The Effect of Irradiation Dose and Temperature on Irradiation Hardening and Microstructure Study of Hot-Forged V-4Cr-4Ti Under Self-Ion Irradiation

Abstract

After being self-ion implanted at 400 °C and 550 °C, the microstructure and irradiation hardness of kilogram-scale V-4Cr-4Ti alloys were studied using a transmission electron microscope (TEM) and nano-indentation test technology. Irradiation was performed using self-ions (V²⁺) at 2.5 MeV with design influences of 1.15 × 10¹⁵ ions cm⁻², 4.59 × 10¹⁵ ions cm⁻², and 9.17 × 10¹⁵ ions cm⁻², so that the peak damages of V-4Cr-4Ti alloys are 1, 4, and 8 dpa, respectively. Compared with the 400 °C samples, the 550 °C samples exhibited a higher-density number of dislocation loops and increased hardness and reached saturation at lower irradiation doses. The irradiation temperature was mainly responsible for these differences, and the potential mechanism for its effect on the irradiation behavior was discussed.

1. Introduction

The service lifetime of future fusion reactors is greatly impacted by the study of structural materials. However, these materials are confronted with significant challenges due to the extreme environment in fusion reactors. V-4Cr-4Ti has been regarded as the leading candidate because of its balanced high-temperature strength, low-temperature ductility, workability, and other factors [1,2,3]. During their service life, structural materials are subjected to long-term high-energy neutron irradiation, which causes the formation of point defects like interstitial atoms and vacancies. These point defects can aggregate to form the larger defect, such as the dislocation loop and cavity. These defects that lead to material swelling and hardening cause a significant reduction in mechanical properties like strength and toughness. In the present, while the radiation resistance of V-4Cr-4Ti has been widely studied, most of the V-4Cr-4Ti alloys were at the laboratory scale. However, the manufacturing of blanket systems components requires large-scale vanadium alloy ingots. In the past, the USA and Japan have developed several hundred kilograms or tons of V–4Cr–4Ti alloys, and Southwest Institute of Physics of China prepared a 30 kg V-4Cr-4Ti alloy using the melting method [4,5,6]. As a team with advanced large-scale vanadium alloy preparation technology, it is crucial for us to understand the irradiation properties of the materials we produce.

The V-4Cr-4Ti used in this study was prepared by the Southwest Institute of Physics from kilogram-scale high-purity vanadium alloy ingots through multiple high-temperature hot-forging procedures. As shown in Figure 1, the kilogram-grade vanadium alloy ingot has a diameter of 180 mm, a height of 200 mm, and a weight of 30 kg. Vanadium alloy is a kind of material that is very prone to oxidation, especially at high temperatures. Therefore, it is necessary to protect it during the hot-forging process. There are currently two conventional protection methods; one is vacuum sealing with a protection sheath, and the other is using inert gas protection. We mainly use the vacuum-sealing method, which is not only to prevent material oxidation, but also to prevent contamination and the propagation of surface cracks on the material. As is well known, hot forging can reduce defects in the as-cast structure, refine grains, and improve the mechanical properties and workability [7]. Meanwhile, the hot-forging process can theoretically improve the radiation resistance of the material. Research has shown that smaller grains generate more grain boundaries, which can be sinks for adsorbing and combining defects, such as interstitial atoms, vacancy clusters, and dislocation loops generated by irradiation [8,9,10,11] What kind of radiation resistance does the hot-forged vanadium alloy have under different irradiation doses and temperatures is worthy of in-depth study.

Studying the behavior of metallic materials subjected to neutron irradiation has long been a challenging problem due to the limited availability of neutron sources, high radioactivity of irradiated samples, and long experimental period and high cost. Because of the advantages of a short time, high damage level, and no radioactivity, ion irradiation is widely used to simulate neutron irradiation [12,13]. Meanwhile, self-ion irradiation can avoid introducing impurity elements and better simulate neutron irradiation [14]. In this study, the effect of irradiation fluence and temperature on the dislocation loops and hardening by irradiation were studied by irradiating V-4Cr-4Ti alloys with V²⁺ ion at 400 °C and 550 °C, with irradiation fluence of 1.15 × 10¹⁵ ions cm⁻², 4.59 × 10¹⁵ ions cm⁻², and 9.17 × 10¹⁵ ions cm⁻², respectively.

