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The Refinement Effect of Alumina–Silica Sol‐Coated ZnO Particles for Cast Magnesium
Author(s) -
Cao Rui,
Meng Meng
Publication year - 2020
Publication title -
advanced engineering materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.938
H-Index - 114
eISSN - 1527-2648
pISSN - 1438-1656
DOI - 10.1002/adem.202000329
Subject(s) - materials science , nucleation , coating , sol gel , chemical engineering , scanning electron microscope , magnesium , grain size , crystal (programming language) , metallurgy , composite material , nanotechnology , chemistry , organic chemistry , computer science , engineering , programming language
Based on edge‐to‐edge matching crystallographic model, ZnO is identified as an effective grain refiner for magnesium. However, the reduction of ZnO particles by molten Mg destroys the crystal structure and restricts its refinement efficiency. Herein, alumina/silica sol–gel coatings are prepared on the surface of micrometer‐sized ZnO, forming a ZnO core sol‐coating shell structure to improve the thermal stability of the ZnO. Experimental observations indicate that the refining effect of ZnO is significantly enhanced by the presence of alumina‐silica sol (ASS) coatings and the core‐shell structure. The addition of 1.5 wt% ASS‐coated ZnO particles to Mg reduced the grain size from ≈3200 to 65 μm. The grain size of the Mg‐3wt%Al‐1 wt%‐Zn alloy (AZ31) can also be refined from 145 to 82 μm. Scanning electron microscopy and X‐ray powder diffraction reveals that the sol coating partly maintain the crystal structure of ZnO. More unreduced ZnO and related solid redox products are reproducibly observed within the refined Mg grains, which promote heterogeneous nucleation of Mg. Zn, and Al solute produced by relative reduction and unreduced ZnO on grain boundaries also contribute to refinement. In addition, the ASS coating also increase the refining efficiency of ZnO for Mg and AZ31 at various cooling rates.