Excimer Laser Processing of Crystalline Iron-Boron Alloys
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EXCIMER LASER PROCESSING OF CRYSTALLINE IRON-BORON ALLOYS
J. W. McCamy*, A. J. Pedraza* and D. H. Lowndes** Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 ** Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056
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ABSTRACT Crystalline Fe-B alloys in the concentration range of 5-25 at. pct. boron were processed by slightly (- 5%) overlapping single pulse XeCl excimer laser. The influence of the initial microstructure and phases upon the structure that results after laser processing (LP) were studied. The specimens were made by a rapid solidification procedure that permits readily glass-forming alloys to be obtained with structures ranging from fully amorphous to fully crystalline. The technique is briefly described. Two crystalline specimens of Fe-20 pct. boron, one in the as-cast condition and the other annealed, were laser treated. While the melted layer of the former was amorphous, the latter remained crystalline, exhibiting a similar phase content as before irradiation. Observations of both samples by scanning electron microscopy (SEM) prior to LP revealed a difference in particle size of one order of magnitude. A mathematical model of diffusion is developed for studying the homogenization process. It is concluded that the time for diffusion in the liquid state in the sample containing the larger particles is insufficient for homogenization to occur. Similar results regarding the influence of particle size were obtained with all the other samples. INTRODUCTION Pulsed laser processing of metals is a surface modification technique that offers a number of advantages as a result of recent technological advances in ultraviolet (UV) excimer lasers. Because the reflectivity of metals at UV wavelengths is generally low, both deeper melts (1 - 1.5 0m) and lower surface damage can be produced 111, in comparison with results obtained using pulsed visible and near-IR solid state lasers. Therefore, when a given surface is laser-scanned, harmful phase transformations in the heat-affected overlapped zones are avoided. Moreover, the quenching process is so fast (1010 K/s) that thermally activated solid state reactions are suppressed [1]. Uniform amorphous films of thickness 1.5 Pm are readily produced when samples are scanned under the excimer laser beam on a computerized stage. Studies of pulsed laser treatment of Fe-B alloys were conducted by several authors (see for instance the review by Draper and Poate [2] and references therein). Lin and Spaepen 131 were able to amorphize Fe-5 at.pct. boron thin films using a picosecond pulsed laser. In these experiments, the film was completely melted, and thus for crystallization to occur both nucleation and growth were necessary. In the experiments reported here, a much greater thickness was melted by using a nanosecond Research sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-ACO5-840R21400 with Martin Marietta Energy Systems, Inc. and in part by the Center
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