Reactive sintering of boron-doped Ni76Al24 intermetallic

Abstract

A preliminary investigation into the formation of boron-doped nickel-rich Ni3Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni3Al) from elemental powders by reaction synthesis was carried out. The application of reaction synthesis was seen as a low-energy alternative to the production of Ni3Al/boride composite suitable for wear applications. X-ray diffraction, Neutron diffraction, SEM/EDS,WDS, Image analysis, Archimedes principle and Rockwell hardness measurements; were used to study the effect of boron addition on the final microstructure, average grain size, bulk density and hardness of as-prepared Ni76Al24. Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni3Al, however, at a boron content of 0.5 wt% an apparent transition from a single phase microstructure to a two-phase intermetallic/boride composite microstructure was observed, which dominated when the boron content increased, up to 2 wt%. The two-phase microstructure was identified as Ni3Al (particles) within an Ni41Al5B12 boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni3Al via two mechanisms. Below the solubility limit, the increase in hardness was due to solution hardening. Above 0.5 wt%B, solution hardening in addition to the formation of the harder boride phase, were found to amount to up to 50% increase in the hardness compared with boron free Ni3Al. The extrusion of semi-molten beads at the surface of the compact at high B-content may be a limiting factor, in the formation of Ni3Al/boride composites via this route.