EFFECT OF LASER REPETITION RATE ON THE MELTING AND ABLATION OF NI24ZR76 ALLOY RIBBON

An amorphous Ni24Zr76 metallic ribbon 35 mu m thick, prepared by planar flow casting, was furnace recrystallized and subsequently laser annealed in a vacuum of 2 x 10(-3) Pa. The fluences of the laser pulses were 1.4 and 1.9 J cm(-2), the number of pulses directed to the same irradiation site was between 1 and 300. The repetition rate of the laser pulses varied between 5 and 100 Hz. The temperature evolution in the irradiated samples in one- and multipulse regimes were numerically calculated. The samples were analysed by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy and differential scanning calorimetry. A strong increase of the laser damage level, manifested by melting and ablation with the increase of the pulse repetition rate, was observed. This effect is attributed to the heat accumulation in the sample at higher pulse repetition rate. Additional effects, such as roughening of the sample surface, increase of the optical absorption, decrease of the thermal diffusivity, and decrease of the thickness of irradiated sample can also amplify this effect.