Elastic Anisotropy in Polycrystalline Superelastic Nitinol

The phase transformation in superelastic Nitinol is a non-load bearing process in which the material undergoes a significant amount of reversible deformation, or strain, without an increase in stress. However, this is a bulk phenomenon and the local micromechanical deformation state of the material is not well understood. Diffraction techniques are capable of measuring strain at the local level. Until recently, diffraction was only viable for materials in which the crystallite size was larger than the incident beam size. In here, we use a high energy x-ray diffraction technique to analyze the full second rank strain tensor along different crystallographic directions during the phase transformation in polycrystalline superelastic Nitinol. It can be seen that certain directions bear more load during the transformation process while others are shielded from the load. This leads to a better understanding of the martensite variants that form and strain environment surrounding the newly formed phase.