Molecular Dynamics Simulations Of Vitrification And Martensitic Transformation In Cu-ti(-zr) Systems

Metallic glass (or amorphous alloy) was first obtained in the Au-Si system by liquid melt quenching (LMQ) in 1960 and since then a number of experiments and theoretical studies have been carried out to investigate one of the basic issues in the field, i.e. glass-forming ability (GFA). For a specific alloy, the GFA is commonly estimated by a number of empirical parameters, such as the reduced temperature Trg, stability parameter S and Tl-Tg. Practically, a quantitative measure of the GFA is glass-forming composition range (GFR), within which amorphous alloy can be obtained via some glass-producing technique. In theoretical studies, Liu et al. have proposed an atomistic method to determine the GFR of a binary metal system directly from the interatomic potential of the system through molecular dynamics (MD) simulations. In this thesis, we first study the GFRs of the Cu-Ti binary system with Liu's method, and then extend the same idea to the Cu-Zr-Ti ternary system. The tight binding (TB-SMA) many-body potentials for the Cu-Ti and Cu-Zr-Ti systems were constructed and applied in MD simulations to study crystalline-to-amorphous transition and GFR in both binary and ternary systems, as well as an hcp-to-fco martensitic transformation in the Cu-Ti system.Based on solid solution models, the MD simulations using the constructed potentials show:The GFR of the Cu-Ti system is predicted to be 22at.%~71at.%Cu, in good agreement with experimental results. Coordination number and common-neighbor analyses clarified the physical mechanism responsible for the energy difference between solid solution and amorphous phase as follows: with increasing the solute atom content, the coordination number and unlike bond of amorphous phases increase greatly than those of solid solutions, leading to a cross-over point between their energy curves because of a fast energy drop of amorphous phases than the solid solutions. These results show that the relative stability between solid solution and amorphous phase is correlated to their microstructure.The GFR of Cu-Zr-Ti system is located in an approximate distorted quadrilateral region, and the compositions of the four vertexes of the quadrilateral are Cu22Zr78Ti0, Cu24Zr0Ti76, Cu56Zr0Ti44 and Cu72Zr28Ti0, respectively. In addition, the simulation results are in accordance with Egami's glass-forming and Liu's structural difference empirical rules.An hcp-to-fco martensitic transformation was observed in Ti lattice, upon dissolution of Cu atoms. Common-neighbor analysis showed that the new formed fco phase features bcc-like structure. Based on the detailed analysis of different manifestations observed in simulations, the hcp-to-fco phase transformation mechanism is proposed as follows: the hcp lattice elongates along the <100> close-packed directions and slightly shrinks along [001] direction, accompanying with a relative movement (or slide) along <120> directions between the adjacent close-packed planes and the lattice constants adjustment to form the new fco phase.