Influence of Phase Composition on Stress-Corrosion Cracking of Ti-6Mo-5V-3Al-2Fe-2Zr Alloy in 3.5% NaCl Solution

Abstract

The metastable β titanium alloys used in marine engineering applications suffered from stress-corrosion cracking in seawater. The different phase composition leads to the distinct stress-corrosion cracking behaviors of the alloy. In this work, the influence of the phase composition on the stress-corrosion cracking of a novel metastable β titanium alloy Ti-6Mo-5V-3Al-2Fe-2Zr was investigated. The alloys with different phase compositions were prepared by three types of thermal-mechanical processing, i.e., the single β phase (assigned as M_(β)), the β phase plus fine α phase (assigned as M_(β+fα)), and the β phase plus coarsened α phase (assigned as M_(β+cα)). The electrochemical tests and constant-stress loading tests were performed, and the phase composition and microstructure were analyzed by XRD and SEM. The M_(β) alloy exhibits the best corrosion resistance as well as the compact properties of oxide films, followed by the M_(β+fα) alloy and the M_(β+cα) alloy. Tear ridges and a flat facet with an undulating surface were observed on the stress-corrosion cracking fracture surface, which indicated the occurrence of high-degree dislocations movement and localized plastic deformation. Absorption-induced dislocation emission (AIDE) and hydrogen-enhanced localized plasticity (HELP) are the primary mechanisms for the stress-corrosion cracking of the alloy. The increased amount of β phase has a beneficial effect on stress-corrosion cracking resistance. For the alloy with β and α phases, the α phase with wider spacing has an adverse effect on stress corrosion performance.

1. Introduction

Specific strength, sufficient fatigue resistance, and good corrosion resistance are necessary for the metallic materials of marine engineering applications [1,2,3]. Titanium alloys have been widely used in marine engineering because of the advantages mentioned above [4,5]. With the development of marine engineering from shallow sea to deep sea, the higher mechanical properties of titanium alloys are required. Stress-corrosion cracking (SCC) is one of the main failures of titanium alloys in marine environments. Thus, many efforts have been made to investigate the SCC of titanium alloys in the seawater environment.

The stress corrosion sensitivity of the Ti6321 titanium alloy in an 3.5% NaCl solution has been investigated in the literature [6]. The authors found that the microstructure with interlaced acicular or lamellar exhibited different stress-corrosion behavior from the bimodal microstructure with the equiaxed α phase and the transition β phase. The stress-corrosion cracking of TC4 ELI alloy with varying microstructures in the 3.5% NaCl solution was investigated by slow a strain rate test (SSRT) with an in-site electrochemical experiment in the literature [7]. The work confirmed that the stress corrosion sensitivity of the lamellar microstructure is significantly higher than that of the equiaxed microstructure. The higher α phase content can promote an anodic reaction and accelerate the hydride formation, resulting in a higher hydrogen embrittlement tendency. The literature [8] has reported that the β-Processed Ti-6Al-4V alloy was cooled at different cooling rates, resulting in different phase compositions. It has been found that the SCC susceptibility of the β-processed Ti-6Al-4V alloy in an aqueous NaCl solution decreased with a fast cooling rate, which was particularly substantial under an anodic applied potential. Chi et al. [9] prepared three types of microstructure of Ti-6Al-4V alloy through heat treatment, i.e., widmanstatten, bimodal, and trimodal specimens. It has been found that the trimodal specimen exhibits the lowest SCC susceptibility in 3.5% NaCl solution due to the larger thickness and different orientation of lamellar α. The widmanstatten microstructure is not conducive to improving the SCC resistance of the alloy. Sun et al. [10] found that a region with a short lamellar α phase would significantly change the fracture strength and elongation of the Ti-6Al-4V alloy during slow strain rate tests in the 3.5% NaCl solution. Li et al. [11] investigated the stress-corrosion cracking behaviors of the Ti-4Al-1.5Mn alloy with different grain morphologies in simulated seawater. The elongated grains could increase the electrochemical reaction and decrease the SCC resistance. Hence, these works indicate that the different phase compositions of titanium alloys strongly affect the susceptibility and resistance of SCC. The results of the literature [12,13,14,15] also confirmed that titanium alloys with different phase compositions of α and β exhibit different SCC behaviors.

