Superelasticity of slim hysteresis over a wide temperature range by nanodomains of martensite

By nanostrain-domain engineering of shape memory alloys (SMAs) via impurity doping, we show a new mechanism that leads to superelasticity with slim hysteresis across a wide temperature range. Three-dimensional computer simulations using the Landau theory of phase transformations and Khachaturyan's microelasticity theory predict the formation of randomly distributed nanosized, single-variant martensitic domains in Fe-doped NiTi SMAs. These nanoscale martensitic domains are frustrated and cannot evolve into long-range-ordered, internally twinned structures (i.e. long-range strain ordering). Such a structural state is found to evolve gradually upon loading and unloading or heating and cooling across a wide temperature range with narrow hysteresis. The simulation predictions have been confirmed by experiments carried out by doping a conventional SMA, Ti₅₀Ni₄₈Fe₂, with extra Fe into a new composition of Ti₅₀Ni₄₄Fe₆.