Abstract
ABSTRACTBy using a thermodynamic model of nanocrystalline alloys the grain size effect on the solubility of carbon in α-iron is calculated. More specifically the enrichment at grain boundaries is predicted to result in a solubility enhancement. An experimental setup is devised to measure carbon solubility in nanocrystalline iron powder in equilibrium with graphite. At 390 °C a solubility of 0.514 at% is determined for nanocrystalline iron with a grain size of 23 nm.
Highlights
An enhancement of solid solubility has been found in many bulk nanocrystalline materials, for instance [1, 2]
Ab initio methods based on density-functional theory (DFT) allow for highly accurate simulation of carbon solution in α-iron
Good agreement with the strain dependence of excess enthalpies computed by DFT and a modified embedded-atom method (MEAM) is observed
Summary
An enhancement of solid solubility has been found in many bulk nanocrystalline materials, for instance [1, 2]. Quantifying the impact of grain refinement on equilibrium solid solubility is important for understanding the behavior of nanocrystalline alloys upon thermal activation. In this context equilibrium shall be constrained by the assumption of a stable grain size. The scope of this work is to develop a thermodynamic model of nanocrystalline alloys It has to comprise both the nanoscale grains and the grain boundary regions, since the fraction of atoms located in those cannot be neglected. In pure iron the nanocrystalline state is retained even after prolonged annealing at 650 K [7] At this temperature and a time of 3600 s, the diffusion length of carbon in bulk iron exceeds 50 μm [8]. Since the carbon diffusivity will be larger in nanocrystalline iron, this enables chemical equilibration without strong grain growth
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