Microstructures and mechanical properties of a directionally solidified NiAl-Mo eutectic alloy

A NiAl-Mo ternary eutectic alloy, having the nominal composition Ni-45.5Al-9Mo (at.%), was directionally solidified in a high-temperature optical floating zone furnace. Well-aligned rod-like microstructures were obtained, consisting of NiAl matrix and 14% (by volume) continuous Mo fibers having a square cross-section. With increasing growth rate (from 20 to 80 mm/h), the spacing and size of the Mo fibers decreased, from ≈2 to 1 μm (fiber spacing) and 800 to 400 nm (fiber size). X-ray microprobe analyses revealed that the NiAl matrix contained essentially no Mo (<0.1 at.%) and had the composition Ni-45.2Al (at.%), while the composition of the Mo fibers was Mo-10.1Al-3.9Ni (at.%). From electron backscatter diffraction patterns, the following orientation relationships were obtained: 〈1 0 0〉NiAl//〈1 0 0〉Mo, and {0 1 1}NiAl//{0 1 1}Mo. The growth directions and Mo/NiAl interface boundaries were found to be parallel to 〈1 0 0〉 and {0 1 1}, respectively, in both Mo and NiAl. Nanoindentation was used to probe the mechanical properties of the individual phases in the eutectic microstructure and the modulus and hardness of the NiAl matrix were determined to be 180 and 2.9 GPa, respectively, and those of the Mo fibers 270 and 4.8 GPa. Tensile tests were used to investigate the temperature dependence of the strength and ductility of the composite. Its ductile-to-brittle transition temperature was found to be ∼675 °C, and its yield strength about 25-30% higher than that of 〈1 0 0〉 NiAl single crystals over the temperature range 600-1000 °C.