2. Materials and Methods

2.1. Preparation of Samples

In this study, the electron-beam-melting method was used to prepare the V-4Cr-4Ti ingot. The ingot was canned into a stainless steel container by electron-beam welding, and the canned ingot was repeatedly hot-forged. The chemical composition analysis of the vanadium alloy was conducted at the National Non-ferrous Metals and Electronic Materials Analysis and Testing Center, and the results are listed in

Table 1. The chemical composition of the V-4Cr-4Ti alloys (wt.%).

Simple	V (wt.%)	Cr (wt.%)	Ti (wt.%)	O (wt.%)
V-4Cr-4Ti	92.64	4.40	3.89	0.021

. The purities of the raw power used on the alloy are 99.95% (V), 99.99% (Cr), and 99.99% (Ti).

2.2. Irradiation Experiments

The sheets of 3 mm × 3 mm × 1 mm were cut from the V-4Cr-4Ti samples. The samples were first rough-polished with 400 #, 800 #, 1000 #, 2000 #, and 3000 # silicon carbide (SiC) paper successively and then finishing-polished with 1 #, and 0.5 # grinding paste successively. The final process involved electrochemical polishing with 25% sulfuric acid and 75% ethanol polishing solution (in volume percentage) at room temperature under 10V for 50 s. In this paper, the 2 × 1.7 MV tandem accelerator in the Accelerator Laboratory of Wuhan University was used to conducted the ion-irradiation experiments with 2.5 MeV V²⁺ ions at 400 °C and 550 °C. The irradiation fluences were 1.15 × 10¹⁵ ions cm⁻², 4.59 × 10¹⁵ ions cm⁻², and 9.17 × 10¹⁵ ions cm⁻², respectively. The vacuum degree was kept at 2.2 × 10⁻⁴ Pa, and the temperature error was kept at ±5 °C during the irradiation.

According to the SRIM 2013 (Stopping and Range of Ions in Matter) [15] calculation results of the model of “Ion Distribution and Quick Calculation of Damage”, the peak damages of 1.15 × 10¹⁵, 4.59 × 10¹⁵, and 9.17 × 10¹⁵ ions cm⁻² are 1, 4, and 8dpa, respectively (corresponding to 885 nm from the surface). And the displacement threshold energy of V, Cr, and Ti is 40 eV, 40 eV, and 30 eV, respectively [16]. Lattice-binding energy and surface-binding energy are the default value of the SRIM 2013 software. At the irradiation damage peak (885 nm), the damage dose rate is 3.0 × 10⁻⁴ dpa/s. Samples are named according to irradiation conditions as shown in

Table 2. Irradiation conditions.

Simple	Temperature (°C)	Fluence (Ions cm−2)	Peak Damage (dpa)
1dpa-400 °C	400	1.15 × 1015	1
4dpa-400 °C	400	4.59 × 1015	4
8dpa-400 °C	400	9.17 × 1015	8
1dpa-550 °C	550	1.15 × 1015	1
4dpa-550 °C	550	4.59 × 1015	4
8dpa-550 °C	550	9.17 × 1015	8

. The corresponding SRIM calculation results and a cross-sectional view of an irradiated sample at 550 °C and 9.17 × 10¹⁵ ions cm⁻² are shown in Figure 2.

2.3. Preparation and Observation of TEM Samples

The irradiated samples were prepared using a focused ion beam (FIB) method, which was performed with a Thermo Scientific Scios 2 Dual Beam (Thermo Fisher Scientific, Waltham, MA, USA). The steps were as follows: (1) A Pt film was deposited in the selected area, then a 30 keV Ga⁺ beam was used to groove the deposited area. (2) The 2 µm thin sheet was then cut from the samples and attached to the TEM grid. (3) The sheet was thinned to a thickness of 100 nm using 30, 15, 10, and 5 keV Ga⁺ ion beams sequentially. Through a series of lower-energy ion beams, the thin layer was reduced to a thickness of 100 nm. (4) The FIB defects were minimized by cleaning it with a 2 keV Ga+ beam at the end. The size of the FIB sample in this paper was 4 µm × 4 µm. All of the FIB samples were studied with TEM in JEM-2100 (JEOL, Tokyo, Japan) from the Analysis and Testing Center of Wuhan University.

2.4. Nano-Indentation Test

All nano-indentation tests were performed on the Nano Indenter G200 produced by Agilent Technologies at Wuhan University, Wuhan, China. Continuous stiffness measurement (CSM) mode was used to obtain the continuous curve of nano-indentation hardness (H) relative to indentation depth (nm). The berkovich tip was used in the nano-indentation test. The indentation depth was set as 1200 nm, the strain rate was set as 0.05 s⁻¹, the test temperature was room temperature (25 °C), and a minimum of 10 indentation points per sample were tested. To avoid the interaction between different indents, the distance between them needed to be greater than 50 μm.

