Precipitates and lamellar microstructures in NiTi films

The crystallization process in thick sputtered amorphous films was examined using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Crystalline grains always nucleated first at the surface, rapidly grew laterally until impingement, and then grew inward to form columnar grains. Surface roughness delayed the onset of surface nucleation. For very smooth surfaces, crystallization of columnar grains nucleated quickly, but after lateral impingement, inward growth was more sluggish. Multiple DSC exotherms observed in these films suggested that additional nucleation events occurred, and TEM study of partially transformed films confirmed that secondary nucleation events resulted in distinct microstructual layers. These were perfectly stratified in the plane of the film, with single grain aspect ratios that were radically different from those of surface-nucleated columnar grains. After crystallization was largely complete, Ti-rich particles, Ti2Ni, nucleated in high concentrations at the columnar/plate grain interface. These precipitates were mostly observed in the columnar grains with a maximum density at the columnar/plate grain interface. It is proposed that excess titanium was driven into the amorphous region ahead of the crystal/amorphous interface during crystallization, leading to a high concentration of titanium and, thus, precipitates at the columnar/plate grain interface.