Atomistic structural evolution with cooling rates during the solidification of liquid nickel

In this paper, molecular dynamics simulations based on the second-moment approximation of tight-binding scheme have been performed to investigate the effect of cooling rate on solidification microstructures of liquid Ni. Two transitional structures characterized by one-dimensional (1D) and 2D periodicities respectively between the crystal and amorphous states have been found as the cooling rates range from 6 × 1012 K/s to 1 × 1013 K/s. As the cooling rates Q ≥ 1.5 × 1013 K/s, an amorphous structure can be obtained, whilst crystal structures are formed when Q ≤ 4 × 1012 K/s. Moreover, our results reveal that the intensity ratio (g21/g22) of the two subpeaks, which split from the second peak on the pair distribution function for the amorphous state, can act as a structural indicator to differentiate amorphous, translational structure and crystalline states. As such, one may determine the structure state of a material by estimating the value of g21/g22 from its pair distribution function.

Molecular dynamics (MD) simulated instantaneous atomic images of the Ni system at 300 K resulted from different cooling rates. (a) Amorphous structure at Q3 = 1.5 × 1013 K/s. (b) 1D periodic structure at Q4 = 1 × 1013 K/s, the green arrow denotes the direction of the 1D periodic structure. (c) Detailed atomic configuration of the selected plane (blue atoms) in (b); (d) 2D periodic structure at Q6 = 6 × 1012 K/s, the green and blue arrows indicate the two directions of the 2D periodic structure. (e) 3D crystal structure at Q9 = 5 × 1011 K/s. All the insets in the top right corner of these figures display the corresponding fast Fourier transformation (FFT) patterns of the atomic configurations for different cooling rates.Figure optionsDownload as PowerPoint slideResearch highlights
► Relationship between solidification microstructures and cooling rates of Ni has been established.
► Two transitional structures characterized by 1D and 2D periodicities respectively have been revealed.
► The fcc- and hcp-type close-packed planes constitute the 1D and 2D structures.
► Three different g21/g22 values correspond to the three structural states.
► The value of g21/g22 could act as a parameter to characterize the structural state.