3. Results

3.1. Microstructure After Irradiation

Figure 2 shows the cross-sectional view of the ion-irradiated samples. An irradiation band appears at the depth of peak damage and can be clearly observed at sample “8dpa-550”. This phenomenon is not observed under other irradiation conditions.

The TEM images of the dislocation loops produced by irradiation are shown in Figure 3. To avoid surface effect and interstitial ions implantation effect, the depth range of 400–600 nm from the surface (the three groups of samples, respectively, correspond to 0.5 dpa–0.7 dpa, 2.0 dpa–2.8 dpa, and 4.0 dpa–5.5 dpa) was selected for the dislocation loops statistics. The TEM photos were displayed using the software “ImageJ V1.8.0.112”. According to the invisibility criterion of g·b = 0, the number of observed dislocation loops varies under different g conditions. We consistently chose to perform statistics at g = 110 to avoid the impact of diffraction direction on dislocation loops. At least five regions of each sample were selected for the statistics of dislocation loops size and volume number density, and the area of each region was 200 nm × 200 nm × 100 nm. As shown in Figure 3a–c, the irradiation-induced defects were dominated by dislocation loops at 400 °C, the size of dislocation loops increased significantly with the damage dose. At 4dpa and 8dpa, a large number of dislocation loop aggregates formed by the merging and entanglement of appearing dislocation loops. Conversely, at 550 °C, the dislocation loops exhibit higher density and smaller size, while only a small number of dislocation loop aggregates are generated with increasing irradiation dose.

The statistical results were listed in

Table 3. Statistical results of dislocation loop.

Simple	Number Density (1022 m−3)	Average Size (nm)
1dpa-400 °C	5.4 ± 0.7	4.8 ± 0.8
4dpa-400 °C	7.2 ± 0.6	5.0 ± 0.9
8dpa-400 °C	3.6 ± 0.4	6.5 ± 1.2
1dpa-550 °C	12.0 ± 0.9	4.0 ± 0.9
4dpa-550 °C	10.4 ± 0.6	4.3 ± 0.8
8dpa-550 °C	9.1 ± 0.6	5.3 ± 1.1

. The results indicate that the size of dislocation loops increases with increasing irradiation dose, and this trend is further endorsed by the size distribution of dislocation loops depicted in Figure 4. Furthermore, it is noteworthy that the 550 °C samples showed larger dislocation loop size and smaller density at the same dose, indicating a significant relationship between the temperature and the characteristics of the dislocation loops.

3.2. Irradiation Hardening

A nano-indentation test is capable of measuring mechanical properties on the ion-irradiated surface because of the highly accurate depth-sensing loading method [12]. Figure 5a shows the average curve of nano-indentation hardness with depth. Compared to the unirradiated samples, the ion-irradiated samples clearly showed increases in hardness in the depth investigated in the present work. For the unirradiated samples, the decrease in hardness with increasing indent depth was observed at the indentation depth of h > 50 nm. Such depth dependent hardness behavior has been noticed as an indentation size effect (ISE) [5]. The hardness data within h < 100 nm are ignored due to indentation size effect (ISE). Nix–Gao model [17,18] is widely utilized to eliminate the influence of the indentation size effect (ISE). The specific methods involve using the model’s equations to correct or interpret the indentation data, taking into account factors such as the indenter geometry, material properties, and indentation depth, and the specific methods are as follows:

$${\mathrm{H}}^2={{\mathrm{H}}_0}^2(1+{\displaystyle \frac{{\mathrm{h}}^{\ast }}{\mathrm{h}}})$$

(1)

where H₀ is the hardness at infinite depth, H is the hardness measured at depth h, and h^* is the characteristic length related to the material and the shape of the indenter. According to Equation (1), we can plot the square of nano-indentation hardness (H²) and the reciprocal of depth (h⁻¹) to obtain the curve, as shown in Figure 5b. The results for the 550 °C specimens show different trends before and after the critical indentation depth of 250 nm. The curve between 100 and 250 nm is relatively smooth. Above 250 nm, H² decreases rapidly with a decrease of 1/h; that is, the hardness decreases rapidly with the increase in depth. This phenomenon can be attributed to the soft substrate effect (SSE). Specifically, above the critical indentation depth, the unirradiated area undergoes plastic deformation before the indentation arrives, thereby making the influence of the unirradiated region on the hardness measurement non-negligible [19]. For samples irradiated at 550 °C, data in the depth range of 100 to 250 nm were fitted, and the fitted curve is shown in Figure 5b. The fitting result is extrapolated to infinity depth (intersection with the y-axis) to obtain the hardness data. Conversely, the unirradiated and 400 °C samples demonstrate only a linear relationship within the range of h > 100 nm, potentially attributed to minor irradiation hardening, resulting in a weak SSE. Therefore, the true hardness of V-4Cr-4Ti irradiated at 400 °C and unirradiated cannot be obtained by using the Nix–Gao model. To exclude ISE and SSE, an average hardness value between 100 and 250 nm was used to represent them.

