greater in β phase than α phase [26]. β-annealed Ti64 alloy with the microstructure of acicular α platelets separated by continuous network
Corresponding author. E-mail address: [email protected] (S. Kim).
https://doi.org/10.1016/j.ijfatigue.2018.02.013 Received 9 October 2017; Received in revised form 7 February 2018; Accepted 9 February 2018 Available online 10 February 2018 0142-1123/ © 2018 Elsevier Ltd. All rights reserved.
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acicular α phases, aligned along certain directions forming colonies, within large prior β grain boundaries. The FCP tests were conducted on CT specimens in air and 0.6 M NaCl on a servo-hydraulic testing machine (Instron model 8516) at a frequency of 10 Hz. For the fitting on NASGRO equation, positive R ratios of 0.1 and 0.7 were chosen for the tests [35]. The FCP tests in 0.6 M NaCl solution were performed while the CT specimens were polarized under an anodic (+0.05 V vs Ecorr) and a cathodic (−0.1 V vs Ecorr) applied potential, respectively, using a potentiostat (Wonatech model, WPG100E). The Ecorr value of β-annealed Ti64 alloy was measured in 0.6 M NaCl solution at a scan rate of 1 mV s−1 in three-electrode cell using a potentiostat (PAR model versa stat II). The open circuit potential of −0.51 VSCE was used as a reference value for polarizing the specimen. The length of fatigue crack in air and 0.6 M NaCl solution was measured by using a DCPD (direct current potential drop) technique, following an ASTM E647 standard [38]. Detailed set-up for an FCP test in aqueous environment is schematically described in Fig. 3. After the FCP tests, fracture surfaces and crack paths of tested specimens were examined by using an SEM and an optical microscope to identify any change in FCP mechanism with different environments and R ratios. Ultrasonic cleaning was utilized in distilled water for those specimens tested in NaCl environment to remove any calcareous deposits on the fracture surface.
Fig. 1. The schematic illustration of compact tension (CT) specimen used in this study. All the units are mm.
of β phase can therefore be more sensitive to hydrogen embrittlement [26]. The hydrogen diffused through the network of β phase may induce brittle fracture in β-annealed Ti64 alloy along α/β interfaces, either by the formation of hydrides at α/β interface or by the reduction in interatomic bond strength [20,21,26]. The environmental FCP behavior of β-annealed Ti64 alloy can also be enhanced by hydrogen-assisted separation along α/β interfaces [27–30]. Conversely, gaseous hydrogen in mill-annealed Ti64 alloy may induce brittle fracture in α phases, resulting in the crack deflection and the increase in FCP resistance [31]. It has been reported that β-annealing heat treatment enhances the FCP resistance of Ti64 alloy by inducing crack branching in the structure of acicular α phases [13,14]. Unfortunately, the research on the FCP behavior of β-annealed Ti64 alloy has mostly been done in the 70es and 80es [10,14,32,33]. Therefore, the advances in metal processing and metallurgical techniques have not been incorporated in the FCP characterization of β-annealed Ti64 alloy. Moreover the understanding on environmental FCP behavior has not been progressed since that time [10,14,32–34]. Despite the improvement in FCP prediction methodologies, such as NASGRO [35] and LAPS1 [36], factors affecting environmental FCP behavior of β-annealed Ti64 alloy have not been well established. In this study, the environmental FCP behavior of β-annealed Ti64 alloy was examined in an aqueous NaCl solution under controlled potentials. The FCP tests were conducted on β-annealed Ti64 alloy at two different stress (R) ratios of 0.1 and 0.7 in air and 0.6 M NaCl solution under anodic and cathodic applied potentials. The mechanisms associated with environmental FCP behavior of β-annealed Ti64 alloy was discussed based on the micrographic and fractographic observations.
