CHEMICAL AND MORPHOLOGICAL CHANGES AT Al_2O_3/NiAl INTERFACES AND THEIR RELATIONSHIP TO SCALE ADHESION

摘要

Ni-(40,50)at%Al alloys with different C and S contents were oxidized at 1000 -1150℃ for various times in oxygen. Auger electron microscopy was used to study the interface chemistry after scale spallation in ultra high vacuum. The interfacial failure stresses were determined with a tensile pull tester and they were related to the interfacial pore density. Results show that sulfur did not segregate to the Al_2O_3/Ni50Al interface even after extended oxidation times. Small amounts, however, segregated to the Al_2O_3/Ni40Al interface. The difference in behavior may be related to the surface energy difference between Ni50Al and Ni40Al. On the interfacial void faces of Ni50Al, C first segregated, then it was replaced by S after longer oxidation times; the amount of segregants varied with different crystallographic orientation of the void face. On Ni40Al, S segregated much earlier on the void faces due to a faster diffusion rate in the Ni-rich NiAl. The apparent S diffusivity in Ni50Al and Ni40Al at 1000℃ was determined to be 10~(-9) and 6x10~(-9) cm~2/s respectively. Excess sulfur in Ni40Al greatly increased the interfacial pore density. Preliminary results on interfacial failure stress showed that it decreased with increasing pore density, regardless of whether S was present at the interface, indicating that the major detrimental effect of S on scale adhesion may be to enhance interfacial pore formation.