In situ observations of grain growth and recovery in cold sprayed Ni-Y2O3 and CP-Ni coatings using hot-stage TEM

Abstract

Cold spray is a severe plastic deformation-based processing technique. The material undergoes extreme environments and a large energy can be assumed to be stored in material in form of strain, dislocations, and vacancies. The thermal stability and structural integrity of cermet coatings processed by cold spray for elevated temperature applications are rather important but not well understood. Here, commercially pure-Ni (CP-Ni) and Ni-Y 2 O 3 cermet coatings were fabricated and annealed by an in-situ hot-stage transmission electron microscopy (TEM) technique to investigate and compare the microstructural evolution of these two materials including strain relaxation, sub grain boundary and grain boundary motion which as otherwise impossible to probe using ex situ annealing. The as-deposited CP-Ni exhibits crystallized ultra-fine grains (UFG), while the Ni-Y 2 O 3 coatings show a combination of nanocrystalline (NC) and UFG grains and a non-uniform distribution of Y 2 O 3 in the Ni matrix. The hot-stage TEM results show a decreased strain contrast and limited grain growth for both materials annealed at 350 °C for 2 h. When the materials are continuously heated from 25 °C to 800 °C in 19 min, the presence of nano Y 2 O 3 particles shows pinning effects at grain boundaries, resulting in improved thermal stability of Ni-Y 2 O 3 coatings compared with CP-Ni coatings. • The as-deposited cold sprayed CP-Ni exhibits crystallized ultra-fine grains (UFG). • Ni-Y 2 O 3 coatings show a combination of nanocrystalline and UFG grains and a non-uniform distribution of Y 2 O 3 in the Ni matrix. • Grain growth in the two-dimensional specimen is more sluggish compared to that in the three-dimensional specimen. • Nano Y 2 O 3 addition shows pinning effects at grain boundaries, resulting in improved thermal stability of Ni-Y 2 O 3 coatings.