Compared with other types of titanium alloys, the metastable β titanium alloy has the most potential to achieve high strength, which can compensate for the phase composition by thermal mechanical processing [16,17,18]. For the metastable β titanium alloy, the α phase with a hexagonal close-packed structure exhibits different deformation behavior and corrosion behavior to the β phase with a body-centered cubic structure [19,20]. Thus, the phase composition would greatly influence the SCC behavior of the metastable β titanium alloy in 3.5% NaCl solution. However, less attention has paid to this point. Moreover, the Ti-6Mo-5V-3Al-2Fe-2Zr alloy is a novel metastable β titanium alloy with great potential for high strength [21]. The α/β phase ratio, as well as the size and morphology of the α phase, can be significantly tailored by thermal mechanical processing to adjust for the properties of the alloy. However, there is limited information on the relations between the phase composition and SCC behaviors of the alloy. Thus, this work investigated the influence of phase composition on the stress-corrosion cracking of the Ti-6Mo-5V-3Al-2Fe-2Zr alloy in 3.5% NaCl solution. The objective of the work is to provide meaningful references for the marine engineering application of the alloy, as well as to enrich the theory of the relationship between the phase composition and SCC for metastable β titanium alloys.

2. Materials and Methods

Pure Ti, Mo-Al alloy, V-Al alloy, pure Fe, and pure Zr were used to make electrodes for vacuum arc remelting. The ingots of the Ti-6Mo-5V-3Al-2Fe-2Zr alloy were prepared by triple-vacuum arc remelting. The following three types of thermal mechanical processing (TMP) were used to process the alloy ingot to obtain the particular microstructure with three different phase compositions.

TMP (1) Forging in α + β phase zone;

TMP (2) Forging and solution treatment in β phase zone;

TMP (3) Forged in α + β phase zone and isothermal treated at a high temperature of 680 °C for 8 h.

The phase compositions were analyzed by using X-ray diffraction (XRD, XRD-7000). The microstructures were observed by using a scanning electron microscope (SEM, ZEISS GeminiSEM300).

Potentiodynamic polarization and electrochemical impedance spectroscopy were measured to evaluate the corrosion-resistance of the alloys. Electrochemical measurements were performed using the VSP-300 electrochemical experimental system in a 3.5 wt.% NaCl solution. The specimen was used as the working electrode. A saturated calomel electrode was used as a reference electrode. A carbon rod was used as the counter electrode. The scanning rate of the potentiodynamic polarization curves was 0.5 mV/s. The electrochemical impedance spectroscopy measurement was performed using a frequency range from 100 kHz and 10 mHz at a 10 mV amplitude over the open-circuit potential.

Constant-stress loading tests in 3.5 wt.% NaCl solution were performed on a stress corrosion testing machine (WDML-10) to study the SCC behaviors. Before tests, the specimens were immersed for 12 h to ensure a stable surface condition. Except for the standard distance area, the rest was sealed with epoxy resin, installed in the corrosion solution tank, and further sealed with silicone rubber. The loading stress was selected as 85%, 90%, and 95% of yield stress (YS), and the loading time was 15 days. If the specimens do not fracture within the loading time, a room temperature tensile test will be carried out to let them be fractured. Three tests shall be conducted for each stress value to ensure the accuracy of the experimental results.

In order to investigate the SCC mechanism, SCC fracture morphologies were observed by using SEM (S-3400N). The SCC fracture was obtained by the slow strain rate tensile test (SSRT). The SSRTs were performed on a stress-corrosion testing machine (WDML-10) with a strain rate of 1 × 10⁻⁶ s⁻¹ in air and in 3.5% NaCl solution, respectively.

3. Results

3.1. Phase Composition and Microstructure

The phase compositions of the alloys after TMP are shown in Figure 1. After TMP1 (forging in α + β phase zone) or TMP3 (forging in α + β phase zone and isothermal treatment at high temperature of 680 °C for 8 h), the alloy microstructure is composed of the β phase plus the α phase. After TMP2 (forged and solution-treated in the β phase zone), the alloy microstructure exhibits a single β phase structure.

