Molecular dynamics determination of the time-temperature-transformation diagram for crystallization of an undercooled liquid Ni50Al50 alloy

Molecular dynamics simulations with an embedded-atom method potential are used for the first time to determine the time temperature-transformation diagram for crystallization of stoichiometric intermetallic NiAl. This diagram shows a typical nose-like shape. It is found that the shortest time, about 1 ns, needed for the beginning of the crystallization process of the undercooled liquid Ni50Al50 alloy is at a temperature about 41% below the melting temperature and becomes longer as the temperature either increases or decreases. Analysis of the diagram demonstrates that the critical cooling rate of liquid Ni50Al50 alloy to produce an amorphous state is very high, about 10(11)-10(12) K s(-1), and is comparable with similar estimations for pure metals. These findings reveal a low glass-forming ability of the undercooled liquid Ni50Al50 alloy and suggest why there is a lack of experimental data on synthesizing of amorphous and nanocrystalline structures of this intermetallic by rapid solidification. At the same time, these findings support the possibility of the reaction synthesis of nanostructured intermetallic NiAl starting from elemental Ni and Al nanopowders which is in good agreement with recent experimental results (Moussa SO, El-Shall MS. Mater Chem Phys 2008;112:1015). (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.