Microstructure, phase constitution and tensile properties of Co-Ni-Ti-Al base multi-phase 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, L1₂ and L2₁ (Heusler), and the primary solid solution denoted as (Co, 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/Li₂, L1₂/L2₁ and B2/L1₂/(Co, Ni) phases after annealing at 1273 K. It was found that limited tensile ductility can be achieved in the B2/L1₂ and B2/L1₂/(Co, Ni) alloys but not in the L1₂/L2₁ 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 L1₂ phase, (3) separation of B2/L1₂ 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 L1₂/B2/(Co, Ni) three phases was found to exhibit over 20% room temperature ductility, with a yield strength of over 600 MPa.