Phase transformation during annealing of rapidly solidified Al- rich Al- Fe- Si alloys
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I.
INTRODUCTION
IRONand silicon are the most common impurities in commercial aluminum. In cast structures they are partially in solid solution in aluminum but occur mostly in interrnetallic phases. According to the equilibrium phase d i a g r a m f '21 different complex intermetallic phases can be formed depending on the degree of local segregation. In addition to the numerous equilibrium phases in this system, a variety of metastable phases readily forms when solidification conditions deviate from equilibrium. A search of recent literature shows that more than 20 intermetallic A1-Fe and A1-Fe-Si phases have been detected.iS-l~ Most of the phases have large unit cells with complex crystal structures. Table I gives a list of some of these phases, their designations used in the present work and in some other publications, composition, and available crystallographic data. In addition to these complicated crystalline phases, incommensurate I51and quasiperiodic Ill1 phases were also reported. The observation of so many phases in a relatively close compositional range raises a question about possible structural interrelations among them. Another important aspect of Al-based A1-Fe-Si alloys is their response to rapid solidification (RS). Recently, it was demonstrated that certain RS AI-(Fe, V)-Si alloys, consoli-
L.A. BENDERSKY, A.J. McALISTER, and E S. BIANCANIELLO are with Institute of Materials Science and Engineering, National Bureau of Standards, Gaithersburg, MD 20899. Manuscript submitted February 26, 1988.
Table I.
dated via hot isostatic pressing (HIP) and hot extrusion, contained uniformly distributed, small ( - 5 0 nm) dispersoids of an intermetallic compound which revealed high coarsening resistance, mj These alloys may have a potential as a structural aluminum alloy for elevated temperature application. Small additions of transition elements such as vanadium to A1-Fe-Si alloys seem to have an important effect on the stability of these dispersoids. From this engineering point of view it is important first to understand the thermal decomposition of rapidly solidified ternary A1-Fe-Si alloys. The purpose of the present study is to investigate phase transformations during annealing of rapidly solidified A1-Fe-Si alloys. To do this, differential thermal analysis (DTA) and transmission electron microscopy (TEM) were employed. In this study the emphasis is placed on crystallography of the observed phases and possible structural interrelation among them. Results on initial microstructures of RS melt-spun material were presented elsewhere 1~31and are briefly reviewed.
II.
R E V I E W OF M I C R O S T R U C T U R E OF AS-SOLIDIFIED AI-Fe-Si RIBBONS tt31
For all previously studied alloys I131(including three compositions of the present study) two morphologically distinct as-solidified microstructures have been observed. The first type of microstructure, generally observed for alloys with Fe content less than 12 wt pct, is cellular, with aluminum
Phases, Compositions, and Crystallographic Data
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