Improvement in room temperature ductility of intermetallic alloys through microstructural control

A summary is presented of the recent advances in the development of multiphase intermetallic alloys towards improved room temperature ductility investigated by the present authors. The alloy systems of interest are Ni-Al-Be, Co-Al-Ni-Ti, and Co-Al-C, in which two- or three-phase alloys can be designed involving the Ll2, B2 and the primary solid solution, hereby denoted as (Ni) or (Co,Ni) in the first two systems, while the B2, E21, and the primary solid solutions, denoted as (Co) in the latter system. It is shown that by choosing a proper combination of phases with particular morphologies, good room temperature ductility can be obtained not only by compression and bending tests but also by tensile tests. Such a strategy has been so far most successful in the Co-Al-Ni-Ti system, where room temperature ductility of near to 20% is achieved in a B2/Ll2/(Co,Ni) three-phase alloy. It is pointed out that keys to design ductile multi-phase intermetallic alloys would be: (1) refinement of the microstructure utilizing solid state phase transformations such as an invariant reaction: (2) adjustment of amounts and compositions of the constituent phases, which is not possible in the binary system, but is in a multi-component system, and (3) hot fabrication to reduce solidification defects.