Metastable phases formed in the combustion synthesis of Al/Ni and TiO2/Al/C

The self-propagating high-temperature synthesis process has significantly improved the production efficiency of various advanced materials due to its short processing time and low power consumption. The high combustion temperature, however, can alter the crystal structure of the synthesized product, including the generation of the metastable phase. This work investigated the microstructural change generated by the high-temperature reaction during the combustion synthesis of Ni/Al and TiO₂/Al/C in various compositions. Induction heating was used to ignite the combustion process of the compressed mixture under an Argon gas. The temperature profile of the reaction was measured using an infrared thermometer and pyrometer. The X-ray diffraction test was used to identify the product's phases. ImageJ supported the Scanning electron microscopy image to identify the synthesized products. The results show that combustion synthesis of the Ni/Al and TiO₂/Al/C generates AlNi, TiC, and Al₂O₃ as the primary phase, Al steinhardtite and C carbon as unreacted products, and the metastable phase comprising C lonsdaleite and C diamond. High temperature and pressure generated by the second reaction are responsible for developing Al steinhardtite, C lonsdaleite, and C diamond. This work delivers a more comprehensive understanding of the products generated during the synthesis of Ni/Al and TiO₂/Al/C while acknowledging the presence of unreacted and metastable phases.