A pre-formed yttria alumina garnet (YAG) and Al₂O₃ mixed oxide layer induced by continuous laser surface melting in air for inhibiting β depletion in a CoNiCrAlY bond coat alloy

This work investigated the influence of pre-formed yttria alumina garnet (YAG) and Al₂O₃ mixed oxide layer induced by laser surface melting (LSM) on the oxidation behaviour of a CoNiCrAlY bond coat alloy. As-cast CoNiCrAlY alloy was vacuum heat treated at 1100 °C for 2 h and LSM was conducted at a laser power of 400 W and scanning speeds from 200 to 800 mm/min in air. Both heat-treated (HT) and LSM CoNiCrAlY alloys were subsequently oxidised at 1100 °C for 100 h. It was found that the β phases were refined and redistributed along the γ phase boundaries after LSM. In addition, an initial mixed oxide layer composed of YAG and Al₂O₃ was developed on the surface during LSM, which was due to the high oxygen affinity of Y and Al. It was further demonstrated that the pre-formed YAG/Al₂O₃ mixed oxide layer at scanning speeds of 200–600 mm/min effectively suppressed oxide growth and inhibited β depletion during oxidation. This was attributed to the fact that the stable YAG acted as grain boundary barriers for oxygen inward diffusion. However, a newly formed Al₂O₃ layer, associated with a corresponding β depletion zone, appeared underneath the mixed oxide layer at the highest scanning speed of 800 mm/min. This was due to that the spallation of the mixed oxide layer and refined γ phases allowed the oxide growth at the alloy surface and subsequently corresponding β depletion occurred.