Influences of chromium and hafnium additions on the microstructures of β-nial coatings on superalloy substrates

In this study, Hf-doped and Cr/Hf-modified NiAl coatings were deposited onto René N5^® substrates via direct current magnetron sputtering. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) showed the as-deposited coatings to be single phase with B2 crystal structures. Post-deposition annealing at 1000 °C in an Ar + 5%H₂ atmosphere resulted in the formation of nanometer-sized precipitates along grain boundaries and within grain interiors. TEM analyses showed most of the precipitates to be β′-Ni₂AlHf. Three-dimensional atom probe tomography (3D-APT) also revealed the presence of α-Cr precipitates within the NiAlCrHf coatings after annealing. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.