Delineation of the reaction mechanism of Ti2AlC MAX-phase formation during spark plasma sintering synthesis

Despite several attempts to synthesize Ti₂AlC MAX-phase using different processes and input materials, a comprehensive understanding of its formation mechanism is still lacking. This study reports, for the first time, a novel reaction mechanism of Ti₂AlC MAX-phase formation, based on extensive microstructural characterizations and density functional theory (DFT)-based ab initio calculations. A systematic investigation was carried out up to 1400°C temperature to understand the phase transformation behavior in stoichiometric 2Ti:Al:C elemental powder mixture during spark plasma synthesis of Ti₂AlC MAX-phase. An integrated approach was adopted to establish correlations between results obtained through differential scanning calorimetry, x-ray diffraction, scanning electron microscopy–energy-dispersive spectroscopy–electron backscattered diffraction characterizations, and DFT calculations. Microstructural examination revealed that the formation of Ti₂AlC MAX-phase was preceded by successive transformation of Ti powder particles via a series of transient Ti–Al intermetallic formation between 660 and 1200°C, until the Ti particles get transformed into a core–shell structure of Ti₂Al surrounded by TiAl intermetallic. The formation of Ti₂AlC MAX-phase was observed to occur via two different reactions. Initially, the peripheral TiAl intermetallic reacts with TiC to form Ti₂AlC MAX-phase in the temperature range of 1000–1200°C. At a later stage, the inner Ti₂Al phase transforms to Ti₂AlC MAX-phase through inward carbon diffusion till 1350°C.