Nb-Based Nb-Al-Fe Alloys: Solidification Behavior and High-Temperature Phase Equilibria
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Nb-Based Nb-Al-Fe Alloys: Solidification Behavior and High-Temperature Phase Equilibria FRANK STEIN and NOAH PHILIPS High-melting Nb-based alloys hold significant promise for the development of novel high-temperature materials for structural applications. In order to understand the effect of alloying elements Al and Fe, the Nb-rich part of the ternary Nb-Al-Fe system was investigated. A series of Nb-rich ternary alloys were synthesized from high-purity Nb, Al, and Fe metals by arc melting. Solidification paths were identified and the liquidus surface of the Nb corner of the ternary system was established by analysis of the as-melted microstructures and thermal analysis. Complementary analysis of heat-treated samples yielded isothermal sections at 1723 K and 1873 K (1450 C and 1600 C). DOI: 10.1007/s11661-017-4334-0 The Author(s) 2017. This article is an open access publication
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INTRODUCTION
NB-BASED intermetallic alloys have been of considerable interest for structural applications at very high temperatures.[1–13] The basic concept relies on obtaining a multiphase microstructure consisting of the Nb solid solution and a high-melting, strengthening intermetallic phase (or phases). The aluminides Nb3Al and Nb2Al have been reported as interesting candidates due to both high-temperature strength and the potential to form a protective alumina scale, but respective alloys with further additions of Ti, V, Cr, or Hf were found to suffer from microstructural instability or internal oxidation with precipitation of alumina.[3,5] The present work focuses on an investigation of the combined alloying effect of Al and Fe. It appears that the simultaneous addition of Fe and Al increases the solubility limit in the Nb solid solution as well as expands the Nb2Al phase field.[14] To support the development of Nb-based intermetallics, the Nb-rich corner of the ternary Nb-Al-Fe system was investigated with a focus on solidification paths, high-temperature phase equilibria, and expanded ternary solubilities. In a preceding study, the Nb-lean part of the Nb-Al-Fe system (less than 60 at. pct Nb) was investigated in detail including isothermal sections at 1273 K, 1423 K, and 1573 K (1000 C, 1150 C, and 1300 C) as well as a liquidus surface projection.[14] The portion of
FRANK STEIN is with the Max-Planck-Institut fu¨r Eisenforschung, Max-Planck-Str. 1, D 40237 Du¨sseldorf, Germany. Contact e-mail: [email protected] NOAH PHILIPS is with ATI Specialty Alloys and Components, 1600 NE Old Salem Road, Albany, OR 97321. Dedicated to Dr. Martin Palm on the occasion of his 60th birthday. Manuscript submitted June 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the ternary with Nb greater than 60 at. pct is hypothesized as no experimental data were available and other information from the literature about phase relations in the Nb corner of the system is very scarce (as was reviewed in the previous work). Isothermal sections of the ternary system were provided by Burnashova et al.[15] for 1073 K (800 C) and by Raman[16] and Zelaya Bejarano
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