Size distribution of pores in metal melts at non-equilibrium cavitation and further stretching, and similarity with the spall fracture of solids


DOI: 10.1016/j.ijheatmasstransfer.2018.08.053

Non-equilibrium cavitation in liquids is similar in many aspects to the spall fracture of solids. The dynamics of cavitation and further evolution of melt with cavities are determined by the processes of nucleation and size variation in the ensemble of pores. Exploration of the statistical distribution of the pore sizes and its evolution in the course of the melt stretching is important for in-deep understanding of the spall fracture of melts and formulation of adequate models. Size distributions of pores in pure Al, Ni, Cu, Ti and Fe melts under stretching with ultra-high strain rates are investigated with the help of molecular dynamics simulations and analysis of the obtained atomic configurations by an algorithm of pore searching. The size distribution of pores fits well to exponential one at the stage of active homogeneous nucleation of pores in all considered cases. This form of distribution is explained by the exponential growth of the nucleation rate together with the decrease in the size of critical pores at the decrease in pressure. A transition from the exponential distribution to normal one in the course of stretching is found out for melts of most metals except Fe at the strain rates above 30/ns. The transition to the normal distribution is explained by depopulation in the area of small pores due to their collapse in conditions of the increase in pressure. The transition occurs near the pore number maximum, because the reaching the maximum indicates that the collapse becomes predominant over the nucleation. The collapse of small pores is accompanied by enlargement of the largest pores and rapid widening of the size distribution with increase in the mean pore size. This is a mediated mechanism of the volume redistribution from the small pores to the largest ones; this mechanism is the most pronounced in Al melt and the most suppressed in Fe melt. In most cases, it is accompanied by direct merging of pores that is, vice versa, the most active in Fe and the most delayed in AI. The merging knocks out pores from the distribution depending on both the pore size and the mutual position of pores; therefore, predominant depopulation of the area of small pores does not happens at merging. The prevalence of merging over the collapse in Fe melt at the strain rates above 30/ns prevents the transition to the normal distribution in this melt, and the size distribution remains exponential after the stopping of nucleation. The merging also leads to corruption of the size distribution at the late stages of stretching for all melts. Time evolution of the distribution parameters reflects the distribution widening and shows the beginning of the distribution corruption. It is shown for the case of Al melt that the general behavior of the size distribution of pores remains the same within the range of strain rates from 100/ns down to lins. In the case of Fe melt, the transition to the normal size distribution arises for the strain rates below 10/ns because of the delaying of merging. Additional simulations for solid Ni and Al at room temperature show that the statistical behavior of the pore ensemble in solid metals at high-rate stretching is very similar to that is observed in the case of metal melts. (C) 2018 Elsevier Ltd. All rights reserved.

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