Manuscript submitted to TMS 2019 148th Annual Meeting & Exhibition Supplemental Proceedings 1 Revealing the heterogeneous nucleation and growth behaviour of grains in inoculated aluminium alloys during solidification Yijiang Xu 1 , Daniele Casari 2 , Ragnvald H. Mathiesen 2 , Yanjun Li 1* 1 Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway 2 Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway *Corresponding author: E-mail address: yanjun.li@ntnu.no Abstract An in-situ X-ray radiographic study on the grain nucleation and grain growth of inoculated Al-10Cu and Al-20Cu alloys during isothermal melt solidification and directional solidification conditions with constant cooling rates has been carried out. The influence of addition level of inoculation particles, cooling rates, and temperature gradient on the nucleation rate and growth kinetics of grains have been quantitatively studied. The deterministic nature of the heterogeneous nucleation of aluminium grain on inoculant particles is revealed. Numerical microstructure models have been developed to simulate the nucleation and growth behavior of aluminum grains and a good agreement between the experimental results and simulation results has been achieved. Keywords: Heterogeneous nucleation, Grain growth, Solidification, Aluminium alloy 1. Introduction Grain refinement by inoculation is a common practice during the casting and solidification of Al and Al alloys. As summarized in several review papers [1-4], the grain refinement mechanisms have been much understood through extensive studies in the past decades. Lots of experimental works have been made to study grain refinement behaviour in inoculated Al alloys, such as thermal analysis [5, 6], post-solidification characterization of cast samples, in-situ X-ray diffraction [7-9] and in-situ X-radiography study [10-13]. However, comprehensive in-situ studies on the heterogeneous nucleation and grain growth under dedicated solidification conditions are still needed to reach an in-depth understanding on the nucleation kinetics under different solidification conditions (cooling rate, temperature gradient) and different solute/grain refiner addition levels, the nucleation ceasing mechanisms, and the grain growth kinetics. Besides, many numerical and analytical grain size prediction models have been developed [14-28] to better understand the nucleation and grain growth behaviour. However, most of the grain size prediction models [14, 16, 17, 24, 26, 29], are based on the assumption of spherical/globular grain growth kinetics. Thus, a more sophisticated model including globular to dendritic transition (GDT) and dendritic growth kinetics needs to be developed, by which the influence of grain morphology transition on the nucleation kinetics and predicted grain size can be quantitively investigated. Furthermore, most of the published models intended for directional solidification [20, 24-26], are still based on a local isothermal melt solidification assumption. Therefore, new models with a rigorous treatment of grain nucleation on inoculant particles under the temperature gradient effect during is still demanded. In the present work, an integrated study by in-situ X-radiography and numerical modelling will be carried out to reveal the heterogeneous nucleation and growth behaviour of grains during solidification of inoculated Al alloys. 2. Experimental The materials used in the experimental study are Al-10Cu and Al-20Cu (wt.%) alloys prepared by melting 5N (99.999 wt.%) purity aluminum and 4N (99.99 wt.%) purity copper in a clay graphite crucible using a Nabertherm melting furnace. After complete melting and mixing of the raw metals, different levels of commercial Al-5Ti-1B (wt.%) master alloy were added to the melt and finally cast into a copper mold. Thin plate samples with dimension of 5 × 50 × 0.2 ±0.01 mm (X×Y×Z) are prepared from the cast ingots by cutting, grinding and polishing.