Two-Zone Microstructures in Al-18Si Alloy Powders
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AL-SI alloys containing from 4 to 22 wt pct Si comprise more than 90 pct of all Al-based castings. Parts manufactured with these alloys have a wide range of industrial applications due to their excellent castability, crack resistance, good corrosion resistance, good weldability, low density, and high strength.[1–3] Hypereutectic Al-Si alloys are widely used and researched in the automotive and aerospace sectors due to their low coefficient of thermal expansion, high temperature resistance, good wear resistance, and high strength.[3,4] As an alternative to the conventional hypoeutectic Al-Si, there is growing interest in hypereutectic Al-Si alloys as a candidate material for high-quality engines because a higher volume fraction of silicon would result in superior properties.[5,6] Regarding the increasing applications of these alloys in the automotive and aeronautical industries, the weight factor of Si is directly related to improved energy efficiency of engines.[7,8] By the virtue of their high thermal conductivity and a thermal expansion coefficient (7 to 9 9 106 C1) similar to pure silicon,[9] hypereutectic Al-Si alloys show a broad
J.E. SPINELLI is with the Department of Materials Engineering, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP 13565-905 Brazil and also with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2G6 Canada. Contact e-mail: [email protected] A.-A. BOGNO and H. HENEIN are with the Department of Chemical and Materials Engineering, University of Alberta. Manuscript submitted June 21, 2017.
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range of commercial applications also in electronic packaging industries. These characteristics of hypereutectic Al-Si alloys have also aroused interest in the fields of rapid solidification technologies, such as fusion casting technology, powder metallurgy, and spray deposition method.[10,11] Again, the low thermal expansion and uniform distribution of oxides on the powder surface are among the advantages of these alloys. Understanding the microstructure evolution in these powders and the corresponding processing conditions is critical to developing a knowledgebase for their efficient and effective utilization involving rapid solidification. Determination of solidification conditions might yield a specified shape of Si in hypereutectic Al-Si alloys. This shape is able to tailor the alloy mechanical properties. Observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of gas-atomized 18 wt pct Si alloy powders indicate a continuous dendritic–cellular–microcellular transition that occurs when particle size is decreased.[12] Even though a range of morphologies was observed, each morphology was associated with a certain range of powder size. In contrast, Boettinger and collaborators[13] observed the presence of two microstructural zones in single rapidly solidified powders of Al-8 wt pct Fe alloy generated by gas atomization. These authors observed the occurrence of different morphologies emanati
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