Alpha-to-gamma reverse phase transformation of molten removal in alumina ceramics laser processing

Generally speaking, in the melting process of alumina ceramics, all the metastable transitional phase will be gradually transformed into the stable α phase with rising temperature due to an irreversible lattice reconstruction. However, under the action of pulsed laser cutting, the lattice structure of α-alumina needs to withstand the great stress from instantaneous high energy impact. Thus, a slip dislocation of 1/4 at. diameter is generated on the atomic plane of each second basal layer to produce γ-alumina. In this work, a ceramic plate comprised of α-alumina is taken as the object of study. The α-to-γ reverse phase transformation of the molten removal is verified by the observed microstructure from scanning electron microscope (SEM) and transmission electron microscope (TEM). Simultaneously, the X-ray diffraction (XRD) investigation shows that the basis of molten removal after laser cutting is still α-alumina. The micrographs of octahedral corners or hexagons are shown in different observation directions. There is no γ-alumina phase transformation product in the spherical granular removal at each vapor-to-melt ratio (rvmr). In contrast, γ-alumina is detected in the adhering slag with a micro-morphology of cubic phase. The mass fraction is increased with the rising of vapor-to-melt ratio. At the same time, θ-alumina also exists in some spattering removal as intermediate product between α and γ phase. It is indicated that the generation of α-to-γ reverse phase transformation occurs mainly in the melting process of temperature rise, and the maintenance of phase transformation results mainly occurs during cooling process of condensation.