Microstructures and Mechanical Properties of Ni-(50-x)Al-xSc Alloys Solidified in Water-Cooled Copper Mold

Ni-(50-x)Al-xSc (at%) alloys were prepared by using a vacuum arc smelting furnace with a water-cooled copper mold. The microstructures of the alloys were observed by optical microscope and scanning electron microscope (SEM). The phases of the alloys were analyzed by X-ray diffraction (XRD) and energy spectrum (EDS). The solidification processes of the alloys were analyzed based on scanning calorimetry (DSC) results. The mechanical properties of the alloys were analyzed by micro Vickers hardness test and nanoindentation test. With the two-phase system simplified model, the overall elastic moduli of the alloys were estimated. The results show that the sub-rapid solidification structures of Ni-50Al, Ni-45Al-5Sc, Ni-40Al-10Sc and Ni-35Al-15Sc alloys are NiAl, NiAl+AlNi2Sc, NiAl+AlNi2Sc and NiAl+AlNi2Sc+(AlNi2Sc+Ni-16.93Al-21.53Sc), respectively. The precipitation of each phase in the alloys is in the sequence of NiAl, AlNi2Sc and (AlNi2Sc+Ni-16.93Al-21.53Sc) accordingly. The value of crystal growth Jackson factor of AlNi2Sc compound is alpha=0.2, much smaller than 2, which means AlNi2Sc phase grows in a continuous manner and a coarse solid/liquid interface. The hardness of the primary NiAl phase is increased by alloying with Sc element and the hardness of the secondary phase AlNi2Sc is greater than that of the primary NiAl phase. Therefore, the hardnesses of the Ni-(50-x)Al-xSc alloys increases. Alloying with Sc can reduce the elastic modulus of the primary NiAl phase, and the overall elastic moduli of Ni-(50-x)Al-xSc alloys decrease compared with that of the NiAl intermetallic compound.