KINETICS OF LASER-INDUCED PHASE-TRANSITIONS IN NI-AL ALLOYS

Time-resolved transmission electron microscopy (TEM) was applied to study phase transitions in free-standing Ni0.75Al0.25 films, induced by pulses from an Nd:YAG laser (20 ns, 532 nm). At fluences above 0.23 J/cm2 melting occurred within the laser pulse, the liquid was set into violent motion during 300 ns after melting, and the film solidified 1-2 mus later. The liquid motion was triggered by expansion of the melting film. Solidification proceeded by heterogeneous nucleation of crystals at the periphery of the laser molten spot and by centripetal growth of large plates with velocities 0.9-2.4 m/s, from which a supercooling of 2-20 K at the liquid/crystal interface is inferred. The final texture consisted of large gamma and gamma' crystals plus fine-grained Al precipitates. The dynamics were either continuously traced or intermediate stages were visualized by double-frame TEM with exposure times of some 10 ns.