Spark Plasma Sintering Processing for the Evaluation of Cryomilled CoNiCrAlY Alloys for High Temperature Applications in Oxidizing Environment

The aim of this work is to investigate the use of Spark Plasma Sintering (SPS) as a rapid and efficient approach to evaluate possible benefits deriving from cryogenic mechano-treatments of MCrAlY systems for high temperature applications in oxidizing conditions. Specifically, following our previous investigation, the influence of cryomilling on Co–32Ni–21Cr–8Al–0.5Y (wt.%) alloy characteristics is systematically analyzed in this work. It is shown that mechanical milling in liquid N2 produces morphology changes, particle size increase, crystallite size decrease down to nanometric level (about 20 nm sized) and the formation of oxide nanodispersoids in the processed powders. These features are found to be responsible for the significant influence of the cryogenic treatment on the sintering behavior of resulting powders. Specifically, the densification process is progressively favoured if cryomilling intensity does not exceed a certain level. In contrast, powders sintering becomes relatively more difficult with severely milled powders. It is shown that a significant microstructure refinement (down to 350 ± 150 grain sized) is obtained in SPSed samples prepared from cryomilled powders. When cryomilled CoNiCrAlY SPSed specimens are exposed to an oxidizing environment at 1100 °C, an α-Al2O3 layer tends to be selectively produced on their external surface, thus limiting the formation of less protective mixed oxides observed when the unmilled system is oxidized under the same condition. Al depletion from the bulk of the sample is correspondingly reduced. Therefore, the obtained results provide a useful contribution for the identification of the optimal cryomilling conditions leading to high-temperature oxidation resistance improvement in thermal barrier coatings of gas turbine engines