^{1}

^{*}

^{1}

^{1}

By using LAMMPS of the open source software, the micro-structures of Al30Co10 alloys were studied. Based on the average atomic volume, pair distribution function and bond-angle distribution functions, Honeycutt-Andersen (HA) bond-type index analysis shows that Al30Co10 alloy system begins to transform into a glass state with the temperature rapidly decreasing to 900 K. The process temperature is decreased from 900 K to 300 K, the radial distribution function g(r) the first peak height with increased, width as decreasing temperature, and the system is an amorphous alloy when second peak appears obvious splitting. The bond angle distribution function showed second peaks when the temperature dropped to 300 K, so that the alloy atoms become orderly. Meanwhile the 1551 bond pairs increase to 35% with decreasing temperature; it implies that the Al30Co10 alloy system can be well transformed into the glass state.

With the long-range disorder and short-range characteristics of amorphous alloy, it is also called the metallic glass (MGs). The amorphous alloys are of great interest owing to their unique combination of high specific strength and ductility, good corrosion resistance and abrasive resistance, representing an intriguing class of potential structural materials [_{30}Co_{10}. It is hoped that more data will be provided for experiment by the molecular dynamics simulations to create the high temperature, high pressure and other extreme conditions.

In this article, we selected the Al as the main backing material to study the glass formation process and associated structural evolution by MD simulations. A detailed analysis of local atomic structure has been done during the glass formation of the amorphous Al_{30}Co_{10} by various structural characterization methods, including pair distribution function [_{g}) and micro-structures.

In this work, the MD simulation is carried out using the open code LAMMPS program [^{ }

where _{ij}, and F(ρ_{i}) the embedding energy of atom with the electron density [^{12} K/s. During the simulation processes, system volume is automatically adjusted to correspond to the given temperature and zero pressure conditions.

As is known that the Liquid transform to crystalline state is a first order phase transformation; it is typical of the System volume will be sudden changes in the crystalline temperature. ^{12} K/s. As shown in _{g}, one can yield a crossover at which the T_{g} was determined to be 900 K. In other words, the system of Al_{30}Co_{10} liquid alloy will be transform to amorphous with the T_{g} = 900K.

It is one of the most important function, revealing the structural characteristics of the system as Pair distribution function (PDF) analysis, most particularly for amorphous and liquids systems. The Pair distribution function (PDF) is defined by the probability of finding an atom in the spherical shell where particle i is taken as the center. Expressed in a formula, this law can be written as follows.

where ρ(r) the number of atoms around a central atom within the distance interval between r and r + dr, which is atomic average statistical distribution. By simulating the PDF of the liquid Al, we found that the PDF of the liquid Al are good agreement with the experimental data (Ref. 84) as shown in

In liquid system, atoms do as random motion, which cannot be described by the fixed coordinate; we compared the PDF for the liquid system with amorphous state, it is seen that the PDF are very similar in almost every way. However, the atom can be vibrate in a certain range in amorphous, which can be described by the fixed coordinate; the first peak of the PDF is relatively narrow and sharp; the second peak will be split into two peaks, which one peak is higher than the other peaks, which hump shaped is also one of the most important judgement of amorphous typical characteristics.

The PDF are shown in _{30}Co_{10} liquid alloy. It can be seen that the first peak of PDF gets higher as the temperature decreases, which means that atomic ordering in the first coordination shell increases as the temperature decreases. The second and third broad peaks become more pronounced as the temperature decreases to 1200 K. Meanwhile the temperature decreases to 900 K, the second peak start to splits into a tiny peak at the right; and then the second peak splits into two obvious sub peaks when the temperature further decreases to 300 K. In summary, the system of Al_{30}Co_{10} liquid alloy were start to transform to amorphous at the temperature decreases to 900 K, which is accord to the upward analysis that the T_{g} of the simulated system is 900 K.

In amorphous system, the local structure was more comprehensive described by bond-angle distribution functions. Every barber knows that if a system is dominated by icosahedral structures, there will be the two main peaks at 63.4 and 113.4 in bond-angle distribution functions for the cluster structures in the system; meanwhile, there will be obvious degrees of 60, 90 and 120 in bond-angle distribution functions, it is shows that the FCC or HCP system are dominate in the system. For ^{12} K/s, the capacity of degrees in the vicinity of 60 and 120 has obvious increased with the decrease of temperature; it is obvious that the angle degrees tend to be about 63.4 and 113.4, which means that the dominated local structures are icosahedral structures in the amorphous Al-Co system.

Furthermore, we get more information about micro-structures of Al_{30}Co_{10} alloy, which is analyzed using the Honeycutt-Andersen (HA) bond-type index and give a detailed three-dimensional image of the local configurations in the cooling process. The HA bond-type index is able to give information about the number and properties of common nearest neighbors of each atom pair which can be used to characterize the local environment

surrounding atomic pair under consideration. Each pair of atoms is classified by the relation among their neighbors with four indices of integer. The first integer indicates whether the considered atomic pair is closer than a specified cutoff distance, chosen to be the nearest-neighbor distance determined by the first valley in the pair correlation function. If the pair is bonded, the first integer is 1, or else, 2. The second integer is the common- neighbor number of the considered pair; the third represents the number of bonds among the common neighbors; the fourth is used to distinguish the atomic pair when the former three integers are not sufficient. While, the some pairs of 1551, 1541, 1431 types are shown because they represented local icosahedron and defect icosahedron, respectively. As shown in

8.5%, 13% at the temperature of 2100 K, respectively. For the 1551 pairs, the fraction increased from 5% to 27.5% with temperature rapidly decreasing from 2100 K to 300 K. The fraction of 1551 pairs, a modest figure, with the temperature of decreasing from 2100 K to 900 K; however, it was booming growth to 27.5% when the temperature of decreasing from 900 K to 300 K because the system have started to transform to amorphous at 900 K. The fraction of 1431 pair did not change much, while increasing from 13% to 15.7% in the whole temperature range; the fraction of 1541 pairs is 14.6% at 300 K.

We presented the results of an MD study of the glass transition for Al_{30}Co_{10}. The pair correlation function, bond-angle distribution functions and HA bond-type index at different temperatures are obtained by our simulation. The PDF illustrates that the shortage of medium structure orderings is enhanced with decreasing temperature. Meanwhile, when the temperature decreases to 900 K, the second peak starts to split into a tiny peak at the right, and then the second peak splits into two obvious sub peaks when the temperature further decreases to 300 K. In summary, the system of Al_{30}Co_{10} liquid alloy starts to transform into amorphous alloys when the temperature decreases to 900 K, which is according to the upward analysis that the T_{g} of the simulated system is 900 K. The HA bond-type result shows that the local icosahedron and defect icosahedron are decreased in cooled alloy liquid during the rapid solidification processes, while the 1551 pairs, the fraction increases from 5% to 27.5% with temperature rapidly decreasing from 2100 K to 300 K.

The authors thank the financial support of the Natural Science Foundation of Yong Chuan (Grant 2014nc4002) and the foundation of Chongqing University of Arts and Science (Grants Z2013KJ17).

ChunpingFu,LingtaoSun,ZhengfuCheng, (2015) Molecular Dynamics Simulation of Glass Forming Ability of Al_{30}Co_{10 }Amorphous Alloy. Open Journal of Applied Sciences,05,552-558. doi: 10.4236/ojapps.2015.59053