What Are the Different Types of Vanadium Metal?

Vanadium alloy is an alloy composed of vanadium and other alloying elements. The fast neutron absorption cross section of vanadium alloy is small, has good corrosion resistance to liquid metal lithium, sodium, potassium, etc., as well as good strength and plasticity, good processability, resistance to radiation embrittlement, and resistance to radiation swelling It has good dimensional stability under irradiation and is an important reactor structural material. Typical vanadium alloys are V-15Ti-7.5Ct, V-15Cr-5Ti, V-10Ti, V-20Ti, V-9Cr-3Fe-1.5Zr-0.05C. These vanadium alloys are used as fuel jackets and structural elements for liquid metal cooled fast breeder reactors.

The fast neutron absorption cross section of vanadium alloy is small, and it has good corrosion resistance to liquid metals such as lithium, sodium, potassium, etc.

The pure metal is used to smelt the alloy in a magnetic levitation furnace. 5% The purity of other raw materials are greater than 99.5%. After the raw materials are mixed uniformly, they are cold pressed into 50 mm ingots and placed in a water-cooled copper crucible in a magnetic levitation furnace. Under the heating of high-frequency induction current, the spindles are rapidly melted from the surface and the inside, and further homogenization is achieved under magnetic stirring. Ar gas protection is used in the smelting process to prevent alloy oxidation. Each ingot is smelted 3 times to ensure uniform alloy composition. After forging, hot rolling and cold rolling, the alloy ingot became a thin plate of 0.5 to 1 mm, and was finally annealed under vacuum at 1100 ° C for 1 h, and the degree of vacuum was better than 4 × 10-4 Pa. Recrystallize the tissue. Forging temperature is 950 1150 , and hot rolling temperature is 850 . All thermal processing is performed in the air. In order to prevent the alloy from absorbing oxygen, nitrogen and other impurity atoms, the surface coating and copper sheath technology are used.

Using the EDM cutting method, the hardness test specimens (HT) and tensile specimens (TT) were cut from the alloy plate. Using a stamping method, a TEM sample was punched from a thin plate having a thickness of 0.25 mm. The sizes of the three samples are: 10 mm × 5 mm × 1 mm (for HT), 8 mm × 3 mm × 0.5 mm (gauge distance for TT samples) and 3 mm. The length of the tensile specimen is parallel to the rolling direction of the plate.

The temperature and time characteristics of aging hardening of the alloy were studied respectively. For the former, Ih isothermal annealing is used for 1 h at a temperature of 200 to 1100 ° C and at intervals of 100 ° C. Isochronous annealing is performed in a vacuum furnace, and the degree of vacuum is better than 1.33 × 10-4 Pa, which is cooled with the furnace. For the latter, the samples were coated with Zr foil, placed in a vacuum-sealed quartz tube, and aged in a common resistance furnace. The aging temperature is 600 , and the holding time is from 1 to 393 h. After the holding, the quartz tube is taken out of the furnace and air-cooled to room temperature. A microhardness tester was used to test the room temperature hardness of the alloy, the indenter load was 41903N, and the action time was 30 s. At room temperature, the tensile test was performed on a MTS810 material mechanical property testing machine with a strain rate of 3.5 × 10-4 s-1. The microcomputer system records the displacement curve of tensile load 2 and determines the tensile strength and elongation of the alloy, the deformation strengthening index, and the work energy of the sample (tensile fracture toughness) ^[2]

Hardening occurs at 500 to 800 ° C. Different alloys have different results. The hardening peak of V24Cr24Ti appears at 700 , while the results of V26W24Ti and V26W21Ti are similar, appearing at 600 . Moreover, the strengthening peak of V24Cr24Ti is significantly higher than the other two alloys. This difference in hardening strength and temperature effect explains the effect of the alloying element Cr, which makes the aging strengthening of vanadium alloys appear at higher temperatures. Around 900 , the hardness of V24Cr24Ti and V26W24Ti alloys decreased, which should be caused by the reduction of solid solution strengthening due to aging.

Before aging, the three alloys were all in SA (1100 / 1h annealed) state, and the hardness was 112, 145 and 146 HV, respectively. At the beginning of aging, the hardening of the alloy increases approximately with time. For V24Cr24Ti, it reaches a peak at about 10 h, and then it decreases with the increase of aging time. For V26W21Ti and V26W24Ti, a plateau appeared during about 10 to 135 h, and hardening hardly changed significantly with time, but after 135 h, the hardening effect was still weakened. These results show that the aging strengthening of V2Cr (W) 2Ti alloy is unstable at 600 . Comparing their aging strengthening effects, there is almost no difference between V26W21Ti and V26W24Ti, indicating that changes in Ti between 1% and 4% cannot change the aging strengthening behavior of the alloy. On the other hand, the strengthening effect of the V24Cr24Ti alloy is significantly stronger than the other two alloys, so the alloying element Cr should have a promoting effect of improving the aging strengthening of the alloy ^[3]

Aging increases the overall stress-strain level of the alloy. When not aging, immediately after the stage of elastic deformation, yielding occurs, the stress suddenly decreases, and then the slow deformation strengthens. For aging-treated alloys, the tensile yielding phenomenon disappears, and immediately after the elastic deformation stage, significant deformation strengthening occurs immediately. According to the classic plastic deformation theory, the disappearance of the yield phenomenon should be attributed to the precipitation of the precipitated phase caused by aging, which consumes the solid solution C, N and O interstitial impurity atoms in the alloy.

