Comparison of the hydrogen damage of different rolling surfaces of TC4 Ti alloy sheet

1 Introduction

The corrosion resistance of Ti alloys is excellent due to the existence of a dense passivation film on their surface (Cui et al. 2011; Wang et al. 2020a,b; Wu et al. 2020; Zhao et al. 2021). However, Ti alloys are hydrogen-sensitive materials (Vezvaie et al. 2013; Zeng et al. 2010). With the widespread application of Ti alloys in the marine environment, the hydrogen embrittlement of Ti alloys in chlorides and salt solutions has also been reported (Liu et al. 2020), which can result in the extreme destructiveness to metallic materials (Sun et al. 2022). The main mechanism of hydrogen embrittlement of Ti alloys is considered to be the precipitation of brittle hydrides, which leads to the cracking of the Ti substrate (Claeke et al. 1997; Liu et al. 2023a). Two kinds of titanium hydrides are formed in the process of hydrogen embrittlement, that is, TiH_1.5 belongs to γ hydride with FCC structure, and TiH_1.971 belongs to δ hydride which has the same crystal structure and larger lattice parameters as γ hydride (Azkarate et al. 2009; Liu et al. 2023a). Since the volume of titanium hydrides is 20 % larger than that of α-Ti matrix, a large stress will inevitably be generated, resulting in the emergence of microcracks, further the microcracks will expand and penetrate the Ti substrate to cause hydrogen-induced cracking.

The hydrogen sensitivity of different Ti alloys presents great difference. The Ti alloys can be divided into three categories based on the annealing microstructures (Lu et al. 2014; Zhan et al. 2022; Zhao et al. 2009): (1) α-Ti with hexagonal close-packed (HCP) structure below the transition temperature of 882 °C; (2) β-Ti with body-centered cubic (BCC) structure above the transition temperature; (3) α + β Ti alloys with the coexist of two kinds of microstructure. It is found that the solubility of hydrogen in different Ti alloys varies in a large scope. At room temperature, the solubility of hydrogen in pure Ti is only 0.9 ppm. The solid solubility of hydrogen in α-Ti alloys varies with the composition of the alloying elements, generally 20–200 ppm. The solid solubility of hydrogen in β-Ti alloys can be as high as thousands to tens of thousands of ppm (Liu et al. 2022; Wang et al. (2020a,b), which is much higher than that in α-Ti alloys. Due to the different solubility of hydrogen in α-Ti and β-Ti, the hydrogen sensitivity of Ti alloys is associated with their microstructures. Azkarate et al. (2009) studied the hydrogen-induced cracking sensitivity of three Ti alloys, and concluded that the content of β phases plays the most important role.

TC4 Ti alloy with α + β dual phase structure shows good comprehensive properties, including good microstructure stability, toughness, plasticity and high temperature deformation properties, which is also the most widely used Ti alloy (Hu et al. 2020, Shi et al. 2021; Zhao et al. 2023). The production process of TC4 Ti alloy mainly includes melting, forging, rolling, and so on. At present, the TC4 rolling sheet is widely used, which includes three rolling directions, namely, horizontal rolling directions, sheet thickness direction and vertical rolling direction, corresponding to three rolling surfaces. It was found that the coarse grains on the original surface of TC4 are broken to exhibit the banded structure after rolling treatment, and the α phases are elongated and β phases are thinned (Chuvil’deev et al. 2019). Although the diffusion ability of hydrogen in Ti alloys is strong (Yang et al. 2023), its diffusion rate in different rolling directions is associated with the microstructure of Ti substrate. Thus, the resistance ability of different rolling surfaces of Ti alloy sheet to the destructiveness of hydrogen is related to the hydrogen diffusion depth in the corresponding rolling directions. For convenient description, the destructiveness of hydrogen to Ti alloy is named as “hydrogen damage”. Once the hydrogen diffusion depth is determined, the hydrogen sensitivity of different rolling surfaces can be clarified, which can provide theoretical guidance for selecting the appropriate processing methods of TC4 in practical applications. However, the current investigation pays little attention to the hydrogen sensitivity of different rolling surfaces of TC4 sheet.

Therefore, the electrochemical cathodic hydrogen charging method was used to study the destructiveness of hydrogen to the three rolling surfaces of TC4 alloy, and the related mechanism was analyzed.

