Surface segregation in MCrAlY alloys
- PDF / 1,559,397 Bytes
- 10 Pages / 594 x 774 pts Page_size
- 108 Downloads / 232 Views
I.
INTRODUCTION
OXIDATION of iron and nickel base alloys has been studied for many years because it often limits applications of these alloys at high temperatures. As a result of this research several statements can be made. One is that chromium and aluminum additions to the alloy help provide protection against oxidation of the base metal through the formation of Cr203 and AI~O3 on the surface. Of these two oxides alumina is preferred because Cr20 3 is subject to volatilization above approximately 1000 ~ Another fact is that yttrium and hafnium additions to these allo~s reduce spallation of the oxide when the metal is cooled.I I Many mechanisms have been proposed to explain why the additions of yttrium and hafnium improve adherence of the oxide, and they are summarized in Reference 2. None of these models is consistent with all of the experimental data. Recently, several researchers 13-6j have proposed that these two elements have a beneficial effect because they retard sulfur segregation to the metal surface through the formation of yttrium and hafnium sulfides in the matrix. They point out that, since sulfur is a strong embrittler of grain boundaries in iron and nickel base alloys, IT-l~ it might also weaken the metal-oxide bond if it is present on the metal surface. Therefore, in this model spallation is primarily attributed to the presence of sulfur at the oxide-metal interface. More generally, this proposal would suggest that spallation resistance depends on the composition of the alloy surface rather than the composition of the bulk alloy. As a result of this proposal it has become essential to study surface segregation in these alloys. In this paper we report a study of segregation in NiCrAI and FeCrAI type alloys. We have measured this segregation by Auger electron spectroscopy. The results will show that sulfur and yttrium are clearly enriched on the surface and that cobalt and chromium are depleted. Aluminum appeared to be slightly segregated in most cases, but because Auger sensitivity factors are rather uncertain, one must be careful in interpreting this result. C.L. BRIANT and K, L. LUTHRA are Staff Scientists with General Electric Company, Research and Development Center, P.O. Box 8, Schenectady. NY 12301. Manuscript submitted May 27, 1987.
METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL
The MCrAI(Y) alloys used in this study were fabricated from the metals of various purities listed in Table I.* The *Unless otherwise indicated, the alloys are of commercial purity. The alloy compositions are expressed in weight percent, and the surface compositions are expressed in atomic percent.
alloys were prepared by casting in graphite or copper molds and were subsequently machined to produce samples of the desired size. In addition to the MCrA1Y type alloys we also prepared samples of the metals Ni, Cr, A1, and Y containing 100 ppm S. We used Auger electron spectroscopy to monitor the composition of the alloy surfaces at temperatures similar to those used in the oxidation experiments. For the Auger experime
Data Loading...