3. Results and discussion 3.1. FCP behavior of β-annealed Ti64 in NaCl at an R ratio of 0.1
To examine the environmental FCP behavior of β-annealed Ti64 alloy, 12.7 mm thick compact tension (CT) specimens with L (longitudinal)-T (transverse) orientation were prepared from the central portion of 105 mm thick plate. Fig. 1 shows the schematic illustration of CT specimen used in this study. The β-annealing treatment was conducted by heating at 1050 °C for 1 h and furnace cooling. The specimens were then heated again at 730 °C for 2 h and furnace cooled for the purpose of stabilization [9]. For a micrographic observation, β-annealed Ti64 alloy was polished and etched by using a Kroll’s reagent (85 mL H2O + 3 mL HNO3 + 5 mL HF) [37]. Fig. 2 shows the microstructure of β-annealed Ti64 alloy examined by using (a) an optical microscope and (b) an SEM (scanning electron microscope), showing
Fig. 4 shows the da/dN vs ΔK (stress intensity factor range) curves of β-annealed Ti64 alloy at an R ratio of 0.1 in air and 0.6 M NaCl solution under an anodic applied potential of +0.05 V vs Ecorr and a cathodic applied potential of −0.1 V vs Ecorr, respectively. Since a metallic structure is designed to spend the most of its fatigue life at low ΔK regime, the near-threshold ΔK (ΔKth) value, below which fatigue crack does not grow, provides an excellent indication of FCP resistance of metal [39–44]. The ΔKth value of β-annealed Ti64 alloy was 8.8 MPa√m in air which was substantially greater than the reported value of 3.9 MPa√m for the mill-annealed counterpart [8]. The improved FCP resistance of Ti64 alloy with β-annealing has been attributed to crack branching, as a crack tends to grow along α/β interfaces within colonies of acicular α phases [8,13,14,45]. Yoder and co-workers have proposed that crystallographic crack branching can occur in β-annealed Ti64 alloy when the size of cyclic plastic zone2 is smaller than the size of relevant structural barriers (i.e., widmanstatten packet size, colony boundaries, α phase grain size and prior β grain size) [10,32]. Crack branching is then expected to decrease the FCP rates by reducing effective ΔK as a result of dispersing strain field energy of crack among multiple crack tips [8,9,32]. It has been suggested that fatigue crack in β-annealed Ti64 alloy grows basically in crystallographic manner, trespassing a bundle of acicular α platelets [8,10,32]. When a growing crack meets the structural barriers, it may also follow the boundaries of these barriers [8,10,32,45]. The α/ β interfaces can play a similar role as that of colony boundaries and β grain boundaries, when acicular α platelets are positioned to be roughly perpendicular to the direction of crack growth [8,10,32,45]. These two favorable crack growth mechanisms competing in β-annealed Ti64 alloy, i.e., crystallographic cleavage and structure-sensitive fracture, are believed to induce crack branching [8,10,13,14]. It was also noted in Fig. 4 that the FCP rates of β-annealed Ti64 alloy was lower in air than those in NaCl solution below the ΔK value of approximately 25 MPa√m, while the difference in FCP rates between anodic and cathodic applied potential was not significant. The ΔKth
1 The simplified fatigue life equation formulated from low cycle fatigue data of smooth specimen and the Rice-Kujawski-Ellyin asymptotic field.
2 Yoder et al. utilized the following equation to calculate cyclic plastic zone size, ryc = 0.132(ΔK/2σcy)2.
2. Experimental procedures
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Fig. 2. The microstructure of β-annealed Ti64 alloy examined by using (a) an optical microscope and (b) an SEM.
Fig. 3. The fatigue crack propagation testing system in aqueous environment under controlled potential.
value decreased from 8.8 MPa√m in air to 6.5 MPa√m @ +0.05 V vs Ecorr and 5.7 MPa√m @ −0.1 V vs Ecorr, representing the reduction of 26% and 35%, respectively. For the case of mill-annealed Ti64 alloy, no notable reduction in ΔKth value has been reported in NaCl solution under both anodic and cathodic applied potentials [31,46]. It is therefore clearly suggested that β-annealed Ti64 alloy with the microstructure of acicular α phases is more sensitive to environmental FCP in NaCl solution than mill-annealed counterpart with the microstructure of equiaxed α phases [20,21,26]. At present, the reason for the increased sensitivity with the microstructure of acicular α phases has not been clearly understood [32,47–49]. The ease of hydrogen-transport may vary with different microstructural arrays of α and β phases, since each phase has different hydrogen diffusivity [27,31,48]. Continuous network of β phases between acicular α platelets in β-annealed Ti64 alloy can provide a path for easy hydrogen-transport [18,26,50]. Discontinuous nature of β phases in mill-annealed Ti64 alloy, on the other hand, is not a favorable microstructure for hydrogen-transport. It has been well established that hydrogen influences the FCP behavior of metals [24–30]. Even in NaCl solution under an anodic applied potential, the chemical dissociation at crack tip enables hydrogen generation and absorption causing an intrinsic hydrogen-damage [25,50]. To understand the environmental FCP behavior of β-annealed Ti64 alloy in NaCl solution, the SEM fractographic analysis was
Fig. 4. The da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.1 in air and 0.6 M NaCl solution under controlled potentials.
