Effect of Various Thermal Treatments and Cu Contents on Precipitation Mechanism, Martensitic Growth, and Cold Workability of Ti50Ni50-xCux Ternary Shape Memory Alloys

摘要

Ti50Ni50-xCux (x = 10, 15, 20, 25) ternary shape memory alloys (SMAs) were developed in an arc melting furnace and characterized by differential scanning calorimetry (DSC), atomic force microscopy (AFM), scanning electron microscopy (SEM), and x-ray diffraction (XRD) techniques. The transformation temperatures and thermal hysteresis showed a decreasing trend with the successive addition of Cu. The SEM and XRD results showed the formation of nano-scaled Ti(Ni,Cu)(2) type precipitates along with small densities of Ti2Ni type precipitates. The precipitation of fine Ti(Ni,Cu)(2) type precipitates was found to be sensitive toward the presence of lattice defects. It is expected that in TiNiCu based SMAs, the precipitation mechanism of Ti(Ni,Cu)(2) type precipitates depends upon the relative ease of diffusion of Cu toward the heterogeneous nucleation sites provided by the lattice defects. Surface topographic characteristics of the alloys were also investigated by the AFM. The AFM results showed that the fine acicular martensite structure transformed to coarser acicular martensite with successive addition of Cu followed by different thermal treatments. Hardness showed direct correlation with the Cu content and the thermal treatment employed, whereas cold workability was shown to decrease with the successive addition of Cu in TiNiCu based SMAs.