Figure 6 shows the hardness values (H_nano) before and after the irradiation of different samples obtained via the above method. It can be observed that the H_nano values of the V-4Cr-4Ti alloy at the irradiation temperature of 550 °C are much higher than those at 400 °C for different irradiation doses. In addition, H_nano increases linearly with the increasing irradiation dose at 400 °C. In contrast, there was no significant change in the H_nano values after irradiation at 550 °C. The increment in irradiation hardness, denoted as ΔH_nano (ΔH_nano = H_irr – H_unirr), is shown in

Table 4. The irradiation hardness increment calculated using the DBH model and measured by nano-indentation.

Simple	∆HDBH (Gpa)	Hnano (Gpa)	ΔHnano (Gpa)	ΔHnano/Hunirrnano (%)
unirr	-	3.02	-	-
1dpa-400 °C	0.96	4.00	0.98	33
4dpa-400 °C	1.13	6.10	3.08	102
8dpa-400 °C	0.91	7.57	4.55	151
1dpa-550 °C	1.30	11.12	6.91	228
4dpa-550 °C	1.26	11.38	6.65	220
8dpa-550 °C	1.30	11.47	6.67	221

. Similarly, the irradiation-hardening value and hardening rate at 550 °C are much higher than those at 400 °C.

4. Discussion

4.1. The Effect of Different Irradiation Fluence and Temperatures on Dislocation Loops

As depicted in Figure 7a, alterations in density and size trends in dislocation loops are evident. A progressive increase in the average dislocation loop size corresponds to augmenting irradiation damage in conditions of 400 °C and 550 °C. The increase in size is attributed to the growth of dislocation loops by irradiation. Loop growth was via the absorption of mobile defect clusters and point defects or the merger of multiple dislocation loops. Moreover, with the increases in irradiation damage, a notable increment in intersecting or merging loops is observed, resulting in an evolution into dislocation loop aggregates and networks. This evolution, in turn, leads to a reduction in the loop number density. Especially in sample “4dpa-400 °C” and “8dpa-400 °C” (Figure 3b,c), many dislocation loop aggregates were observed. The change is a typical microstructural evolution in irradiated metallic materials [13,20,21].

Figure 7b demonstrates that the number density of the dislocation loops is highly dependent on the temperature. At the same irradiation dose, it is evident that the 400 °C sample exhibits larger dislocation loop sizes but a smaller density compared to the 550 °C samples. At 550 °C, the density reaches a saturation level at a low irradiation damage and decreases slightly with increasing dpa, indicating that the nucleation of dislocation loops primarily occurs before reaching 1 dpa. This is followed by growth and merging of dislocation loops into larger loops, resulting in a reduction in the number density. In contrast, at 400 °C, the number density of dislocation loops reaches the saturation late, indicating that nucleation continued to occur in large numbers even when the radiation damage reaches 4dpa. Considering that the fundamental cause of dislocation loop growth is defect aggregation, the temperature effect is an essential factor for the mobility of defects [13,20,22]. That is, the higher mobility of defects due to thermal effects leads to easier nucleation. This assertion is supported by Christien’s study, which found that higher irradiation temperatures lead to a shorter irradiation time required for the dislocation loop to reach saturation [23]. Therefore, irradiation at 550 °C induces higher density of small dislocation defect clusters and loops, and the average size of the defects may be limited by the densely distributed dislocation loops or networks, thereby explaining why the average size of loops is smaller compared to irradiation at 400 °C.

4.2. Irradiation Hardening

These findings suggest that the irradiation hardening reaches a saturation point before 1 dpa at 550 °C, while at 400 °C, the irradiation hardening may still be unsaturated at 8 dpa. It is noteworthy that the ion-irradiation-hardening saturation of vanadium alloys has been reported in prior studies as well [12,24]. The plastic deformation of a material is determined by the slip of dislocations, and the defect clusters generated by irradiation can block the movement of dislocations, thereby inducing radiation hardening [25,26,27,28]. Obviously, in our study, irradiation temperature is the main cause of differences in irradiation hardening.

