Structure and Properties of NiTi Shape Memory alloys After Severe Plastic Deformation

Results presented here concern two NiTi alloys (near-equiatomic NiTi and Ni-rich alloy) subjected to plastic deformation by compression combined with reversion oscillating torsion. The maximal strain obtained was epsilon(c) = 6.20. After the deformation process the alloys were annealed at the temperature range 350 - 500 degrees C for 1 hour. Structure of the as-prepared alloys was studied with the use of temperature X-ray diffraction and TEM observations. Also the DSC and bend and free recovery ASTM tests were carried out. It was found that the structure consists of a mixture of highly deformed B2 parent phase and B19' martensite. The TEM studies revealed some amorphous areas in the most strained region of the samples. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500 degrees C specimens. A multi-stage transformation was observed in the samples annealed at lower temperatures.