1. 1. FURI Proposal for Fall 2015 Spring 2016 Investigating the Formation of Nanostructures in Alloys Produced by a High-Energy Ball Milling at Cryogenic Temperature Introduction The development of high performance vehicles in the automotive and aerospace industries requires new, lightweight, inexpensive materials that do not sacrifice performance. To this end, research and development of nanocrystalline metal alloys has gained high interest due to the enhanced mechanical properties made available with grain size reduction combined with the development of large scale material processing techniques such as ball milling of fine powders. It has been found that nanocrystalline pure metals such as Al, Sn, Pb, Zn, and Mg exhibit extensive grain growth at room temperatures. Similarly, pure metals with higher melting points such as Co, Ni, and Fe, show growth at a relatively low temperature range (220-450C) [1], [2]. Thus the applications for pure, nanocrystalline metals are very limited, and a nanocrystalline alloy that is resistant to grain growth at a higher temperature range is required for practical application of these alloys. Using a ball mill to combine metallic powders is a process called mechanical alloying. Ball milling produces nanostructures by structural decomposition of coarser-grained structures as the result of severe deformation of powders [3]. Mechanical alloying is the process of cold welding amorphous, nanocrystalline, and/or intermetallic powders to synthesize nanostructures at high energy levels [4]. By milling at liquid-nitrogen temperatures, cold deformation is achieved and grain refinement during the milling process is dominated by the total lattice microstrain. Mechanical alloying of ductile metals is therefore performed in a ball mill at cryogenic temperatures to reduce the grain size of the powders [3]. Milled alloys are typically harder than their coarse grained counterparts, but the nanocrystalline grain boundaries are unstable and tend to grow from the nano regime (1 m) size scales. Atwater and Darling [1] found that additions of certain solutes can pin grain boundaries to prevent growth under high temperature or dynamic loading. In fact, Atwater et al. [5] further investigated the thermal stability of a Cu-Ta alloy and found that the alloy is stable at up to 600 C. In this FURI project, other nanocrystalline alloys will be analyzed through the modification of a standard SPEX ball mill to incorporate cryogenic capability. Specimens with composition determined by the findings of Atwater and Darling [1] will be manufactured using the newly developed cryogenic ball mill and will be heated to elevated temperatures and analyzed using transmission electron microscopy (TEM) to investigate the nanostructures of the alloys produced. Objective The primary objective of this research is to investigate the formation of nanostructures in alloys produced by high-energy ball milling at cryogenic temperature. Approach The design of the cryomill will be based on the specifications of the cryomill at the Army Research Laboratory at the Aberdeen Proving Grounds in Maryland. Alloy powder, Teflon tubing, and a vacuum pump will be purchased and combined with an existing SPEX mixer/mill to incorporate cryogenic capability. Once the cryomill is constructed, specimens will be synthesized based on the findings of Atwater and Darling [1] through the combinations of various alloy powders.
2. 2. The cryomill works on the principle of cold welding and fracture through impact, a metal vial will be filled with multiple stainless steel ball bearings (0.25 inch diameter) then the measured amounts of each powder for the alloy will be added to the vial. The metal vial will be sealed under argon to reduce oxide formation, and then the metal vial will be placed into a larger polymer vial designed to allow liquid nitrogen to flow around the metal vial. The vial and the cylinder will be rigorously vibrated such that the ball bearings will pulverize the powder and cold weld the particles so that they become a homogeneous alloy. Figure 1. a) SolidWorks design showing the polymer vial that will house the b) smaller steel vial. Once the nanostructures are synthesized, the powder will be examined using TEM both before and after thermal stabilization tests. The images produced from TEM will show the grain size of the alloy at the nano level. The results from TEM will show the microstructural change, including grain growth, resulting from thermal stabilization tests. Thermal stabilization tests will subject the synthesized powders to elevated temperatures typical for inducing grain growth in the pure nanocrystalline counter-part of the alloy. If the grain sizes of the nano structures are resistant to growth at the elevated temperature, that alloy will be further examined in future work to analyze the effectiveness of the alloy as a structural material. Expected outcomes The purpose of this project is to investigate the stability of multiple nanocrystalline alloy systems. We will synthesize different alloys chosen through the work of Atwater and Darling [1] by way of mechanical alloying. Through this we hope to determine several microstructurally stable nanocrystalline alloys for future structural testing. This research will provide more efficient materials for use in machinery such as automobiles and aircraft. Alloys found to exhibit minimal grain growth will offer the enhanced mechanical behavior of nanocrystalline samples while maintaining the microstructural stability of coarse grained materials thereby enhancing the vehicle performance including enhanced mission capability and lower engine emissions as a result of the lower mass required from the structure, without sacrificing strength or safety. Impact In keeping with the Ira A. Fulton School of Engineering research themes of sustainability, energy, and security, developing samples from high- energy ball milling at a cryogenic state will result in development of enhanced materials capable of replacing existing, heavier structures. Lightweight vehicles are more efficient which translates to lower engine emissions thereby reducing the impact of engine byproducts on the environment. The enhanced efficiency also suggests lower fuel usage despite the growing need for rapid transportation. Lighter vehicle structures also translates to enhanced performance especially in the aerospace industry where aircraft ceiling, maneuverability, and payload capacity are limited by the need to overcome the weight of the aircraft. Enhanced performance provides a significant advantage to defense forces ensuring national security. This research will open the door to the development of new alloys which will lead to enhancement of current technologies.
