Structural transition and glass-forming ability of the Ni-Hf system studied by molecular dynamics simulation
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tight-binding Ni–Hf potential is constructed by fitting some of the ground-state properties, such as the cohesive energy, lattice constants, and the elastic constants of some Ni–Hf alloys. The constructed potential is verified to be realistic by reproducing some static and dynamic properties of the system, such as the melting points and thermal expansion coefficients for the pure Ni and Hf as well as some of the equilibrium compounds, through molecular dynamics simulation. Applying the constructed potential, molecular dynamics simulations are performed to compare the relative stability of the face-centered-cubic (fcc)/hexagonal close-packed (hcp) solid solutions to their disordered counterparts as a function of solute concentration. It is found that the solid solutions become unstable and transform into the disordered states spontaneously, when the solute concentrations exceed the two critical solid solubilities, i.e., 25 at.% Ni for hcp Hf-rich solid solution and 18 at.% Hf for fcc Ni-based solid solution, respectively. This allows us to determine that the glass-forming ability/range of the Ni–Hf system is within 25–82 at.% Ni. Interestingly, simulations also reveal for the first time, that two mixed regions exist in which an amorphous phase coexists with a crystalline phase, and at about 18 at.% Ni, the hcp lattice turns into a new metastable phase identified to be face-centered orthorhombic structure.
I. INTRODUCTION
Amorphous alloys (namely metallic glasses) frequently feature novel properties for practical applications. For example, the electrical resistivity of the Ni–Hf alloys in an amorphous state was reported to be rather high and vary only slightly with temperature.1 Rathnayaka et al. observed that the Hall coefficient of an amorphous Ni–Hf alloy changes its sign from positive to negative while the Ni concentration increases and features weak temperature dependence.2 Consequently, much attention has been paid, in recent decades, to study the crystal-to-amorphous transition in the Ni–Hf system by both experimental and theoretical methods. For instance, Buschow et al. have produced amorphous alloys in the Ni–Hf system by melt spinning.1 Rossum obtained some amorphous Ni–Hf alloy films by solid-state reactions.3 Thompson also observed a single amorphous phase in the Ni–Hf system by mechanical alloying.4 In theoretical study, one of the important and fundamental issues in the field of metallic glass is to develop a relevant model capable of predicting, for a binary metal a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0453 J. Mater. Res., Vol. 19, No. 12, Dec 2004
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system, the composition range, in which the metal glasses can be obtained. Such composition range is often named glass-formation range (GFR), which reflects the glass-formation ability (GFA) of the system. In early 1980s, Egami and Waseda proposed a simple empirical formula to predict the GFR based on the consideration of atomic size/volume e
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