An in situ transmission electron microscope investigation into grain growth and ordering of sputter-deposited nanocrystalline Ni₃Al thin films

The grain growth kinetics and ordering behavior of direct-current magnetron sputter-deposited Ni_75at.%Al_25at.% alloy films were investigated using in situ isothermal annealing in a transmission electron microscope. Both normal and abnormal grain growth modes were observed. The normal grain growth kinetics under isothermal heating from 300 to 700 °C were found to comply with the Burke law d = K/dⁿ⁻¹, where d is grain size and K and n are constants with respect to time. The grain boundary mobility parameter K was found to obey an Arrehnius rate law with an apparent activation energy of 1.6 eV, and n was found to increase gradually from 5.2 at 300 °C to 8.7 at 700 °C. Abnormal grain growth occurred at 500 °C or higher, and grain coalescence was identified as an important operative mechanism. It was also observed that the initially as-deposited state of the films was crystalline with a disordered face-centered-cubic structure, but ordering into the equilibrium L1₂ intermetallic structure followed from annealing at temperatures above approximately 500 °C.