2 Materials and methods

The material used in this experiment was a hot-rolled α + β dual phase TC4 (Ti6Al4V) rolling sheet, which was hot-rolled to the thickness of 19 mm at a temperature below the transition temperature of 50 °C. The chemical composition of the TC4 rolling sheet was shown in Table 1, which was the same as reported in our previous work (Liu et al. 2023a,b).

The sketch maps of the TC4 rolling sheet and the sampling surfaces were shown in Figure 1a. R was the rolling direction, N and T were the sheet thickness direction and vertical rolling direction, respectively. The samples for microstructure characterization were cut from the rolling sheet, with a dimension of 10 mm × 10 mm × 10 mm. The surfaces of R-T, N-T, and R-N marked by blue arrows were polished to 5000# sandpapers until there were no visible scratches, and then polished with SiO₂ suspension to mirror brightness, rinsed with distilled water and ethanol, dried in cold air. Before the observation of microstructure, the polished samples were etched using the Kroll reagent (5 mL HF + 15 mL HNO₃ + 80 mL deionized water) for 3–5 s.

Figure 1:

Sketch map of the TC4 rolling sheet and sampling surface (a); and three-dimensional metallographic microstructure of three rolling surfaces (b).

The samples for cathodic hydrogen charging were sampled as the same as microstructure characterization. The N-T or R-T surfaces in Figure 1b was exposed as hydrogen charging surface, and the other sides were connected with the copper wire. The samples were sealed into a PVC tube with epoxy resin, and the working area of 1 cm² was exposed. After polished by the same method as the microstructure observation, the samples were charged hydrogen. In our previous work (Liu et al. 2023a), the hydrogen charging treatment with the constant current density of 50, 100, and 100 mA cm⁻² was compared, and found that the TC4 presented a moderate damage degree under the current density of 100 mA cm⁻². Thus, the sample in this investigation was charged hydrogen in 3.5 wt% NaCl solution at room temperature (25 ± 2 °C) with the current density of 100 mA cm⁻² for 24 h. During the hydrogen charging process, the sample was connected with the cathode of the power supply, and the platinum plate was connected with the anode of the power supply. Abundant adsorption hydrogen atoms H_ad were formed on the surface of TC4 during the hydrogen charging process. Part of the H_ad can diffuse into the interior of the TC4 substrate resulting in hydrogen-induced cracking, and the other H_ad can combine to form hydrogen bubbles escaped from the electrolyte (Vezvaie et al. 2013). After hydrogen charging treatment, the samples were taken out from the resin, and then the corrosion morphologies in different rolling directions were compared to investigate the hydrogen diffusion law.

The metallographic microstructures of three rolling surfaces of TC4 sheet before and after hydrogen charging treatment were observed by optical microscope (OM, Olympus, DSX 500, Japan). The morphologies of the samples after hydrogen charging were observed by scanning electron microscope (SEM, Pillips XL30FEG). The spot diameter was 2–3 μm, and the detection depth was consistent with the spot diameter.

3 Results

3.1 Microstructures of the three rolling surfaces of TC4 alloy

The microstructures of the three rolling surfaces of TC4 rolling sheet along the corresponding rolling directions are shown in Figure 1b. It is clear that the size, morphology, and distribution of α phases and β phases in the three surfaces of R-T, N-T, and R-N are different. Furthermore, the microstructure of the three surfaces is characterized in detail as shown in Figure 2. It can be seen that the R-T rolling surface (Figure 2a, b) exhibits a typical dual-phase equiaxed grain structure, containing the large-size of bright α phases and dark β phases. The volume fraction of α phases is much larger than β phases, and the width of the β phases on the R-T surface is about 5–10 μm. Figure 2c–f shows the microstructure of the rolling surfaces in the sheet thickness direction (N-T) and the vertical rolling direction (R-N), which are very different from the microstructure of the R-T surface. This is due to the deformation of the grains during the rolling process under the action of extrusion force. The size of the α phases on the N-T surface varies in a large scope, the small one is less than 1 μm, whereas the large one is tens of microns. Also, the shape of the α phases is irregular, including the striped and subcircular shape, and the β phases look like black fine filaments with a width of approximately 1–2 μm. The microstructure of the R-N surface is relatively uniform, the α phases are distributed in a similar striped shape, and the continuous distribution of β phases are fine with a width below 1 μm. Previous studies (Liu et al. 2023a,b) indicated that the solubility of hydrogen in α phases is low while it is high in β phases. The microstructure of the three rolling directions and surfaces show great differences in the size, number and distribution of α phases and β phases, which will greatly affect the dissolution and diffusion process of hydrogen. Thus, it can be speculated that the resistance ability of three rolling surfaces of TC4 sheet to hydrogen damage is also different.

