In Situ Observation on Bubble Behavior of Solidifying Al-Ni Alloy Under the Interference of Intermetallic Compounds
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bubbles, as a major defect of Al castings, impair the properties and applications of castings, in particular on the ductility, fatigue resistance, and surface quality of castings.[1,2] They nucleate and grow when the alloy solidifies from the liquid state due to the remarkable difference in the solubility of hydrogen between the liquid and solid phases.[1] It has been generally accepted that the bubble behavior mainly depends on solidification velocity, thermal gradient, and initial concentration of hydrogen.[3,4] However, the state-of-the-art investigations reveal that the interaction between the solidification front and the bubbles also greatly influences the dynamics of bubble evolution, typically as the bubble interacting with dendritic arms.[5–13] It has been found that the bubbles would normally grow, move, distort,
SIYU SUN, QIAODAN HU, WENQUAN LU, ZONGYE DING, MINGXU XIA, and JIANGUO LI are with the Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted September 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
disappear, and finally leave the pores in the solids due to the pushing force of solidifying front and dendritic arms.[5,6] Karagadde et al. [12,13] proposed some models about bubble growth, engulfment, and its interaction with solidifying front, which were validated with experimental observations. Wu et al.[7] developed a LBM-CA model to describe the bubble behavior of Al-4 wt pct Cu alloy during unidirectional solidification which elegantly illustrates the bubble deformation phenomenon under squeezing of the neighboring dendrites. Wang et al.[8] adopted MCA-FD model to simulate the interactive and competitive growth of dendrites and porosities and concluded that the solid dendrites would alter the spherical morphology of gas pores. Additional models elaborating on the microporosity have been reviewed by Lee et al.[14] The nucleation of bubbles in front of the planar, cellular, and dendritic solidification interface was also investigated both experimentally[15–17] and theoretically.[13] It was concluded that the rate of solidification plays a key role in the engulfment of bubbles by the moving solidification interface. However, only limited work deals with the interaction between bubbles and intermetallic compounds (IMCs).[18–20] Similar to dendrites, IMCs precipitated during the solidification at the solidification front may also alter the bubble behaviors to a large extent similar to dendritic arms. But the sizes, morphology, as well as its growth kinetics of IMC are significantly different from primary dendritic arms. Therefore, it is of great importance to investigate the effects of growing IMCs on the evolution of bubbles. In this investigation, synchrotron radiography was subject to examine the growth behavior of hydrogen bubbles and their interaction with IMCs during the directional solidification of Al-12 wt pct Ni Alloy. The mor
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