Properties of monolithic and composite NiAl processed by hydrostatic extrusion and vacuum hot-pressing

Abstract

Nickel aluminide intermetallics and composites have been processed by hydrostatic extrusion or vacuum hot-pressing. Hydrostatic extrusion at both 298 K and 573 K was used to process nominally stoichiometric cast and previously extruded NiAl, while vacuum hot-pressing of prealloyed NiAl (45 at.% Al) powders was additionally used to process NiAl and NiAl+TiB 2 particulate composites. The microstructures, hardnesses, and compressive yield strengths of the processed materials were evaluated. It is shown that NiAl can be hydrostatically extruded at both temperatures, although substantial cracking was observed in the material extruded at 298 K. Hydrostatic extrusion at 573 K leads to significant hardness and strength increases without producing fracturing around the hardness indentations. Reinforcement with TiB 2 in the hot-pressed materials similarly provided an increase in both the compressive strength and hardness, although cracking around the hardness indents was observed. The fracture toughnesses of the hydrostatically extruded cast/extruded NiAl, the vacuum hot-pressed powder processed monolithic NiAl, and the composite NiAl intermetallics were determined in accordance with standard testing techniques. The effects of TiB 2 reinforcement particle size and distribution at 10 vol.% loading on the composite fracture toughness were also characterized. Monolithic NiAl demonstrated toughness values of about 5 MPa√m. Reinforcement additions to NiAl resulted in a toughness increase while the range of reinforcement distributions and sizes tested did not significantly change the measured composite toughness values. Significant increases in toughness and R-curve behavior were obtained in NiAl hydrostatically extruded at 573 K. The fracture behavior of the monolithics and composites is discussed in light of the deformation and fracture mechanisms operating at the crack tips, aided with in situ fracture studies and post-failure analyses.