MICROSTRUCTURE, PHASE CONSTITUTION AND TENSILE PROPERTIES OF CO-NI-TI-AL BASE MULTIPHASE INTERMETALLIC ALLOYS

Three multi-phase alloys were selected in the Co-Ni-Ti-Al system with microstructures consisting of combinations of such intermetallic phases as the B2, L1(2) and L2(1) (Heusler), and the primary solid solution denoted as (Go, Ni). They were hot fabricated after drop casting and were heat treated to obtain the multi-phase structures. Optical metallography and electron microprobe analyses revealed that the alloys contained B2/L1(2), L1(2)/L2(1) and B2/L1(2)/(Co, Ni) phases after annealing at 1273 K. It was found that limited tensile ductility can be achieved in the B2/L1(2) and B2/L1(2)/(Co, Ni) alloys but not in the L1(2)/L2(1) alloy. Fracture analyses on the former two alloys showed that the causes for the brittle fracture include: (1) cleavage fracture in the B2 phase, (2) intergranular fracture of the polycrystalline L1(2) phase, (3) separation of B2/L1(2) interfaces, and (4) environmental effect. Based on such analyses, a new alloy was designed to minimize the above factors for ductility improvement. The alloy consisting of L1(2)/B2/(Co, Ni) three phases was found to exhibit over 20% room temperature ductility, with a yield strength of over 600 MPa.