| Grain Size Limit of Nanocrystalline Materials Obtained by Annealing Bulk Metallic Glasses |
Arun Pratap, Ashmi T. Patel, Heena R. Shevde and Kirit N. Lad
pages 219-221
DOI:10.1080/0371750X.2013.772734 |
| Abstract |
Polycrystalline materials with nanometer sized grains, termed nanocrystalline materials, can be formed by crystallizing completely amorphous solids under proper heat treatment conditions termed as ‘annealing’. Experimentally it is found that the smallest grain size is obtained when the glasses are annealed at a crystallization temperature which is nearly half of the melting temperature in metallic glasses. At this temperature the Gibbs free energy difference between the amorphous and crystalline state is maximum. However, in case of bulk metallic glasses (BMG), the maximum in Gibbs free energy, G, is observed to fall below Tm/2, because the Kauzamann temperature (TK), at which G is maximum falls below Tm/2. Therefore, annealing BMG’s at temperatures close to their TK is likely to lead to the smallest grain size. In this paper, we have calculated the minimum grain size for different metallic glasses by considering a simple thermodynamic relation that suggests a general thermodynamic lower limit of grain size of metallic nanocrystalline materials. Two different G relations are used for calculations. The grain size is also calculated using the experimental values of G. The results show that the values of grain size calculated by using theoretical G are in reasonable agreement with those calculated using experimental G.
[Keywords: Grain size, Nanocrystalline materials, Gibbs free energy, Bulk metallic glasses] |
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