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Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy
Accepted Manuscript Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy Q. Wang, J.Q. Ren, Y.K. Wu, P. Jiang, J.Q. Li, Z.J. Sun, X.T. Liu PII:
S0925-8388(19)30768-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.02.302
Reference:
JALCOM 49734
To appear in:
Journal of Alloys and Compounds
Received Date: 11 December 2018 Revised Date:
25 February 2019
Accepted Date: 26 February 2019
Please cite this article as: Q. Wang, J.Q. Ren, Y.K. Wu, P. Jiang, J.Q. Li, Z.J. Sun, X.T. Liu, Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.02.302. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy Q. Wang a, *, J.Q. Ren b,*, Y.K. Wu a, P. Jiang a, J.Q. Li a, Z.J. Sun a, X.T. Liu b Luoyang Ship Material Research Institute, Luoyang, 471003, China
b
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals,
Lanzhou University of Technology, Lanzhou 730050, China
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a
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*Corresponding authors. Tel.: 86-0931-2976688, Fax.: 86-0931-2806962
E-mail address: [email protected] (Q. Wang); [email protected] (J.Q. Ren)
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Abstract: The fatigue crack growth (FCG) behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy were investigated comparably. The FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially at the low stress intensity factor range (∆K<40MPa·m1/2). The fatigue crack growth path, deformed sub-structure and surface damage morphology were observed by SEM and TEM. For the fully-lamellar
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structure, because the same orientation of α lamellae and Burgers relationship between α and β lamellas in the α colony, the fatigue cracks often grow along a straight line in the α colony by cutting the β lamellas. For the bi-lamellar structure, because the resistance of
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secondary α (αs) precipitations in the β lamellas to dislocation slips, the frequency of crack propagation along the α/β interface increases. The fatigue cracks often grow along a zigzag
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in the α colony, which results in the fatigue crack growth path is more circuitous than that of the fully-lamellar structure in α colonies, and this is the main reason that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure. This research has an important engineering significance to improve the low cycle fatigue (LCF) performance of the coarse lamellar structured Ti alloys. Key words: Titanium alloy, Fatigue crack growth, Lamellar structure
1. Introduction Titanium alloys have a potential application in the marine engineering due to its high -1-
ACCEPTED MANUSCRIPT specific strength (the ratio of strength and density) and excellent corrosion resistance to seawater [1-2]. These properties are of great significance for the reduction of marine weight and the cost of corrosion protection. Thus, the application of titanium alloys in the marine engineering is gradually developed from the local parts to the body structure. For
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instance, the pressure-resistant hulls of manned submersible are recently constructed by titanium alloys [3-4].
The titanium alloys used in the marine engineering are generally the near-α titanium alloy or the extra low interstitial (ELI) α/β titanium alloy due to their high fracture
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toughness and excellent weldability [ 5 - 6 ], such as Ti-6Al-2Zr-1Nb-1Mo [ 7 ], Ti-5Al-1V-1Sn-1Zr-0.8Mo [8] and Ti-6Al-4V ELI alloy [9]. Four different structures can
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be obtained for those kinds of titanium alloys depending on their heat treatment processes, namely, bimodal, lamellar, basket-weave and equiaxed structure [10]. Among them, the bimodal structure containing 20% primary α phase (αp) is usually selected as the final structure in the engineering application due to its good balance of strength and ductility [11]. The lamellar structure obtained by cooling from the β region [12] often has high
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impact toughness and resistance to fatigue crack propagation, and its properties are sensitive to the thickness of α lamellas [13-14]. In order to investigate the effect of the microstructure (β grain size, α/β colony size and width of α lamella) on the fatigue crack growth behavior of lamellar Ti-6Al-2Sn-4Zr-xMo (x=2, 4 and 6) alloys, the fatigue crack
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growth rate curves, crack paths and plastic zone ahead of the crack tip were compared [15], they found that the resistance of fatigue crack growth decreases from Ti6242 to Ti6246 due
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to the decreasing α/β colony size. Since the existence of Burgers Orientation Relationship (BOR), i.e., 110 ∥ 0002 ， 〈11 1〉 ∥ 〈112 0〉 , the dislocation slip can easily pass through the β lamellas in a straight line or a deflection of 10.5° during plastic deformation[16,17,18], which result in the fatigue strength and the resistance to crack propagation of lamellar structure decreased, especially in the coarse lamellar structure, such as the casting structure. In order to improve the resistance of β lamellas to dislocation slip in the coarse lamellar structure, a bi-lamellar structure was processed by aging treatments [19-20], which is characterized by a large -2-
ACCEPTED MANUSCRIPT number of secondary α precipitations (αs) in β lamellas. It is found that the bi-lamellar structure shows the higher yield strength and fatigue strength in comparison with the fully-lamellar structure due to dislocation slips impeded by the β lamellas with the αs precipitations. To date, the studies of bi-lamellar structure mainly focused on high-cycle
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fatigue (HCF) performance [21-22], and there is little research on its low-cycle fatigue (LCF) performance. In practice, LCF fracture is the most common failure mode of the marine engineering equipment [23-24]. Because the fatigue crack propagation life accounts for more than 90% of the total LCF life [25], improving the resistance to fatigue crack
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propagation can obviously increase its LCF life.
In this study, a Ti-6Al-3Nb-2Zr-1Mo (Ti6321) alloy was selected as the research
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material. This alloy was developed by the Luoyang Ship Materials Research Institute on the basis of Ti-6Al-2Zr-1Nb-1Mo alloy in order to improve its impact toughness, which has been widely used in the marine engineering. In this paper, the fatigue crack growth behaviors of bi-lamellar and fully-lamellar Ti6321 alloy were comparatively studied in order to analyze the effect of αs precipitations on the FCG rate and the mechanism of
2. Experimental
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fatigue crack propagation.
