Simulation Study On Forming And Atomic Arrangements Of Strengthening Phases In Highstrength Magnesium-rare Earth Alloys

Precipitation strengthening is an important method to enhance the mechanical properties of magnesium alloys. In Mg-RE binary alloy systems, the ?'-phase has been widely recognized as the most important strengthening phase. Recently its structural variant, named ?_t'-phase, was found by our group, which could precipitate together with the ?'-phase somewhere during the aging precipitation process. In order to account for the precipitation of the ?_t'-phase under definite promotion conditions instead of the ?'-phase, it is quite essential to determine their difference between formation energy.On the other hand, in Mg-RE-X ternary alloy systems?RE=rare earth elements, X=Al, or transition metal elements?, various long-period stacking ordered?LPSO? structures are widely concerned strengthening phases of the other type, whose formation can effectively enhance the properties of the alloys at many aspects. Although there are already many studies on the formation and stacking features of LPSO structures, the detailed atomic arrangements in LPSO structures is still an open question. For instance, Yokobayashi et al. proposed an atomic arrangement model of the LPSO structure in the Mg-Gd-Al system, in which the positions of Gd atoms were given according to HAADF observations, while those of Al atoms just via conjecture. In order that the formation conditions and intrinsic structural features of LPSO can be clearly revealed, it is very important to understand where are the preferred positions for the alloying atoms and what are options for atomic configurations when different alloying atoms involving in formation of the LPSO structure.In this study, First-principle calculations based on the density functional theory was carried out with an attempt to solve the problems mentioned above, during which the research work done includes two aspects, that is, the formation energies for the ?'-phase and the ?_t'-phase in the Mg-Gd alloy under different stress states have been calculated and compared, and the atomic arrangements of alloying atoms in ternary Mg alloys investigated. The main results are summarized as follows.It has been revealed that formation of the ?_t'-phase can be affected by the stress state. Under the isotropic compression condition along three axes, the ?'-phase is more stable than the ?_t'-phase within a wide stress range from compression. However under the uniaxial stress condition?parallel to the c-axis?, the situation is reversed at compression stress of 1GPa. This indicates that the formation of the ?_t'-phase can be energically favorable under compressive stress. Moreover, bulk modulus for both the ?_t'-phase and the ?'-phase have also been determined via fitting their p-V state equations. The results show that the bulk modulus of the ?_t'-phase is slightly higher than that of the ?'-phase.As for the atom arrangements of alloying atoms in the LPSO structure in Mg-RE-X alloy systems, the calculation results have shown that, in the case of the symmetry of the faulted layer being preserved, the theoretically predicted positions of the rare earth atoms coincide well with those of experimental observation, while those of X alloy atoms may have two sets of configurations whose numbers should satisfy constituent ratio Mg_xRE₈X₆ or Mg_xRE₈X₁₂, depending on the sort and content of X element. The Mg-RE-X alloy prefers to form a LPSO structure of Mg_xRE₈X₆ type when X is Zn, but will turn to Mg_xRE₈X₁₂ type when X is Al. In particular, in the MgY-Al system, either Mg_xY8Al6 or Mg_xY8Al12 is most probably the type of LPSO structure, whose selection closely depends on contents of alloying atoms.On the other hand, the effects of different X elements on the stability of LPSO structure in the Mg-RE-X system have also been investigated, through which an insight into structural features of the LPSO can be associated with the empirical rules for LPSO formation. When Fe or Zr is selected as LPSO-forming element, the LPSO structure will be in high energy state, meaning that the formation of the LPSO structure is not conducive. In contrast, selection of Ag element is allowable for LPSO formation, but the stability of the LPSO structure formed turns out to be low. Ni element can form stable LPSO structure.In addition, in order to improve calculating efficiency, a simplified calculation scheme by adopting 4N stacking fault unit model instead of 6N stacking unit model has been proposed in this study. It has been proved that this simplified calculation model is adequate to obtain the key features of a LPSO structure more quickly.