Molecular dynamics simulation of crystal growth in Al${}_{50}$Ni${}_{50}$: The generation of defects

The ordering processes in the interface of a solidifying binary alloy (Al${}_{50}$Ni${}_{50}$) are studied by molecular dynamics computer simulation. At various temperatures below the melting point, inhomogeneous systems with planar crystal-melt interfaces in (100) orientation are prepared. The growth of a new crystalline Al or Ni layer proceeds through different time-delayed ordering processes. Before the onset of crystallization, there is a segregation process of Al and Ni atoms in the region where a new layer forms. We show that the interplay between segregation and crystallization supports the formation of a high nonequilibrium concentration of point defects.

Figure 1

Order parameter profile of the interfacial region with a section of the corresponding atomic configuration at $T=1500$ K (blue particles are Ni and white ones Al; the green lines mark the $z$ range depicted in the profile plot). The inset shows the interface velocity $v_{\mathrm{I}}$ as a function of undercooling $\Delta T$. The solid line is a linear fit to the data for small $\Delta T$.

Figure 2

Concentration $x_{\alpha }$ ($\alpha =\mathrm{Ni},\mathrm{Al}$) and order parameter ${\left(q_6{q}_6\right)}^{*}$ (see text) for (a) Al and (b) Ni as a function of rescaled time $v_{\mathrm{I}}t$ for different undercoolings, as indicated. Note that $x_{\mathrm{Al}}$ and ${\left(q_6{q}_6\right)}^{*}$ for Al and Ni do not reach the value of a perfect crystal because of structural defects. The bold arrows on the $x$ axes mark the time $t=320$ ps at $\Delta T=95$ K (see corresponding snapshots in Fig. 3).

Figure 3

Snapshots of crystalline Ni (left) and Al layers (right) at $t=192$, 320, and 700 ps (top down) resulting from the planar growth from the melt at $\Delta T=95$ K. Blue and white spheres correspond to Ni and Al atoms, respectively. To provide a visual accentuation of crystalline configurations, particles with $q_6{q}_6<0.6$ (liquid) are depicted semitransparent.

Figure 4

Percentage of defects per layer as a function of undercooling. For an explanation of the different types of defects, see text. The scattering of the values from different simulation runs is indicated by error bars.