The Ti-6Al-3Nb-2Zr-1Mo alloy used in this study is a hot-rolled plate with a thickness
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of 28mm, and its chemical composition is 6.5%Al, 3.18%Nb, 2.04%Zr, 1.17 %Mo, 0.0019%H, 0.114%O and the balance Ti (in wt%). The initial microstructure is a typical
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bimodal structure, as shown in Fig. 1. Two pieces of the materials with the size of 160 × 80 × 28 mm were cut from the hot-rolled plate along the rolling direction. In order to obtain a fully-lamellar structure, one piece was solid solution treated at 1020 oC for 0.5 hour (above its β-transus temperature of 998 oC) and then cooled to room temperature in air. The other piece was solid solution treated at 1020 oC for 0.5 hour and then cooled to 950 oC in the furnace with a rate of 1 oC/min, and subsequently aged at 500 oC for 5 hours to obtain a bi-lamellar structure. The FCG rate (da/dN) of bi-lamellar and fully-lamellar Ti6321 alloy was tested using the compact tensile (CT) specimens. The CT specimens were cut from the above -3-
ACCEPTED MANUSCRIPT heat-treatment materials using electrical discharge machining, and the crack propagation direction is parallel to the rolling direction. The preparation process of CT specimens is schematically shown in Fig. 1. The test of FCG rate was performed in accordance with GB Standard T 6398-2000 using an INSTRON 1343-250 kN electro-hydraulic servo fatigue
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machine. Three specimens were tested for each structure. The samples were first fatigue precracked for a crack length of ~ 2mm from the notch tip at a loading frequency of 20 Hz. Subsequently, the actual FCG rate tests were conducted on the fatigue precracked samples. The tests were controlled by the load with its maximum of 13.5 kN under the stress ratio
recorded by measuring the crack opening distance.
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(R) of 0.1 and the load frequency of 10 Hz. The length of fatigue crack growth was
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The microstructures of fully-lamellar and bi-lamellar Ti6321 alloy before and after tests were observed using Leica DMI 5000M optic microscopy (OM) and JEM-2100 transmission electron microcopy (TEM). TEM samples were prepared using a twin-jet electro-polishing technique at -35oC and 40V, using the following reagent: 60% methanol, 35% butyl alcohol, and 5% perchloric acid. In order to reveal the effect of microstructure
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on the mechanism of fatigue crack propagation, the side surface of one fully-lamellar and bi-lamellar structural CT sample was firstly mechanically polished and etched using Kroll’s reagent for 10 seconds before the fatigue crack growth test, as shown in Fig.1. After test, the fatigue crack propagation path and surface damage morphology of both structures
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3. Results
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were observed using QUANTA FEG 450 scanning electron microscope (SEM).
3.1 Microstructure
Figure 2 shows the microstructures of the Ti6321 alloy with fully-lamellar and bi-lamellar. As shown in Fig. 2a and d, both structures are consisted of the coarse prior β grains and the α colony, and each α colony is characteristic of with similar aligned α lamellas separated by β lamellas. The size of prior β grains and α colony were measured using a mean intercept method and the thickness of α and β lamellas were estimated by taking the average distance over fifteen lamellas in different α colonies. It is found that the average size of prior β grains in fully-lamellar and bi-lamellar structure is approximately equal with -4-
ACCEPTED MANUSCRIPT the size of ~ 400 µm, but the thickness of α and β lamellas in the bi-lamellar structure is obviously larger than that of the fully-lamellar structure, as shown in Fig. 2b and e. Further observation by TEM revealed that, for bi-lamellar structure, there are a large number of fine αs precipitations in β lamellas, as shown in Fig.2 c and e.
