Low-Temperature XRD Study of Phase Transformations in NiTi and TiNiCo Shape Memory Alloys Used for the Preparation of the Prototypes of Medical Implants

The NiTi shape memory alloys with ternary additions of Co are being considered for future applications in the construction of medical products. In this study the commercially available Ni50.8Ti49.2 medical alloy and Ti50Ni48.7Co1.3 alloy obtained by the conventional VIM technique, were used to prepare shape memory and superelastic staples. The phase transformations in the wires of those alloys after various thermo-mechanical and thermal treatments have been defined by the differential scanning calorimetry (DSC) method and three-point bending and free recovery ASTM F2082-06 tests. In this work the courses of phase transformations in the studied alloys were investigated by means of the low-temperature X-ray powder diffraction method. In both alloys after cold working and annealing during cooling two phase transformations occur: from parent B2-phase to rhombohedral R-phase and monoclinic B19’ martensite phase. Such phase transformations are fully reversible during heating and the obtained characteristic temperatures from DSC and X-ray powder diffraction measurements are in good agreement.