3. 3. References [1] M. Atwater and K. Darling, A Visual Library of Stability in Binary Metallic Systems: The Stabilization of Nanocrystalline Grain Size by Solute Addition: Part 1, Army Research Laboratory, Aberdeen Proving Ground, MD, Final ARL-TR-6007, 2012. [2] U. Klement, U. Erb, A. M. El-Sherik, and K. T. Aust, Thermal stability of nanocrystalline Ni, Mater. Sci. Eng. A, vol. 203, no. 12, pp. 177186, Nov. 1995. [3] F. Zhou, D. Witkin, S. R. Nutt, and E. J. Lavernia, Formation of nanostructure in Al produced by a low-energy ball milling at cryogenic temperature, Mater. Sci. Eng. A, vol. 375377, pp. 917921, Jul. 2004. [4] D. L. Zhang, Processing of advanced materials using high-energy mechanical milling, Prog. Mater. Sci., vol. 49, no. 34, pp. 537560, 2004. [5] M. A. Atwater, D. Roy, K. A. Darling, B. G. Butler, R. O. Scattergood, and C. C. Koch, The thermal stability of nanocrystalline copper cryogenically milled with tungsten, Mater. Sci. Eng. A, vol. 558, pp. 226233, Dec. 2012. PERSONAL STATEMENT: Motivation My motivation for the success for this research derives from my hobby of constructing longboards from scratch. I truly enjoy my time not only researching and developing new materials, but also bringing ideas from paper to the physical world. Through the experience gained from working in Dr. Solankis lab, I hope to improve my current knowledge of structures and materials while making a scientific impact that may one day benefit society. Through my time as a research assistant, I hope to leverage these skills that I have picked up to benefit me in industry post-graduation. Career Planning As an aspiring aerospace engineer, my goal is to pick up the valuable research skills only obtainable through first-hand experience. With my experience in one hand and my tenacious mind set in the other, this project will be on the first stepping stones for my journey to enter the aerospace industry. Coursework This semester I began to take specialized aerospace and materials courses and have begun to truly enjoy the material. I found that learning a topic both in the laboratory and classroom truly improved my learning experience and helped me understand and retain the knowledge. This opportunity will further my craving in gaining knowledge that will put me above my peers and competitors. Work Experience After working as a research assistant, an engineering Community Assistant, and balancing FURI, I have picked up the critical time management skills required to succeed in and out of the laboratory. For this project I will be relying on several different experiences, from constructing longboards, to my current research experience in tensile testing, to my fabrication experience, and finally my pure ingenuity. I will use these tools to construct and develop the cryomill, synthesize nanostructures, and run thermal stability tests. With the success of this project I will gain new grounds in the field of high energy ball-milling at cryogenic state propelling me that much further towards my aspirations Summary This project really interests me because of the massive fabrication and development processes needed in order to produce quality nanostructures. I am eager to face and solve challenges in the lab when constructing, synthesizing, and testing materials. It soothes my mind when I am in an engulfed state of mind solving a problem, assembling parts, or running tests awhile waiting for the unknown outcome. My sheer passion and mental tenacity will lead to this projects success, and my experience gained from this project will project me further into my journey in becoming a true and successful engineer.
4. 4. Timeline: Tasks Timeline (week) 2 4 6 8 9 10 12 14 16 18 20 22 24 26 28 30 Preliminary literature review Cryomill setup Synthesizing Nanostructures Thermal Stability Tests TEM analysis Data analysis Report and presentation