Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti3AlC2 powder is prepared in a very mild condition via Ti3Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti3AlC2 can be obtained at 900 degrees C with the 1:1 salt-to-material ratio. The formation mechanism of Ti3AlC2 through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti3Al and C -> TiAl and TiC -> Ti2AlC and TiC -> psi and TiC -> Ti5Al2C3 and TiC -> Ti3AlC2, where the key psi, a modulated Ti2AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti2AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti5Al2C3 through topotactic transition, followed by Ti5Al2C3 converting into Ti3AlC2 by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti2AlC (0 0 0 2)//psi (0 0 0 1), psi (0 0 0 1)//Ti5Al2C3 (0 0 0 3), Ti5Al2C3 (0 0 0 3)//Ti3AlC2 (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.