Superelastic polycrystalline NiTi is one of the most popular "shape memory alloy"(SMA) in the world, it has been widely used in engineering, and medical devices. But some of its basic properties have not been investigated completely, for example the stress drop during macroscopic martensitic band nucleation. The thesis will focus on this property....[ Read more ]

Superelastic polycrystalline NiTi is one of the most popular "shape memory alloy"(SMA) in the world, it has been widely used in engineering, and medical devices. But some of its basic properties have not been investigated completely, for example the stress drop during macroscopic martensitic band nucleation. The thesis will focus on this property.

Basic properties of the material like grain size, transformation temperatures, surface profile and composition were found by different material characterization method. Also, the deformation field during stress-induced martensitic transformation was first studied by using high sensitivity Moire Interferometry.

Uniaxial tensile tests were conducted for different shaped NiTi of different sizes, different lengths and different constraints using different machines to find the characteristics of the martensitic band nucleation and propagation.

The size effect, which includes the wire diameter effect and the length effect, was first investigated, the experiments were then conducted systematically from macroscopic observation to mesoscopic observation. The experimental discovery showed that the stress-strain response is strongly dependent on the size of the material.

To explain the results of the size effect, experiments like tensile test and infrared camera imaging were conducted, pre-peak martensitic transformation was found in the results of the experiments and detailed discussions were given.

Key words : shape memory alloy, superelastic, stress-induced martensitic transformation, pre-peak martensitic transformation, uniaxial tensile test, Moire interferometry, deformation field, size effect.