Microstructural Investigation and Phase Relationships of Fe-Al-Hf Alloys
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INTRODUCTION
IRON aluminides have been studied widely as potential candidates for structural applications at high temperatures.[1,2] Their high temperature corrosion and oxidation resistance and lower density are the main advantages of iron aluminides compared to many other commercial Fe-based materials.[1–8] Moreover, iron aluminides offer relatively high melting temperatures, low material cost, and conservation of strategic elements such as chromium.[9–11] However, the major drawbacks of iron aluminides are their poor ductility and toughness at ambient temperatures, limited fabricability, and poor strength and creep resistance at temperatures above 873 K (600 °C).[4,8–11] Alloying element addition is one of the most effective ways of improving the insufficient properties of iron aluminides.[7] Such alloying additions can help strengthen the Fe-Al-based alloys by different mechanisms such as solid solution hardening, coherent/incoherent precipitates, and ordering.[5,7,12] The solubility of the ternary alloying element X in the Fe-Al alloys plays an important role in determining which of these strengthening mechanisms can actually operate or is hindered. Alloying additions such as Nb, Zr, Hf, Ta, or Ti that have limited solubilities in Fe-Al-based phases often lead to formation of a secondary phase.[7,8,13–15] This secondary phase is a hard and brittle ternary intermetallic compound, and generally tends to form a eutectic phase mixture with the MEHMET YILDIRIM, Ph.D. Student, M. VEDAT AKDENIZ and AMDULLA O. MEKHRABOV, Professors, are with the Novel Alloys Design and Development Laboratory (NOVALAB), Department of Metallurgical and Materials Engineering, Middle East Technical University, 06800 Ankara, Turkey. Contact e-mail: ymehmet@ metu.edu.tr Manuscript submitted August 29, 2013. Article published online April 2, 2014 3412—VOLUME 45A, JULY 2014
Fe-Al-based phase.[7] Mechanical properties of these Fe-Al-X systems where X leads to formation of a ternary phase strongly depend on the microstructural features including the volume fraction, size, and morphology of the existing phases.[14–16] For example, strength can be improved by controlling the amount and size of the constituent phases.[14,15] Room temperature ductility, on the other hand, can be improved by impeding the shear localization by obtaining a bimodal microstructure having micrometer size primary dendrites and a nanoscale eutectic mixture.[16–20] Alloying elements with different solubilities often result in significantly different microstructural changes in Fe-Al-X alloys, which subsequently lead to a wide spectrum of mechanical properties. For instance, among the alloying additions, Zr has been reported to have a very limited solid solubility compared to the other elements in the Fe-Al alloys, independent of the Al content and temperature.[21] This makes Fe-Al-Zr alloys extremely brittle at room temperature due to formation of plentiful amount of ternary intermetallic compound. On the other hand, Ta and Ti show larger solid solubilities, and they have positive e
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