Microstructures and mechanical properties of directionally solidified NiAl–Mo and NiAl–Mo(Re) eutectic alloys

The room temperature fracture toughness of a directionally-solidified (DS) NiAl–9 at.%Mo eutectic alloy was evaluated. It was found that the toughness of NiAl-refractory metal eutectic composites may be limited by the low ductility of the reinforcing bcc metal phase at room temperature and weak interfaces. The effects of the residual interstitial impurities and solid–solution-strengthening of Mo fibers by Ni and Al which partition to the fibers from the matrix on the mechanical properties were identified. The potential of rhenium (Re) addition, which is known to solid–solution soften and lower the ductile-to-brittle transition temperature of various bcc metals, to enhance the ductility and toughness of DS NiAl–9Mo eutectic alloy was also investigated. Re partitioned to the bcc metal phase and formed a substitutional solid solution. The interface morphology was changed from a faceted to a non-faceted one. Re alloying caused softening of the Mo fibers, and as a result the NiAl–Mo(Re) alloys were softer in compression and had ∼20% higher fracture toughness values as compared to the transverse orientation toughness of NiAl–9Mo alloy. The toughness of the NiAl–Mo(Re) alloys was lower than the longitudinal orientation toughness of the NiAl–9Mo alloy due to the poor alignment of the Mo(Re) phase with the growth direction. Processing conditions were successfully changed to produce plane front solidification resulting in completely eutectic microstructures with the fibers well-aligned with the growth direction in the NiAl–9Mo alloy; similar effort is needed for NiAl–Mo(Re) alloys to achieve higher toughness in the longitudinal orientation. The toughening mechanisms operative in these alloys are discussed.