Abstract
Homogeneous nucleation from an undercooled iron melt is investigated by the statistical sampling of million-atom molecular dynamics (MD) simulations performed on a graphics processing unit (GPU). Fifty independent instances of isothermal MD calculations with one million atoms in a quasi-two-dimensional cell over a nanosecond reveal that the nucleation rate and the incubation time of nucleation as functions of temperature have characteristic shapes with a nose at the critical temperature. This indicates that thermally activated homogeneous nucleation occurs spontaneously in MD simulations without any inducing factor, whereas most previous studies have employed factors such as pressure, surface effect, and continuous cooling to induce nucleation. Moreover, further calculations over ten nanoseconds capture the microstructure evolution on the order of tens of nanometers from the atomistic viewpoint and the grain growth exponent is directly estimated. Our novel approach based on the concept of “melting pots in a supercomputer” is opening a new phase in computational metallurgy with the aid of rapid advances in computational environments.
Highlights
One of the remaining problems in computational metallurgy is how to treat nucleation
We have developed our own code for carrying out molecular dynamics (MD) simulations on a graphics processing unit (GPU), which enables the handling of one million atoms in MD simulations over a period of nanoseconds and has a computation time of several days[24,25]
Further calculations along the continuous cooling transformation (CCT) lines in the TTT diagram yielded glassy and crystalline structures depending on the cooling rate, in agreement with the prediction based on the TTT diagram
Summary
One of the remaining problems in computational metallurgy is how to treat nucleation. It is not yet straightforward to investigate spontaneous homogeneous nucleation, even by MD simulation, without an inducing factor Under such circumstances, we have developed our own code for carrying out MD simulations on a graphics processing unit (GPU), which enables the handling of one million atoms in MD simulations over a period of nanoseconds and has a computation time of several days[24,25]. Using this code on a GPU architecture, we successfully revealed the spontaneous evolution of anisotropy in a solid nucleus during the solidification of iron comprising one million atoms[19] These new insights obtained using the powerful MD tool inspired the idea of directly capturing the nature of nucleation by statistical sampling of a large-scale MD simulation. Spontaneous nucleation from an undercooled melt of iron and the subsequent microstructure evolution are investigated by the statistical sampling of isothermal MD calculations with one million atoms performed on the GPU supercomputer, TSUBAME2.5
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