Compared to conventional engineering materials, NiTi shape memory alloys deform via a wide range of deformation mechanisms owing to the B2⇔B19’ martensitic transformation, including twinning in martensite and plastic deformation by dislocation slip. A detailed understanding of the functional properties of NiTi requires comprehensive knowledge of all deformation processes possibly activated in thermomechanical loads. A stress-temperature diagram (Fig. 1c), constructed from the results of isothermal (Fig. 1a) and isostress (Fig. 1b) tensile tests on superelastic NiTi wire (Fig. 1a,b), provides basic information on the critical stress and temperature conditions at which individual deformation/transformation processes are activated in thermomechanical loads. The σ-T diagram is a very useful tool in NiTi research since it defines stress and temperature conditions under which martensitic transformation occurs and plastic deformation is avoided. Problems arise when multiple deformation processes are activated simultaneously, and one cannot be sure which deformation mechanism is activated. In such cases, in-situ experimental methods (e.g., in- situ electric resistivity, in-situ ultrasonic methods, in-situ x-ray diffraction) are beneficially employed. In this work, we report on the application of in-situ Dynamic Mechanical Analysis (DMA) to detect and distinguish the activation of various deformation/transformation processes during the tensile thermomechanical loading of nanocrystalline NiTi wires, particularly upon isostress heating under a wide range of tensile stresses up to fracture (Fig. 1b,d).

Deformation-transformation mechanisms in NiTi shape memory alloys (SMAs) wires subjected to tension can be investigated using various methods and techniques, often through either isothermal tensile tests or isostress thermal loading. Excluding the R-phase, there are essentially five deformation-transformation processes (elastic deformation, B2 ⇔ B19’ martensitic transformation, martensite reorientation, plastic deformation of martensite, and plastic deformation of austenite) that can be studied under both isothermal and isostress loads.

The late John Boylan and his team in Temecula, CA, filed a patent in the 1990’s that included a variety of radiopaque ternary additions to a nitinol alloy. Several of these elements including Pt, Pd, and Au, hold potential to not only raise x-ray attenuation, but also to perfect the interface between parent and daughter phases thereby increasing certain material performance aspects. Work is needed to understand and apply specific Ni-Ti-Pt chemistries and understand their practical utility. As early as 1988, one way shape memory, superelasticity, and austenitic transformation temperatures from about 0 to 1000°C were demonstrated for the Ni-Ti-Pt system by Lindquist and colleagues at University of Illinois Urbana-Champaign. Under equilibrium, stress-free conditions, the material composition for maximum austenite-martensite interface compatibility is approximately Ti 50 Ni 42.5 Pt 7.5 . Further study is warranted since improved crystallographic compatibility may improve the structural and functional performance of critical medical and industrial subcomponents where performance gains could pay for higher initial material costs.