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3.2 Fatigue crack growth rate The da/dN curves of fully-lamellar and bi-lamellar structure are shown in Fig. 3a. It is found that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially at the low stress intensity factor range (∆ <40MPa·m1/2). This
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suggests the influence of αs precipitations on FCG rate was mainly reflected in the initial stage of crack growth. When the fatigue crack growth to a certain length, the effect of
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microstructure on the FCG rate gradually became weak and the stress state became the main influencing factor. Figure 3b shows the corresponding fatigue crack length as a function of the cyclic number. It can be seen that, as the cyclic number increasing, the crack length of both fully-lamellar and bi-lamellar structure increases slowly at the beginning, and subsequently increase rapidly. It is worthy to note that the fatigue crack of
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bi-lamellar structure begins to grow rapidly at the cyclic number of 2.5×105, which is obviously larger than that of fully-lamellar structure at the cyclic number of 1×105, although both of the structure begin to grow rapidly at the same length of ~ 18 mm. 3.3 Fatigue crack propagation path
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As the above results demonstrated, the effect of αs precipitations on FCG rate mainly reflected in the low ∆ . Thus, the fatigue crack propagation characteristics of
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fully-lamellar and bi-lamellar structure at the same ∆ of ~20 MPa·m1/2 (corresponding to the crack length of ~11.5 mm for fully-lamellar structure and ~14.5 mm for bi-lamellar structure) are shown in Fig. 4. It can be found that, for both fully-lamellar and bi-lamellar structure, the fatigue crack propagated by two modes, i.e., along the interface of α colonies (Fig. 4a and d) or by traversing the entire α colony (Fig. 4b-c and e-f). The crack propagation path of 68 and 45 α colonies in fully-lamellar and bi-lamellar structure were observed. The statistic results reveal that the traversing α colony is the dominant crack propagated mode for fully-lamellar and bi-lamellar structure, which is consistent with the previous results [26]. Further focusing on this propagated mode, it is found that, for -5-
ACCEPTED MANUSCRIPT fully-lamellar structure, the fatigue crack mostly pass through α colony along a relatively straight line (Fig. 4b), although sometimes the bifurcation of the crack path was also observed, as shown in Fig. 4c. Different from the straight crack path in fully-lamellar structure, the step-wise crack paths are frequently observed in bi-lamellar structure, as
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shown in Fig. 4d. Figure 5 shows the crack propagation characteristics of fully-lamellar and bi-lamellar structure at ∆ of ~50 MPa·m1/2 (corresponding to the crack length of ~24 mm for fully-lamellar structure and ~26 mm for bi-lamellar structure). It is found that the crack
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propagation path of fully-lamellar structure in the α colonies did not change significantly as the ∆ increased, as shown in Fig. 5a-b. However, the crack propagation paths of
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bi-lamellar structure in the α colonies change from the step-wise path to a straight path. As shown in Fig. 5c-d, the fatigue cracks grow through the entire α colony along a straight path, which direction is either perpendicular or parallel to the direction of α lamellas. In other words, when the ∆ increased to a critical value, the fatigue crack growth mechanism of fully-lamellar structure and bi-lamellar structure tends to be the same. This
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may be the underlying reason for that the FCG rate of bi-lamellar structure and fully-lamellar structure tends to be same in high ∆ (∆ > 50 MPa·m1/2).
4. Discussion
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It can be seen from Fig. 3a that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially in the low ∆ . It indicates that the fine αs precipitation
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in β lamellas decreased the FCG rate of the lamellar-structured titanium alloys in the initial stage of fatigue crack growth. It is worth emphasizing that, in this experiment, the average thickness of α lamellas in the bi-lamellar structure was significantly larger than that in the fully-lamellar structure (Fig. 2b and Fig. 2e). Because the FCG rate of lamellar-structured titanium alloys is sensitive to the thickness of α lamellas, and the FCG rate decrease obviously with decreasing the thickness of α lamellas [27-28]. It can be expected that, if the thickness of α lamellas in bi-lamellar structure in this experiment was decreased to the same value of fully-lamellar structure, the FCG rate of bi-lamellar structure will be further lower than that of the fully-lamellar structure. -6-
ACCEPTED MANUSCRIPT In addition, it should be emphasized that, the thickness of β lamellas must be wide enough to precipitate a large number of αs precipitations in β lamellas [29]. As shown in Fig. 2, the thickness of β lamellas in the bi-lamellar structure is significantly larger than that of the fully-lamellar structure. This conclusion was also verified by our supplementary
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experiment, as shown in Fig. 6. It was found that, because the thickness of the β lamellas was too narrow, few αs phase was precipitated in the fully-lamellar structure after aging at 500 °C for 5h. Therefore, when this technology is adopted to improve the fatigue crack growth resistance of the lamellar-structured titanium alloys, the most priority is to ensure
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the β lamellas approaching a certain thickness.
The resistance of fatigue crack propagation of the bi-lamellar structure is better than that
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of the fully-lamellar structure at the initial stage of fatigue crack growth, which mainly results from the different mechanisms for fatigue crack propagation. For the fully-lamellar Ti6321 alloy, since the α phase is softer than the β phase [30] and its Al equivalent is greater than 6%, 0002〈112 0〉 basal slips take place preferentially in the α lamellas during cyclic deformation [ 31 ], as shown in Fig. 7a. In addition, because the
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crystallographic orientations of the parallel α lamellas in the α colony are same, and the α and β lamellas completely obey the Burgers orientation relationship, the direction of 0002〈112 0〉 basal dislocation slips in the different α lamellas is same [32-33], and the dislocations could continuously pass through the β lamellas, as shown in Fig. 7a. As a
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consequence, the slip bands in the α colony are straight and parallel, which can cut the β lamellas into several sections, as shown in Fig. 7b-c. As the cyclic deformation continues,
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the fatigue cracks will tend to propagate along the slip bands in each α colony. For the bi-lamellar structure, because the strength of the β lamellas is significantly improved by the fine αs precipitations, the 0002〈112 0〉 basal dislocations in the α lamellas are difficult to pass through the β lamellas, and gradually pile up at the α/β interface, as shown in Fig. 7d. As a consequence, the second slip system with the direction parallel to the α/β interface was activated, as shown in Fig. 7e-f. As the cyclic deformation continues, the fatigue cracks will tend to propagate along the slip bands or the α/β interface, which results in a more circuitous fatigue crack propagation paths of the bi-lamellar structure than that of fully-lamellar structure, as shown in Fig. 4b and f. -7-
ACCEPTED MANUSCRIPT It is well known that the additional energy needs to be consumed when the fatigue crack occur deflect and bifurcate during its propagation, which can decrease the FCG rate [34-35]. The schematic diagrams of the mechanism for fatigue cracks propagation in fully-lamellar and bi-lamellar structure are illustrated in Fig. 8. It is found that, for
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fully-lamellar structure, the 0002〈112 0〉 basal slip in the α lamellas can directly pass through the thin β lamellas due to the BOR between the α lamellas and the β lamellas, the fatigue cracks often propagate along the slip bands with a straight line in the α colony. The deflection of fatigue cracks mainly occur at the interface of the different α colonies, and the
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degree of deflection depends on the size of α colony and the crystallographic orientation of the adjacent α colonies [ 36 ], as shown in Fig. 8a. For bi-lamellar structure, the
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0002〈112 0〉 basal slip in the α lamellas are hindered by the thick lamellas due to the fine αs precipitations, the fatigue crack growing along the α/β interface became the other propagated mechanism besides the cross β lamellas. Thus, the frequency of fatigue crack deflection in bi-lamellar structure is significantly larger than that of fully-lamellar structure, as shown in Fig. 8b.
