Preparation And Characterization Of Ni-Ti-Hf High Temperatuer Shape Memory Alloy Thin Films

The films of Ni Ti Hf high temperature shape memory alloy were prepared by DC magnetron sputtering deposition with annealed. Element composition of films, phase structure, crystallization behavior, martensite transformation and microstructure were studied by XRD, SEM,EDS, DSC and TEM.In experimental, The varieties of sputtering work and sputtering time, Ar gas pressure, changed the composition of films, especially that of sputtering work. By the rise of sputtering work, it has been found that Ni content of the thin films increased about 5 at.%; Ti content keeped stablely, while Hf content dropped. In compared, the influence of gas pressure and sputtering time was weak. It found that Ni content of the thin films were very higher, mostly about 3~6 at.% than that of the Ni Ti Hf alloy targets, while the content of Ti were lost. Finally, we get the amorphous thin films of Ni Ti Hf shape memory alloy, with 50 at.% of Ni content by the target of Ni44Ti41Hf15 alloy, when was deposited with 150 W of sputtering work, 0.12 Pa of Ar gas pressure and 2h of sputtering time.Crystallized during the DSC heating at the same rate, the different compositions of Ni Ti Hf thin films show different peak temperature. And we found that the higher content of Ni resulted in the higher peak temperature. One can see that peak temperature of Ni Ti Hf thin film was associated with the heating rate in such anisothermal experiments. Besides, the effective activation energy can be calculated by the Kissinger’s equation. The crystallized activation energy of E were 348.79 k J/mol of Ni54.94Ti31.38Hf13.68 a alloy thin film and 315.67 k J/mol of Ni46.83Ti33.56Hf19.61 alloy thin film. After annealing, we acquired Ni Ti Hf shape memory alloy thin films with B19′ martensite matrix at room temperature, without second phase appeared.In addition, it exists broad peaks and more than one DSC peak through one heating process in Ni46.83Ti33.56Hf19.61 alloy thin film annealed at temperature lower than 650 oC, which we can explain with inhomogeneous about distribution of the structure and crystal partical. The one step phase transformation that was from Austenite(B2) to B19′ Matensite( monoclinic B19′ type) is observed. The phase temperature increase with the raise of the annealing temperature, and we get the highest transformation temperature at 650 oC annealing, which is the 112 oC of Ms.We also observed that the martensite variants exhibited self-accommodated morphology and had spear-like, mosaic-like characteristics. The substructure of them were thin slabs in each matensite variant, which were(001) compound twins.