Microstructural properties of Nb-Si alloys investigated using EBSD at large and small scale
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I. INTRODUCTION
ALLOYS in the Nb-Si system have been found to be good candidates for high-temperature applications,[1,2] especially for future increased temperature jet engines. The poor toughness and oxidation resistance of binary alloys can be improved by alloying, leading, e.g., to the NbTiHfCrAlSi MASC family,[3] and by tuning the microstructure, through appropriate processing routes (casting, directional solidification, powder metallurgy, hot-working, etc.) and heat treatments.[4] An accurate characterization of the microstructure is thus necessary at various steps of the alloy processing cycle. The microstructure of arc-melted binary alloys before and after heat treatment (mostly at temperatures between 1500 °C and 1700 °C) has been widely described in the literature[5] depending on the silicon content, e.g., for hypoeutectic, eutectic (Si 18.7 pct; all compositions are given in at. pct), and hypereutectic compositions, as far as phase identification and morphology are concerned. However, texture and crystallographic orientation relationships have been less studied, despite the fact that texture can have a significant effect on mechanical properties. Orientation relationships between Nb3Si precipitates and matrix within Nb dendrites in hypoeutectic binary alloy (Nb14Si) have been described by Grylls et al.[6] using transmission electron microscopy (TEM). Cockeram et al.[7] have defined, for a eutectic binary alloy, the crystallographic orientation relationships between Nb rods and Nb3Si matrix for different processing routes (as cast, directionally solidified, and extruded).
Arc-melted Nb-Si alloys exhibit microstructural features at both large and small scales: the width of dendrites ranges roughly from 10 to 100 m, while the Nb or the silicide phases extend in the eutectic phase over about 1 m. Electron backscattering diffraction (EBSD) is a microstructure characterization technique complementary to scanning electron microscopy (SEM) and TEM, giving access to local crystallographic information, such as orientation relationships; this technique is well suited to study microstructures at small and large scale, with a spatial resolution lower than 1 m, and with the capacity of analyzing fields extending over several hundreds of micrometers. The EBSD results have been reported for various compositions and processing routes, except for arc-melted binary Nb-Si alloys. In an extruded Nb-10Si alloy, Sutliff and Bewlay[8] found a [110] fiber texture parallel to the extrusion direction in the Nb phase, and a [001] direction in Nb5Si3 perpendicular to the extrusion direction. For a directionally solidified Nb-33Ti-16Si alloy, Sutliff and Bewlay[9] reported a strong texture for both phases: for Nb3Si, an alignment of [001] with the growth direction, and for the metallic phase, a broader alignment of [113] with that direction. This study investigates texture and crystallographic orientation relationships in arc-melted binary Nb-Si alloys. The EBSD is used here in conventional conditions, i.e., at relatively high sp
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