Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Raymond A. Fournelle
Bobbie L. Richardson
Martin A. Seitz
The fatigue behavior of an Fe-.3C-4Ni-1Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, and x-ray line broadening and stress relaxation measurements.
The three different heat treatments produced the following structures: (I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, (II) a microstructure) of high dislocation density, coarse carbides and fine coherent NiAl precipitates and (III) a highly tempered microstructure with a recovered dislocation substructure, coarse carbides, equiaxed grains and a fine coherent NiAl precipitate.
In low cycle, strain controlled fatigue cyclic softening for Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain.
Treatment II which remained cyclically stable during the initial portion (approximately 10%) of the fatigue life showed little change in the internal stress and dislocation density, and a slight increase in lattice microstrain. Treatment III which initially cyclically hardened exhibited a rise in internal stress, lattice microstrain and dislocation density. The above behavior is attributed in part to the presence of the fine precipitates which appear to homogenize the deformation substructure and in part to the reduced tendency of the "as heat treated" dislocation substructure (which is of lower apparent density) to rearrange into a cell structure during fatigue.
In high cycle stress controlled fatigue Treatment III showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation. Improved resistance to crack initiation can be attributed to the relative stability of the microstructure which contains the precipitate.