Heterogeneous Nb-Based Nuclei for the Grain Refinement of Al-Si Alloys

L Bolzoni,N Hari Babu

doi:10.1007/s11837-016-1855-8

Abstract

Nb-based intermetallics are, generally, low-density high-temperature materials used for structural applications or cryogenic superconductors. In this work, we report the development of an Al(96)-Nb(2)-B(2) master alloy where in situ-formed micrometric Nb-based intermetallics (i.e. NbB2 and Al3Nb) are used for a completely different purpose: to promote the refinement of Al-Si alloys by taking advantage of enhanced heterogeneous nucleation. Nb-based intermetallics have the right characteristics, like low density, stability at high temperature and good lattice match, to be used as heterogeneous nucleation substrates. It was found that the addition of these Nb-based intermetallics permits the significant refinement of the microstructural features of the Al-Si alloy studied. The enhanced heterogeneous nucleation makes the grain size of the material far less dependent on the cooling rate, which is one of the critical parameters influencing the variation of the properties of the alloy.

Highlights

Niobium is a transition metal characterized by some specific properties such as superconductivity at the cryogenic temperature of 9.2 K
Examining the master alloy with the scanning electron microscopy (SEM) permitted the confirmation of the formation and nature of the Nb-based intermetallics (i.e. NbB2 and Al3Nb) due to the reaction of niobium with boron or aluminium, respectively
Micrometric Nb-based intermetallics were formed in situ in an Al(96)-Nb(2)B(2) master alloy in order to take advantage of their intrinsic characteristics such as low density and high-temperature stability

Summary

Introduction

Niobium is a transition metal characterized by some specific properties such as superconductivity at the cryogenic temperature of 9.2 K. NbB2 has been extensively studied and it has been reported that it exhibits superconductive transition behaviour depending on the stoichiometry of the compound.[1,2,3] Niobium aluminides, Al3Nb, were developed due to promising attributes for hightemperature applications such as high melting point and low density,[4,5] high strength over a wide range of temperatures and good oxidation resistance. These properties made them strong candidates for advanced aeropropulsion systems and for other high-temperature components, either as a coating or as structural materials. In our first works,[16,17] in which the Nb-based intermetallics were formed (Published online April 1, 2016)

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