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The casting is widely used in the manufacturing of components due to its low cost in comparison with the forging. However, the microstructure of the as-cast titanium alloys is generally a coarse lamellar structure [37-38], the large size of α colony and the wide α and β lamellas resulting in a significant decrease in the fatigue crack resistance and decrease
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the service safety of the component. According to the present results, the resistance to fatigue crack growth of the as-cast titanium alloys can be improved by the aging treatment.
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Therefore, the treatment process has a good application prospect to improve the fatigue performance of the coarse lamellar structured titanium alloys.
5. Conclusions
1) The fatigue crack growth rate of the bi-lamellar Ti6321 alloy is lower than that of the fully-lamellar structure, especially in the low stress intensity factor range. 2) The crack length of fully-lamellar and bi-lamellar structure increases slowly at the beginning as the cyclic number increase and then increase rapidly. In the case of the same crack length, the cyclic number of the bi-lamellar structure is 2.5 times than that -8-
ACCEPTED MANUSCRIPT of the fully-lamellar structure. 3) Because the fatigue crack growth of bi-lamellar structure was impeded by αs precipitations, the frequency of propagation along the α/β interface obviously increases in comparison with the fully-lamellar structure. As a consequence, in the α
circuitous than that of the fully-lamellar structure.
Acknowledgements
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colony, the fatigue crack propagation paths of the bi-lamellar structure is more
This project is financially supported by the Key projects to strengthen the foundation
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(YK180701) and the National Natural Science Foundation of China (51701189, 51775055 and 51601084).
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Captions Fig. 1 The initial microstructure of the hot-rolled Ti6321 alloy plate and the preparation process diagram of the compact tension (CT) specimen for the FCG rate testing. Fig. 2 The characteristics of fully-lamellar and bi-lamellar Ti6321 alloy: (a-b) Optic
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images of fully-lamellar structure; (c) TEM image of fully-lamellar structure; (d-e) Optic images of bi-lamellar structure; (f) TEM image of bi-lamellar structure.
Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321 alloy: (a)
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the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of cycles.
Fig. 4 SEM images of the fatigue crack propagation path in fully-lamellar (a-c) and bi-lamellar structure (d-e) at the same ∆ of ~20 MPa·m1/2.
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Fig. 5 SEM images of fatigue crack propagation path of the fully-lamellar (a-b) and bi-lamellar (c-d) structure at ∆ of ~50 MPa·m1/2.
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a) OM image, (b) TEM image.
Fig. 7 TEM and SEM images near the fracture surface together with the schematic diagram
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of deformation mechanism of fully-lamellar structure (a-c) and bi- lamellar structure (d-f) after fatigue crack growth rate testing.
Fig. 8 Schematic diagram of the mechanism for the fatigue crack propagation in
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fully-lamellar and bi-lamellar structure.
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Figures Fig.1
Fig. 1 The initial microstructure of the hot-rolled Ti6321 alloy plate and the
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preparation process diagram of the compact tension (CT) specimen for the FCG rate
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testing.
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Fig. 2 The characteristics of fully-lamellar and bi-lamellar Ti6321 alloy: (a-b) Optic images of fully-lamellar structure; (c) TEM image of fully-lamellar structure; (d-e)
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Optic images of bi-lamellar structure; (f) TEM image of bi-lamellar structure.
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Fig. 3a
Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321
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alloy: (a) the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of
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Fig. 3b
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Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321 alloy: (a) the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of
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Fig. 4 SEM images of the fatigue crack propagation path in fully-lamellar (a-c) and
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bi-lamellar structure (d-e) at the same ∆ ܭof ~20 MPa·m1/2.
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Fig. 5 SEM images of fatigue crack propagation path of the fully-lamellar (a-b) and
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Fig. 6a
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a)
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OM image, (b) TEM image.
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Fig. 6b
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a)
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Fig. 7
Fig. 7 TEM and SEM images near the fracture surface together with the schematic diagram of deformation mechanism of fully-lamellar structure (a-c) and bi- lamellar
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Fig. 8
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Fig. 8 Schematic diagram of the mechanism for the fatigue crack propagation in fully-lamellar and bi-lamellar structure.
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The fatigue crack growth rate is decreased by the αs precipitations in β lamellas. The treatment has a good application for improving the fatigue performance. The fatigue crack growth mode of fully- and bi-lamellar structure are established.
