Faceted-nonfaceted growth transition and 3-D morphological evolution of primary Al 6 Mn microcrystals in directionally s
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hong Li, Yanqing Su, Jingjie Guo, and Hengzhi Fu Department of Materials Engineering, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China (Received 27 February 2014; accepted 1 May 2014)
A comprehensive understanding of the growth pattern of intermetallic compounds (IMCs) during solidification is critical to both the crystal-growth theory and its property optimization. In this article, growth pattern and three-dimensional (3D) morphology of primary Al6Mn IMC were investigated in directionally solidified Al–3 at.% Mn alloy at a wide range of growth rates. A transition from faceted (,60 lm/s) to nonfaceted growth (.100 lm/s) was observed with increasing growth rates. Correspondingly, 3D morphologies of primary Al6Mn change from a solid polyhedron to a hollow structure, and then to a dendrite. This kind of change is associated with the competitive growths of different crystal planes determined by the crystallographic anisotropy and growth kinetics of Al6Mn. A growth model based on atomic cluster attachment is proposed to reveal the growth transition, and a growth-rate ratio between different crystal planes is used to appropriately reveal the formation mechanism of different morphologies at low rates.
I. INTRODUCTION
The melt-growth control of intermetallic compounds (IMCs) is of practical interest because of the significance of IMCs with appropriate morphologies, sizes, distributions, and volume fractions introduced in alloys for optimizing properties.1–6 However, IMCs exhibiting complex crystal structure and directional bonding commonly show a faceted growth with a strong anisotropy during solidification.7,8 Growth discrepancies of different crystal planes, originated from the inherent crystal structure and external environment, usually lead to complicated morphologies of IMCs. Especially, a potential transition is predicted theoretically from faceted IMCs to nonfaceted IMCs then quasicrystals or noncrystals with increasing cooling rates.9,10 However, further accumulation of experimental evidences is required for understanding the detailed pattern formation of melt-growth IMCs. Several attempts7,8,11–15 have been made for this purpose. For instance, deep-etching and extracting techniques were used to characterize the three-dimensional (3D) morphologies of primary Al3Sc IMCs in casting aluminum–scandium (Al–Sc) alloys.11 The growth of primary Mg2Si IMCs in casting aluminum–magnesium–silicon (Al–Mg2Si) alloys was highly dependent on its crystal structure and growth a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.111 1256
J. Mater. Res., Vol. 29, No. 11, Jun 14, 2014
http://journals.cambridge.org
Downloaded: 16 Dec 2014
conditions.7 Recently, we have investigated the 3D morphologies of Al6Mn IMCs at a growth rate of 1 lm/s in directionally solidified Al–3 at.% Mn alloys.8 It is found that directional solidification is an appropriate technique to reveal the growth of IMCs due to the defined conditions of crystal growth. In this ar
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