Amorphization along interfaces and grain boundaries in polycrystalline multilayers: An x-ray-diffraction study of Ni/Ti multilayers

Diffusion-induced solid-state amorphization (SSA) was studied in (fcc Ni)/(hcp Ti) multilayers at 523 K. The multilayers had a modulation length of 24.4 nm and an overall composition of ${\mathrm{Ni}}_{40}$ ${\mathrm{Ti}}_{60}$. They were polycrystalline without coherency between the Ni and Ti sublayers. The phase changes, composition changes, and strain development were studied for annealing times up to 220 h, primarily using x-ray-diffraction methods. Upon annealing, an amorphous phase developed, concurrently with dissolution of Ti in crystalline Ni. The amorphous phase not only formed along the Ni/Ti interfaces, but also along the grain boundaries in the sublayers. Continued amorphization along the original grain boundaries on prolonged annealing implies that fast-diffusion paths in the amorphous phase remained active. Diffusion coefficients were determined, using methods described in a separate paper. The interdiffusion coefficient for the amorphous phase is smaller than the tracer-diffusion coefficient of Ni in hcp Ti and larger than the chemical diffusion coefficient in fcc (Ni,Ti) solid solutions. The reaction virtually stopped after 16 h, when appreciable amounts of crystalline Ni and Ti were still present, which is ascribed to ordering in the crystalline (Ni,Ti) solid solution. Both the dissolution of Ti in crystalline Ni and the amorphization are associated with the development of stress profiles in the multilayer. These are quantitatively discussed and analyzed using the x-ray-diffraction and Fizeau interferometric data.