S0925-8388(19)30768-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.02.302
Reference:
JALCOM 49734
To appear in:
Journal of Alloys and Compounds
Received Date: 11 December 2018 Revised Date:
25 February 2019
Accepted Date: 26 February 2019
Please cite this article as: Q. Wang, J.Q. Ren, Y.K. Wu, P. Jiang, J.Q. Li, Z.J. Sun, X.T. Liu, Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.02.302. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy Q. Wang a, *, J.Q. Ren b,*, Y.K. Wu a, P. Jiang a, J.Q. Li a, Z.J. Sun a, X.T. Liu b Luoyang Ship Material Research Institute, Luoyang, 471003, China
b
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals,
Lanzhou University of Technology, Lanzhou 730050, China
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a
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*Corresponding authors. Tel.: 86-0931-2976688, Fax.: 86-0931-2806962
E-mail address: [email protected] (Q. Wang); [email protected] (J.Q. Ren)
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Abstract: The fatigue crack growth (FCG) behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy were investigated comparably. The FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially at the low stress intensity factor range (∆K<40MPa·m1/2). The fatigue crack growth path, deformed sub-structure and surface damage morphology were observed by SEM and TEM. For the fully-lamellar
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structure, because the same orientation of α lamellae and Burgers relationship between α and β lamellas in the α colony, the fatigue cracks often grow along a straight line in the α colony by cutting the β lamellas. For the bi-lamellar structure, because the resistance of
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secondary α (αs) precipitations in the β lamellas to dislocation slips, the frequency of crack propagation along the α/β interface increases. The fatigue cracks often grow along a zigzag
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in the α colony, which results in the fatigue crack growth path is more circuitous than that of the fully-lamellar structure in α colonies, and this is the main reason that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure. This research has an important engineering significance to improve the low cycle fatigue (LCF) performance of the coarse lamellar structured Ti alloys. Key words: Titanium alloy, Fatigue crack growth, Lamellar structure
1. Introduction Titanium alloys have a potential application in the marine engineering due to its high -1-
ACCEPTED MANUSCRIPT specific strength (the ratio of strength and density) and excellent corrosion resistance to seawater [1-2]. These properties are of great significance for the reduction of marine weight and the cost of corrosion protection. Thus, the application of titanium alloys in the marine engineering is gradually developed from the local parts to the body structure. For
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instance, the pressure-resistant hulls of manned submersible are recently constructed by titanium alloys [3-4].
The titanium alloys used in the marine engineering are generally the near-α titanium alloy or the extra low interstitial (ELI) α/β titanium alloy due to their high fracture
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toughness and excellent weldability [ 5 - 6 ], such as Ti-6Al-2Zr-1Nb-1Mo [ 7 ], Ti-5Al-1V-1Sn-1Zr-0.8Mo [8] and Ti-6Al-4V ELI alloy [9]. Four different structures can
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be obtained for those kinds of titanium alloys depending on their heat treatment processes, namely, bimodal, lamellar, basket-weave and equiaxed structure [10]. Among them, the bimodal structure containing 20% primary α phase (αp) is usually selected as the final structure in the engineering application due to its good balance of strength and ductility [11]. The lamellar structure obtained by cooling from the β region [12] often has high
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impact toughness and resistance to fatigue crack propagation, and its properties are sensitive to the thickness of α lamellas [13-14]. In order to investigate the effect of the microstructure (β grain size, α/β colony size and width of α lamella) on the fatigue crack growth behavior of lamellar Ti-6Al-2Sn-4Zr-xMo (x=2, 4 and 6) alloys, the fatigue crack
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growth rate curves, crack paths and plastic zone ahead of the crack tip were compared [15], they found that the resistance of fatigue crack growth decreases from Ti6242 to Ti6246 due
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to the decreasing α/β colony size. Since the existence of Burgers Orientation Relationship (BOR), i.e., 110 ∥ 0002 ， 〈11 1〉 ∥ 〈112 0〉 , the dislocation slip can easily pass through the β lamellas in a straight line or a deflection of 10.5° during plastic deformation[16,17,18], which result in the fatigue strength and the resistance to crack propagation of lamellar structure decreased, especially in the coarse lamellar structure, such as the casting structure. In order to improve the resistance of β lamellas to dislocation slip in the coarse lamellar structure, a bi-lamellar structure was processed by aging treatments [19-20], which is characterized by a large -2-
ACCEPTED MANUSCRIPT number of secondary α precipitations (αs) in β lamellas. It is found that the bi-lamellar structure shows the higher yield strength and fatigue strength in comparison with the fully-lamellar structure due to dislocation slips impeded by the β lamellas with the αs precipitations. To date, the studies of bi-lamellar structure mainly focused on high-cycle
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fatigue (HCF) performance [21-22], and there is little research on its low-cycle fatigue (LCF) performance. In practice, LCF fracture is the most common failure mode of the marine engineering equipment [23-24]. Because the fatigue crack propagation life accounts for more than 90% of the total LCF life [25], improving the resistance to fatigue crack
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propagation can obviously increase its LCF life.
In this study, a Ti-6Al-3Nb-2Zr-1Mo (Ti6321) alloy was selected as the research
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material. This alloy was developed by the Luoyang Ship Materials Research Institute on the basis of Ti-6Al-2Zr-1Nb-1Mo alloy in order to improve its impact toughness, which has been widely used in the marine engineering. In this paper, the fatigue crack growth behaviors of bi-lamellar and fully-lamellar Ti6321 alloy were comparatively studied in order to analyze the effect of αs precipitations on the FCG rate and the mechanism of
2. Experimental
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fatigue crack propagation.
