The near threshold fatigue behavior of a nickel base superalloy and a wrought 2024 aluminum alloy is examined as a function of specimen geometry. Experimental results revealed that for a given value of AK in Region I, crack growth rates were observed to increase as the specimen loading became more symmetric with respect to the load line. Compact tension type specimens exhibited lower crack growth rates than the more symmetrically loaded center cracked tension specimens. Consideration of the observed discrepancies is given in terms of the near field crack tip stress distribution and deformation behavior of the material affecting crack advance.
The nature of the second phase particles associated with LC Astroloy prepared using powder metallurgy techniques has been examined. The individual particles have been identified using energy dispersive X-ray analysis and convergent beam electron diffraction. Four distinct types of particles have been observed: a cubic MC carbide in which M is either titanium or titanium plus molybdenum, a monoclinic phase ZrO2, a trigonal ~-AI203 and a tetragonal M3B 2 phase in which M is molybdenum or molybdenum and chromium. The observations indicate that, although some MC carbides are associated with the ZrO2 phase, the majority of the prior particle boundary precipitates are separate entities. Hot isostatic pressing or subsequent heat treatments above or below the ~' solvus were observed to have little effect on the incidence or distribution of the precipitation associated with the prior particle boundaries. In contrast, heat treatments above the 7'solvus resulted in the dissolution of the M3 B2 phase and its preferential precipitation on the grain boundaries.
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