Masters Theses

Date of Award

12-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Antonio J. Pedraza

Committee Members

D. H. Lowndes, B. F. Oliver

Abstract

Samples of crystalline iron-boron alloys containing 5-30 at%B were processed by irradiating them with slightly overlapping single pulses from a XeCl excimer laser to study the extent to which amorphization occurs at extremely high heating and cooling rates. The influence of the initial microstructure and phases upon the structure that results after laser processing was also studied.

A surface sensitive x-ray diffraction (SSXD) technique was developed to study the structural changes in the modified near-surface region. With this technique both the melt depth and the volume fraction of crystalline material present in the amorphous layer could be determined. The layer was further characterized using a grazing angle diffraction geometry.

The specimens were prepared using the hammer and anvil method of splat quenching. It was found that the inital microstructure could be determined by controlling the sample weight. An amorphous structure was obtained with sample weights less than 0.04 grams, while fully crystalline structures were produced when the sample weight was greater than 0.08 grams.

Amorphization was observed when as-quenched Fe-B alloys with 10-30 at%B were processed with the excimer laser. However, only those samples with 15-20 at%B showed complete amorphization; all other compositions showed only partial amorphization. Processing of as-quenched Fe-5at%B alloys produced a bcc phase with a lattice parameter 0.9% less than that of pure iron.

Amorphization was not observed in Fe-B samples which were annealed prior to laser processing. Comparision of the microstructures showed that the grain size of the annealed material was an order of a magnitude larger than that found in the as-quenched material. An analytical diffusion model was developed which showed that the initial microstructure plays a fundamental role during pulsed laser processing inasmuch as compositional inhomogeneities that may remain in the liquid determine the resulting structure. These inhomogeneities occur because the time of diffusion in the liquid state in samples with large grain sizes is insufficient for homogenization to occur. The experimental evidence, together with the developed model, gives 5.3 at%B as the lower limit for amorphization. Extrapolation of experimental data yields an upper limit of 40 at%B.

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