An in-depth understanding of thermal behavior and phase evolution is required to apply heterostructured nanowires (NWs) in real devices. The intermediate status during the vaporization process of InAs NWs in an Al₂O₃ shell was studied by conducting quenching during in situ heating experiments, using a transmission electron microscope. The formation of As clusters in the amorphous Al₂O₃ shell was confirmed by analyzing the high-angle annular dark field images and energy-dispersive X-ray spectra. The As clusters existed independently in the shell and were also observed at the end of the InAs pieces obtained after quenching. The formation process of the As clusters was demonstrated from a theoretical perspective. Moreover, an ab initio molecular dynamics simulation (AIMD) was conducted to study the atomic and molecular behaviors.