Thermal-expansion behavior of a directionally solidified NiAl–Mo composite investigated by neutron diffraction and dilatometry

The thermal expansion of directionally solidified NiAl–Mo eutectic alloys consisting of nanoscale Mo fibers embedded in a NiAl matrix was analyzed by neutron diffraction and dilatometry. From room temperature to 800°C, perpendicular to the fiber direction, the NiAl and Mo phases expand independently with average coefficients of thermal expansion (CTEs) of 16.0×10−6°C−1 and 5.8×10−6°C−1, respectively. Parallel to the fiber direction, they coexpand up to 650°C with an average CTE of 12.8×10−6°C−1, but above this temperature the Mo fibers expand more than the NiAl matrix. This anomalous behavior is the result of the load transfer to the Mo fibers when the NiAl matrix softens. The average CTE of the composite parallel to the fiber direction was determined by dilatometry to be 13.0×10−6°C−1, which is approximately 11% lower than the value predicted by a simple rule of mixtures using the CTEs of the constituent phases.