Typically, the relationship between dislocations and radiation hardening can be described using the dispersed barrier hardening (DBH) model, in which irradiation hardening is described as the resistance to dislocation gliding. The hardness can be estimated from yield stress through the relation $H\approx 3\mathrm{\Delta }{\mathrm{\sigma }}_{\mathrm{y}}$, which can be obtained with the DBH:

$$\mathrm{∆}{\mathrm{\sigma }}_{\mathrm{y}}=\mathrm{M}\mathrm{\alpha }\mathrm{\mu }\mathrm{b}\sqrt{\mathrm{N}\mathrm{d}}$$

(2)

where M is the Taylor factor (3.06 for BCC metal); α is the barrier strength (α = 0.5) [12]; b is Burgers vector of dislocation (0.262 nm for vanadium alloy); N is number; and d is density and size of loops; μ is the shear modulus (49.5 GPa), which may be estimated from the Young’s modulus, E, and Poisson’s ratio, ν, by assuming isotropic elasticity, i.e., $\mathrm{\mu }=\mathrm{E}/\left(2\right(1+\mathrm{\nu }\left)\right)$. E can be obtained from nano-indentation test (E = 135 GPa). The value of ν is 0.365 for vanadium. This Equation indicates that the hardness change is proportional to the square root of the product of the defect density and size. The irradiation hardness increment calculated by DBH is shown in Table 4. The results from the nano-indentation test and DBH model (ΔH_nano and ΔH_DBH) comparison, as depicted in Figure 8, indicate that the DBH model underestimates the radiation hardening caused by dislocation loops. In general, the hardness increment predicted by the DBH model is smaller than the results of the nano-indentation test [29,30]. Considering the invisibility criterion (g × b = 0), the density of the dislocation loops using g = <110> near pole [001] is less than the real density of the dislocation loops. Because of the limited resolution of the TEM [31], the dislocation loops whose size is smaller than 2 nm cannot be reliably counted. And the DBH model is used to estimate hardening based on the sizes and number density of loops only in a specific area of the irradiated layer (between 400 and 600 nm), but the plastic deformation will take place in much bigger volumes in the nano-indentation test. Moreover, the barrier strength parameter (α) in the DBH model has not been precisely determined. The contribution of dislocation loops to radiation hardening may be greater than the predicted value of the DBH model, due to potential undercounting of small loops and plastic deformation in larger volumes during the nano-indentation test. Nevertheless, it is noteworthy that the ΔH^DBH value at 550 °C surpasses that at 400 °C, and the ΔH^DBH values of the three samples at 550 °C exhibit no significant differences, which aligns with the saturation trend in hardness in the nano-indentation test results. Possible mechanisms for significant increase in irradiation hardening at 550 °C are the processes that higher temperatures of ion irradiation enhanced nucleation of irradiation defects, increased the number density of the loops, and thus limited dislocation slips even more. This idea is supported by the study by Chen et al. [32], who believe that higher-density dislocation loops will act as a stronger obstruction. Therefore, compared to 400 °C, the high-density dislocation loops are a significant factor contributing to the severe irradiation hardening at 550 °C.

5. Conclusions

The effect of irradiation dose and temperature on the microstructure and irradiation hardening in V-4Cr-4Ti alloys under self-ion (V²⁺) irradiation was investigated. The conclusions are as follows:

• Dislocation loops were observed in all V-4Cr-4Ti alloy samples after irradiation. Compared with the 400 °C sample, the 550 °C sample has a larger dislocation loop number density but a smaller size. It was noted that the number density of dislocation loops reached saturation at 4 dpa at 400 °C, while saturation occurred before 1 dpa at 550 °C.
• All V-4Cr-4Ti alloy samples showed significant irradiation hardening, with the irradiation hardening at 550 °C being significantly higher than that at 400 °C. Additionally, it was found that the irradiation hardening reached saturation before 1 dpa at 550 °C, while it remained unsaturated at 8 dpa at 400 °C.
• The irradiation temperature was the primary cause of the variations in irradiation hardening. It was suggested that the higher temperature of ion-irradiation enhanced the nucleation of irradiation defects, leading to an increased number density of the loops. In the end, this further limited dislocation sliding, which improved the irradiation hardening of the V-4Cr-4Ti alloys.