Zhao-Yang Hou

Formation and Evolution of Metastable bcc Phase during Solidification of Liquid Ag: A Molecular Dynamics Simulation Study

On the basis of the quantum Sutton-Chen potential, the rapid solidification processes of liquid silver have been studied by molecular dynamics simulation for four cooling rates. By means of several analysis methods, the competitions and transitions between microstructures during the cooling processes have been analyzed intensively. It is found that there are two phase transitions in all simulation processes. The first one is from liquid state to metastable (transitional) body-centered cubic (bcc) phase. The initial crystallization temperature T(ic) increases with the decrease of the cooling rate. The second one is from the transitional bcc phase to the final solid phase. This study validates the Ostwalds step rule and provides evidence for the prediction that the metastable bcc phase forms first from liquid. Further analyses reveal that the final solid at 273 K can be a mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) microstructures with various proportions of the two, and the slower the cooling rate is, the higher proportion the fcc structure occupies.

Cooling rate dependence of solidification for liquid aluminium: a large-scale molecular dynamics simulation study.

The effect of the cooling rate on the solidification process of liquid aluminium is studied using a large-scale molecular dynamics method. It is found that there are various types of short-range order (SRO) structures in the liquid, among which the icosahedral (ICO)-like structures are dominant. These SRO structures are in dynamic fluctuation and transform each other. The effect of the cooling rate on the microstructure is very weak at high temperatures and in supercooled liquids, and it appears only below the liquid-solid transition temperature. Fast cooling rates favour the formation of amorphous structures with ICO-like features, while slow cooling rates favour the formation of FCC crystalline structures. Furthermore, FCC and HCP structures can coexist in crystalline structures. It is also found that nanocrystalline aluminium can be achieved at appropriate cooling rates, and its formation mechanism is thoroughly investigated by tracing the evolution of nanoclusters. The arrangement of FCC and HCP atoms in the nanograins displays various twinned structures as observed using visualization analysis, which is different from the layering or phase separation structures observed in the solidification of Lennard-Jones fluids and some metal liquids.

Short-range and medium-range order in Ca7Mg3 metallic glass

A molecular dynamics simulation has been performed on the rapid quenching processes of Ca7Mg3 alloy including 100u2009000 atoms. The structures of short-range order (SRO) and medium-range order (MRO) in Ca7Mg3 metallic glass are investigated by means of several structural analysis methods. It is found that the SRO in Ca7Mg3 metallic glass can be modeled by neither a uniquely prescribed stereo-chemical structure nor five Bernal polyhedra but rather various types of basic clusters in which the icosahedron is dominant. The local energy together with the geometrical constraint plays very important roles in the favorable local structure in metal glasses. The MRO in Ca7Mg3 metallic glass is characterized by certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or dendrites, which is different from the fcc or icosahedral building schemes for the MRO in metallic glasses with significant chemical SRO. The size distributions of these MRO clusters present a magic numbe...

Formation mechanism of critical nucleus during nucleation process of liquid metal sodium

To deeply understand the formation mechanism of a critical nucleus during the nucleation process of liquid metal sodium, a system consisting of 10 000 Na atoms has been simulated by using molecular dynamics method. The evolutions of nuclei are traced directly, adopting the cluster-type index method. It is found that the energies of clusters and their geometrical constraints interplay to form the favorable microstructures during the nucleation process. The nucleus can be formed through many different pathways, and the critical size of the nucleus would be different for each pathway. It is also found that the critical nucleus is nonspherical and may include some metastable structures. Furthermore, the size of the cluster and its internal structure both play a crucial role in determining whether it is a critical nucleus, and this is in agreement with the simulations by computing the free energy of the Lennard-Jones system [D. Moroni, P. R. ten Wolde, and P. G. Bolhuis, Phys. Rev. Lett. 94, 235703 (2005)].

Formation and evolution properties of clusters in liquid metal copper during rapid cooling processes

Abstract Based on the quantum Sutton-Chen many-body potential, a molecular dynamics simulation was performed to investigate the formation and evolution properties of clusters in liquid Cu with 50u2008000 atoms. The cluster-type index method(CTIM) was used to describe the complex microstructure transitions. It is demonstrated that the amorphous structures are mainly formed with the three bond-types of 1551, 1541 and 1431 in the system, and the icosahedral cluster (12 0 12 0) and other basic polyhedron clusters of (12 2 8 2), (13 1 10 2), (13 3 6 4), (14 1 10 3), (14 2 8 4) and (14 3 6 5) play a critical and leading role in the transition from liquid to glass. The nano-clusters formed in the system consist of some basic clusters and middle cluster configurations by connecting to each other, and distinguish from those obtained by gaseous deposition and ionic spray. From the results of structural parameter pair distribution function g ( r ), bond-types and basic cluster-types, it is found that the glass transition temperature T g for liquid metal Cu is about 673 K at the cooling rate of 1.0×10 14 K/s.

