The ongoing collaborative research between Auburn University and Johnson Controls was described. A Ti rich binary NiTi alloy and two ternary alloys having the compositions Ni 49 Ti 51- x Hf x with 1% and 3%(more fraction) Hf were investigated. The influence of Hf content, cold working(CW) and heat treatment temperature (HT) on the martensitic and R phase transformations was thermally analyzed using differential scanning calorimetry (DSC). The crystal structures of the alloys were examined by X ray diffraction. Results showed that at low HT, the martensitic transformation temperature (TT) decreases with increasing CW for all the alloys studied. In this alloy, for a given CW, the martensite TT increases with increasing HT above 350 ℃. This phenomenon is enhanced by the presence of Hf. Different mechanisms can be responsible for the observed behavior. These mechanisms may involve interaction between phase transformations, dislocations and internal stresses. The effect of Hf substitution is likely due to the atomic size difference between Ti and Hf. A secondary phase,identified as Ti 2Ni in binary NiTi and (TiHf) 2Ni in the two ternary alloys, was observed. The influence of cold work, heat treatment and thermal cycling on the R phase (R) transformation in three alloys was also thermally analyzed. Results showed that such transformation depends on composition as well as CW and HT conditions in a complex manner. Effort was made to identify the microstructure and the factors responsible for the observed behavior in the R phase transformation.
展开▼
