Crystallization, Phase Transformation And Shape Memory Properties Of TiNiCu Melt-spun Ribbon

TiNi-based shape memory alloys have been widely used as actuators and sensors because of their excellent shape memory effect as well as superelasticity. Rapid solidification by melt spinning can introduce changes in the microstructure and other characteristics in comparison to the bulk material, as the reduction of the grain size (even obtaining fully amorphous as cast ribbons under a high cooling rate). Proper annealing process can flexibly modify the final crystalline structure, and thus some properties of the martensitic transformation for the amorphous ribbon. Recently, the technique is used to produce TiNiCu ribbon with high Cu content. However, Research mainly focuses on the Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon, and few works on Ti-rich TiNiCu ribbon, which constrains the application domain of TiNiCu alloy ribbon.In this paper, several Ti-rich TiNiCu ribbons were fabricated by single-roll melt spinning technique. The microstructure of as-cast and annealed TiNiCu ribbons is studied, and the crystallization parameters and kinetic mechanism of amorphous ribbon are presented. Moreover, the effect of annealing treatment and thermal cycling on the marentsitic transformation characteristics is explored. Finally, the mechanical properties and its shape memory properties are also investigated based on the tensile experiments and thermomechanical cycling experiment.The results showed that, the microstructure of Ti_53.5Ni_22.8Cu_23.7 ribbon is fully amorphous; however, the as-cast Ti_51.7Ni_24.5Cu_23.8 ribbon was mainly amorphous embedded with a small amount of crystalline particles composed of B2 or B19. B19 martensite variants show various morphologies such as needle, coarse plate, strip and hook. With increasing annealing temperature, the structure of Ti_53.5Ni_22.8Cu_23.7 ribbon evolutes from B2—>B19—>B19+ precipitates at room temperature. The morphology of martensite varies from nanocrystalline to twinned martensite. The plate precipitates coherent with the matrix were formed for the ribbon annealed at 450 ℃, and Ti₂(Ni+ Cu) precipitates were formed when annealing temperature is higher than 550℃. With the ribbon annealing at 650℃, moire fringe can locally be formed due to high density of overlapping irregular structured grain boundaries. The crystalline for Ti_51.7Ni_24.5Cu_23.8 ribbon annealed at 400 ℃ is mainly B19 martensite with a single-pair variant in most of the grains. There exits two pairs of variants self-accommodated with perpendicular orientation in some particles. The hook-like martensite variants were found to disappear and new martensite preferred to nucleate at the grain boundary.The crystallization onset temperature of amorphous Ti_53.5Ni_22.8Cu_23.7 ribbon was 432°C by continuous DSC heating at 10°C/min. The apparent crystallization activation energies for the ribbon calculated by Kissinger's method and Ozawa method are 441.5 kJ/mol and 453.5 kJ/mol, respectively. However, the data are overestimated the local crystallization activation energy determined by Doyle method. The Avrami exponent n determined by isothermal DSC experiments at the temperature range from 435 to 450℃ is from 2.34 to 3.92, suggesting that the crystallization is three-dimensional diffusion controlled process. Different from the polymorphic crystallization of Ti₅₀Ni₂₅Cu₂₅ alloy ribbon, the ribbon exhibit a primary reaction. In-situ XRD analysis shows that the texture of B2 evolutes from B2-(200) to B2-(110) and then keeps constant with increasing annealing temperature.The martensitic transformation temperature of Ti_53.5Ni_22.8Cu_23.7 ribbon annealed at 450～650°C increases with annealing temperature. Its average transformation hysteresis is 13.3℃ and the transformation heat is typical lower than 10J/g. The transformation hysteresis, which is stabilized about 13～15°C, is not sensitive to holding time when the ribbon was annealed at 450～500 °C for 1 to 10h. Thermal cycling can stabilize martensitic transformation temperature and hysteresis. The hysteresis for the ribbon annealed at 450 °C for 10min and 1h are 15.3 and 21°C, respectively.The tensile fracture stress and strain of as-cast Ti_53.5Ni_22.8Cu_23.7 ribbon and Ti_51.5Ni₂₅Cu_23.5 are 1257 MPa, 3.85%, and, 1068 MPa, 3.7%, respectively. The morphology of fracture shows typical vein fringe as well as amorphous featureless zone. The tensile stress decreases after annealing treatment, and the fracture features show equi-axis crystalline and dimple. After the tensile experiments of Ti_51.5Ni₂₅Cu_23.5 ribbon annealed at 450°C for 1h, it can exhibit detwinning behavior, by which the unity of martensite variants morphology is destroyed, and the moire fringe can be formed with irregular structured grain boundaries overlapping.Based on the thermomechanical cycling experiment, it shows that the martensitic transformation temperature and transformation strain increased with increasing applied tensile stresses. The obtained transformation hysteresis is obviously higher than that achieved from the DSC experiment with free stress. Ti_51.5Ni₂₅Cu_23.5 ribbon annealed at 450℃ for 1h exhibits the transformation strain up to 3%, which can be recovered 100% completely when applied stress is from 3～6N, showing great shape memory properties. Ti_53.5Ni_22.8Cu_23.7 annealed at 450℃ for 10min to 1h exhibits the transformation strain up to 2%. The data of da / dT of the annealed ribbon varied from 6.47 to 9.3 MPa/℃.