Abstract
The molecular dynamics method was used to study the formation of free volume at grain boundaries and triple junctions during crystallization using nickel as an example. To simulate the crystallization process, crystalline nuclei were used — small ideal crystals with a fixed lattice. The orientation of the crystal lattice in the nuclei was set randomly, but so that tilt boundaries with the misorientation axis <100> or <111> were formed in the result of crystallization. It is shown that the free volume during crystallization is predominantly concentrated at grain boundaries and triple junctions, and more so in the latter ones, i. e. where the last solidification of the structure occurs and crystallization fronts are encountered. The reduced density of the structure at triple junctions compared with grain boundaries is explained by the fact that when three crystallization fronts meet, the volume is fixed, where the liquid phase having a density lower than that of the crystal solidifies. The velocity of crystallization front motion is several tens of times lower than the speed of sound in a metal, therefore, defects, as a rule, formed only in the last turn — when the fronts meet, i. e. at grain boundaries and triple junctions. Disclinations at triple junctions were not observed in this work. During crystallization, relatively small subgrains with an orientation different from the neighboring grains, and usually in a stretched state, sometimes appeared in the region of triple junctions. But they “healed” relatively quickly, being absorbed by neighboring growing grains.
Highlights
The triple junction of grain boundaries is a linear defect along which three grain boundaries are conjugated
In [12], we showed that the most probable reason for the formation of excess free volume at the triple junctions obtained during crystallization is the “locking” of the density of the liquid phase at the meeting of three crystallization fronts, which causes the concentration of excess free volume in the triple junction after solidification
It is shown that the free volume during crystallization is predominantly concentrated at grain boundaries and triple junctions, especially at the latter, i. e. where crystallization fronts are encountered and solidification occurs last
Summary
The triple junction of grain boundaries is a linear defect along which three grain boundaries are conjugated. The systems of grain boundaries and their triple junctions in metals obtained as a result of crystallization from a melt and during plastic deformation differ very much In the latter case, the proportion of nonequilibrium boundaries and excess defects is high both at the boundaries and at the triple junctions [1,2,3]. The simulation results confirmed that the free volume can be locked at triple junctions in this way It was found in [12] that, in some cases the formation of relatively small subgrains with different orientations from the joined grains and in the stretched state is possible during crystallization in the triple junction region. The use of OpenCL technology in our own program MDR allowed us to increase the size of the computational cells by an order of magnitude
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