Oxide Phase Formation in Excimer Laser Processed Ti Alloy
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OXIDE PHASE FORMATION IN EXCIMER LASER PROCESSED TI ALLOY T. R. Jervisa, K. M. Hubbardb, and T. G. Zoccob, a. Center for Materials Science, b. Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM ABSTRACT Excimer laser surface processing of Ti-6A1-4V results in substantial surface oxygen incorporation as measured by the narrow resonance in the elastic scattering of a particles at 3.05 MeV. Single pulse processing at 1.0 J-cm-2 results in transformation from the original a + p grain structure to a martensite structure. Multiple pulse processing results in oxygen incorporation proportional to the number of pulses during processing, but this is primarily reflected in the depth of the oxygen containing layer. A maximum oxygen to titanium ratio of 1.0 was observed in the surface layer. This was confirmed by transmission electron microscopy (TEM) which showed the development of a fine-grained equiaxed TiO surface layer from a phase mixture of Ti alloy with TiO precipitates. Concentration vs. depth profiles show that this layer is approximately 400 nm thick after 40 pulses. Further processing with a total of 60 pulses results in coarsening of the grains and an increase in the thickness of the layer. The TiO precipitate concentration increases gradually, becoming apparent in electron diffraction patterns after 30 pulses. No other oxide phases are observed, the TiO developing directly from the metallic phases. In particular, we do not observe the formation of Ti2 0 prior to the formation of the TiO, or the formation of Ti0 2 . INTRODUCTION Titanium alloys, in particular Ti-6A1-4V, are useful engineering alloys in a number of applications. However, their wear and corrosion properties restrict their use in extreme environments such as those encountered in space or surgical implant applications. Surface processing by ion implantation and mixing has been successful in improving the mechanical properties of the alloy. Laser surface processing also has the potential for improving the properties of the alloy through microstructural changes or surface alloying. Previous work has concentrated on laser nitriding [1], production of dispersoids [2], and surface finish [3] using lasers operating in the infra red (IR) portion of the spectrum. Excimer laser processing, using light in the ultra-violet (248 nm wavelength), offers a rapid and efficient alternative to IR techniques for surface processing of Ti. We have demonstrated the formation of an amorphous B-Ti alloy layer by excimer alloying[4]. In a previous publication[5] we reported initial results on the effect of excimer laser surface processing on the properties and microstructure of Ti-6A1-4V. In that work we observed an initial transformation to martensite (Widmanstatten structure) with single pulse processing and no further microstructural change up to 10 pulses of laser energy. We also compared the effect of laser processing and different polishing techniques on the surface hardness of the alloy,. We found that the transformation to martens
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