Microstructural characterization of rapidly solidified Al-Fe-Si, Al-V-Si, and Al-Fe-V-Si alloys
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study of rapidly solidified melt-spun Al80Fe14Si6 Al80V14Si6, and Al80Fe10V4Si6 alloys by electron microscopy techniques, x-ray diffractometry, and differential scanning calorimetry leads to a number of microstructural results. Coexistence of a micro-quasicrystalline state of an icosahedral phase with monoclinic –Al13Fe4 and hexagonal –Al6V in Al–Fe–Si and Al–V–Si alloys, respectively, is reported. Also, the growth morphology of the icosahedral phase surrounded by a crystalline ring was investigated in an Al–Fe–V–Si alloy. The crystalline ring has the particles of the cubic ␣–Al12(Fe,V)3Si silicide phase. Evidence of irrational twinning of cubic crystals, giving rise to a symmetry not deviating much from icosahedral symmetry was found in this alloy. In all the three alloys crystalline intermetallics were elucidated in the context of rational approximants of an icosahedral quasicrystal. It was noticed that while the icosahedral phase in Al–Fe–Si and Al–V–Si alloys transforms to crystalline intermetallics at about the same temperature (approximately 610 K), the transformation of icosahedral phase in Al–Fe–V–Si alloy occurred at a relatively lower temperature (540 K). The origin of different metastable microstructures and their stability at elevated temperatures, in these alloys, are compared and discussed.
I. INTRODUCTION
Aluminium alloys produced by rapid solidification routes have provided a new class of materials strengthened by dispersoids of aluminium-rich intermetallic compounds. The importance of these alloys has been elucidated by Das1 in terms of their excellent mechanical and corrosion properties arising from a number of metastable intermetallic compounds that exhibit unique structure, thermal stability, and desirable properties in an aluminium matrix. Addition of ternary and quaternary elements in Al-transition metal binary systems increases the thermal stability of intermetallics formed in these alloys. Examples are Al–Fe-based Al–Fe–Ce and Al– Fe–V–Si alloys. In this series, other Al-transition metalbased alloys are Al–Mn, Al–Cr, Al–Zr, and Al–V.1,2 The strength and stability of these alloys are dependent on the metastable microstructures and intermetallics formed during rapid solidification and their thermal stability at elevated temperatures. These alloys have proved their suitability for elevated temperature applications. Two commercial alloys developed by Allied Signal, namely FVS0611 (Al–6.5Fe–0.6V–1.3Si by wt%) and FVS0812 a)
Present address: Electron Microscope Section, National Physical Laboratory, New Delhi 110 012, India J. Mater. Res., Vol. 16, No. 7, Jul 2001
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(Al–8Fe–1.4V–1.7Si by wt%), have been redesignated as 8022 and 8099, respectively, by the Aluminium Association.1 This paper describes a detailed study of microstructural characteristics of Al–Fe–Si, Al–V–Si, and Al–Fe–V–Si alloys subjected to rapid solidification. The discovery of metastable quasicrystalline phase formations in Al-transition metal alloys was a major break throu
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