The Ti-6Al-3Nb-2Zr-1Mo alloy used in this study is a hot-rolled plate with a thickness
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of 28mm, and its chemical composition is 6.5%Al, 3.18%Nb, 2.04%Zr, 1.17 %Mo, 0.0019%H, 0.114%O and the balance Ti (in wt%). The initial microstructure is a typical
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bimodal structure, as shown in Fig. 1. Two pieces of the materials with the size of 160 × 80 × 28 mm were cut from the hot-rolled plate along the rolling direction. In order to obtain a fully-lamellar structure, one piece was solid solution treated at 1020 oC for 0.5 hour (above its β-transus temperature of 998 oC) and then cooled to room temperature in air. The other piece was solid solution treated at 1020 oC for 0.5 hour and then cooled to 950 oC in the furnace with a rate of 1 oC/min, and subsequently aged at 500 oC for 5 hours to obtain a bi-lamellar structure. The FCG rate (da/dN) of bi-lamellar and fully-lamellar Ti6321 alloy was tested using the compact tensile (CT) specimens. The CT specimens were cut from the above -3-
ACCEPTED MANUSCRIPT heat-treatment materials using electrical discharge machining, and the crack propagation direction is parallel to the rolling direction. The preparation process of CT specimens is schematically shown in Fig. 1. The test of FCG rate was performed in accordance with GB Standard T 6398-2000 using an INSTRON 1343-250 kN electro-hydraulic servo fatigue
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machine. Three specimens were tested for each structure. The samples were first fatigue precracked for a crack length of ~ 2mm from the notch tip at a loading frequency of 20 Hz. Subsequently, the actual FCG rate tests were conducted on the fatigue precracked samples. The tests were controlled by the load with its maximum of 13.5 kN under the stress ratio
recorded by measuring the crack opening distance.
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(R) of 0.1 and the load frequency of 10 Hz. The length of fatigue crack growth was
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The microstructures of fully-lamellar and bi-lamellar Ti6321 alloy before and after tests were observed using Leica DMI 5000M optic microscopy (OM) and JEM-2100 transmission electron microcopy (TEM). TEM samples were prepared using a twin-jet electro-polishing technique at -35oC and 40V, using the following reagent: 60% methanol, 35% butyl alcohol, and 5% perchloric acid. In order to reveal the effect of microstructure
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on the mechanism of fatigue crack propagation, the side surface of one fully-lamellar and bi-lamellar structural CT sample was firstly mechanically polished and etched using Kroll’s reagent for 10 seconds before the fatigue crack growth test, as shown in Fig.1. After test, the fatigue crack propagation path and surface damage morphology of both structures
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3. Results
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were observed using QUANTA FEG 450 scanning electron microscope (SEM).
3.1 Microstructure
Figure 2 shows the microstructures of the Ti6321 alloy with fully-lamellar and bi-lamellar. As shown in Fig. 2a and d, both structures are consisted of the coarse prior β grains and the α colony, and each α colony is characteristic of with similar aligned α lamellas separated by β lamellas. The size of prior β grains and α colony were measured using a mean intercept method and the thickness of α and β lamellas were estimated by taking the average distance over fifteen lamellas in different α colonies. It is found that the average size of prior β grains in fully-lamellar and bi-lamellar structure is approximately equal with -4-
ACCEPTED MANUSCRIPT the size of ~ 400 µm, but the thickness of α and β lamellas in the bi-lamellar structure is obviously larger than that of the fully-lamellar structure, as shown in Fig. 2b and e. Further observation by TEM revealed that, for bi-lamellar structure, there are a large number of fine αs precipitations in β lamellas, as shown in Fig.2 c and e.
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3.2 Fatigue crack growth rate The da/dN curves of fully-lamellar and bi-lamellar structure are shown in Fig. 3a. It is found that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially at the low stress intensity factor range (∆ <40MPa·m1/2). This
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suggests the influence of αs precipitations on FCG rate was mainly reflected in the initial stage of crack growth. When the fatigue crack growth to a certain length, the effect of
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microstructure on the FCG rate gradually became weak and the stress state became the main influencing factor. Figure 3b shows the corresponding fatigue crack length as a function of the cyclic number. It can be seen that, as the cyclic number increasing, the crack length of both fully-lamellar and bi-lamellar structure increases slowly at the beginning, and subsequently increase rapidly. It is worthy to note that the fatigue crack of
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bi-lamellar structure begins to grow rapidly at the cyclic number of 2.5×105, which is obviously larger than that of fully-lamellar structure at the cyclic number of 1×105, although both of the structure begin to grow rapidly at the same length of ~ 18 mm. 3.3 Fatigue crack propagation path
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As the above results demonstrated, the effect of αs precipitations on FCG rate mainly reflected in the low ∆ . Thus, the fatigue crack propagation characteristics of
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fully-lamellar and bi-lamellar structure at the same ∆ of ~20 MPa·m1/2 (corresponding to the crack length of ~11.5 mm for fully-lamellar structure and ~14.5 mm for bi-lamellar structure) are shown in Fig. 4. It can be found that, for both fully-lamellar and bi-lamellar structure, the fatigue crack propagated by two modes, i.e., along the interface of α colonies (Fig. 4a and d) or by traversing the entire α colony (Fig. 4b-c and e-f). The crack propagation path of 68 and 45 α colonies in fully-lamellar and bi-lamellar structure were observed. The statistic results reveal that the traversing α colony is the dominant crack propagated mode for fully-lamellar and bi-lamellar structure, which is consistent with the previous results [26]. Further focusing on this propagated mode, it is found that, for -5-
ACCEPTED MANUSCRIPT fully-lamellar structure, the fatigue crack mostly pass through α colony along a relatively straight line (Fig. 4b), although sometimes the bifurcation of the crack path was also observed, as shown in Fig. 4c. Different from the straight crack path in fully-lamellar structure, the step-wise crack paths are frequently observed in bi-lamellar structure, as
