Functional fatigue is one of the most significant problems to be solved for applications of superelastic alloys. Although they often have an ordered structure and highly ordered alloys tend to have less degradation of superelasticity due to cycling, it is not easy to achieve both ductility and functional fatigue resistance.

Cu-Al-Mn superelastic alloys show good ductility and cold-workability, and they have recently been used as seismic damping members in housing. The superelasticity of the Cu-Al-Mn is good until about 100 cycles, which is sufficient for normal earthquakes. However, the development of seismic damping members for long-period earthquakes over 100 cycles requires an improvement in functional fatigue resistance. The purpose of this study is to improve functional fatigue resistance in Cu-Al-Mn by the precipitation of NiAl.

Cu-Al-Mn-Ni alloys with various Al contents were prepared. Fine NiAl precipitates with the B2 structure were observed by TEM, and it was also found that the matrix changes from A2 to L21 structure with increasing Al content. This change in the ordered structure was accompanied by an increase in Vickers hardness and a decrease in cold-workability. The Cu-Al-Mn-Ni single crystals exhibited improved functional fatigue resistance compared to the Cu-Al-Mn single crystal, which is due to the particle dispersion strengthening. Increasing the Al content decreased residual strain by cyclic tensile deformation, but also decreased ductility. Therefore, it can be concluded that microstructure with the B2 particles precipitated in the A2 matrix is suitable for both ductility and fatigue properties.

Dr. Toshihiro Omori