Actuator Response of Improved Two-Way Memory TiNi Wires Evaluated by Weight Fraction Diagrams

This paper experimentally studies the improvement in the actuator response of TiNi shape memory wires brought about by thermal treatments. Heat-treated TiNi wires were thermally cycled at zero stress before being trained by constant stress to develop the two-way shape memory effect. Subsequently, the work output of these two-way memory TiNi shape memory alloys are measured during repeated thermomechanical cycling under various levels of constant stress. Changes in the phase transformation behavior in two-way memory and thermomechanically cycled TiNi shape memory alloy wires are quantified by x-ray diffraction as a function of temperature. The weight fraction diagrams of TiNi wires thermally cycled at zero stress before they were trained suggests that during constant stress training they develop a lower quantity of R-phase than samples that have not been thermally cycled at zero stress before being trained. This gives thermally cycled TiNi samples higher levels of transformation strain and work output during thermomechanical cycling than samples that have not been thermally cycled before training. These results suggest that for the best material performance—that is, significant transformation strain and, consequently, substantial work output—the TiNi wire should be thermally cycled at zero stress before training.