Figure 2:

Metallographic microstructure of three rolling surfaces of TC4 rolling sheet: R-T surface (a, b); N-T surface (c, d); R-N surface (e, f).

3.2 Comparison of hydrogen diffusion behavior along R and N rolling directions

During the rolling process, the TC4 sheet is not only subjected to the tensile force in the R rolling direction but also the extrusion force in the N thickness direction. Thus, the diffusion of hydrogen into the interior of Ti substrate from the thickness surface (N-T) and rolling surface (R-T), respectively, may present some difference. In order to clarify this case, the hydrogen damage of R-N surface charged hydrogen from N-T surface and R-T surface, respectively, are compared. The optical images of hydrogen diffusion depth on R-N surface along R direction and N direction, respectively, are shown in Figure 3. It can be found that the color of Ti substrate gradually changes from dark to light, and the dark areas can be attributed to the precipitation of hydrides (Liu et al. 2023a). It indicates that hydrogen gradually diffuses from the surface layer to the interior of Ti substrate, accompanying with the generation of hydrides (Qiao et al. 2021). According to the previous research results (Liu et al. 2023a), the hydrides precipitated in the TC4 substrate should be TiH_1.971. The precipitation of hydrides results in the variation of the microstructure of Ti substrate.

Figure 3:

Optical images of hydrogen diffusion depth on R-N surface charged hydrogen from N-T surface (a) and R-T surface (b), respectively.

Based on the color difference of the R-N surface charged hydrogen from R and N rolling directions, three regions are defined. The color of the area near the hydrogen charging surface is the darkest, which is defined as the hydride region. The color of the area far away from the hydrogen charging surface is the lightest, which is defined as the unaffected region. The area between the hydride region and unaffected region is defined as the transition region. The approximate positions of the three regions are marked by white lines in Figure 3 according to the color difference. It can see that the depth of the hydride region along the R direction is approximately 180 μm, whereas it is approximately 110 μm along the N direction. It is obvious that hydrogen is easier to diffuse along the R rolling direction during the hydrogen charging process.

The hydrogen damage characteristics of R-N surface charged hydrogen from N-T surface and R-T surface, respectively, is compared by SEM as shown in Figure 4. The microstructure shows that the R-N surface along the R direction is seriously damaged, and large cracks are visible near the hydrogen charging surface. Differently, although the microstructure of R-N surface along the N direction also changes, the cracks are relatively small. Comparing the hydrogen diffusion behavior on R-N surface along the R and N rolling directions, it can be seen that the hydrogen damage degree along the R direction is much severer than that along the N directions, which can be greatly associated with the different distribution status of β phases in both directions, and detailed reason will be analyzed below.

Figure 4:

SEM morphologies of hydrogen diffusion depth on R-N surface charged hydrogen from N-T surface (a) and R-T surface (b), respectively.

3.3 Comparison of hydrogen damage behavior of the R-T and R-N rolling surfaces of TC4 alloy

The above results indicate that the hydrogen diffusion behavior in different rolling directions presents great differences. Furthermore, the precipitation law and distribution state of the hydrides on different rolling surfaces were investigated. The microstructures of the R-T surface and R-N surface exhibit the greatest difference, so the both surfaces are chosen for comparison. Figure 5 shows the distribution of hydrides on the R-T surface where the hydrogen is charged from N-T surface and diffuses along R direction. Figure 6 shows the distribution of hydrides on the R-N surface where the hydrogen is charged from R-T surface and diffuses along N direction. The morphologies of the hydride region, transition region, and unaffected region are observed. It is found that the status of the α and β phases on both surfaces are changed in comparison with the uncharged hydrogen TC4 sheet. The hydride regions in Figures 5a, b and 6a, b show that there are hydrides at the boundaries of α and β phases. Since the hydrides tightly wrap the β phases, it makes the β phases difficult to be distinguished. Thus, it seems that the size of β phases becomes much larger in comparison with the uncharged hydrogen TC4, especially for the R-T surface. Moreover, the pin-like hydrides named as acicular hydrides are visible in the α phases, and the size of the acicular hydrides varies in a large scope, the maximum can even penetrate the α phases, resulting in the α phases are divided into small dimension. Thus, the volume fraction of α phases in the charged hydrogen sample is reduced in comparison with the sample before hydrogen charging. In addition, it is found that the size and number of the hydrides in the interior of α phases on the R-N surface is larger than that on the R-T surface. This case should be relative to the different state of β phases on both surfaces.

