Influences of annealing and hafnium concentration on the microstructures of sputter deposited β-NiAl coatings on superalloy substrates

This study was conducted to establish the influences of high hafnium concentrations and high temperature annealing on the microstructures of β–NiAl coatings. Binary NiAl and ternary NiAl-1.0 at.% Hf coatings were deposited onto CMSX-4® superalloy substrates via unbalanced DC magnetron sputtering. Investigations were conducted using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, X-ray diffraction, and three-dimensional atom probe tomography in an effort to determine the influences of annealing on the coating microstructures and coating/substrate interdiffusion. All as-deposited coatings were found to consist of a B2 structured solid solution consistent with the Ni-rich β–NiAl. Post-deposition annealing at 1000 °C resulted in significant coating-substrate interdiffusion and in the formation of an interdiffusion zone, the extent/thickness of which decreased with the addition of Hf. Annealing additionally produced γ′–Ni3Al precipitates in the binary coatings and nanometer-sized Hf-rich precipitates in Hf-containing coatings. The majority of these precipitates were identified as the equilibrium Heusler (i.e., Ni2AlHf) phase; however, when Hf-containing specimens were annealed in a high carbon environment, significant concentrations of carbon were incorporated into the coatings. This sometimes resulted in the formation of HfC precipitates. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.