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Fig. 5. Low- and high-magnification SEM fractographs of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in near-threshold ΔK regime.
solution under both applied potentials. A number of literatures have suggested that the secondary cracks on the out of plane are the result of crack branching, which tend to decrease the FCP rates by reducing effective ΔK at crack tip [8,9,13,14]. As compared to that in air, the fracture surface appeared to be smoothened in NaCl solution under both applied potentials. The change in crystallographic cleavage plane from low-index to high-index plane in aggressive environment has often been attributed to “smoothen” the cleavage facets [51–53]. To identify any change in crystallographic cleavage plane with exposure to NaCl solution, the EBSD technique was utilized.
conducted. Fig. 5 shows low- and high-magnification SEM fractographs of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the fractographs were documented in near-threshold ΔK regime. Several things were notable on the fracture mode of β-annealed Ti64 alloy. First of all, a number of secondary cracks were observed on the specimen tested in air, while not in NaCl solution. Fig. 6 shows high-magnification SEM fractographs matching those represented in Fig. 5. It was clearly demonstrated that the tendency for secondary cracking was substantially reduced in NaCl
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Fig. 6. High-magnification SEM fractographs showing secondary cracks on the fracture surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were documented in near-threshold ΔK regime.
Fig. 7. The SEM fractographs and the EBSD orientation image maps of metallographically polished surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
3.2. EBSD analysis on fracture surface of β-annealed Ti64
orientation image maps of each specimen indicated the change in facet orientation at grain boundaries. For clear interpretation, facet crystallographic analysis and pole figure analysis were conducted. Fig. 8 shows the pole figures, inverse pole figures and facet crystallography results plotted on an irreducible triangle projection of β-annealed Ti64 specimens tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. The pole figure analysis indicated that texture was moderately strong in between rolling direction (RD) and transverse direction (TD), while the intensity of texture varied for each specimen.
Fig. 7 shows the SEM fractographs and the EBSD orientation image maps of metallographically polished surface of β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in near-threshold ΔK regime. Each specimen was sectioned normal to the direction of crack growth, and grain orientation was measured from the surface adjoining to the fracture surface by using an EBSD technique. The EBSD studies on
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Fig. 8. Pole figures, inverse pole figures and facet crystallography results plotted on an irreducible triangle projection on β-annealed Ti64 specimen tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
From the analysis of inverse pole figure, preferred facet orientations were determined. The maximum intensity of texture was close to {2 1 1 0 } pole for the specimen tested in air along with a weak texture close to {0 0 0 1} pole, indicating low-index planes. In NaCl solution under anodic and cathodic applied potentials, on the other hand, the maximum intensity of texture was measured in between {0 0 0 1} and {2 1 1 0 } pole, indicating high-index planes. To identify the change in cleavage facet plane from low- to high-index planes with the exposure to NaCl solution, the facet orientations, located exactly adjacent to fatigue cracked plane, are marked by blue circles on an irreducible triangle projection. The HCP (hexagonal close packed) crystal lattices for some selected facets are also demonstrated in Fig. 8. It is shown that the orientations of facets exactly match to the maximum texture intensity regime, confirming the change in tendency of crack growth from low- to high-index planes with the presence of aggressive environment.
boundaries), it tends to bifurcate [8,9,13,14]. The crack growing in NaCl solution, on the other hand, has a tendency of following α/β interfaces, as shown in Fig. 9(b) and (c). The change in preferred crack path in NaCl solution is possibly associated with hydrogen-enhanced cracking along α/β interfaces. As mentioned previously, hydrogen diffusivity and solubility are greater in β phase than α phase [26]. The ribs of β phase in α/β interlamellar structure can therefore provide easy crack path in hydrogen-generating environment [19,26,27]. The fracture surface of crack growing through the interface of α/β interlamellar structure may be confused with the facets of crystallographic cleavage crack. High-magnification SEM fractographic analysis was conducted on these facets to identify any difference in facet morphology, as shown in Fig. 10. Featureless surface morphology was observed on the facets in air, representing the crystallographic cleavage faceting (Fig. 10(a)). On the other hand, fine steps were observed on the facets in NaCl solution as a result of separating acicular α phase and interfacial β phase (Fig. 10(b) and (c)). The crack growing mechanism would change from crystallographic cleavage cracking in air to microstructure-sensitive cracking in hydrogen-generating environment, restraining crack branching. This supports the proposition of Irving and Beevers that crack branching-related transition phenomenon in da/dN vs ΔK curves of Ti alloys only occurs in relatively inert environment, but not in aggressive environment [33]. The present study strongly suggests that environmental effect on the FCP behavior of β-annealed Ti64 alloy may be related to crack branching, the extent of which is influenced by different environment.
3.3. Crack branching phenomenon in β-annealed Ti64 Fig. 9 shows low- and high-magnification crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and 0.6 M NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were taken in low ΔK regime. It was notable that crack branching was significant in air, while the extent decreased considerably in NaCl solution. In air, the crack tends to cut through the bundle of acicular α platelets, due to strong tendency for crystallographic cleavage cracking. When the crack meets the structural barriers (i.e., α/β interface, colony boundaries, prior β
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Fig. 9. Low- and high-magnification crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively. All the pictures were documented in low ΔK regime.
