Formation of porosity in ferritic ODS alloys on high temperature exposure

In order to investigate the formation, distribution, growth behavior and volume fraction of porosity at high temperatures in ferritic Oxide Dispersion Strengthened (ODS) superalloys MA 956, ODM 751 and PM 2000 were exposed at 1100 and 1200 degrees C in air and in 2% oxygen containing nitrogen atmospheres for up to 8760 h. All samples exhibited small amounts of porosity (% < 0.1) in the as-received condition. During the early stages of exposure (up to 10 h) pores were generally located close to the surface of the samples. Later they were found in the central region of the samples and the volume fraction of porosity and the mean pore size increased. After exposure for 240-1000 h, mainly depending on sample thickness, the size and volume fraction of porosity reached a maximum value and then decreased. SEM and EDS analysis results showed that some pores contained small amounts of Ti, Al and Y rich particles which were distributed over the internal surface of pores. It was noted that when the volume fraction of porosity and the size of pores began to decrease the pores had filled with the matrix material. A relation was established between the production environment of the materials and the growth behavior of the pores. (C) 2005 Springer Science + Business Media, Inc.