Characterization of Laser Deposited Niobium and Molybdenum Silicides
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Sandia National Laboratory in the early 90's. This technology utilizes a Nd:YAG laser to fuse metallic powders into fully dense parts. Computerized motion controls allows extremely complex shapes to be deposited directly from a CAD computer file. Prealloyed or elemental blended powders can be used for feedstock. Prealloyed powders have the benefit of having a constant composition, thus producing a very homogeneous deposit. These powders, however, are not readily available and can be very expensive to make. Elemental powders, while being easier to procure, can produce segregated deposits that are not fully alloyed. The goal of this work is to produce in-situ alloys of structural silicide composites from elemental powder blends utilizing LENSTM technology. EXPERIMENTAL Several cylinders of each composition were deposited using the Optomec LENS machine. One alloy was an elemental powder blend consisting of 70 atomic percent molybdenum, 10 atomic percent silicon, and 20 atomic percent boron The second alloy was an elemental powder blend consisting of 65 atomic percent niobium and 35 atomic percent
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Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
Figure 1: Backscattered electron micrographs of laser deposited molybdenum-boron-silicon.
silicon. These powders were obtained from Alfa Aesar and had a size distribution of-60 to +325 mesh. The powders were combined in a plastic bottle and mixed thoroughly before being loaded into the powder feeder. The LENSTM unit was then used to fuse these powders into small samples, roughly 1/2 inch in diameter and 3/8 inch high. Approximately 400 watts output power from the laser was used to achieve full density in the samples. The deposited cylinders were then cross-sectioned and samples were mounted in conductive resin for examination in the scanning electron microscope (SEM). A Philips XL-30 FEG SEM was used to perform the analysis. Transmission electron microscope (TEM) foils were made by EDM core drilling through the cross-section of the sample. These cross-sectional cores was then sliced into thin sections, dimple ground and thinned using an ion mill. The analysis was done on a Philips CM200 microscope with an EDAX energy dispersive spectroscopic (EDS) attachment. X-ray diffraction (XRD) analysis was performed on a Scintag-PAD-V X-ray diffraction unit.
Figure 2: Backscattered electron micrographs of laser deposited niobium-silicon.
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Figure 3: X-ray diffraction scans of niobium-silicon alloy (left) and molybnenum-boron silicon alloy (right).
RESULTS AND DISCUSSION The equilibrium ternary phase diagram for the Mo-Si-B system predicts three phases should be present, Mo, Mo 5SiB 2, and Mo 2B, at the given composition [5]. Cross-sectional backscattered electron SEM images of the as-deposited Mo-Si-B samples (Figure 1) show that only two phas
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