Reactive sintering of boron-doped Ni sub 76 Al sub 24 intermetallic

• Published in: Journal of materials science 2002-05, Vol.37 (10), p.2005-2009
• Main Authors: Moussa, S O, Morsi, K
• Format: Article
• Language: English
• ISSN: 0022-2461

A preliminary investigation into the formation of boron-doped nickel-rich Ni sub(3)Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni sub(3)Al) 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 Ni sub(3 )Al/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 Ni sub(76)Al sub(24). Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni sub(3)Al, 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 Ni sub(3)Al (particles) within an Ni sub(41)Al sub(5)B sub(12) boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni sub(3)Al 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 Ni sub(3)Al. 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 Ni sub(3)Al/boride composites via this route.