X-ray diffraction studies of NiTi shape memory alloys

Abstract

Purpose: The purpose of this paper is to present the results of the investigations of phase transitions of TiNiCo and Ni-rich NiTi shape memory alloys designed for medical applications. Design/methodology/approach: Temperature X-ray diffraction (TXRD), differential scanning calorimetry (DSC), electrical resistivity (ER) and the temperature shape recovery measurements in three-point bending ASTM 2082-01 tests were used. Findings: It has been found in this work that ageing after solution treatment and annealing below the recrystallization temperature after cold working in the alloys studied create separate reversible B2↔R↔B19’ transformations. During thermomechanical cycles characteristic temperatures of the reversible B2↔R phase transition remain stable. It was concluded that ageing after solution treatment or recovery during annealing after cold working causes the precipitation process and the changes of the defect structure of the alloys promote transitions with the R-phase contribution. Research limitations/implications: The results of the courses of transformations and their characteristic temperatures obtained by TXRD, DSC and ER techniques have a good correlation. Future TXRD research with the use of automatic rapid recording of diffraction patterns during cooling and heating are necessary. The course of phase transitions of the studied alloys determine their applications. Practical implications: The obtained results can be applied into the practice of processing and thermomechanical treatments of NiTi alloys designed for the production of shape memory medical implants and devices which act under the influence of the human body heat. Presented are the conditions of thermomechanical treatment to obtain a wide temperature range for the R-phase existence in the investigated alloys. Originality/value: The paper presents new results of optimization of the thermal treatment of NiTi shape memory alloys to obtain the reversible B2↔ R transformation used to prepare new shape memory implants andmedical devices which exhibit shape recovery at a narrow temperature range below the human body temperature. In this paper it was shown that the temperature X-ray diffraction method can be used for the visualization of transformation courses and obtaining characteristic temperatures of transformations.