Microstructure and martensite transformation in aged Ti-25Ni-25Cu shape memory melt spun ribbons

The phase transformations and microstructure of Ti-25at.%Ni-25at.%Cu alloy melt-spun at 26 m/s (50 μm thick, 3 mm wide) were investigated using DSC and TEM. The as received ribbon was nearly fully amorphous with a few spherical β phase grains (of size up to 15 μm). The heating (20°C°min.) of this ribbon resulted first in restart of growth of existing large grains and later in homogenous nucleation and growth of new small β phase crystallites within the amorphous matrix. The crystallization peak temperature of amorphous parts in this ribbon was determined to be 460°C. Further heating with the same rate to higher temperatures causes precipitation predominantly at grain boundaries with maximum of its thermal effect around 600°C. Isothermal aging of crystallized ribbon cause precipitation of thin plates of a tetragonal phase. The size and density of these plate-like precipitates depends on local microstructure and differs most drastically within large primary grains. The in situ TEM experiments confirmed the presence of only B2–B19 martensite transformation in the investigated ribbon. However, in situ TEM observations showed also that this transformation was significantly retarded in areas with small grains and even more in areas of high dislocation density as compared to primary large grains. Therefore, the delay in martensitic transformation caused by differences in the microstructure was the reason for the peak splitting in DSC curves.