222X.Z. Liang, M.F. Dodge, S. Kabra, J.F. Kelleher, T.L. Lee, H.B. Dong, Effect of hydrogen charging on dislocation multiplication in pre-strained super duplex stainless steel, Scripta Materialia, Volume 143, 15 January 2018, Pages 20-24, ISSN 1359-6462.


ABSTRACT The effect of hydrogen charging on dislocation multiplication in super duplex stainless steel was investigated. Steel samples were pre-strained and charged with hydrogen for 10 days. Dislocation density was then measured using neutron diffraction. It is found that dislocation density multiplies by about one order of magnitude in samples with less than 5% pre-strain, but remains the same level in samples with pre-strain level of 10% and above.


30454ZHENG, L., LEE, T.L., LIU, N., LI, Z., ZHANG, G., MI, J. & GRANT, P.S. Numerical and physical simulation of rapid microstructural evolution of gas atomised Ni superalloy powders, Materials and Design 117 (2017) 157-167.


ABSTRACT The rapid microstructural evolution of gas atomised Ni superalloy powder compacts over timescales of a few seconds was studied using a Gleeble 3500 thermomechanical simulator, finite element based numerical model and electron microscopy. The study found that the microstructural changes were governed by the characteristic temperatures of the alloy. At a temperature below the γ’ solvus, the powders maintained dendritic structures. Above the γ’ solvus temperature but in the solid-state, rapid grain spheroidisation and coarsening occurred, although the fine-scale microstructures were largely retained. Once the incipient melting temperature of the alloy was exceeded, microstructural change was rapid, and when the temperature was increased into the solid + liquid state, the powder compact partially melted and then re-solidified with no trace of the original structures, despite the fast timescales. The study reveals the relationship between short, severe thermal excursions and microstructural evolution in powder processed components, and gives guidance on the upper limit of temperature and time for powder-based processes if desirable fine-scale features of powders are to be preserved.

My research on the 3D characterisation of the microstructures and stresses in spray formed steels using X-ray computed tomography and neutron diffraction was featured in the Faculty of Science and Engineering (University of Hull) 2016 Research Brochure.Research in Focus

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51Unn2mxyNL._SY344_BO1,204,203,200_TAN, D., LEE, T., KHONG, J., CONNOLLEY, T., FEZZAA, K. & MI, J. 2015. High-Speed Synchrotron X-ray Imaging Studies of the Ultrasound Shockwave and Enhanced Flow during Metal Solidification Processes. Metallurgical and Materials Transactions A, 46, 2851-2861.


ABSTRACT The highly dynamic behavior of ultrasonic bubble implosion in liquid metal, the multiphase liquid metal flow containing bubbles and particles, and the interaction between ultrasonic waves and semisolid phases during solidification of metal were studied in situ using the complementary ultrafast and high-speed synchrotron X-ray imaging facilities housed, respectively, at the Advanced Photon Source, Argonne National Laboratory, US, and Diamond Light Source, UK. Real-time ultrafast X-ray imaging of 135,780 frames per second revealed that ultrasonic bubble implosion in a liquid Bi-8 wt pct Zn alloy can occur in a single wave period (30 kHz), and the effective region affected by the shockwave at implosion was 3.5 times the original bubble diameter. Furthermore, ultrasound bubbles in liquid metal move faster than the primary particles, and the velocity of bubbles is 70 ~ 100 pct higher than that of the primary particles present in the same locations close to the sonotrode. Ultrasound waves can very effectively create a strong swirling flow in a semisolid melt in less than one second. The energetic flow can detach solid particles from the liquid–solid interface and redistribute them back into the bulk liquid very effectively.


LEE, T. L., MI, J., ZHAO, S. L., FAN, J. F., ZHANG, S. Y., KABRA, S. & GRANT, P. S. 2015. Characterization of the residual stresses in spray-formed steels using neutron diffraction. Scripta Materialia, 100, 82-85.


ABSTRACT Neutron diffraction was used to characterize the residual stresses in an as-sprayed tube-shaped steel preform. The measured residual stress distributions were compared with those simulated using finite element method by taking into account the effects of the thermal history, porosity and different phases of the sprayed preform. The porosity was measured using X-ray microcomputed tomography. The study revealed for the first time the correlation between the distribution of  porosity and residual stress developed in the as-sprayed preform.

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MI, J., TAN, D. & LEE, T. 2014. In Situ Synchrotron X-ray Study of Ultrasound Cavitation and Its Effect on Solidification Microstructures. Metallurgical and Materials Transactions B, 1-5.

ABSTRACT Considerable progress has been made in studying the mechanism and effectiveness of using ultrasound waves to manipulate the solidification microstructures of metallic alloys. However, uncertainties remain in both the underlying physics of how microstructures evolve under ultrasonic waves, and the best technological approach to control the final microstructures and properties. We used the ultrafast synchrotron X-ray phase contrast imaging facility housed at the Advanced Photon Source, Argonne National Laboratory, US to study in situ the highly transient and dynamic interactions between the liquid metal and ultrasonic waves/bubbles. The dynamics of ultrasonic bubbles in liquid metal and their interactions with the solidifying phases in a transparent alloy were captured in situ. The experiments were complemented by the simulations of the acoustic pressure field, the pulsing of the bubbles, and the associated forces acting onto the solidifying dendrites. The study provides more quantitative understanding on how ultrasonic waves/bubbles influence the growth of dendritic grains and promote the grain multiplication effect for grain refinement.

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