In situ Investigation of the Heterogeneous Nucleation Sequence in Al-15 Weight Percent Cu Alloy Inoculated by Al-Ti-B
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INTRODUCTION
THE efficiency of heterogeneous nucleation is generally assumed to be affected by the cooling rate, solute content, and inoculant particles.[1–7] For potent grain refiners such as Al-Ti-B, the required undercooling to initiate nucleation is determined by the radius of an inoculant particle.[8,9] The growth of grains is restricted by the solute content and promoted by the cooling rate.[10,11] As a result, with the given cooling rate, solute content, grain refiner addition level, and size distribution of inoculant particles, the number of nuclei can be calculated by the free growth model[9,12–14] or interdependency model.[15–17] To reduce the melt convection and minimize the effect of gravity, Xu et al.[18,19] performed a laboratory-based in situ X-radiography study by placing the thin plate samples in a horizontal YIWANG JIA, DONGHONG WANG, ANPING DONG, GUOLIANG ZHU, and DA SHU are with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] YANAN FU is with the Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, CAS, Shanghai 201204, China. BAODE SUN is with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University and also with the State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China. Manuscript submitted September 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
direction. The results were consistent with an optimized grain size prediction model based on the free growth model. However, several other factors such as the agglomeration[20] and settlement of inoculant particles,[21] the poisoning phenomenon of certain elements,[22–25] and the effect of gravity[26–28] should also play an important part in heterogeneous nucleation under certain conditions. Because those factors mainly act on the primary phase after nucleation, they are difficult to analyze through postmortem methods and are seldom considered in numerical or analytical models.[29] Consequently, a detailed understanding of the dynamic process during solidification through in situ observation is critical for examining the effect of those factors. Chen et al.[30] reported that the refining performance of Al-5Ti-1B could be greatly weakened by the agglomeration of TiB2 induced by a rotating magnetic field. Murphy et al.[27,28] studied the effect of buoyancy on the equiaxed grain nucleation via in situ X-radiography. They observed that the grain buoyancy caused a significant increase in the number of nucleation sites due to the cessation of primary dendritic growth and early developed grain coherency. Bogno et al.[31,32] investigated the growth and interaction of dendritic equiaxed grains. They found that the grain falling under gravity made a nearby dendritic arm temporally stop growing. Prasad et al.[33,34] reported the study of
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