Study of the multi-length scale structure of metallic glasses using synchrotron X-rays and phase-field crystal modelling

• Main Author: Zhang, Wei
• Other Authors: Mi, Jiawei
• Published: University of Hull 2016
• Subjects: 620.1; Engineering
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.693806

Metallic glasses and composites are a new class of metal alloys which have unique mechanical and functional properties. The structure of metallic glasses and composites are key to understand the glass formation ability and also their unique properties. However it is often very challenging to determine the structure of such amorphous materials using conventional experiment methods. The research studies the multi-length scale structures of metallic glasses and composites with different composition and processing conditions. A novel experimental apparatus were designed made, and commissioned for in-situ PDFs studies. The composition dependent atomic structure of binary and ternary metallic glasses were characterised using synchrotron X-ray pair distribution functions. And a binary vacancy phase-field crystal model and a ternary model developed based on it, were successfully used to interpret the atomic structure changes with composition for the binary alloys, CuZr and NiTi, and also the ternary alloys, CuZrAl and TiCuNi. Systematic in-situ experiments were conducted using this novel apparatus at the synchrotron X-ray diffraction beamline, I12, Diamond light source. A large amount of real-time diffraction patterns of Vit1 (Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀Be₂₂.₅) with different thermal conditions, were acquired in this research. A highly thorough procedure of data reduction, processing and analysis enables to get high quality in-situ PDFs and extract the key information for understanding the local structure evolution during the crystallisation from supercooled liquid and annealed amorphous state, and also that during the glass formation with different cooling conditions at atomic scale. The significant advantage of bulk metallic glass matrix composites (BMGMC) is the strong atomic bond across the amorphous–crystalline interface which is formed in-situ during solidification. The transition from an ordered to disordered atomic structure plays a critical role in determining the mechanical properties in atomic scale. Several HRTEM image based methods, fast Fourier transformation, auto correlation function, and 2D pair distribution function, etc, were used to characterise the local ordering from short range to long inter-atomic range. Furthermore, the mechanical properties of such BMGMCs are often strongly related to the complex crystalline dendrites, and a well-understanding of 3D morphologies of the dendrites is also very important. However, the 3D structures and their morphologies of such composite at nano and micrometre scale have never been reported before. The high density electric currents were used to thermally shock a Zr-Ti based BMGMC (DH3) to different temperatures, and used X-ray micro-tomography, FIB-SEM nanotomography and neutron diffraction to reveal the morphologies, compositions, and volume fractions of the nano and microstructures of the thermally-shocked composites and their thermal stabilities under rapid heating by electric currents. The key research findings are: • A binary vacancy phase-field crystal model and a ternary model have been developed and successfully used to interpret the atomic structure changes with composition for the binary alloys, CuZr and NiTi, and also the ternary alloys, CuZrAl and TiCuNi. As the concentration of Cu and Ti increases, the populations of Cu-Cu atomic pairs / Ti-Ti atomic pairs increase, which leads to increasing peak intensity of the first / second peak. The atomic size ratio significantly affects the peak positions and peak shape of PDFs for binary and ternary alloys. The peak separation of the first PDF peak for Zr₅₅Cu₄₀Al₅ is mainly due to the increasing population of Zr-Cu and Zr-Al pairs. • An dedicated experimental apparatus were designed, made, and commissioned for the total scattering experiments using Synchrotron X-rays, which can be used to thoroughly acquire high-quality X-ray scattering data for real time studies of atomic structure using pair distribution functions. • The crystallisation of Vit1 during heating is very different to those found during slow cooling. The crystallisation induced by heating is very rapid, and the crystalline structure is formed in 6 s, while that occurred in cooling is fairly slow (∼ 21 s), and the atomic structure are largely determined by the cooling rates. During the crystallisation induced by heating at 1.5 K/s, Zr₂Cu is formed at 270 °C and ZrBe₂ is formed 457 °C. While Zr₂Cu and ZrBe₂ are formed together during the crystallisation induced by slow cooling. • A novel 2D HRTEM image based pair distribution function method was proposed to characterise the local structure transition within amorphous-crystalline interface at atomic level. The 2D PDFs, which are very sensitive to ordered and disordered structure, can provide an alternative approach to characterise the local ordering using HRTEM. • The 3D nano and microstructures of the DH3 at different thermal shock conditions were studied and characterised using X-ray microtomography and FIB-SEM nanotomgraphy. It is found that the ductile β-Zr crystalline dendrites are interlocked 3D structures with complex morphology of a few hundreds of micrometres. They are not the “globular” particles of a few to tens of microns in length, as previously inferred from 2D imaging. The amorphous to crystalline transition at the interface under thermal shock by applying electric current is very different to that occurred in isothermal heating conditions. The large difference in electric resistivities between the amorphous matrix and the crystalline dendrites resulted in differential heating across the amorphous-crystalline interface, which led to the nucleation of new crystalline phases (Zr₂Cu and ZrBe₂) preferably at the interface, rather than within the amorphous matrix. They grew concurrently to form 3D eutectic network as revealed by using the FIB-SEM nanotomography.