International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016E-mail: wdenggxu.edu.cn National Natural Science Foundation of China (Grant No.), Guangxi Scientific Research and Technology Development Program (No. 0480004) and Guangxi University Key Science and Technology Research Project (No. 2003ZD04 ) Funding for 3D electron defects in TiAl alloys. Positrons obliterate information about 3D electrons and defects in these metals and alloys. The results show that the more the number of electrons in the 3d orbits of the transition metal elements Ti, V and Cu atoms, the stronger the 3d signal of Doppler broadened spectrum of positron annihilation radiation. The signal density of binary TiAl alloy and the signal of 3d electrons are low, and the open space of grain boundary defects is large. Adding V or Cu to the TiAl alloy enhances the 3d electron signal in the alloy, and the electron density at the matrix and grain boundaries increases. The Doppler broad-spectrum 3d electronic signal of Ti50Al48Cu2 alloy is higher than that of Y-TiAl alloy, which is ordered L1. It has the characteristics of low density, high melting point, high elastic modulus, good oxidation resistance and high temperature strength, etc. It is considered to be a high-temperature structural material with application prospects. However, TiAl alloy is very brittle at room temperature, which hinders its practical use. In order to ascertain the deformation mechanism of TiAl alloy, a lot of research has been conducted on TiAl alloy in recent years. By alloying and optimizing the structure of the alloy, the mechanical properties and corrosion resistance of the alloy can be significantly improved. Some research groups have studied the effect of adding a third component to the binary TiAl alloy on the material properties. We assume that the binary TiAl alloy is metal bonded and the injected positrons are evenly distributed in the alloy. This assumption will lead to the following results: the probability of positrons appearing in the Ti and Al atom positions in the alloy lattice should be 50%, respectively, and the commercial spectrum of Ti50Al5. The alloy should be comparable to the CTiAl commercial spectrum. But ours.
Table 1 Positron lifetime spectrum parameters of T1A1 based alloy, matrix and defect state electron density alloy analysis Positron lifetime spectrum of fully annealed pure element samples Al, Si, Tl, V and Cu, obtained a single lifetime, indicating that the sample Most of the defects have been replied. The matrix positron lifetime, positron annihilation rate, electron density, electron configuration and atomic radius of these pure element samples are shown in Table 2. Our life spectrum measurement results are consistent with the reports.
Table 2 Al, Si, Ti, V and Cu element matrix positron lifetime, positron annihilation rate, electron density and electron configuration element electronic state Ti5Al5 alloy matrix valence electron density (b (TiAl) = alloy, electron density decreases Because there are two unpaired 3d electrons in the Ti atom, when the Ti atom and the Al atom form a bond, the Al atom provides its valence electron and the Ti ’s 3d electron will form a local covalent bond (Ti and Al electrons) The configuration is shown in Table 2), which leads to a reduction in the number of valence electrons participating in the formation of metal bonds in the alloy matrix. This is consistent with the results obtained by the Doppler broadening spectrum of positron annihilation radiation.
The positron lifetime increases with the increase of the defect opening space. The life of positron in the defect state of binary TiAl alloy T (TiAl) = (Table 1) is greater than the life of positron in Al vacancy Tv (Al) = 240ps. This shows that there is a large open space in binary TiAl alloy Defects. This defect structure characteristic is attributed to TiAl having a long-range ordered L1. Type structure and high order energy. For polycrystalline TiAl alloys with high order energy, the arrangement of atoms in the grains is highly ordered, and the atoms at the grain boundaries are not prone to hesitation, which leads to defects with larger open spaces at the grain boundaries. Therefore, T2 of the TiAl alloy is larger. Moreover, the valence electron density d (TiAl) at the grain boundary of the TiAl alloy = 1.03X10-2a.u. (Table 2) is very low. It can be seen that the TiAl alloy not only has a weak bonding force between atoms at the grain boundaries, but also has a strong bonding force between atoms in the matrix. For example, TiAl is an intrinsically brittle intermetallic compound, and its single crystal and polycrystal are both brittle at room temperature.
The data in 1 shows that the V of b (Ti5Al48V2) (atomic fraction) increases the electron density of the alloy matrix. It can be seen from Table 2 that since b (V) = 0.04576au is greater than b (Al) = 0.0317au or b (Ti) = 0.0366au, when V replaces Al or Ti atoms, they all provide more Many valence electrons participate in the formation of metal bonds, increasing the free electron density in the alloy, and d (Ti50Al48V2) 1). This shows that adding 2% (atomic fraction) of V to TiAl alloy can reduce the open space of grain boundary defects and increase the electron density at the grain boundary. The positron lifetime of the base metal Cu is also shorter than that of the metal Al or Ti (Table 2). Adding 2% (atomic fraction) of Cu to the TiAl alloy also increases the valence electron density at the alloy matrix and grain boundaries, ie b (Ti50Al48Cu2)> d (Ti50Al50) (Table 1). The addition of Cu to TiAl alloy has a similar effect to the addition of V.
3. Conclusion The average electron momentum of the transition metal elements Ti, V and Cu atoms increases with the increase of atomic number. The greater the number of electrons in the 3d orbit of a metal atom, the stronger the 3d signal of Doppler broadened spectrum of positron annihilation radiation.
Due to the hybridization of 3d electrons of Ti and 3p electrons of Al, the electron density of the binary TiAl alloy and the signal of 3d electrons are low. There is a defect in the alloy grain boundary with a large open space.
The addition of V and Cu elements in the TiAl alloy increases the valence electron density at the alloy matrix and grain boundaries, and the 3d electron signal increases. The Doppler broad spectrum 3d electronic signal of Ti50Al48Cu2 alloy is higher than that of Ti50Al48V2 alloy. Compared with adding V, adding Cu to TiAl alloy is more effective to increase 3d-3d electron effect.
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