J.Y. Qin

The relationship between the stability of glass-forming Fe-based liquid alloys and the metalloid-centered clusters

This article considers the roles of metalloids in two typical Fe-based glass former alloys (Fe78Si9B13 and Fe80P11C9) in liquid state by using ab initio molecular dynamics simulations. It presents that Fe78Si9B13 alloy is mainly composed of B-centered prism-like clusters and bcc-like Fe-Si solid solution, while P-centered antiprism-like and C-centered prism-like clusters dominate in Fe80P11C9 alloy. The different liquid stabilities of the two alloys can be attributed to the different local environments around P and Si atoms and the different avoidance of metalloid atoms. The appearance of supercooled liquid region for metallic glasses has gained a better understanding based on these models.

Correlation between local structure of melts and glass forming ability for Fe78M9B13 (M=Nb, Si, and Zr) alloys

The structures of Fe78M9B13 (M=Nb, Si, and Zr) melts have been studied by ab initio molecular dynamics simulation. The essential features of the structures are the same in these melts. Prism-type local atomic arrangements dominate around B atoms and icosahedral-like clusters are frequently found around Fe atoms. However, the structures of the three melts differ in the degree of structural distortion caused by introducing M atoms to the prisms. The structural and dynamical properties of these melts change with different M elements, suggesting that they are closely correlated with their glass formation. The degree of complexity of molten structure caused by the interactions among Fe, M, and B may play an important role in their glass forming ability.

Crystallization pathways of liquid-bcc transition for a model iron by fast quenching.

We report simulations on the local structural evolution in the liquid-bcc transition of a model iron. Fourteen main Voronoi polyhedra are chosen as the representatives of short-range orders (SROs) and their transformations during crystallization are also investigated. Thus, the crystallization pathways for the main SROs are drawn. Our results also show that the transformations between two SROs in the crystallization pathways can be classified into two categories, first the enlargement of coordination number, second the transformation of local symmetry from five-fold to four-fold. The former reduces the potential energy while the latter increases it. It is found that the potential energy cannot decease monotonously whatever crystallization pathway is chosen to transform the icosahedral SRO to bcc SRO. Therefore, the latter transformation might provide the energy barrier of crystallization. We propose two transformation styles among SROs. All the transformations in the crystallization pathways can be achieved according to the styles. Moreover, the two transformation styles indicates that the bcc structure is more similar to liquid than other crystals. That might be the reason why the first phase nucleated during a rapid cooling process should be bcc crystal.

Chemical and topological short-range order evolution of Mg65Cu25Gd10 alloy in the process of rapid solidification

The structural evolution of Mg65Cu25Gd10 alloy from liquid to glass state was investigated by ab initio molecular dynamics simulations. From 500 K, the splitting of the second peaks of the three partial pair correlation functions related to Mg has been perceived which implicates that short-range order is strengthened owing to the atomic rearrangement during the quenching process; medium-range order up to 7.59 A has been observed from gcc(r) curves. The Gd-centered Voronoi polyhedron with index of 〈 0, 1, 10, 5 〉 was observed to increase drastically to an extraordinary level as the melt transformed to glass at 500 K, suggesting that it plays important role to keep the amorphous system stable. The variation of diffusion coefficients indicates that the solidification occurred at the temperature between 750 and 500 K.

The role of rare earth elements in the structures of FeB-based glass forming liquid alloys

The structures of liquid Fe72RE6B22 (RE=Sc, Er, Ho, Dy, Y, Sm, Gd, Nd, and Ce) alloys were investigated by ab initio molecular dynamics simulation. The results show that the chemical affinity of Fe-RE and RE-B may influence the glass forming ability more than the atomic size of RE atom in these alloys. As expected, the ⟨0,3,6,0⟩ polyhedron dominates around B atoms and is significantly enriched by adding RE elements to the liquid Fe78B22 alloy. The good glass formers do not correspond to the larger percentages but to more RE atoms in the shell of ⟨0,3,6,0⟩ polyhedron. These features suggest that the effect of the chemical composition of ⟨0,3,6,0⟩ polyhedron on the glass forming ability may be larger than that of its quantity in these alloys.