Ultrahigh cycling stability and large R-phase thermal hysteresis in heterostructure TiNi/Ag film via introducing constrained layer

A novel strategy introducing an Ag constrained layer on TiNi films is proposed to develop high functional stability and longtime intervals for microactuator applications. Thermal expansion mismatch between TiNi and Ag causes residual compressive stress in TiNi film, resulting in B2→R→B19′ diverse transformation upon cooling and high energy barrier of B2→R transformation, thereby obtain large R phase thermal hysteresis (∼19°C). An increase in the Ag layer thickness caused the expansion of the interface influence zone, which induced a stronger constraint effect. This effect refines grains and stabilizes R phase, suppressing R→B19′ transformation and dislocation generation. Consequently, an ultra-small shift in transformation temperature (∼0.30°C) is achieved after 100 cycles. This new strategy provides distinct insight for the design of micro actuator materials with large output displacement, high reliability and longtime intervals.