Face-centered Cubic Ordered Alloy Films Disorder Phase Transition And Phase Diagram Study

The knowledge of order-disorder phase transition and phase diagrams is important to the study of the structure, thermodynamic properties and phase transitions of alloy films. It is also the basis for the study of the dynamic properties of alloys. We have calculated the order-disorder phase transitions and phase diagrams of alloy films under surface fields using the mean field method.Based on the ground state structures, we calculate the order-disorder phase transitions of the 6 and 7 layer fcc alloy films in the (001) direction with the CVM method. Due to the combined effects of surface segregation and finite size confinement, the order-disorder phase transitions of the thin films are much more complicated than those of the bulk. The main results of this thesis are as follows: 1) The computational results show that the order-disorder phase transitions of thin films are determined by the parity of the layer number of films. The thin films with different parity will show different phase structures and thermodynamic properties. Under different surface field, the odd number layer films have different phase structures and phase sequences with the even number layer ones. For the even number layer films, there are anti-phase boundary phenomena induced by the the missing of symmetry and surface effect, which causes the intermediate temperature phase emerged during the process of order-disorder phase transitions. While for the odd number layer films, the order-disorder phase transitions are similar with that of the bulk structures. 2) for the odd number layer films, in the chemical potential region corresponding to the bulk concentration x=0.5, there is an AB(AB)_nA phase which does not show order-disorder phase transition with the increase of temperature due to the combined effect of the surface field and finite size. While for the even number layer films, the corresponding phase structures are AC(CC)_nCA. or AB(CC)_nBA. The phase transitions can be determined by the variation of specific heats ororder parameters. 3) there are some critical points in the phase diagrams both in the odd and even number layer films. 4) Generally, for the fee alloy films, when the surface field is greater than a certain critical surface field, an N layer system undergoes a phase separation between its surface and the inner (N-2) layers, In the phase diagrams, both the ordered phase and the disordered phase can exist as the temperature is down to zero.