3.4. FCP behavior of β-annealed Ti64 in NaCl at an R ratio of 0.7
ratio of 0.7. No notable difference in fracture morphology was observed between different R ratios of 0.7 and 0.1, suggesting that the effect of environment was similar for both high and low R ratios. It has been previously demonstrated that crack branching in β-annealed Ti64 alloy is restrained in air above the ΔK value of approximately 25 MPa√m at an R ratio of 0.1 and/or at high R ratio of 0.7 [9]. This study also shows that crack branching is largely suppressed at an R ratio of 0.7 in NaCl solution. It is therefore suggested that the extent of crack branching with different environments (Fig. 9) and different R ratios (Fig. 12) is an important parameter to determine the FCP resistance of β-annealed Ti64 alloy. It is hypothesized that two competing crack growth mechanisms, including crystallographic cleavage cracking and microstructure-sensitive cracking, are responsible for the crack branching in β-annealed Ti64 alloy. The crack branching is suppressed, as the tendency increases for microstructure-sensitive cracking, such as cracking along α/β interfaces, colony boundaries and β grain boundaries. The present fractographic analysis suggests that microstructuresensitive cracking in β-annealed Ti64 alloy is encouraged in NaCl solution and at high R ratios. Resultantly, crack branching is restrained in NaCl solution and/or high R ratio, increasing the FCP rates of β-
The FCP tests of β-annealed Ti64 alloy in hydrogen-generating environment were conducted at an R ratio of 0.7 to understand how the environmental sensitivity varied with different R ratios. It is because hydrogen intake through adsorption at crack tip can be encouraged with greater CTOD (crack tip opening displacement) at high R ratios [54,55]. Hydrogen transport by means of moving dislocations at the crack tip can also be enhanced with increasing mean stress [16,54,55]. Fig. 11 shows the da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.7 in air and NaCl solution under a cathodic applied potential of −0.1 V vs Ecorr. Unlike those at an R ratio of 0.1, environmental effect was negligible at high R ratio of 0.7. To identify the reason for the apparently reduced environmental effect at an R ratio of 0.7, the fractographic and micrographic analysis were conducted. Fig. 12 shows the SEM fractographs and the matching crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.7 in (a) air and (b) 0.6 M NaCl solution under an applied potential of −0.1 V vs Ecorr, documented in the near-threshold ΔK regime. As for the case of R ratio of 0.1, relatively smoothened fracture surface was also observed in NaCl solution at an R
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Fig. 10. The High-magnification examination on the facets of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.1 in (a) air and NaCl solution under an applied potential of (b) +0.05 V vs Ecorr and (c) −0.1 V vs Ecorr, respectively.
4. Conclusions The environment-enhanced FCP behavior of β-annealed Ti64 alloy was investigated at R ratios of 0.1 and 0.7 in air and 0.6 M NaCl solution under anodic and cathodic applied potentials, and the following conclusions were drawn. (1) β-annealed Ti64 alloy was sensitive to environmental FCP in NaCl solution at an R ratio of 0.1. The EBSD analysis of fracture surface and the SEM observation of crack path suggested that crystallographic cleavage crack growth was suppressed in NaCl solution, while microstructure-sensitive crack growth along α/β interfaces was encouraged. (2) Significant extent of crack branching in β-annealed Ti64 alloy, as known to increase the resistance to FCP by reducing effective ΔK, was observed in air at an R ratio of 0.1. On the other hand, crack branching was largely suppressed in NaCl solution and/or at an R ratio of 0.7, reducing the resistance to FCP. (3) Two competing crack growth mechanisms, including crystallographic cleavage cracking and microstructure-sensitive cracking, were responsible for the crack branching in β-annealed Ti64 alloy. The presence of NaCl solution and/or high R ratios in β-annealed Ti64 alloy tended to encourage the tendency for microstructuresensitive cracking, restraining crack branching during fatigue loading. (4) Environmental effect on the FCP of β-annealed Ti64 alloy was almost negligible at an R ratio of 0.7. It was therefore suggested that the extent of crack branching, rather than hydrogen-damage, played an important role in determining the environmental FCP behavior of β-annealed Ti64 alloy at high R ratio.
Fig. 11. The da/dN vs ΔK curves of β-annealed Ti64 alloy at an R ratio of 0.7 in air and 0.6 M NaCl solution under a cathodic applied potential of −0.1 V vs Ecorr.
annealed Ti64 alloy. The trends in Figs. 11 and 12 suggested that the extent of crack branching, rather than hydrogen-damage, played an important role in determining the environmental FCP behavior of βannealed Ti64 alloy.
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Fig. 12. The SEM fractographs and the matching crack paths of β-annealed Ti64 specimens, FCP tested at an R ratio of 0.7 in (a) air and (b) NaCl solution under an applied potential of −0.1 V vs Ecorr. All the pictures were documented in near-threshold ΔK regime.
Acknowledgments
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