Materials Frontier No.155
Title: High Resolution TEM of Metal/Inoculant Interfaces and Mechanism of heterogeneous Nucleation
Speaker: Dr Yun Wang, EPSRC-LiME Centre, BCAST, BrunelUniversityLondon,UxbridgeUB8 3PH,UK
Inviter: Prof Mingxu XIA
Dr Yun Wang obtained his PhD in materials science atUniversity of Sheffield,UKin 1998 and worked as a research fellow atUniversityofSheffieldandUniversity 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.
Grain refinement is usually desirable for solidification processing of metallic materials since it not only improves the mechanical properties of the solidified materials but also facilitates the casting process itself by reducing the cast defects and improving casting efficiency. Addition of grain refiners, which are usually in the form of particulates called heterogeneous nucleation inoculants, is a common industrial practice in both continuous and shape casting processes. For instance, the most widely used approach for grain refinement of Al is inoculation using TiB2 particles provided by the addition of Al-Ti-B master alloys prior to solidification. Although the addition of the heterogeneous nuclei was first proposed in 1940s, the mechanism of heterogeneous nucleation resulting grain refinement is not fully understood. Direct investigation of the metal/inoculant interfaces in atomic resolution using advanced analytical electron microscopy is therefore required to establish the atomic configuration across the interfaces in order to achieve a deep understanding of the heterogeneous nucleation mechanisms, which is the theoretical basis for grain refinement. In the present work, Mg/MgO and Al/TiB2 interfaces in solidified Al- and Mg-based alloys, both original and modified interfaces by dopant atoms such as Ca, Zr and Ti, were examined by high resolution TEM (JEOL 2100F) and atomic resolution STEM (Cs-corrected Titan and Nion UltraSTEM). Figs 1 and 2 show two examples of such interfaces observed by the HRTEM. Well-defined orientation relationships were found between the inoculants and metal crystals, indicating the potency of MgO and TiB2 particles for heterogeneous nucleation of Al and Mg, respectively. In particular, Ti, Zr and Si dopant atoms were located at the interface using atomic resolution EELS mapping across the interface. Heterogeneous nucleation mechanism was discussed in terms of the resultant poisoning or promoting effect on the TiB2 and MgO particles by the dopant atoms.