Calculation of Glass-Forming Ability in the Ni-Zr and Ni-Ti Systems from Interatomic Potentials
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Calculation of Glass-Forming Ability in the Ni-Zr and Ni-Ti Systems from Interatomic Potentials W.S. Lai and B.X. Liu Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, CHINA ABSTRACT For the Ni-Zr and Ni-Ti systems, Molecular-dynamics (MD) simulations are conducted to compare the relative stability of the terminal solid solutions versus the corresponding amorphous states as a function of solute concentrations. It turns out that the terminal solid solutions transform into an amorphous state spontaneously when the solute concentrations are beyond the maximum allowable values, i.e. the critical solubilities, determined to be 14 at.% Zr in Ni and 25 at.% Ni in Zr for Ni-Zr system and 38 at.% Ti in Ni and 15 at.% Ni in Ti for the Ni-Ti system, respectively. The glass-forming ranges are therefore deduced to be within the respective critical solubilities, i.e. 14-75 at.% Zr and 38-85 at.% Ti for the Ni-Zr and Ni-Ti systems, respectively, which are compatible with those from experiments and/or from the generalized Lindemann criterion. Moreover, MD simulation also reveals that solid-state amorphization does take place and that the growth of the amorphous interlayer follows exactly a t1/2 law. Besides, a solubility criterion is proposed that the lower the maximum solid solubility the less stable is the lattice of the metal upon solid-state reaction and it can explain the fact that the growing speed of amorphous interlayer toward Ni (melting point = 1528 K) is greater than that directed to the Zr (2128 K) lattice, while it is smaller than that to Ti (1941 K) side. INTRODUCTION Since the first amorphous alloy was obtained in 1960 by liquid melt quenching [1], some other techniques have been developed to produce amorphous alloys (metallic glasses) [2,3]. In practice, a quantitative measure of the glass-forming ability (GFA) of a binary metal system is of glass-forming composition range (GFR), in which amorphous alloys can be obtained. It is well known that when a melt has a composition within the range of a stable solid solution, rapid solidification usually can not quench the melt into a glass and therefore the GFR should exclude the terminal solid solutions. Naturally, a binary metal system should have its own GFA or GFR, as a specific atomic configuration of a disordered state is directly determined by its intrinsic characteristics of the system. In the present work, we employ MD simulation to derive the critical solid solubilities of the Ni-Zr and Ni-Ti systems for assessing the GFR of the systems. Asymmetric growth of an amorphous interlayer in multilayers during solid-state amorphization reaction (SSAR) has been revealed by MD simulations [4-6] and /or observed in experiments [7,8] in several systems. It seems that the metal with a lower melting temperature becomes disordered earlier than its partner with a higher melting point. It is of interest to test if L4.2.1
such correlation is also true for the Ni-Ti system, in which Ti has a higher melting poin
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