Mechanics of Large, Uniaxial Deformations of Martensitic and Superelastic Nickel-Titanium

Existing knowledge of elasticity, twinning, and slip was used to develop novel diffraction data analysis techniques. These techniques allow one to empirically quantify the contribution of individual deformation mechanisms to macroscopic stress-strain behaviors of phase transforming crystalline solids. These methods were applied in analyzing neutron diffraction data of tensile & compressive deformations to ~ 18% true strain of monoclinic NiTi (i.e., thermal martensite) and subsequent thermal induced phase transformation events. Transformation and deformation twinning events are directly observed and individual contributions of each deformation twinning and slip to the total polycrystalline strain of these materials is quantified for the first time; specifically it was found that 10% to 30% of the plastic response is due to deformation twinning and 70% to 80% due to slip. This study is also the first to document deformation twinning during compressive loading of a martensitic SMA.