The chemical and mechanical processes of thermal fatigue degradation of an aluminide coating
- PDF / 3,900,306 Bytes
- 14 Pages / 594 x 774 pts Page_size
- 42 Downloads / 185 Views
INTRODUCTION
ALUMINIDE coatings are widely used for oxidation and hot-corrosion protection of gas-turbine airfoils manufactured from Ni-base superalloys. These coatings are simultaneously exposed to high-temperature oxidizing or corrosive gases and severe thermal/mechanical strain histories, ti] For airfoil temperatures in excess of 1000 ~ oxidation and scalloping (sometimes referred to as "wrinkling") are the dominant mechanisms of aluminide coating degradation, t2-51 For airfoil temperatures below 950 ~ hot corrosion can severely limit the life of aluminide coatings, t6'7"81 Although the oxidation behavior and degradation mechanisms of aluminide coatings and NiA1 alloys have been widely studied under various isothermal and cyclic conditions, t6-13j the synergistic effect of oxidation and thermal/mechanical strains on coating degradation has not been investigated in a quantitative manner. The present paper makes a start in this direction by examining aluminide coating degradation in an oxidizing environment in relation to the temperature and strain history experienced by the coating. II.
EXPERIMENTAL PROCEDURE
A. Specimen Geometry, Substrate, and Aluminide Coating Stepped-disk oxidation specimens (Figure l(a)) were machined from a monocrystalline rod of REN]~* N4 J.W. HOLMES, Assistant Professor, is with the Department of Mechanical Engineering and Applied Mechanics, The University of Michigan, Ann Arbor, MI 48109. F.A. McCLINTOCK, Professor, is with the Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted December 21, 1987. METALLURGICAL TRANSACTIONS A
(composition given in Table I). Specimens were removed from the crystal with their faces normal to the [001] growth direction of the crystal (Figure l(b)). To simulate the sharp radii found along the leading and trailing edges of gas-turbine airfoils, the disks were machined with a periphery radius of 0.45 mm. Pack aluminization was used to form a coating approximately 70-/xm thick along the periphery of the disks. Further details concerning specimen heat treatment and the pack aluminization process are given by Holmes et al. I141
B. Test Apparatus and Control of Specimen Strain History Only a brief discussion of the test apparatus is given here; for further details, the reader is referred to earlier publications by the authors, t2'14-161 Cyclic strains were produced in the substrate and coating by induction heating of the specimen periphery, followed by high-velocity air cooling (a schematic of the test apparatus is given in Figure 2). A 2.5 kW, 450 kHz induction generator was used for specimen heating. At 450 kHz, the depth of heating in RENE N4 is approximately 1 mm. II41 As a consequence of this shallow heating depth, the periphery of the specimen heats before its center, generating compressive circumferential mechanical strains as the periphery tries to expand relative to the cooler core of the specimen. The magnitude of the transient compressive strain developed along the periphery was con
Data Loading...
