Elinvaric behavior of cold rolled NiTi shape memory alloys

The macroscopic Young's modulus E of NiTi SMA's are strongly sensitive to temperature, which limits the application in high-precision devices. In our study, it is discovered that temperature insensitive modulus or elinvaric behavior can be obtained over large temperature span in the cold rolled nanocrystalline equiatomic NiTi sheets. The effects of grain size, texture and thermal-induced phase transition of B2 and B19' crystallites on the macroscopic E(T) behavior are investigated. It is shown that the temperature coefficient of Young's modulus (dE(T)/dT) of the sheets can be controlled by controlling the volume fractions and textures of martensite B19' and austenite B2 crystal through different cold rolling and heat treatment procedures. A very small dE(T)/dT (0.003 GPa/K) in a large temperature range of -100K -300K is obtained in the cold-rolled nano-crystalline NiTi sheet. The in-situ TEM and XRD results show that the nano-scaled B2 and B19'crystallites have high stability against the phase transition due to the grain size reduction and the defect pinning in the cold rolling, so that thermal induced B19'to B2 and B2 to B19' phase transitions are significantly suppressed during heating and cooling. It is shown that it is the compensation of the intrinsic positive (B2) and negative (B19') temperature coefficient of elastic constants of the co-exiting two crystals that results in the observed elinvaric behavior of the cold rolled nanocrystalline NiTi sheets over the wide temperature span.