Gradient deformation behavior of NiTi alloy by ageing treatment

• The suitable temperature range to create gradient plateau is between 400 °C and 460 °C.
• The gradient plateau exhibited similar stress interval to the isothermal curves.
• The transformation plateau produced a liner slope of 130 MPa stress interval.

The applicability of ageing treatment in creating gradient thermomechanical behavior in a Ni-rich NiTi shape memory alloy is investigated in this work. Based on the deformation behavior of isothermal ageing, the NiTi wire was designed to have a near-perfect gradient pseudoelastic behavior by means of gradient ageing treatment. The linear slope on both forward and reverse transformations was achieved by ageing a wire specimen in a temperature gradient of 400–460 °C. The gradient-aged wire exhibited martensitic transformation over a linear slope, producing 130 MPa of transformation stress interval on both forward and reverse. The slope illustrates a gradient thermomechanical property of the wire specimen, and corresponds to the effect of gradient ageing temperature. Such gradient pseudoelastic behavior provides an activation stress of 130 MPa for progressive motion of NiTi apparatuses, especially in control and actuation applications.