In-situ transmission electron microscopy investigation of surface-oxide, stress-relief mechanisms during melting of sub-micrometer Al–Si alloy particles

In-situ heating in a transmission electron microscope was used to investigate the stress-relief mechanisms occurring in the 3–4 nm thick aluminum-oxide films present on the surfaces of submicron Al–11.6% Si powder particles during melting. Experimental observations show that the aluminum-oxide shell undergoes creep to relieve the high stress generated by the expanding liquid during melting. Surface oxides that contain defects do not undergo uniform creep and are prone to cracking, which causes ejection of some of the pressurized liquid inside the particles through the crack. Calculations were performed to estimate the stresses generated on the aluminum-oxide shell by the expanding liquid and the possible effect of the stress generated by the confining oxide shell on the melting temperature of the Al–Si alloy. The calculations show that the oxide film creeps to avoid the high tangential stress (on the order of 15 GPa) produced by the expanding liquid and that this creep mechanism prevents the oxide from markedly changing the melting behavior of the Al–Si particles.