The microstructures of the alloys after TMP are shown in Figure 2. After TMP1, the fine acicular α phase is formed in β matrix (Figure 2a). Moreover, the α phase is obviously coarsened after the high temperature isothermal treatment of TMP3 (Figure 2b). In addition, the TMP2 can lead to a single β phase without the α phase (Figure 2c). As a result, three types of microstructure with different phase compositions are obtained, as shown in

Table 1. Numbers of microstructure composed of three different phases.

TMP Number	Thermal Mechanical Processing	Phase Composition	Microstructure Number
TMP1	Forged in α + β phase zone	β phase and fine α phase	M(β+fα)
TMP2	Forged and solution-treated in β phase zone	Single β phase	M(β)
TMP3	Forged in α + β phase zone and isothermal-treated at high temperature of 680 °C for 8 h	β phase and coarsened α phase	M(β+cα)

.

3.2. Polarization Curves and EIS Analysis

The potentiodynamic polarization curves are shown in Figure 3. The corrosion potential E_corr and corrosion current density I_corr have been determined using Tafel extrapolation as listed at

Table 2. Corrosion potential and corrosion current density.

Microstructure Number	Ecorr (mV)	Icorr (μA/cm2)
M(β+fα)	−391	0.727
M(β)	−352	0.359
M(β+cα)	−447	1.015

. Compared with the M_(β+fα) and M_(β+cα) alloys, the M_(β) alloy exhibits the highest corrosion potential value and the lowest current density value. Meanwhile, the M_(β+cα) alloy has the lowest corrosion potential value and highest current density value. It can be speculated that the M_(β) alloy exhibits the best corrosion resistance, followed by the M_(β+fα) alloy and the M_(β+cα) alloy.

Nyquist and Bode plots are shown in Figure 4. According to the Nyquist plots shown in Figure 4a, the capacitive loop radius of the M_(β) alloy is larger than that of the M_(β+fα) and M_(β+cα) alloys, and that of the M_(β+fα) alloy is larger than that of the M_(β+cα) alloy. According to the Bode plots shown in Figure 4b, the impedance modulus |Z| at the frequency of 0.01 Hz for the M_(β) alloy is bigger than that of the M_(β+fα) and M_(β+cα) alloys, while that of the M_(β+fα) alloy is larger than that of the M_(β+cα) alloy. The impedance modulus can reflect the characterization of the corrosion protection of the oxide films on the alloy surface. It can be inferred that the compact properties of the oxide films on the M_(β+cα) alloy surface are the worst among the three alloys with different phase compositions, while the M_(β) alloy has the best compact properties of the oxide films.

It can be seen from the above analysis that the corrosion resistance and the passivation behavior may be affected by the different phase compositions.

3.3. Constant-Stress Loading Test Results

Constant-stress loading tests (CSLT) are often used as the experimental basis for the stress-corrosion behavior research of steel, aluminum alloy, and titanium alloy [22,23,24]. For the steel tested in the NaCl solution, the fracture may occur within a short time due to its relatively low corrosion resistance, and the fractured time is used to evaluate the SCC resistance [23]. In this work, all specimens did not fracture during CSLT in 3.5% NaCl solution within 15 days because of the excellent corrosion resistance of the alloy. However, the strength and ductility of the specimens under different constant loads have decreased to varying degrees. The average ultimate tensile strength (UTS) and average elongation after fracture (EL) of the specimens before and after CSLT are shown in

Table 3. The UTS and EL of the alloys before and after constant-stress loading test.

Microstructure Number	Constant Stress	UTS (MPa)	St. dev (MPa)	EL (%)	St. dev (%)
M(β+fα)	/	1215	8.2	6.8	0.2
	85% YS	1186	8.3	5.9	0.2
	90% YS	1170	7.7	5.5	0.3
	95% YS	1133	7.2	4.9	0.2
M(β)	/	812	6.1	10.1	0.8
	85% YS	796	5.8	9.3	0.6
	90% YS	787	5.6	8.3	0.4
	95% YS	779	5.7	7.7	0.3
M(β+cα)	/	1014	7.8	8.9	0.6
	85% YS	982	7.4	7.3	0.2
	90% YS	969	6.8	6.9	0.3
	95% YS	942	6.6	6.3	0.3

. The loss of strength and ductility after CSLT is used to quantitatively evaluate the SCC resistance, which is defined by the decreased amplitudes of UTS and EL, as shown in Equations (1) and (2), respectively.

