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M: Metallphysik

M 32: Phasenumwandlungen II

M 32.1: Fachvortrag

Thursday, March 29, 2001, 11:45–12:00, S12

Nanosecond Transmission Electron Microscopy of Laser-Pulsed Chromium Films — •Holger Dömer and Oleg Bostanjoglo — Optisches Institut der TU Berlin, Straße des 17. Juni 135, 10623 Berlin

Melting, evaporation and crystallization, as induced by a laser pulse (20 µm focal diameter, 6 ns) in Cr films, were tracked by short-exposure time (7 ns) electron microscopy and microdiffraction. During the heating laser pulse the crystal lattice constant grows by ≈ 3 % within 10 ns, and linear defects appear prior to melting. A crimped melt is produced. About 500 ns after the laser pulse the lattice constant of heated but not-molten peripheral and of precipitated crystals discontinuously reduces by ≈ 2 % within 100 ns. Simultaneously, the freezing melt is partly detached, splashing drops, and the left solid film disrupted. It is believed that the accelerations of ≈ 10⁸ m/s², resulting from the observed fast lattice deformation cause the reported mechanical damage in laser machined Cr films, and not such mechanisms as thermal stress, brittle-to-ductile transition, or capillary instability. The discontinuous lattice deformation indicates an incomplete bcc-fcc transition during the fast heating and cooling.