HIGH-SPEED TRANSMISSION ELECTRON-MICROSCOPY OF LASER QUENCHING

A time-resolving transmission electron microscope was realized by combining a laser-pulsed thermionic electron gun, fast beam switching, image intensification and electronic storage with electron microscopy and selected area diffraction. Polycrystalline free-standing iron and titanium films were laser quenched within the microscope with 20 ns pulses from a Q-switched Nd:YAG laser. Heating and cooling rates were 10(11) K s(-1) and 3 x 10(8) K s(-1) respectively. The laser-quenched textures consist of a thinned area surrounded by a thick ring. They are generated by the following processes: (1)melting during the laser pulse within 20 ns; (2) oscillatory centrifugal now of the liquid, setting in during fusion and lasting 0.5-1 mu s; (3) solidification occurring 2-3 mu s after the laser pulse within 300 ns and giving micron-sized crystal plates; (4) bending and breaking of the plates within several microseconds after solidification.