δ_S = [(σ_b,before − σ_b,after)/σ_b,before] × 100%

(1)

δ_E = [(A_before − A_after)/A_before] × 100%

(2)

where δ_S is the decrease in the amplitudes of UTS after CSLT, %; δ_E is the decrease in the amplitudes of EL after CSLT, %; σ_b,before is the UTS before CSLT, MPa; σ_b,after is the UTS after CSLT, MPa; A_before is the EL before CSLT, %; and A_after is the EL after CSLT, %.

Obviously, after constant-stress loading tests in aqueous solution, the strength and ductility of all specimens decreased relative to the primitive state, which was confirmed by the decreased amplitudes of UTS and EL. According to Table 3, the decreased amplitudes of UTS δ_S and EL δ_E are calculated and shown in Figure 5. For the alloys with three types of phase composition, the decreased amplitudes of UTS and EL increase with the constant load from 85% YS to 95% YS. According to Figure 5a–c, the larger decreased amplitudes of UTS δ_S and EL δ_E indicate lower SCC resistance. It can be found that the increase in constant load leads to a decrease in SCC resistance for the M_(β), M_(β+fα), and M_(β+cα) alloys. When the constant load is 85% YS, the values of δ_S and δ_E of the M_(β), M_(β+fα), and M_(β+cα) alloys are 1.97% and 7.92%, 2.38% and 13.23%, and 3.15% and 17.97%, respectively (Figure 5a). The decreased amplitudes of UTS and EL of the M_(β) alloy are the most minor, and those of the M_(β+fα) alloy are smaller than those of the M_(β+cα) alloy. The same phenomenon also exists at a constant load of 90% and 95% YS (Figure 5b,c). This indicates that under the combined action of corrosion and stress, the strength and ductility loss for the M_(β) alloy is the least, while that of the M_(β+cα) alloy is the most serious. It can be inferred that the sequence that tended to cause SCC is the M_(β+cα) alloy, the M_(β+fα) alloy, and the M_(β) alloy.

3.4. Fracture Morphology

The fracture morphologies of the alloys after SSRT are shown in Figure 6. As shown in Figure 6a, the fracture surface of the M_(β) alloy tested without corrosive environment exhibits many deep and large dimples. It is easy to find by comparison that the dimples present a small and shallow morphology of the M_(β) alloy under the influence of the corrosion environment (Figure 6b). Moreover, tear ridges surrounded by small and shallow dimples can be observed in the ductile region on the fracture surface of the M_(β+fα) alloy tested with corrosive environment (Figure 6c). In the ductile region, flat facets with undulating surfaces can be observed in addition to the tear ridges for the M_(β+cα) alloy tested in the 3.5% NaCl solution (Figure 6d). The clues to the SCC mechanism of the alloy can be obtained from the appearance of these special fracture morphologies.

4. Discussion

Many efforts have found that the role of absorbed hydrogen is one of the main factors for titanium alloy SCC in the 3.5% NaCl solution [25,26]. The electrochemical actions of the titanium alloy in such a corrosion environment can easily produce hydrogen. One of the electrochemical actions takes place during oxide film formation. Part of the external hydrogen for absorption is produced by the chemical reaction that occurs during oxide film formation, as shown in Equations (3) and (4) [27].

Ti + 2H₂O → TiO₂ + 4H⁺ + 4e

(3)

H⁺ + e⁻ → H

(4)

The other electrochemical action takes place during the dissolution of the bare alloy matrix. Another part of external hydrogen for absorption is produced by the chemical reaction that occurred during titanium alloy matrix dissolution, as shown in Equations (4)–(6) [28].

Ti²⁺ + H⁺ → [Ti³⁺ H] ads

(5)

[Ti³⁺ H] ads → Ti³⁺ + H⁺ + e⁻

(6)

The M_(β) alloy exhibits the best corrosion resistance and the best compact properties of the oxide films. The above-mentioned two electrochemical reactions can be inhibited to minimize the external hydrogen concentration. Instead, the worst corrosion resistance and compact properties of the oxide films for the M_(β+cα) alloy would lead to more intense reactions, resulting in a higher external hydrogen concentration. Thus, it is easier for the M_(β+cα) alloy to absorb more hydrogen.

