Molecular dynamics simulation of alloying in an Al-coated Ti nanoparticle

The alloying reaction of an Al-coated Ti nanoparticle having equi-atomic fractions and a diameter of about 4.8 nm is studied using molecular dynamics simulation in combination with an embedded atom method potential. We demonstrate that the Al-coated Ti nanoparticle is much less reactive than the Ti-coated Al nanoparticle of similar size studied previously (E.V. Levchenko et al., Intermetallics 22 (2012) 193). The reason for this is that the pre-alloying in the vicinity of the interface at moderate temperatures is noticeably more pronounced in the Al-coated Ti nanoparticle. It comes about because of the distorted structure of a rather thin Al shell in the vicinity of the interface with a quite high level of short-range icosahedral order. The distorted structure is a consequence of the influence of the interface between the Ti-core and Al-shell. In the previously studied Ti-coated Al nanoparticle the interface influence was not so pronounced because the larger Ti atoms formed a greater strength shell to confine the Al core. Such a nanoparticle morphology resulted in avoidance of the strong distortions in both core and shell structures. The distorted shell structure of the Al-coated Ti nanoparticle enhances the diffusion mobility of atomic components in the Al shell in the vicinity of the interface at intermediate temperatures, thereby promoting the pre-alloying process. The pre-alloying eventually results in development of a well defined f.c.c. crystal structure in the Al-based shell at high temperatures which has good alignment of the close-packed orientation with the h.c.p. Ti-based core. As a result, the pre-alloyed interfacial layer and improved crystal structure of the Al-based shell serve as an effective reaction barrier in the Al-coated Ti nanoparticle. This slows down the reaction by extending the temperature range of the solid-state interdiffusion process in the Al-coated Ti nanoparticle.