Crystallization and Glass Foriation in Electron and Laser Beam Irradiated Cu-Zr Alloys
- PDF / 2,849,206 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 92 Downloads / 139 Views
CRYSTALLIZATION AND GLASS FORIATION IN ELECTRON AND LASER BEAM IRRADIATED Cu-Zr ALLOYS J. S. Huang, E. N. Kaufmann, M. A. Wall, and B. L. Olsen Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550 ABSTRACT Four Cu-Zr alloys, Cu5 6 Zr 4 4 , Cu5 0 Zr5 0 , Cu4 7Zr 5 3 , and Cu3 3 Zr 6 7 , were surface melted with electron and pulsed laser beams to compare their kinetics of nucleation, growth and glass formation. It was observed that the ease of glass formation increased in the order: Cu3 3 Zr 6 7 , Cu4 7 Zr 5 3 , Cu5 6 Zr 4 4 , and Cu5 oZr 5 o. The nucleation and regrowth produced different metastable phases. At the equiatomic composition, the preferred phase is a CsCl-type (B2) BCC structure. As the composition deviates from this, the preferred phase is either orthorhombic or tetragonal with a much larger unit cell not previously reported in the literature. The maximum growth velocity of these metastable phases was found to be about 0.025 m/s. The slow kinetics are responsible for the ease of glass formation in these systems. INTRODUCTION Surface melting of materials with electron and laser beams is a useful technique for the study of metastable phase formation and crystal-liquid interface kinetics during rapid quenching. The melting produces a thin layer of liquid which is rapidly cooled by conduction into the underlying solid. Metastable phases formed during the rapid quenching can be studied as a function of cooling rate and solidification velocity. In previous work,[1] we studied the nucleation, regrowth and glass formation in the Cu4 7 Zr 5 3 eutectic alloy by electron beam surface processing. In that case, resolidification started with the growth of a highly twinned metastable phase nucleated at the substrate. As the solidification progressed, the melting point isotherm velocity increased, eventually outpacing the crystal-liquid interface to the extent that a glass was formed in the remaining liquid. The maximum growth velocity of the crystalline phase was found to be 0.05 ± .02 m/s. When the scan speed of the electron beam was not fast enough, crystalline particles also nucleated in the liquid. In this work, we extend the study to three other Cu-Zr alloys, i.e. Cu5 0 Zr 5 0 , Cu5 6 Zr 4 4 , and Cu3 3 Zr 6 7. It should be noted that the Cu5 6 Zr 4 4 alloy has an eutectic composition, while the other two have the stoichiometric compositions of intermetallic compounds. Electron-beam surface processing was used primarily, however, when glass formation was not achieved easily, a pulsed laser beam was also used. Results reported here include the nucleation and regrowth kinetics of metastable phases in these alloys. EXPERIMENTAL PROCEDURES Ingots of the alloys were prepared by arc melting. Electron-beam surface processing was conducted with a Hamilton Standard model W2-OS electron-beam welder with beam acceleration voltage and current maintained at 80 KV and 5 mA, respectively. Beam scan speeds were 0.5, 1.0, 1.5, 2.0, and 2.5 m/sec. The focused electron beam had a diameter of 1 mm, and a single
Data Loading...
