Abstract
Unidirectional solidification of eutectic alloys offers a favorable approach for producing metal matrix composites. In this paper, Ni-Ni3Ta eutectic alloys have been unidirectionally solidified over a wide range of growth rate, and the microstructures and the mechanical properties were studied. In this eutectic system, a solubility limit of Ta in Ni (α) phase decreases considerably with temperature, suggesting the possibility of obtaining additional Ni3Ta phase and strength via precipitation processes. Main results obtained here are as follows. (1) The microstructure of unidirectionally solidified Ni-Ni3Ta eutectic alloy consists of roughly parallel platelike Ni3Ta embedded in Ni (α) matrix. Well aligned structure was obtained at the range of solidification rate between 100 mm/hr to 150 mm/hr, and maxima in tensile strength versus solidification rate curve occured in this range. (2) Widmanstätten type Ni3Ta precipitates were produced in Ni (α) matrix after heat treatment, and morphologies of precipitate structures were differed by the heat treatment conditions. Significant strengthening effect was noted in the specimen heat treated at 1, 000°C for 48 hrs. (3) Unidirectionally solidified and heat treated Ni-Ni3Ta eutectic composites exhibited excellent mechanical properties. The tensile strength was 129 kg/mm2 at room temperature and 51 kg/mm2 at 1, 000°C. (4) At room temperature, the unstable fracture of Ni-Ni3Ta eutectic composites was caused by the cleavage of Ni3Ta platelet, whereas at high temperature, the stable fracture of the composites was accompanied with the multiple fracture of the platelets.
