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This study investigated the as-cast microstructure and solidification behavior of a high Al- and Nb-containing low thermal expansion IN783 alloy. By analyzing the as-cast microstructure, the quenched microstructure after soaking at high temperature and differential scanning calorimeter (DSC) results, the solidification sequence of IN783 alloy was determined as follows: L -> L + gamma -> L + gamma + beta -> gamma + beta + Laves. Segregation of Al and Nb promoted the formation of Al-enriched beta phase and Nb-enriched Laves phase, respectively. Several types of beta and Laves phase were observed, which were believed to form by different mechanisms and at different stages during solidification.