Microstructural evolution of a MCrAlY coating upon isothermal annealing

The microstructural evolution of a freestanding γ + β NiCoCrAlY coating, as deposited by electron beam physical vapor deposition, was studied following isothermal annealing at 1373 K. Electron backscatter diffraction (EBSD) was used to characterize the crystallographic texture, grain and phase boundary population and the size and distribution of the grains in the coating as a function of annealing time. The as-deposited coating consisted of small equiaxed grains near the bottom, and large columnar grains near the surface of the coating. A phase separation process inside the columnar grains resulted in a local microstructure consisting of a periodic arrangement of γ and β lamellae, linked to each other according to the Kurdjumov–Sachs orientation relationship. Upon annealing, equiaxed-shaped grains formed within the coating at the expense of the columnar grains. The formation of the equiaxed grains corresponded with an increase of the total interface length per unit area in the coating. Upon prolonged annealing, growth of the equiaxed grains occurred according to a power law with a time exponent n of 2.