Nitinol is extensively used as an advanced structural material because of its superelasticity.  However, among apparently identical samples, the onset of superelasticity, the length of the superelastic plateau, and the amount of work hardening vary dramatically.  Large variation is seen when the modality of loading is changed, for example from tension to torsion.  It is widely believed that these asymmetries arise from the crystallographic texture introduced during fabrication.  But the precise relationship between grain misorientation and mechanical response is still not understood, making it impossible to model the device response accurately.  Herein, based on x-ray diffraction measurements, we present quantitative assessment of the mechanical asymmetry in superelastic Nitinol. 

Superelasticity in Nitinol is driven by a first order transition from cubic austenite to monoclinic martensite.  We show that both the elastic anisotropy in the austenite and the transformation anisotropy in the martensite contribute to the mechanical asymmetry, but the impact of the transformation anisotropy is significantly larger.  The elastic anisotropy in the austenite, measured as the ratio of the <111> to <100> modulus, is 1.3, which is similar to BCC Fe.    However, the onset of superelasticity is significantly more anisotropic.  The transformation of <110> grains begins when the local stress exceeds 300-400 MPa, followed by <211> grains at 500 MPa, then <100> above 1000 MPa.  Furthermore, the transformation relieves some of the elastically stored strain, and consequent perturbation of the local stress field guides subsequent transformation.   The large transformation anisotropy and the transformation driven perturbation of the local stress field explain the large observed mechanical asymmetries associated with texture and load modality, and provide a foundation for the next generation of crystallography based design models.