Analysis of crystallographic high temperature fatigue crack growth in a nickel base alloy
- PDF / 2,196,176 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 6 Downloads / 165 Views
K. SADANANDA and P. SHAHINIAN are with the Thermostructural Materials Branch, Material Science and Technology Division. Naval Research Laboratory, Washington, DC 20375. Manuscript submitted April 22, 1980. METALLURGICAL TRANSACTIONS A
temperature. In the present analysis, this mode of fatigue crack growth is examined in detail at different temperatures as it occurs in Udimet 700, which is a nickel-base superalloy used extensively in turbine applications. There has been extensive work ~s-22on Udimet 700 under low cycle fatigue in the temperature range of 25 to 926 ~ although not much on fatigue crack growth. The alloy exhibited fatigue hardening behavior followed by fatigue softening at both 25 and 760 ~ At 760 ~ and above, both environmental and creep effects become important as they appear to alter the mode of crack nucleation from transgranular to intergranular when frequency is reduced. 22 During the crack propagation stage, fracture was essentially transgranular with predominantly a faceted mode. In our recent study 23 of fatigue crack growth in Udimet 700 using a fracture mechanics type of specimen, we found faceted growth even at a temperature as high as 850 ~ Furthermore, comparison of our data with room-temperature data 24 indicated a large effect of temperature on crack growth. It was not clear whether the effect of temperature was due to environmental interactions since the tests were done in air, or due to creep effects or to the inherent nature of the faceted mode of crack growth. The object of the present work was to investigate further the crack growth behavior of this alloy at room temperature and 850 ~ in air and vacuum in order to determine the role of environment on the mode of crack growth and crack growth rates as well as the causes for the observed effect of temperature.
EXPERIMENTAL DETAILS Compact tension specimens with nominal 1-T dimensions 25 but with 12.7 mm thickness were made from the same bar stock used earlier. 23 A standard duplex heat treatment was given the alloy with anneal at 1180 ~ for 4 h, air cool, intermediate anneal at 1080 ~ for 4 h, air cool, age at 845 ~ for 24 h, furnace cool to 760 ~ and age for 16-h and finally furnace cool
U.S. GOVERNMENT WORK NOT PROTECTED BY U.S. COPYRIGHT
VOLUME 12A, FEBRUARY 1981--343
to room temperature. Figure 1 shows the microstructure of the alloy after heat treatment. Extensive twinning characteristic of superalloys can be seen in the figure. That these twin boundaries form preferred sites for crack nucleation under fatigue has been shown earlier. E~ All specimens were precracked at room temperature in air. All tests were performed on a servo-hydraulic machine at a frequency of 0.17 H z with the specimen enclosed in a chamber for v a c u u m and heated by induction. A vacuum better than 1.33 • l0 -4 Pa was obtained at the test temperature. Crack lengths were measured intermittently through a viewing port using a low magnification travelling microscope. Crack growth rates were determined from the plots of crack length vs time. Correction
Data Loading...