The special SSRT fracture morphologies provide a thread for the analysis of the SCC mechanism. Compared with the deep and large dimples of the alloy fractured in air, the dimples of the alloy fractured in a corrosive environment are shallow and small (Figure 6a,b). It has been proposed that the formation of such fracture characteristics is attributed to the absorption-induced dislocation emission (AIDE) mechanism [29]. The absorbed hydrogen can weaken the intermetallic bond. Then, the dislocation emission from the crack tips is accelerated. The increasing dislocation density around the crack tip leads to a resharpening of the crack tip so that shallow and small dimples are formed [30]. Moreover, because of the formation of the tear ridges and the flat facets, it can be inferred that localized plastic deformation around cracks and high-degree dislocation movements may occur (Figure 6c,d). Such a phenomena have been explained by the hydrogen-enhanced localized plasticity (HELP) mechanism [28]. The continuous β matrix can help the adsorbed hydrogen atom spread to the crack tips. The solute hydrogen concentration around the crack tips increases rapidly in a short time. The dislocation activation is stimulated by hydrogen, resulting in localized plastic deformation. So, the SCC for the alloy that occurred in 3.5% NaCl solution is triggered by the combined action of the AIDE and HELP mechanisms.

For the alloy with β and α phases, the dislocations are more likely to activate in β phase rather than in α phase [31]. When the dislocations move close to the α phase, the α/β interfaces can become an obstacle to the dislocation movement. The dislocation pile-up leads to the acceleration of localized stress concentration during the deformation. For alloys suffering from both corrosion and stress, this phenomenon would continue to intensify under the influence of AIDE and HELP, caused by the interaction between absorbed hydrogen and dislocations. For the titanium alloy, the β phase has a beneficial effect on the structural integrity of the protective oxide film as well as corrosion resistance [32]. In addition, the β matrix without the α phase play.a positive role in delaying the occurrence of localized stress concentration during deformation. As a result, the M_(β) alloy with the single β phase exhibits the lowest SCC tendency. For the alloy containing the α phase, the rod-like α phase can obviously accelerate localized stress concentration during dislocation motion. The literature [31] has confirmed that there is a correlation between the localized stress caused by the α phase and the α phase spacing in the metastable β titanium alloy. The increase in the α phase spacing leads to the increase in the quantity of accumulated dislocations, resulting in the increase in localized stress. Thus, the localized stress of the M_(β+cα) alloy with the wider α phase spacing is higher than that of the M_(β+fα) alloy. Moreover, under the influence of the AIDE and HELP mechanisms, the emission and motion of the dislocations are promoted and the interatomic band is weakened by absorbed hydrogen. As a result, the M_(β+cα) alloy tends to cause SCC more than the M_(β+fα) alloy.

5. Conclusions

The stress-corrosion cracking of the Ti-6Mo-5V-3Al-2Fe-2Zr alloy with multiple phase compositions in the 3.5% NaCl solution was investigated. The following conclusions can be drawn from this work. The microstructures with the single β phase (M_(β)), the β phase plus fine α phase (M_(β+fα)), and the β phase plus coarsened α phase (M_(β+cα)) are obtained by three types of thermal–mechanical processing, respectively. The M_(β) alloy exhibits the best corrosion resistance, followed by the M_(β+fα) alloy and the M_(β+cα) alloy. For the M_(β), M_(β+fα), and M_(β+cα) alloys, the increase in constant load leads to a decrease in SCC resistance. When the constant load is 85% YS, the M_(β) alloy exhibits the smallest UTS decreased amplitudes of 1.97% and the smallest EL decreased amplitudes of 7.92%, and the M_(β+cα) alloy exhibits the largest UTS decreased amplitudes of 3.15% and the largest EL decreased amplitudes of 17.97%. The sequence that tended to cause SCC was the M_(β+cα) alloy, the M_(β+fα) alloy, and the M_(β) alloy. The combined action of the AIDE and HELP mechanisms triggered SCC for the alloy that occurred in the 3.5% NaCl solution. The increased amount of β phase had a beneficial effect on stress-corrosion cracking resistance. For the alloy with the β and α phases, the coarsened α phase with wider spacing had an adverse effect on the stress corrosion performance.