Atom probe tomography of metallic nanostructures
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Introduction We present four examples of contemporary physical metallurgical design and development. Here, the design process of tailoring the alloy composition and the thermomechanical schedule has necessarily required inputs from measurements of the atomic-scale microstructure, such as are only available by atom probe tomography (APT). The physical metallurgical systems we developed have technologically important physical and mechanical properties—Nd-Fe-B permanent magnets, segregation engineering of steels and nickelbased alloys, advanced high-strength steels (AHSSs), and a model Ni-Al-Cr superalloy, which has excellent hightemperature properties. The instrument used to study these alloys was advanced APT, which is used correlatively with other characterization instruments, for example, transmission electron microscopy (TEM), to understand the nanostructures over a range of length scales. The common thread for the four metallic alloys studied is that their nanostructures determine their physical and mechanical properties over a range of temperatures. APT permits their nanostructures to be determined with subnano- to nanoscale spatial resolution and to identify
all of the elements in each alloy studied with equal detection efficiencies. The information garnered via APT cannot be determined by any other characterization instrument.
Grain-boundary chemistry of Nd-Fe-B permanent magnets revealed by APT Nd-Fe-B-based magnets are the strongest permanent magnets, critical for compact powerful motors and generators, such as voice coil motors for hard disk drives, traction motors, generators for hybrid electric vehicles, and high-efficiency wind turbines. Nd-Fe-B magnets consist of a Nd2Fe14B phase and a small fraction (∼10%) of nonferromagnetic phases, see Figure 1a.1 The coercivity is an extrinsic magnetic property, which depends on the size and morphology of the crystal grains and their magnetostatic and exchange interactions; hence, understanding the microstructure–coercivity relationships for Nd-Fe-B permanent magnets is essential for developing a higher coercivity magnet. The typical microstructure of sintered magnets observed with an in-lens secondary ion image in scanning electron microscopy (SEM), shown in Figure 1a, consists of brightly
Kazuhiro Hono, Magnetic Materials Unit, National Institute for Materials Science, Japan; [email protected] Dierk Raabe, Department of Microstructure Physics and Alloy Design, Max Planck Institute for Iron Research, Germany; [email protected] Simon P. Ringer, Australian Institute for Nanoscale Science and Technology, and School of Aerospace Mechanical and Mechatronic Engineering, The University of Sydney, Australia; [email protected] David N. Seidman, Department of Materials Science and Engineering and the Northwestern University Center for Atom Probe Tomography, Northwestern University, USA; [email protected] DOI: 10.1557/mrs.2015.314
© 2016 Materials Research Society
MRS BULLETIN • VOLUME 41 • JANUARY 2016 • www.mrs.org/bulletin
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ATOM PROBE TOMOGRAP
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