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Materials Frontier No.123

Title:High Resolution Electron Microscopy of Interfacial Segregation of Atoms in Al- and Mg-based Alloys

Speaker:Dr. Yun Wang, Senior Research fellow, The EPSRC Centre-LiME, BCAST, Brunel University, Uxbridge UB8 3PH, UK

Date/Time: 2013-12-17,15:00-17:00

Venue: Room.308, Material Building A

Inviter: Prof.Yijie ZHANG


  Dr Yun Wang obtained his PhD in materials science at University of Sheffield, UK in 1998 and worked as a research fellow at University of Sheffield and University of Surrey, UK. With his 20 year experience in research, he is a well-known expert in solidification and light alloys processing, particularly in advanced analytical electron microscopy of solidification microstructure, interfacial characterization and phase transformation. He is now a senior researcher at Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, UK. Dr Yun Wang has published more than 120 research papers including on top academic journals of materials science such as Acta Materilia, J Appl Phys. Metall Trans and Phil Mag. etc. His current research focuses on the investigations of the nature of oxide films and other inclusions in alloy melt, understanding of heterogeneous nucleation mechanisms via inoculations by TiB2 and oxides particles for Al- and Mg-based alloys, and microstructural characterisation of light alloys by analytical electron microscopy, including Cs-corrected scanning transmission electron microscopy and electron energy loss spectrometry.
  Heterogeneous nucleation is a fundamental issue in solidification processing of alloys since a grain-refined uniform structure not only improves mechanical performance of the solidified materials but also facilitates the casting process in terms of reduced cast defects and enhanced casting efficiency. In Al industry, TiB2 inoculants are usually used by the addition of Al-Ti-B master alloys prior to solidification. Due to the difficulties, limited work has been done to investigate the heterogeneous nucleation process, particularly the atomic matching and atomic configuration at the interfaces. By introducing intensive melt shearing prior to solidification, significant grain refinement was observed in various Al- and Mg-based alloys treated by the shearing process. Extensive theoretical and experimental studies were then carried out, including by high resolution TEM and Cs-corrected superSTEM, in order to understand the grain refinement mechanisms involved in the process. In this talk, three issues are covered. 1) Nucleation mechanism of TiB2 in Al alloys. For the first time, the heterogeneous nucleation mechanism of TiB2 was elucidated through the investigation of TiB2/Al interface. Cs-corrected STEM HAADF imaging and atomic resolution EELS mapping across the TiB2/Al interface revealed the presence of an extra Ti-rich atomic monolayer on the faceted (0001) surface of the TiB2. 2) Inoculation by oxide particles in Al- and Mg- alloys. It was shown that intensive melt shearing dispersed the in-situ oxide particles in the alloy melt, such as MgAl2O4 and MgO in Al- and Mg- alloys respectively, which therefore acted as potent nucleation inoculants, resulting in enhanced heterogeneous nucleation and grain refinement. 3) Zr effect on nucleation in Mg alloys. HRTEM, Cs-corrected STEM in particular, of the MgO/Mg interfaces revealed that Zr atoms segregated preferentially on {100} crystal planes of the in-situ MgO particles, and thus improved the heterogeneous nucleation potency of the oxide substrates.

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