Morphologies of Primary Silicon in Hypereutectic Al-Si Alloys: Phase-Field Simulation Supported by Key Experiments
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reutectic and eutectic Al-Si alloys have been widely used in aeronautical and automotive industries due to their low thermal expansion coefficient, high wear resistance, and good castability.[1–3] As an important strengthening phase in Al-Si alloys, primary silicon even forms in hypoeutectic Al-Si alloys at high-cooling rates.[4] The octahedral grains of primary silicon usually solidify from the melt in a faceted manner, and their large size and sharp tip may affect the fracture-sensitive mechanical properties and machinability of hypereutectic Al-Si alloys.[5] Therefore, in order to improve the properties of the hypereutectic Al-Si alloys, it is a prerequisite for comprehensively understanding of the entire growth process of the primary silicon, from which a control of the morphologies of primary silicon becomes feasible. Previous experiments include the characterization of the morphology[6,7] and microstructure evolution[6–8] of primary silicon in hypereutectic
KAI WANG, Master Student, MING WEI, Doctoral Student, and LIJUN ZHANG and YONG DU, Professors, are with the State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China. Contact e-mails: [email protected], lijun. [email protected] Manuscript submitted August 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
Al-Si alloys, but their metallographic[6,7] or scanning electron microscopy (SEM) observations[8] can only provide qualitative two-dimensional (2D) morphology information of primary silicon. Furthermore, Singh et al.[9] realized the reconstruction of a three-dimensional (3D) geometry of primary silicon in a hypereutectic Al-Si alloy by repeating the polish-montage-polish procedure, locating the primary silicon in every montage by means of microhardness indents and processing the images of the montage with an aid of the image analysis software. However, the general disadvantage in experimental investigations lies in that (i) only the morphologies of primary silicon in hypereutectic Al-Si alloys at several time slides can be measured, from which the evolution of primary silicon during the entire growth process cannot be characterized, (ii) the process is a time-consuming one, and (iii) some details about 3D morphologies of primary silicon may not be presented due to the large errors in aligning the montage serial sections. Those drawbacks in the experimental investigations can be easily overcome in quantitative numerical simulations, like the phase-field simulation. However, there has been no any report on numerical simulations on the microstructure evolution of primary silicon in hypereutectic Al-Si alloys. The only existing reports are due to Cantu` et al.[10] and Steinbach,[11] who performed individual phase-field simulations of growth behavior of primary silicon in the pure silicon melt during solidification based on the underlying anisotropy of the interface energy and interface mobility. Unfortunately, not much information on the growth process of primary silicon was given. Moreover, the solute concentration should play an impo
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