Study on Corrosion of Titanium-Al-based Alloy by Zinc Liquid with Positron Annihilation

Corrosion behavior of Ti-50Al and Ti-45Al-8Nb alloys in zinc solution, and the positron lifetime spectra of Ti-50Al and Ti-45Al-8Nb alloys were measured. The positron lifetime parameters were used to calculate the alloy matrix and defect states respectively Free electron density. The free electron density of TiAl alloy is lower than that of metal Ti and metal Al matrix. When Ti and Al constitute TiAl alloy, part of the valence electrons of Ti atom and A1 atom are localized, and metal bonds and their valence bonds coexist in TiAl alloy. The grain boundary defects of TiA1 alloy have a larger open space, the free electron density at the grain boundary defects is lower, and the metal bonding strength is weaker. The addition of Nb element in TiAl alloy increases the free electron density of Ti-45Al-8Nb alloy matrix and grain boundary, which obviously slows down the dissolution rate of zinc solution to titanium aluminum alloy.

CLC classification number: TG Date of receipt of the first draft: 2006-05-22; Date of receipt of the revised draft: 2007-03-11 Fund project: National Natural Science Foundation of China (50274005) 13Al There are 13 electrons outside the nuclear price Electronic 3s23pNb: 4d45sZn: 3d104s2; because we do not have a standard sample of high-purity metal Nb, we can not measure its characteristic spectrum, you can refer to the results.

For the 0 and r samples, from the peak shape and peak height and other information, the CDB diagram is mainly between the Ti and Al single crystal CDB, showing the information of Ti and Al. Due to the existence of vacancies in the sample, the probability of annihilation of positrons and core electrons decreases, and the peak amplitude decreases. At the same time, at 10.7X10-3W0c, the peak information of the d-electron is relatively advanced to the low momentum end relative to Ti, indicating that the positron and core electron are annihilated. Because Nb-4d electrons are less bound to the electron by the nucleus, the momentum is lower, and it is more localized in the momentum space, which causes the 4d momentum distribution of Nb to move toward the low momentum direction relative to Ti-3d.

The CDB curve of 2 samples combined with Zn shows that after the sample is corroded, the vacancy defects where Zn is accumulated in large amounts form vacancy conformance with the defects. The CDB curve shows a large amount of Zn positron annihilation information at greater than 15X10-3W0c Zn contains 10 3d.2 samples. Compared with one sample, after corrosion, the defects become larger and more, resulting in a decrease in the free electron density at the defects.

The life spectrum is decomposed using the POSITRONFIT program for three-life fitting, and the life spectrum fitting results of the positron three-life component life (;) and the corresponding intensity (; / 3) are obtained, and the source components are deducted. The third group lifetime and intensity / 3 of each life spectrum are almost 1.5ns and 1%, which is the result of the positron annihilating on the surface of the sample and positron source. The factors of the sample surface are not considered here. The strength of the life component of the first group and the life component of the second group is as follows: normalize the life of the positron, and obtain the positron life parameters shown in the following table. As well as the average lifetime, the second group of lifetimes corresponds to the annihilation of the defect states of positrons in alloy samples.

At the same time, according to the two-state capture model, the life and annihilation rate of the positron in the alloy matrix can be calculated (4): the second component life T2 is the life of the positron in the alloy defect state, the positron in the alloy defect state annihilation Rate Xd = 1 / T2. Positron lifetime provides information about the electron density of the environment where it was before being annihilated. The positron annihilation rate is a function of electron density. In an ideal crystal, the heated positrons are repelled by positive ions. Before annihilation, they are usually in the gap position of the lattice for most of the time, and then annihilated with the extended Bloch state electrons. In other words, in a metal or alloy lattice, the positrons "see" mainly free electrons. The empirical formula of the root: = (X-2) / 134, using the positron annihilation rates Xb and Xd of the alloy matrix and the defect state, the corresponding free electron density sum can be calculated respectively, where the unit of X is (ns)- 1, the unit is an atomic unit (the symbol is represented by au), for the electron density, 1au = 6.755X1030m-3. Their values ​​are listed in Table 1. The positive in pure Ti, pure Al, pure Nb and pure Zn metal matrix The electron lifetime value, positron annihilation rate and free electron density are listed in Table 2. Since the sample has been fully annealed, most of the vacancies and dislocations have been recovered. The defects in the sample are mainly grain boundaries and phase boundaries, so T2 is mainly positive The lifetime of electrons obliterated in the sample grain boundary or phase boundary. The data in Table 2 shows that the r2 of the tested alloy is greater than the life value of the positron in the single vacancy of the A1 metal rv (A1) = 240ps. The life of the positron in the alloy defect increases with the increase of the open space of the defect. Therefore, the open space of TiA1 alloy grain boundary defects is larger than that of metal A1 single vacancy. This structural feature of the grain boundary defects of TiA1 alloy may be related to the bonding state between the Ti atoms and A1 atoms constituting the alloy. Ti atom (its electronic configuration is 1s22s22p63s23p63d24s2) due to the locality of its 3d electrons, when he and the multivalent element Al (its electronic configuration is) form a TiA1 alloy, (A1) 3p- (Ti) 3d in the alloy The bond level is larger and the directionality is stronger, showing the characteristics of covalent bonds. Similar to other aluminide intermetallic compounds, metal bonds and covalent bonds coexist in TiA1 alloy. Because the covalent bond has a strong binding force and good spatial orientation, TiA1 alloy exhibits long-range order characteristics and usually has a high order energy. In polycrystalline alloys with higher order energy, the arrangement of atoms within the same grain is highly ordered, and atoms at the grain boundaries are less likely to relax, resulting in the formation of larger voids in the grain boundaries and resulting in the price of the grain boundaries. The electron density is low, and the positron has a longer life at the defects of the alloy grain boundaries.

