T. Sawaguchi, K. Ogawa, T. Kikuchi, National Institute for Materials Science, Tsukuba, Japan
In seismic countries, anti-seismic dampers using low-yield-point (LYP) steels have become popular due to their cost efficiency. However, LYP dampers suffer from the shortcomings, namely, degradation due to cyclic hardening and a permanent deformation following a strong earthquake. Very recently, we found that an Fe-28Mn-6Si-5Cr-0.5NbC shape memory alloy (SMA) shows a reversible γ <-> ε transformation under cyclic loading of plastic tension-compression deformation, and absorbs the energy of vibrations with strain amplitudes more than ± 0.1%. Because of the reversible microstructural change, the SMA can be used as a new anti-seismic damping alloy without cyclic hardening, and with longer fatigue lives than LYP steels, which absorb the vibration energy via the irreversible motion of dislocations. The same investigation was made on various Fe-Mn-based alloys. The AFM observations revealed that SMAs with Ms temperatures near or below room temperature, such as Fe-30Mn-6Si, Fe-30Mn-5Si-1Al, Fe-28Mn-6Si-5Cr showed the reversible motion of γ/ε interfaces. It is surprising that cyclic hardening in these alloys is negligible even after ten times tension-compression loading with the strain-amplitude of as much as ± 3%. In TWIP steels such as Fe-30Mn-4Si-2Al and Fe-30Mn-3Si-3Al, cyclic hardening is also very small at the strain-amplitudes less than ± 1%, but it drastically starts to increase when the strain-amplitude exceeds ± 1%. The AFM observation on a TWIP steel showed that compression did not cause any reversal movement of deformation twin boundaries, but produced new deformation twins on a different habit plane from those formed during tension.
Summary: A novel application of Fe-Mn-Si-based shape memory alloys and TRIP/TWIP steels, which undergo γ/ε martensitic transformation, was found in anti-seismic damping of civil constructions. The shape memory alloys with M
sε temperatures near or below room temperature show the reversible motion of γ/ε interfaces under cyclic tension-compression loading without cyclic hardening. These alloys can be used as maintenance free-type anti-seismic dampers in a very wide strain-amplitude range from ± 0.1 to 3.0%. The TRIP/TWIP steels with M
sε far below room temperature also showed very small cyclic hardening at the strain-amplitudes less than ± 1%, but the strain-amplitude more than ± 1% leaded to a significant cyclic hardening.
