Rapid solidification and excimer laser processing studies in iron-based alloys

Pure iron was irradiated using ultraviolet pulsed excimer laser wavelengths of 249nm and 308ntn, in order to study the effects of extremely high heating and cooling rates on the phase transformation sequence and upon the specimen surface. The thickness of the molten layer was found to be a function of the laser power output, reaching 2 a maximum at 1.5μm for 4.5J/cm². Surface damage in the form of the ripples also increased with the laser output power and became very 2 pronounced between 3.5 and 4.7J/cm², the maximum energy density used in the experiments. The optimum energy density, which gives uniform 2 melting with the least surface damage, was determined to be 3.0J/cm and was used for all the subsequent experiments.

A surface sensitive X-ray diffraction (SSXD) technique was developed for studying structural changes in the near surface region (~lym) and was tested on an Fe₈₀B₂₀ alloy which is a ready glass former. oU ZU Using the SSXD technique, the thickness of the amorphous layer formed on the surfaces of alloys by laser irradiation treatment was measured precisely. It was determined that the ability of an alloy to amorphize is strongly dependent on the starting microstructure. Fe₃B metastable phase formed during splat quenching was identified and its role in the amorphization process was analysed. It was concluded that fine microstructure and the presence of the Fe₃B phase increase the glass forming ability of the Fe₈₀B₂₀ alloy system under pulsed laser irradiation.

Fe-Zr alloys with 5, 10, 12 and 15 at% Zr were processed with the 'hammer and anvil' apparatus. With sample weights of 40 mg or less. the 10 and 12 at%Zr alloys amorphize on splat quenching, while 5 and 15 at%Zr remain crystalline independently of the sample weight. With sample weight of 80 mg, all four alloys render crystalline, though metastable, phases upon splat quenching. Ferrite and Fe₂Zr (an intermetallic that is metastable at the compositions used) were identified as the phases present in all of the as splat quenched crystalline samples, using the X-ray diffraction method. In samples that were partially amorphous, three different crystalline phases were identified by TEM and STEM as being embedded in the amorphous matrix.

Pulsed laser irradiation of as splat quenched crystalline samples gives amorphous structure in the composition range of 10 and 15 at% Zr.