Structure and Properties of NiTi Shape Memory Alloys after Severe Plastic Deformation

The 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 εc = 6.20. Finally the alloys were annealed at the temperature range 300 – 500°C for 1 hour. The 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°C specimens. Multi-stage transformation was observed in the annealed at lower temperatures samples.

Structure and Functional Properties of Microcrystalline NiTi Alloy after Severe Deformation and Subsequent Annealing

Thermomechanical treatment was applied to a binary NiTi alloy in order to improve its functional properties by forming nanocrystalline structure of the alloy. The alloy deformation was obtained by cold rolling combined with transverse movement of the rolls. This technique allowed us to obtain high strain (c ≈ 6) for the relatively large specimens. Subsequently, the samples were annealed in the temperature range 300 -500oC in order to form a nano-, submicro –and/or microcrystalline structure. The evolution of the structure and associated changes of the transformation sequences and functional properties were studied with the use of TEM, X-ray phase analysis, DSC and bend and free recovery ASTM tests. A mixed amorphous/crystalline structure was obtained after severe deformation, the martensitic transformation was completely suppressed in the sample. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500oC specimens.