Investigation of High Strength Materials for Achieving Higher Magnetic Fields

1. Investigation of High Strength Materials for Achieving Higher Magnetic Fields Dr. Yan Xin Associate Scholar / Scientist Magnet Science & Technology, Scientific Staff NHMFL Kerry Adams, RET Shadeville Elementary School Crawfordville, FL Logan Crouch, RET Wakulla Middle School Crawfordville, FL 0.7mm Samples Sample Analysis We began examining the samples using an Olympus BX60M optical microscope. We used this to get an overall view of the samples and take images of their surfaces. Later, we used the Zeiss 1540 EsB Scanning Electron Microscope “SEM” to get a closer and clearer look at the samples. We also used the SEM to obtain a composition analysis of the samples so we could identify what percentage of Cu and Ag the samples actually consisted of. After all the images were obtained that we needed from the microscopes, we used a Instron Wilson Tukon 2100 Micro-Indentation machine to test the hardness of the samples so that we could compare the strengths of the CuAg at 0.7mm, prior to rolling, and 0.1mm after the cold-rolling had been completed. (see figures 5-10) Introduction Copper(Cu) is often used in electromagnetism because of it’s high rate of conductivity. Cu is a metal, therefore is malleable. But this has it’s limitations. Composite metals pose a solution to this issue. We explored a sample of a composite that consisted of Copper and Silver(Ag). This material is useful for high field magnets because this composite demonstrates high strength under excessive amounts of stress which are present in the presence of high magnetic fields. Cu and Ag both have a face-centered cubic structure that allows them to fit together like puzzle pieces. This characteristic aids the separate metallic elements in forming a stronger composite. The image to the left represents the horizontally-rolled (parallel to the RD) sample magnified 5000x by the SEM and the image to the right is the same sample seen at 10,000x. Notice the eutectic component areas contrasted by the copper-rich areas. These images are of the vertically-rolled (perpendicular to the RD) samples. The left seen at 5000x magnification and the right at 10,000x. Experimentals The Cu Ag materials were received as-is in a “hot-rolled” condition. Then cold-rolling was performed in a roller, using type 304 1.43mm thick stainless steel plates. Rolling direction “RD” was extremely important to remember because the material is anisotropic. Two samples were included in the puck; both perpendicular and parallel to the rolling direction. The original thickness of the Cu Ag materials were approximately 0.7mm. By the end of the rolling process, we reached a thicknesses of approximately 0.1mm, equaling a total of 86% strain (thickness reduction). For both the ~0.7mm and ~0.1mm samples, we performed a test using an Instron Wilson Tukon Micro-Indentation machine to test the hardness of each sample, respectively. Figure 6 (below): Energy dispersive x-ray spectrum in the SEM shows the elemental components of the sample being Cu and Ag, with 85wt%Cu, and 15wt%Ag. Figure 5 (below): Jeol JSM-6380 Scanning Electron Microscope 0.1mm Samples Preparation: Step 1. The original samples were received as either rolled, flat sheets or cast samples. Small pieces were then cut from each using a Buehler Isomet Low Speed Saw. (see figure 1) Figure 7: Instron Wilson Tukon 2100 Micro-Indentation Microscope. This uses a very small diamond tip to create indentations (see Figure 8) in a sample to measure hardness. The figures shown here were viewed using a EBSDdetector for easier viewing of the eutectic components of the composite metal in the SEM. These images depict the parallel to the RD view of the samples and shear bands (circled in red) are present because of the rolling process and are typical in the parallel regions. Figure 1: Buehler Isomet Low Speed Saw: This saw was used to make precision cuts of the sample to fit it to the proper size for making the “pucks.” Shear Bands Closer view of Shear Band Step 2.Once we had the pieces necessary to complete our first sample, we prepared to create a polymer “puck” using aBuehler Simplimet 3 Mounting Press. A combination of heat and pressure transforms the powder into a solid, encompassing the samples to hold them in place, thus forming the “puck”. The “pucks” are used for easy analysis of samples. (see figure 2) These sample images were taken from the area perpendicular to the RD. Notice that shear bands are not shown in these viewing directions. Ag eutecitc phase were deformed into long thin ribbons. Figure 2: Buehler Simplimet 3 Mounting Press: An instrument that uses polymer compounds to create a mold or “puck” around the sample for easier analysis and handling. Figure 8 (left): The Micro-Indentation Microscope measures the hardness of samples using diagonal measurements of indentations such as these. *This particular sample shows “slips” (circled) or ripples that formed during the indentation process on the top left side of the indentation. Step 3: After creation of the pucks, we had to grind the outer layers of the polymer away from the surface using a Struers Rotopol-2 Grinder. We placedvarious grades of sandpaper on a rotating platform that spun, slowly wearing the surface down exposing the CuAg samples clearly. (see figure 3) Summary Over the course of our work we successfully fabricated Cu0.85Ag0.15 composite sheets with increased strength, with average hardnesses of 2636.2 MPa along parallel rolling direction, and 2658.3 MPa perpendicular to the rolling direction. The strength of the composite increased about 10% after cold rolled with 86% thickness reduction. The materials are found to be anisotropic in two directions. Step 4: The final step before examination of the samples is to place them into the Buehler Vibromet 2 Vibratory Polisher. The purpose of this machine is to polish the surface of the samples using a mix of 0.25 alumina oxide powder and deionized water so that there is a clean surface to view the samples under the microscopes. (see figure 4) Eutectic Areas Figure 9: A cast sample image of the CuAg composite taken by the optical microscope. Figure 10: The same cast sample as seen in Figure 9 viewed by the SEM microscope. Acknowledgements Figure 3 (above): Struers Rotopol-2 Grinder: After the pucks were made, the polymer needed to be ground down to expose the surface of the Cu Ag sample. We started with a rough-grade sandpaper to remove the outer layer of the puck, then finished it with a finer-grade sandpaper to eliminate any large scratches on the surface. We appreciate all the help and support we received during this project. We are grateful for the patience and guidance of our mentor, Dr. Yan Xin. We thank the P.I. of the research project we participated in, Dr. Ke Han, for sharing his knowledge and encouraging us during our experimental processes. We also are appreciative to Kejing, a student working with Dr. Xin, for her expertise in the use of photoshop, Bill Starch for assisting us with the rolling, and Dr. Jun Lu for helping us heat our samples in his lab. We acknowledge the financial support from the NSF programs Research Experiences for Undergraduates and Research Experiences for Teachers hosted by the Center for Integrating Research and Learning (Dir. P. Dixon) at NHMFL. This work was supported by the NSF Cooperative Agreement Grant No. DMR-0654118 and No. NHMFL UCGP 5059, NSF grant No. DMR-0645408 and the Alfred P. Sloan Foundation. Figure 4 (above): Buehler Vibromet 2 Vibratory Polisher: The white circular stage contains the ionized water and alumina oxide solution that is “vibrated” by the machine. The samples were placed in the machine for 5-6 hours in order to prepare the samples’ surface for examination. Sample Observations In the microscope images, eutectic components in the samples are visible in the lighter regions. These areas are where two solids formed one liquid with Ag being the dominate metal of these regions. The darker areas are more Cu rich, noting extensively less amounts of Ag.