On the mechanism that leads to vanishing thermal hysteresis of the B2-R phase transformation in multilayered (TiNi)/(W) shape memory alloy thin films

The film stresses in two-phase (TiNi)/(W) shape memory alloy (SMA) multilayer thin films were evaluated using synchrotron diffraction analysis. The phase transforming B2-TiNi phase is under tensile stress due to the mismatch of the coefficient-of-thermal-expansion (αB2-TiNi > αW > αSi-substrate) and the elastic modulus (EW > ESi > EB2-TiNi) with respect to the bcc-W layers and the Si-substrate. The amount of stress on the B2-TiNi phase increases with increasing W amount in the film, which is proportional to the W layer thickness. This led to important changes in the behavior of the B2-R transformation. On cooling, a B2-R transformation proceeds under increasing tensile stress which increases the transformation start temperature (Rs). Upon transformation to the R phase, the TiNi layers undergo stress-relaxation by reorientation of R phase variants to accommodate the mismatch. During heating the film always starts from a relaxed stress-state, so the reverse transformation proceeds without adversely affecting the reverse transformation temperature (Af). With increasing amount of W in the film Rs increases more on cooling, while Af is not significantly affected on heating, and this leads to vanishing thermal hysteresis (ΔTB2-R = Af − Rs).