Effects of fine porosity on the fatigue behavior of a powder metallurgy superalloy
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ties, particularly low cycle fatigue behavior since turbine disk life is limited by fatigue. A more extensive evaluation of tensile and stress-rupture behavior is presented elsewhere. 3 Fatigue and creep-fatigue tests were conducted on the porous Astroloy at 650 ~ and a variety of strain ranges. Comparable data on sound Astroloy already existed. 4 The failed specimens of both the porous and sound materials were examined by scanning electron microscopy to determine the type and size of defect, if any, which initiated failure for possible correlation with the life of the specimen. Modes of crack initiation and propagation were also noted. Tensile and stress-rupture properties at 650 ~ were measured for both materials also. It is stressed that the limited data presented here do not establish the minimum fatigue behavior. Such determinations require testing of a large volume of material. MATERIALS AND PROCEDURES Materials The material studied in this program was hot-isostatically-pressed, powder-metallurgy, low-carbon Astroloy. Both the sound and porous materials were taken from full scale pressings for an aircraft engine first stage turbine disk. Both were produced according to the engine manufacturer's specifications. The processing methods employed are fully described elsewhere 2 and will be only briefly discussed here. Composition specifications for low carbon Astroloy and the compositions of the sound and porous pressings are presented in Table I. The compositions of both pressings meet the specifications and are very nearly the same. Both materials were produced from - 80 mesh powder from the same manufacturer. The powder was loaded into mild steel containers shaped like the part, hot outgassed, sealed, and hot isostatically pressed for 3 h at 100 MPa and 1190 or 1215 ~ for the sound and porous pressings, respectively. The pressings were solution treated at about 1110 ~ for 3 and 2 h, respec-
U.S. G O V E R N M E N T WORK
NOT PROTECTED BY U.S. COPYRIGHT
VOLUME12A, FEBRUARY 1981--261
Table I. Chemical Specification for Low Carbon Astroloy Aim, Wt Pct (Except as Noted) Element
Min
Max
Actual Wt Pct (Except as Noted) Porous
Carbon 0.02 0.04 0.024 Manganese -0.15 0.001 Silicon -0.20 0.018 Phosphorous -0.015 0.005 Sulfur -0.015 0.006 Chromium 14.00 16.00 14.4 Cobalt 16.00 18.00 17.0 Molybdenum 4.50 5.50 5.3 Titanium 3.35 3.65 3.6 Aluminum 3.85 4.15 4.1 Boron 0.015 0.025 0.021 Zirconium -0.06 0.002 Tungsten -0.05 0.029 Iron -0.50 0.09 Copper -0.10 0.05 Lead -0.0010 (10 ppm) < 1 ppm Bismuth -0.00005 (0.5 ppm) __ 10-4. It can be seen that both materials exhibit very similar behavior under both cycles. The creep-fatigue data also fit this line at the higher strain ranges, however at the lower strain ranges it appears that the stress range may be smaller for the creep-fatigue cycle. The relationship between Ae,, and N I for both materials and cycles is shown in Fig. 4. For the fatigue tests on the porous material at the lowest Ac,, Ae,, is uncertain since it was less than the detectable limit of about 5 • l0 -5. L
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