MSE Colloqium: Julia Mundy, Ferroic engineering of atomic layers to create a room-temperature multiferroic
Transition metal oxides exhibit almost every physical state known including metallic conductivity, (high-temperature) superconductivity, colossal magnetoresistance, photoconductivity, ferroelectricity, and ferromagnetism. Key to both harnessing these exotic phases in device applications and further materials discovery is developing an atomic-resolution understanding of the structural, electronic and magnetic properties. Here I will show how analytical electron microscopy and spectroscopy, in conjunction with advanced thin film deposition, can be used to engineer new materials including a magnetoelectric multiferroic superlattice where ferroelectricity enhances magnetism at all relevant length scales. Starting with a single layer of a ferrimagnet with magnetic spin frustration, we impose sub-Angstrom ferroelectric rumpling to lower the spin frustration and boost the magnetic transition to above room-temperature. As motivated by atomic-resolution electron spectroscopy, we further engineer the ferroelectric domain architecture to move charge through the system to boost the magnetic moment. Our results demonstrate a design methodology for creating higher-temperature multiferroics by exploiting a combination of geometric frustration, polarization doping and epitaxial engineering.
Julia A. Mundy is a President’s Postdoctoral Fellow in the Department of Materials Science and Engineering at the University of California, Berkeley. Her research uses advanced thin film deposition and electron microscopy to design, synthesize and characterize complex oxide heterostructures with sub-Angstrom resolution. Julia earned a B.A. in Chemistry and Physics and an M.A. in Chemistry from Harvard University and a PhD in Applied Physics from Cornell University where she was a National Science Foundation and National Defense Science and Engineering Graduate Fellow. She has also been recognized by the Materials Research Society, the American Physical Society/American Institute of Physics and the Microbeam Analysis Society.
MSE Colloquium: Charles Feng, Improvement of grain boundary tolerance by minor additions of Hf and B in second generation single crystal superalloys
The formation of low angle grain boundary (LAB) defects can dramatically degrade the creep properties of single crystal (SX) superalloys, resulting in lower casting yield and higher cost. Nowdays, to improve grain boundary (GB) tolerance in SX superalloys is still an important and challenging task for physical metallurgists. Minor alloying additions (C, B and Hf) have been observed as a means of improving the damage resistance of grain boundaries in a range of commercial single crystal superalloys. However, the creep strengthening mechanism by their additions is still unclear yet.
In this study, a double-seed solidification technique with two levels of misorientation (5° and 20°) was used to produce a series of second generation SX superalloys with different Hf and B additions. It is the first report that the alloy with the combined additions of Hf and B demonstrated the tolerance to GB as high as 20°misoriention under all the creep conditions of 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. However, the effect of independent additions of Hf or B was not as pronounced as or even much worse than that of their joint additions. In order to understand the influence of Hf and B additions on the creep mechanism in these SX superalloys with GB defects, a detailed characterization of the misorientation related microstructures at the GBs has been carried out. The elemental distribution at the GBs has been analyzed by using Auger electron spectroscopy (AES), time of flight secondary ion mass spectrometry (TOF SIMS) and atom probe tomography (APT) techniques. The bicrystals integrated well with each other, although an almost continuous 5 nm thin layer was identified at the LAB by high angle annular dark field (HAADF) images. Cellular recrystallization microstructures along the high angle grain boundary (HAB) were found in the Hf and B free alloy, but they were significantly suppressed by the minor additions of Hf or B. There was no obvious segregation of Hf, but the segregation of B to HAB was clearly identified by all the analyses using TOF SIMS, AES and APT. Relationships among Hf and B additions, GB microstructural evolution characteristics, segregation behaviors of alloying elements and creep properties as well as their effects on creep behaviors and mechanism will be discussed here as well. This study will be beneficial for understanding the role of Hf and B additions to improve the GB tolerance, and optimizing Hf and B additions in nickel-based single crystal superalloys.
Dr. Qiang (Charles) Feng is the Professor of the State Key Laboratory for Advanced Metals and Materials at the University of Science and Technology, Beijing (USTB). He graduated with the B.S./M.S. degree from USTB in 1991/1994, and received his Ph.D. degree from Polytechnic University at Brookly, NY (currently the Engineering School of NYU) in 2000. He had been working as the Postdoctoral Research Fellow and Senior Research Fellow in the Department of Materials Science and Engineering at the University of Michigan from 2000 to 2005. Professor Feng has authored or co-authored over 100 papers in referred journals and conference proceedings, and his research interests focus on alloy development, microstructure, mechanical property (creep and fatigue) and life prediction of high temperature structural materials, including superalloys, intermetallic alloys and heat resistant steels. Materials systems of interests are mainly applied in aircraft engine and gas turbine industries as well as power generation industries and automotive industries.
