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 phase s 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 metall ography and electron microprobe analyses revealed that the alloys cont ained B2/L1(2), L1(2)/L2(1) and B2/L1(2)/(Co, Ni) phases after anneali ng at 1273 K. It was found that limited tensile ductility can be achie ved 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 th e causes for the brittle fracture include: (1) cleavage fracture in th e B2 phase, (2) intergranular fracture of the polycrystalline L1(2) ph ase, (3) separation of B2/L1(2) interfaces, and (4) environmental effe ct. Based on such analyses, a new alloy was designed to minimize the a bove factors for ductility improvement. The alloy consisting of L1(2)/ B2/(Co, Ni) three phases was found to exhibit over 20% room temperatur e ductility, with a yield strength of over 600 MPa.