Theoretical Study Of Several Physical Properties Of Intermetallic Compounds

In the thesis we investigate the physical and mechanical properties of the binary in-termetallic compounds (Co-Ti, Fe-Al, Ti-Al) using the first-principle calculation method (TB-LMTO). On the basis of the first-principle calculation, we also investigate the phase stability of the Ti-Al system and the lattice vibrational effect on the order-disorder phase transformation by the cluster-variation method (CVM).In order to investigate the mechansim of the ductile difference between Co₃Ti and CoTi, we calculate the electronic structures of Co₃Ti and CoTi. The calculated results show that both of alloys have a pseudogap but the positions of their Fermi surfaces relative to the pseudogaps are different, which will lead to that the valence electrons in CozTi like the free electrons, but the valence electrons in CoTi like the electrons in the covalent-bond crystal. Therefore, the ductility and conductility of Co₃Ti should be better than those of CoTi.We explain why the hydrogen brittlement takes place easily by the calculated results. The results show that the valence electrons of H atoms and the valence electrons of the two kinds of metallic atoms form the covalent bonds in Co₃TiH. The energy of a H atom will lower when the H atom enters into the two inequivalent interstitial sites in Co₃Ti, the volume expansion of Co₃Ti are very small, and the energy barrier between the two interstitial sites is lower. These results indicate that not only H atoms are easily absorbed by the surface of Co₃Ti, but also easily diffuse into Co₃Ti.We investigate the magnetism and its pressure dependence of Fe - Al system in some BCC superstructures, using spin-polarized calculation. The results indicate that the magnetic moment of the BCC Fe — Al alloys in the ferromagnetic state decreases with increasing the concentration of Al in the system and volume-compression. As the concentration of Al in the system is in excess of about 0.5 the magnetism of the system will transform from ferromagnetism to paramagnetism. On the other hand, There are no the pressure induced magnetic phase transitions in the BCC Fe - Al alloys, which is different from in the FCC Fe(γ - Fe) and some other FCC Fe-based alloys.