Prof. Feng currently serves for Chinese Materials Research Society (C-MRS) as the Deputy Secretary-general, and has been served for International Union of Materials Research Society (IUMRS) as the Officer and Secretary from 2009-2012. He is also the former Deputy Director of National Center of Materials Service Safety (NCMS), USTB. Prof. Feng is currently the member of High Temperature Alloys Committee and Mechanical Behavior of Materials Committee for the Minerals, Metals and Materials Society (TMS). He is also the international committee member of Eurosuperalloys 2018 conference, which is held every four years. He also serves on the editorial board of the Progress in Natural Science: Materials International. He has been honored by Chinese Ministry of Education as the New Century Excellent Talents in University, and he received numerous awards including Young Leader Professional Development Award from TMS as well as Research Mentor Award from the College of Engineering at the University of Michigan.
MSE Seminar: Xinpeng Du, The rebuilt of elastic constants from polycrystalline samples by measuring velocities of surface acoustic waves induced by ultrafast laser
Elastic constants are extremely important property to understand mechanical behavior of materials and are also very fundamental and indispensable inputs for physics-based models. Many experimental methods are put forward to measure elastic constants of materials but most of them are on single crystal samples directly, which brings strict requirements on sample size, sample geometry, sample homogeneity and multiple sample orientations in preparations. As a result, experimental values of elastic constants of about 99% solid solutions and compounds (roughly 160,000 kinds) are unavailable, which reveals tremendous limitations of traditional experimental methods. In order to make a breakthrough, we developed an innovative experimental method in rebuilding the elastic constants of a material from its polycrystalline sample by detecting surface acoustic waves induced by ultrafast laser on its surface. This method not only frees us from growing single crystals but also guarantees higher spatial resolution in tests, which makes it applicable in obtaining microstructure or composition dependent elastic constants properties through local measurements. In the end, accurate and repeatable benchmark work are provided to verify this method and several applications are proposed to prove its advantages.
Xinpeng Du got his Bachelor’s degree from Zhejiang University in 2011 majoring in optical engineering. He began his PhD study at OSU in the same year and has worked with professor Ji-Cheng Zhao at MSE since then. Xinpeng’s field is to develop an innovative method in locally measuring elastic constants with high spatial resolution. He works on both simulation and experiments. So far he has completed a benchmark work on several metals of cubic crystal class. He has also extended the application to get the elastic constants as a function of composition by doing experiments on diffusion couples and also has succeeded in obtaining elastic constants from powder samples experimentally. Right now, he is working on measuring elastic constants of intermetallic compound of low-symmetry crystals.
MSE Seminar: Ryan Brune
[To be provided]
MSE Seminar: David Gutschick
MSE Seminar: Timothy Smith, Orientation and Alloying Effects On Creep Strength in Ni-Based Superalloys
Improved modeling of polycrystalline Ni-based superalloys requires a deeper understanding of the effects chemistry and orientation have on different creep deformation modes. Single crystal compression creep tests conducted under varying temperature and stress regimes were performed on two commercially available disk alloys, ME3 and ME501. Utilizing scanning transmission electron microscopy characterization techniques, twinning and superlattice extrinsic stacking faults were determined to be the active deformation mechanisms for the  oriented crystals. Ultra-high-resolution structure and composition analysis using energy dispersive spectroscopy, combined with density functional theory calculations, revealed differences in compositional variation along the extrinsic faults inside the g¢ precipitates between the two alloys. For ME501, this compositional variation corresponds with a shear induced solid-state phase transformation from g¢ to h phase. This nanoscale h phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties and represents a newly discovered strengthening mechanism for Ni-base superalloys.
Tim Smith graduated from Bellefontaine City High School and attended Wright State University, obtaining a bachelor’s degree in Mechanical Engineering. During this time, he had the opportunity to work as a civilian research assistant at Wright-Patterson Air Force Base in the materials directorate. While there, he helped conduct research aimed at improving solid lubricant coatings and thermal interfaces in turbine engines. Afterwards, he decided to pursue his passion for materials science by working towards a PhD under Dr. Michael Mills at The Ohio State University. His thesis concentration is on how crystal orientation and alloying affects creep deformation in Ni-based superalloys through transmission electron microscopy analysis. Other projects that he has contributed to include, dislocation core analysis in high entropy alloys and improvements in characterization techniques using scanning transmission electron microscopy. He is currently a NASA Pathways Intern at the Glenn Research Center working on characterizing additively manufactured superalloys for the SLS engine and plans on working there full-time after graduation. When not working, his hobbies include; weight lifting, playing competitive sports and travelling abroad.
MSE Seminar: Qiaofu Zhang
[To be provided]
MSE Seminar: Steven Hansen
MSE Seminar: Molly Ball
[To be provided]