<p> Quench-aged low carbon iron specimens, containing various distributions of carbide particles were fatigued in tension-compression under low amplitude strain control. </p> <p> Observations of the influence of second phase particles upon the development of dislocation substructure were made using transmission electron microscopy of thin foils. These observations were correlated with the cyclic mechanical response of the material, and with the response of material containing a fatigue saturation substructure of subsequent tensile overstrain. </p> <p> A rationale for the development of cyclic softening in the material is proposed, based on the requirements of continuity in plastically inhomogeneous materials. A source model for the observed instability of the cyclic substructure in tensile overstrain is described. </p> / Thesis / Master of Science (MS)