MICROSTRUCTURAL PROCESSES IN HIGH-TEMPERATURE LOW-CYCLE FATIGUE OF MAR-M200+HF

Microstructural studies in low-cycle fatigue of directionally solidified MAR-M200+Hf superalloy at 975 degrees C showed fatigue response involving internal as well as external oxidation, and precipitate coarsening ahead of a crack tip. Near the crack tip internal oxidation of the material took place due to interstitial diffusion of oxygen. Oxygen combined with reactive Al and Ti elements of the gamma'-precipitates and with Cr atoms of the gamma-matrix by nonmetallic bonding, embrittling the material and enhancing crack propagation. Upon further oxidation, an external layer of loosely-packed oxides developed on the crack surfaces, supported by partial diffusion of Al, Ti, I-If and Nb from the oxidized uniphase layer beneath it. These elements were available as a result of modifications of the gamma'-precipitates in a uniphase layer by internal oxidation. Most of the gamma'-precipitates remained undissolved, but coarsened and tended to coalesce to a uniform layer, following changes in chemical composition. Coarsening took place in the uniphase layer and in bulk material adjoining it, as a means of local internal stress relaxation.