Table 1 The positron lifetime spectrum parameters of samples 0, 1, and 2, the free electron density of the free electron density bond of the matrix and the defect state, the higher the free electron density participating in the bonding, the stronger the metal bonding force between the atoms. The results of this experiment show that the electron density (d) of all tested alloy grain boundary defects is lower than the electron density (b) in the alloy matrix (Table 2), which indicates that the alloy grain boundary is the area where the bonding force is weakened. The bonding strength of the alloy grain boundary depends on the free electron density at the grain boundary. The higher the free electron density at the grain boundary, the stronger the bonding strength at the grain boundary, and vice versa.

Table 2 The positron lifetime, positron annihilation rate and free electron density in pure Ti, Al, Nb and Zn metal substrates are known from Table 1 and Table 2. The free electron density of TiAl alloy substrate is higher than that of pure metal Ti and pure metal A1 substrate Low, the metal bonding force in the alloy matrix is ​​weak; moreover, there is a defect in the TiA1 alloy grain boundary with a larger open space, the electron density at the grain boundary is lower, and the metal bond strength at this point is weaker. In TiA1 alloy, by adding appropriate Nb, it is expected to change the bond structure and grain boundary structure of the alloy.

It can be seen from Table 1 that the free electron density of the Ti-45Al-8Nb alloy matrix is ​​higher than that of the Ti-45Al alloy matrix. This indicates that adding 8% (mole fraction) of Nb to TiAl alloy increases the free electron density of the alloy matrix. For Ti-45Al-8Nb alloy, since the free electron density of Nb metal matrix is ​​higher than that of Ti or Al metal (see Table 2), Nb will provide more than Ti or Al, whether it substitutes Ti or Al in the alloy Valence electrons participate in the formation of metal bonds, increasing the free electron density of the alloy matrix.

Table 3 shows the morphology of the diffusion layer and the element content in the corresponding position after immersing the titanium-aluminum alloy in the zinc solution for the same time. It can be seen from Table 3 that the zinc content of point A is much higher than that of point B. This shows that the zinc solution is more likely to diffuse to the grain boundaries and defects of Ti-50Al alloy. The addition of Nb to the alloy increases the number of free electrons involved in the formation of metal bonds in the alloy, thereby reducing the tendency of covalent bonds to form in the alloy. To homogenize the charge distribution and reduce the order energy of the alloy, so that the alloy grain boundary is easy to relax and the open space of the grain boundary defects becomes smaller; and, when Nb atoms diffuse into the grain boundary, it will increase the free electron density at the grain boundary As a result, the rate of zinc liquid diffusion to the titanium-aluminum-based alloy is further slowed down. Therefore, the zinc solution dissolves the Ti-45Al-8Nb alloy faster than the Ti-50A1 alloy.

3 Conclusion The corrosion of TiAl-based alloy by zinc solution is solution corrosion. The zinc solution gradually diffuses into the alloy matrix, and the formed diffusion layer slowly dissolves into the zinc solution, and so on, until the alloy is completely dissolved. The dissolution rate of the zinc solution to Ti-50Al and Ti-45Al-8Nb alloys is 0.083mm / h and the addition of Nb to the TiAl alloy increases the number of free electrons involved in the formation of metal bonds in the alloy, thereby reducing the formation of covalent alloys Key tendency. To homogenize the charge distribution and reduce the order energy of the alloy, so that the alloy grain boundary is easy to relax and the open space of the grain boundary defects becomes smaller; and, when Nb atoms diffuse into the grain boundary, it will increase the free electron density at the grain boundary, Furthermore, the rate of diffusion of the zinc solution to the titanium-aluminum-based alloy is further slowed down.

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