The plasticity of the alloy also seems to change due to aging treatment, especially after 24 h aging, the elongation of the alloy is significantly lower than that of the SA alloy. At the beginning of aging, the strength of the alloy increases with the aging time, and after 24 to 50 hours, the strength begins to decrease. As for the yield strength, it is slightly different. After the aging time exceeds 50 hours, it basically does not change.

As for tensile elongation, the change trend of aging time is almost opposite to the change trend of strength. At the highest strength, both the uniform elongation (UE) and total elongation (TE) of the alloy are reduced to a minimum. However, the reduction is limited, for UE and TE, it is reduced by about 3% and 4%, respectively. Moreover, when the aging time is long enough (over 50 h), the elongation almost returns to the initial value when it is not aging. As with the hardness change, the tensile properties change with the aging time, which indicates that the aging strengthening is unstable at 600 .

The deformation strengthening ability and strain energy of the alloy are affected by the aging time, where n is the deformation strengthening index, and d S / d is the value of the derivative of true stress (S) to true strain () when is 6%. Intuitively express the deformation strengthening ability of the alloy. The relationship between S and is: S = Kn. As can be seen from the results in the figure, the aging time does not significantly change the value of n, and its fluctuation range is 0.2 to 0.22. However, d S / d shows a strong regularity, similar to the change in intensity. At the beginning of aging, the deformation enhancement increases rapidly with time, but gradually weakens after 50 h, and almost returns to the initial level at 393 h. It should be closely related to the structural changes in the alloy. Ae is measured according to the area under the displacement curve under tensile load 2. To a certain extent, it reflects the fracture toughness of the alloy under tension. It can be seen from the results in the figure that Ae only decreases slightly at ~ 24 h, and after a longer time, it has an increase of more than 10%. Since aging does not improve the plasticity of the alloy, the improvement in tensile fracture toughness should be attributed to the effect of aging strengthening.

The alloy was strengthened at 600 and annealed at 600 to 700 . This strengthening is considered to be caused by precipitates. The deformation strengthening index (n) of the V24Cr24Ti alloy at an aging temperature of 600 , the deformation strengthening rate (d S / d) at 6% true strain, and the absorption work of tensile samples ( Ae) Change over time

The relationship between the size of the precipitates and the temperature is not difficult to find, and there is a good correspondence between the two: the smaller the precipitates, the more significant the hardening effect. Other studies have also found that the same type of precipitates appear in V24Cr24Ti alloys when annealed at 700 and cause hardening of the alloys.

The continuous precipitation phase appeared before ~ 10h, which caused the hardness of the alloy to continuously increase; subsequently, the hardness of the alloy continued to decrease, which should be a process of continuous coarsening of the precipitates.

When aged at 600 , V26W21Ti and V26W24Ti show approximately the same strengthening effect. According to the content of C, N, and O atoms in the alloy, if a Ti2CON precipitate is formed, 1% Ti should be sufficient to consume these impurity elements dissolved in the matrix. Limited by the total amount of these impurity atoms, adding more Ti to V26W2Ti does not change the aging strengthening characteristics of the alloy, but only increases solid solution strengthening. However, in contrast, the V24Cr24Ti alloy exhibits more excellent aging strengthening characteristics, indicating that the alloying element Cr plays an important role in the aging precipitation process of the alloy.

It may be due to the combined effects of Cr and Ti, and there is a tendency to form 2TiCr2 between them, which reduces the high-temperature mobility of Ti in the alloy and moves the precipitation strengthening to higher temperatures. At the same time, the strong interaction between Cr and Ti elements may be beneficial to serve as the center of dispersed nucleation of precipitates, increase the density of precipitates, reduce the size, and increase the contribution of precipitates to alloy strength.

Interstitial impurity atoms C, N, and O are considered to have strong mobility when the temperature exceeds ~ 300 ° C, but Ti has certain mobility only when the temperature exceeds 500 ° C. Therefore, obvious precipitation phase appears above this temperature. On the other hand, the growth of precipitates requires not only the diffusion of interstitial impurity atoms C, N and O, but also the diffusion of Ti, so the precipitates in V2Cr2Ti or V2W2Ti alloys will not grow in the temperature range below 500 , that is, It is thermally stable, and the resulting strengthening is also thermally stable. Based on the results of this experimental study, the V24Cr24Ti alloy has undergone aging treatment at 600 for a long time. Not only is the plasticity not affected, the strength is improved, and the tensile fracture toughness is improved, but all these properties should be stable at 500 . Therefore, there is every reason to believe that the application of this strengthening will reduce the weight and manufacturing cost of vanadium alloy parts and improve the cost performance of alloy parts ^[2]

Investigation and analysis of the precipitation strengthening of alloys such as V24Cr24Ti caused by annealing at different temperatures for 1 h and aging at 600 , their thermal stability and the role of alloying elements are analyzed, and they are summarized as follows:

1. The aging strengthening of V2Cr (W) 2Ti alloy at 600 is caused by fine precipitates. Compared to other Cr-free alloys, the precipitation strengthening of V24Cr24Ti alloy is the strongest. The alloying element Cr seems to enhance the precipitation strengthening of V2Ti alloy. Effect, but the change of Ti in the range of 1% to 4% has no significant effect.

2. When the aging at 600 exceeds 50 hours, the precipitation strengthening does not damage the plasticity of the alloy, but also increases the tensile fracture toughness of the alloy.

3. Aging can greatly increase the strength of the alloy, and it should be thermally stable or effective at temperatures below 500 ° C. Therefore, in the lower temperature area of the fusion reactor, these strengthened vanadium alloys can be used as component materials, which not only reduces weight, but also saves raw material costs ^[1] .