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shown in Fig. 4d. Figure 5 shows the crack propagation characteristics of fully-lamellar and bi-lamellar structure at ∆ of ~50 MPa·m1/2 (corresponding to the crack length of ~24 mm for fully-lamellar structure and ~26 mm for bi-lamellar structure). It is found that the crack
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propagation path of fully-lamellar structure in the α colonies did not change significantly as the ∆ increased, as shown in Fig. 5a-b. However, the crack propagation paths of
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bi-lamellar structure in the α colonies change from the step-wise path to a straight path. As shown in Fig. 5c-d, the fatigue cracks grow through the entire α colony along a straight path, which direction is either perpendicular or parallel to the direction of α lamellas. In other words, when the ∆ increased to a critical value, the fatigue crack growth mechanism of fully-lamellar structure and bi-lamellar structure tends to be the same. This
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4. Discussion
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It can be seen from Fig. 3a that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially in the low ∆ . It indicates that the fine αs precipitation
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in β lamellas decreased the FCG rate of the lamellar-structured titanium alloys in the initial stage of fatigue crack growth. It is worth emphasizing that, in this experiment, the average thickness of α lamellas in the bi-lamellar structure was significantly larger than that in the fully-lamellar structure (Fig. 2b and Fig. 2e). Because the FCG rate of lamellar-structured titanium alloys is sensitive to the thickness of α lamellas, and the FCG rate decrease obviously with decreasing the thickness of α lamellas [27-28]. It can be expected that, if the thickness of α lamellas in bi-lamellar structure in this experiment was decreased to the same value of fully-lamellar structure, the FCG rate of bi-lamellar structure will be further lower than that of the fully-lamellar structure. -6-
ACCEPTED MANUSCRIPT In addition, it should be emphasized that, the thickness of β lamellas must be wide enough to precipitate a large number of αs precipitations in β lamellas [29]. As shown in Fig. 2, the thickness of β lamellas in the bi-lamellar structure is significantly larger than that of the fully-lamellar structure. This conclusion was also verified by our supplementary
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experiment, as shown in Fig. 6. It was found that, because the thickness of the β lamellas was too narrow, few αs phase was precipitated in the fully-lamellar structure after aging at 500 °C for 5h. Therefore, when this technology is adopted to improve the fatigue crack growth resistance of the lamellar-structured titanium alloys, the most priority is to ensure
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the β lamellas approaching a certain thickness.
The resistance of fatigue crack propagation of the bi-lamellar structure is better than that
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of the fully-lamellar structure at the initial stage of fatigue crack growth, which mainly results from the different mechanisms for fatigue crack propagation. For the fully-lamellar Ti6321 alloy, since the α phase is softer than the β phase [30] and its Al equivalent is greater than 6%, 0002〈112 0〉 basal slips take place preferentially in the α lamellas during cyclic deformation [ 31 ], as shown in Fig. 7a. In addition, because the
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crystallographic orientations of the parallel α lamellas in the α colony are same, and the α and β lamellas completely obey the Burgers orientation relationship, the direction of 0002〈112 0〉 basal dislocation slips in the different α lamellas is same [32-33], and the dislocations could continuously pass through the β lamellas, as shown in Fig. 7a. As a
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consequence, the slip bands in the α colony are straight and parallel, which can cut the β lamellas into several sections, as shown in Fig. 7b-c. As the cyclic deformation continues,
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the fatigue cracks will tend to propagate along the slip bands in each α colony. For the bi-lamellar structure, because the strength of the β lamellas is significantly improved by the fine αs precipitations, the 0002〈112 0〉 basal dislocations in the α lamellas are difficult to pass through the β lamellas, and gradually pile up at the α/β interface, as shown in Fig. 7d. As a consequence, the second slip system with the direction parallel to the α/β interface was activated, as shown in Fig. 7e-f. As the cyclic deformation continues, the fatigue cracks will tend to propagate along the slip bands or the α/β interface, which results in a more circuitous fatigue crack propagation paths of the bi-lamellar structure than that of fully-lamellar structure, as shown in Fig. 4b and f. -7-
ACCEPTED MANUSCRIPT It is well known that the additional energy needs to be consumed when the fatigue crack occur deflect and bifurcate during its propagation, which can decrease the FCG rate [34-35]. The schematic diagrams of the mechanism for fatigue cracks propagation in fully-lamellar and bi-lamellar structure are illustrated in Fig. 8. It is found that, for
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fully-lamellar structure, the 0002〈112 0〉 basal slip in the α lamellas can directly pass through the thin β lamellas due to the BOR between the α lamellas and the β lamellas, the fatigue cracks often propagate along the slip bands with a straight line in the α colony. The deflection of fatigue cracks mainly occur at the interface of the different α colonies, and the
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degree of deflection depends on the size of α colony and the crystallographic orientation of the adjacent α colonies [ 36 ], as shown in Fig. 8a. For bi-lamellar structure, the
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0002〈112 0〉 basal slip in the α lamellas are hindered by the thick lamellas due to the fine αs precipitations, the fatigue crack growing along the α/β interface became the other propagated mechanism besides the cross β lamellas. Thus, the frequency of fatigue crack deflection in bi-lamellar structure is significantly larger than that of fully-lamellar structure, as shown in Fig. 8b.
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The casting is widely used in the manufacturing of components due to its low cost in comparison with the forging. However, the microstructure of the as-cast titanium alloys is generally a coarse lamellar structure [37-38], the large size of α colony and the wide α and β lamellas resulting in a significant decrease in the fatigue crack resistance and decrease
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the service safety of the component. According to the present results, the resistance to fatigue crack growth of the as-cast titanium alloys can be improved by the aging treatment.