Formation and evolution characteristics of bcc phase during isothermal relaxation processes of supercooled liquid and amorphous metal Pb

Abstract The formation and evolution characteristics of bcc phase during the isothermal relaxation processes for supercooled-liquid and amorphous Pb were investigated by molecular dynamics simulation and cluster-type index method (CTIM). It is found that during the relaxation process, the formation and evolution of bcc phase are closely dependent on the initial temperature and structure. During the simulation time scale, when the initial temperature is in the range of supercooled liquid region, the bcc phase can be formed and kept a long time; while it is in the range of glassy region, the bcc phase can be formed at first and then partially transformed into hcp phase; when it decreases to the lower one, the hcp and fcc phases can be directly transformed from the glassy structure without undergoing the metastable bcc phase. The Ostwalds “step rule” is impactful during the isothermal relaxation process of the supercooled and glassy Pb, and the metastable bcc phase plays an important role in the precursor of crystallization.

Simulation study on the formation and evolution properties of nano-clusters in rapid solidification structures of sodium

A tracing simulation study has been performed on the rapid solidification process of a liquid metal system consisting of 50 000 Na atoms using the molecular dynamics method. The formation and evolution of nano-clusters in rapid solidification have been investigated by using the cluster-type index method. It is found that the defective icosahedrons, instead of icosahedrons, in liquid and supercooled liquid play a dominant role in the formation of amorphous structures of Na. The formation and evolution of nano-clusters in the rapid solidification process have generally undergone a complicated evolution process: the small cluster is formed dispersedly and aggregates continuously in the liquid, through a middle-cluster including more than one hundred atoms by absorbing some surrounding atoms in the supercooled liquid, and finally the nano-clusters are formed by combining several middle-clusters. The nano-clusters are formed more easily in liquid metal Na than in liquid metal Al, and the configurations of nano-clusters are also obviously different from those obtained by gaseous deposition, ionic spray methods and so on.

Simulation study on non-linear effects of initial melt temperatures on microstructures during solidification process of liquid Mg7Zn3 alloy

Abstract The non-linear effects of different initial melt temperatures on the microstructure evolution during the solidification process of liquid Mg 7 Zn 3 alloys were investigated by molecular dynamics simulation. The microstructure transformation mechanisms were analyzed by several methods. The system was found to be solidified into amorphous structures from different initial melt temperatures at the same cooling rate of 10 × 10 12 K/s, and the 1551 bond-type and the icosahedron basic cluster (12 0 12 0) played a key role in the microstructure transition. Different initial melt temperatures had significant effects on the final microstructures. These effects only can be clearly observed below the glass transition temperature T g ; and these effects are non-linearly related to the initial melt temperatures, and fluctuated in a certain range. However, the changes of the average atomic energy of the systems are still linearly related with the initial melt temperatures, namely, the higher the initial melt temperature is, the more stable the amorphous structure is and the stronger the glass forming ability will be.

Crystallization characteristics in supercooled liquid zinc during isothermal relaxation: A molecular dynamics simulation study.

The crystallization characteristics in supercooled liquid Zn during isothermal relaxation were investigated using molecular dynamics simulations by adopting the cluster-type index method (CTIM) and the tracing method. Results showed that the crystallization process undergo three different stages. The size of the critical nucleus was found to be approximately 90–150 atoms in this system; the growth of nuclei proceeded via the successive formation of hcp and fcc structures with a layered distribution; and finally, the system evolved into a much larger crystal with a distinct layered distribution of hcp and fcc structures with an 8R stacking sequence of ABCBACAB by adjusting all of the atoms in the larger clusters according to a certain rule.

Molecular dynamics simulation of relationship between local structure and dynamics during glass transition of Mg7Zn3 alloy

Abstract The rapid solidification process of Mg 7 Zn 3 alloy was simulated by the molecular dynamics method. The relationship between the local structure and the dynamics during the liquid-glass transition was deeply investigated. It was found that the Mg-centered FK polyhedron and the Zn-centered icosahedron play a critical role in the formation of Mg 7 Zn 3 metallic glass. The self-diffusion coefficients of Mg and Zn atoms deviate from the Arrhenius law near the melting temperature and then satisfy the power law. According to the time correlation functions of mean-square displacement, incoherent intermediate scattering function and non-Gaussian parameter, it was found that the β-relaxation in Mg 7 Zn 3 supercooled liquid becomes more and more evident with decreasing temperature, and the α-relaxation time rapidly increases in the VFT law. Moreover, the smaller Zn atom has a faster relaxation behavior than the Mg atom. Some local atomic structures with short-range order have lower mobility, and they play a critical role in the appearance of cage effect in the β-relaxation regime. The dynamics deviates from the Arrhenius law just at the temperature as the number of local atomic structures begins to rapidly increase. The dynamic glass transition temperature ( T c ) is close to the glass transition point in structure ( T g Str ).