Figure 5:

The distribution of hydrides on the R-T surface: hydride region (a, b); transition region (c, d); and unaffected region (e, f).

Figure 6:

The distribution of hydrides on the R-N surface: hydride region (a, b); transition region (c, d); and unaffected region (e, f).

It is known that plenty of hydrogen atoms H_ad can be generated during charging hydrogen process. If the solubility of hydrogen in the charged material is high, more H_ad can diffuse into the interior of material. Otherwise, the H_ad will escape from the surface as hydrogen gas H₂. As for TC4 Ti alloy, the solubility of H_ad in β phases is much higher than in the α phases. After the absorption hydrogen atoms H_ad diffuse into the Ti substrate, most of hydrogen is preferentially dissolved into β phases. When the concentration of hydrogen in the β phases reaches the critical value of hydrogen solubility in the α phases, hydrides are first precipitated along the phase boundaries due to the concentration gradient (Liu et al. 2023a,b). As a result, abundant hydrides are mainly precipitated at the α and β phase boundaries. In addition, the solubility of hydrogen in α phases is very low, even though a tiny amount of hydrogen diffuses into the α phases, the hydrides can be precipitated quickly. Thus, a small number of hydrides are observed in the interior of α phases.

The microstructures of the transition regions are shown in Figures 5c, d and 6c, d. There are still many hydrides surrounding the β phases, but the hydrides in the interior of α phases decrease. The farther away from the charged hydrogen surface is, the smaller the effect of hydrogen on the microstructure is, and the less the number of hydrides is formed. Figures 5e, f and 6e, f shows the microstructure of the unaffected regions where are far away from the charged hydrogen surface, which is consistent with the microstructure of the Ti substrate without hydrogen charging. The possible reasons for this phenomenon are listed as follow: (1) hydrogen has not penetrated into this depth; (2) the dissolved hydrogen concentration does not reach the critical value of hydride precipitation in TC4 alloy.

In order to clarify the relationship between the microstructures and the precipitation law of hydrides, the high magnification morphologies of the hydride regions on the R-T and R-N surfaces are shown in Figure 7. Comparing the distribution of the hydrides on both surfaces, it can be found that the precipitation of hydrides is closely related to the shape and distribution of β phases in the uncharged hydrogen TC4. For the R-T surface before hydrogen charging, the microstructure is uniform, and both phases are equiaxed structure. Due to the larger size of the β phases on the R-T surface, more hydrogen can be dissolved into β phases, and the fluff-like hydrides named as flocculent hydrides are mainly precipitated along the interface of α and β phases. In addition, the dissolution of hydrogen in the interior of the α phases is low, and only few acicular hydrides are precipitated in the α phases. For the R-N surface before hydrogen charging, the microstructure is relatively uniform, the α phases are distributed in stripped shape, and the β phases are like continuous fine filaments. After hydrogen charging, the flocculent hydrides are mainly precipitated along the interface of α and β phases, showing a filament distribution which is similar to the distribution of β phases. Moreover, more acicular hydrides are precipitated in the interior of α phases in comparison with that on the R-T surface. It can be seen that the distribution and number of hydrides on the R-T and R-N surfaces is greatly associated with the characteristics of α phases and β phases, especially the β phases. Also, in our previous work (Liu et al. 2023b), the difference of hydrogen damage between TA2 (α-Ti) and TC4 (β+α Ti) alloys was compared, and it was found that the hydrogen damage of TA2 with single α phases is more serious than that of TC4, which indicates the key role of β phases in the hydrogen damage of Ti alloys.

Figure 7:

The hydrides on the R-T surface (a) and R-N surface (b).