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Therefore, the treatment process has a good application prospect to improve the fatigue performance of the coarse lamellar structured titanium alloys.
5. Conclusions
1) The fatigue crack growth rate of the bi-lamellar Ti6321 alloy is lower than that of the fully-lamellar structure, especially in the low stress intensity factor range. 2) The crack length of fully-lamellar and bi-lamellar structure increases slowly at the beginning as the cyclic number increase and then increase rapidly. In the case of the same crack length, the cyclic number of the bi-lamellar structure is 2.5 times than that -8-
ACCEPTED MANUSCRIPT of the fully-lamellar structure. 3) Because the fatigue crack growth of bi-lamellar structure was impeded by αs precipitations, the frequency of propagation along the α/β interface obviously increases in comparison with the fully-lamellar structure. As a consequence, in the α
circuitous than that of the fully-lamellar structure.
Acknowledgements
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colony, the fatigue crack propagation paths of the bi-lamellar structure is more
This project is financially supported by the Key projects to strengthen the foundation
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(YK180701) and the National Natural Science Foundation of China (51701189, 51775055 and 51601084).
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[28] N. Verdhan, D.D. Bhende, R. Kapoor, J.K. Chakravartty, Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy, Int. J. Fatigue. 74 (2015) 46-54. [29] P.G. Kubendran, E. Schoof, D. Schneider, B. Nestler, On the globularization of the shapes associated with alpha-precipitate of two phase titanium alloys: Insights from
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phase-field simulations, Acta Mater. 159 (2018) 51-64. [30] F.C. Holden, H.R. Ogden, R.I. Jaffee, Heat treatment, structure, and mechanical
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properties of Ti-Mn alloys, Trans AIME. 200 (1954). [31] I. Bantounas, D. Dye, T.C. Lindley, The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti-6Al-4V, Acta Mater. 57 (2009) 3584-3595. [32] F. Sansoz and H. Ghonem, Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: influence of loading frequency and temperature, Metall. Mater. Trans. A. 34 (2003) 2565-2577. [33] J.R. Seal, M.A. Crimp, T.R. Bieler, C.J. Boehlert, Analysis of slip transfer and deformation behavior across the α/β interface in Ti-5Al-2.5Sn (wt.%) with an equiaxed microstructure, Mater. Sci. Eng., A. 552 (2012) 61-68.
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Captions Fig. 1 The initial microstructure of the hot-rolled Ti6321 alloy plate and the preparation process diagram of the compact tension (CT) specimen for the FCG rate testing. Fig. 2 The characteristics of fully-lamellar and bi-lamellar Ti6321 alloy: (a-b) Optic
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images of fully-lamellar structure; (c) TEM image of fully-lamellar structure; (d-e) Optic images of bi-lamellar structure; (f) TEM image of bi-lamellar structure.
Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321 alloy: (a)
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the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of cycles.
Fig. 4 SEM images of the fatigue crack propagation path in fully-lamellar (a-c) and bi-lamellar structure (d-e) at the same ∆ of ~20 MPa·m1/2.
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Fig. 5 SEM images of fatigue crack propagation path of the fully-lamellar (a-b) and bi-lamellar (c-d) structure at ∆ of ~50 MPa·m1/2.
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a) OM image, (b) TEM image.
Fig. 7 TEM and SEM images near the fracture surface together with the schematic diagram
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of deformation mechanism of fully-lamellar structure (a-c) and bi- lamellar structure (d-f) after fatigue crack growth rate testing.
Fig. 8 Schematic diagram of the mechanism for the fatigue crack propagation in
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Figures Fig.1
Fig. 1 The initial microstructure of the hot-rolled Ti6321 alloy plate and the
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preparation process diagram of the compact tension (CT) specimen for the FCG rate
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testing.
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Fig. 2
Fig. 2 The characteristics of fully-lamellar and bi-lamellar Ti6321 alloy: (a-b) Optic images of fully-lamellar structure; (c) TEM image of fully-lamellar structure; (d-e)
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Optic images of bi-lamellar structure; (f) TEM image of bi-lamellar structure.
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Fig. 3a
Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321
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alloy: (a) the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of
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cycles.
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Fig. 3b
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Fig. 3 Results of fatigue crack growth in the fully-lamellar and bi-lamellar Ti6321 alloy: (a) the curves of FCG rates vs. ∆K; (b) the curves of crack length vs. number of
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Fig. 4 SEM images of the fatigue crack propagation path in fully-lamellar (a-c) and
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bi-lamellar structure (d-e) at the same ∆ ܭof ~20 MPa·m1/2.
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Fig. 5 SEM images of fatigue crack propagation path of the fully-lamellar (a-b) and
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bi-lamellar (c-d) structure at ∆ ܭof ~50 MPa·m1/2.
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Fig. 6a
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a)
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OM image, (b) TEM image.
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Fig. 6b
Fig. 6 The microstructure of fully-lamellar structure after aging at 500 °C for 5h: (a)
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Fig. 7 TEM and SEM images near the fracture surface together with the schematic diagram of deformation mechanism of fully-lamellar structure (a-c) and bi- lamellar
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structure (d-f) after fatigue crack growth rate testing.
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Fig. 8 Schematic diagram of the mechanism for the fatigue crack propagation in fully-lamellar and bi-lamellar structure.
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The fatigue crack growth rate is decreased by the αs precipitations in β lamellas. The treatment has a good application for improving the fatigue performance. The fatigue crack growth mode of fully- and bi-lamellar structure are established.