Dense
-Al
2O
3 condensates, with residual stress up to 3 GPa and ranging from nanometer to an unexpectedmicrometer size, were formed by pulsed laser ablation on Al target under oxygen background gas for a veryrapid heating and cooling effect. Analytical electron microscopic observations indicated such nanoparticlestended to coalesce over {111} facets to form multiple twin and tilt boundary. The micrometer-size particleschanged, upon electron irradiation, into metastable orthorhombic
form full of twin variants and faults parallelto {100}.