4 Discussion

The above results show that the hydrogen damage degree of TC4 alloy along different rolling directions varies greatly, which is closely related to the size and distribution of α and β phases. The possible mechanism is analyzed based on the results above. For the R-N surface where is charged hydrogen from N-T surface and R-T surface, respectively, the diffusion depth of hydrogen along the R direction is much larger than that along the N direction. This is due to that the solubility of hydrogen in β phases is much higher than that in α phases, and the charged hydrogen is mainly dissolved in the β phases. The β phases along the R direction are continuously distributed in a fine filament shape, also most of the β phases are arranged in the same direction as the hydrogen charging direction, that is, there are many continuous channels for the hydrogen diffusion from surface layer to the interior of Ti substrate along the continuous β phases (shown by sketch map in Figure 8a). In the meantime, the hydrides are formed accompanying the diffusion of hydrogen. Thus, hydrogen is susceptible to diffusion into the interior of Ti substrate along the R direction quickly, resulting in the depth of hydride region is larger (Figures 3a and 4a, b). However, the distribution of β phases along the N direction is perpendicular to charging hydrogen direction. When all the β phases near the hydrogen charging surface is full of hydrogen totally, the redundant hydrogen continues to diffuse into the next layer of β phases, that is, the hydrogen diffuses along β phases layer by layer in N direction (shown by sketch map in Figure 8b). Thus, the depth of hydride region along N direction is small in comparison with R direction (Figures 3b and 4c, d).

Figure 8:

Hydrogen damage process on R-N surface where is charged hydrogen from N-T surface and hydrogen diffuses along R direction (a); the hydrogen damage process on R-N surface where is charged hydrogen from R-T surface and hydrogen diffuses along N direction (b); and the precipitation of hydrides on R-T surface (c).

Besides the different hydrogen diffusion behavior along different rolling directions, the precipitation law and distribution state of hydrides on the different rolling surfaces are different. For the diffusion of hydrogen on the R-T surface, the β phases on this surface are coarse and discrete. Due to the absence of continuous β phase diffusion channels, even though the β phases near the hydrogen charging surface is not full of hydrogen totally, the hydrogen can diffuse into the β phases below the surface layer. Moreover, the β phases on the R-T surface are coarse, more hydrogen can be dissolved into them, so more and more hydrides can be gradually precipitated along the interface of β phases and α phases. As a results, the precipitation rate and number of hydrides is large, the hydrides are loose and present a flocculent morphology (Figure 7). Also, abundant hydrides wrap the β phases closely, resulting in that the original of β phases is difficult to discern, and it seems that the dimension of β phases increases. In addition, only a very small number hydrogen diffuses into the α phases, and the precipitation rate and number of the hydrides in α phases is small, so the hydrides are compact and present a pin-like morphology.

For the precipitation of hydrides on the R-N surface along N direction, the hydrogen gradually diffuses from the surface layer to the interior of Ti substrate along the continuous β phases (Figure 8b). On the one hand, the β phases is fine, and the dissolved hydrogen in them is relatively small in comparison with that in the coarse β phases, so the precipitation amount of hydrides along the interface of β phases and α phases is low, it seems that the dimension change of β phases is not dramatical. In addition, the hydrogen dissolution amount in the fine filament of β phases is lower than that in the coarse β phases in a short time, so more redundant hydrogen diffuses into the interior of α phases. As a result, more hydrides are precipitated in the interior of α phases on the hydride region of R-N surface.

5 Conclusions

1. The microstructure of rolling surface (R-T), sheet thickness surface (N-T), and vertical rolling surface (R-N) on TC4 rolling sheet presents greatly different. The R-T surface exhibits a typical dual-phase equiaxed grain structure with coarse α phases and β phases; on the N-T surface, the α and β phases are broken to irregular morphology and size; on the R-N surface, the α phases are distributed in a uniform stripped shape, while the β phases are like continuous filaments surrounding the α phases.

2. Hydrogen diffuses faster along the rolling directions (R) than along the thickness direction (N) due to the existence of continuous β phases as hydrogen diffusion channels, resulting in the more serious hydrogen damage in the rolling direction R.

3. The solubility of hydrogen in β phases is much higher than that in α phases, and the flocculent hydrides are deposited along the phase boundaries and the acicular hydrides are deposited in the interior of α phases. Moreover, the hydrides precipitated in the interior of α phases on the R-N surface are more than on the R-T surface due to the different distribution state of β phases.