Abstract: The effect of the solute (Mo) on the stress development of nanocrystalline Ni and Ni-Mo films upon heating has been investigated in real time using in situ synchrotron X-ray diffraction. The complex and distinct relationship between the film stress and grain boundaries (GBs) has been examined by the evolution of real-time intrinsic stress in combination with the in situ grain growth and thermal characterizations. The different intrinsic stress evolutions in the Ni and Ni-Mo films during the heating process result from the modification of GBs by Mo alloying, including GB amorphization, GB relaxation, and GB segregation. It has been found that GBs play a vital role in the stress development of nanocrystalline films. The addition of a solute can not only inhibit grain growth but also influence the stress evolution in the film by changing the atomic diffusivity at the GBs. This work provides valuable and unique insights into the effect of solutes on stress development in nanocrystalline films during annealing, permitting control of the film stress through solute addition and heat treatment, which is critical for improving the design, processing, and lifetime of advanced nanocrystalline film devices at high temperatures.

Key words: Synchrotron X-ray diffraction, Nanocrystalline film, Solute, Stress development, Grain boundary