Yamoah, Nana Kwame Gyan2013-06-212013-06-212013-06-20vt_gsexam:817http://hdl.handle.net/10919/23253Redesigning heavy components with thinner components is one way to lower automotive weight and improve fuel efficiency. Therefore, replacing thick cast iron exhaust manifolds with thinner heat resistant stainless steel one is a prime example of this approach. Material for a thin exhaust manifold must tolerate cyclic thermal fatigue. In SUS 444, this characteristic is directly related to the influence of microstructure on high temperature strength and the stability of the microstructure at the high operating temperature range. The goal of this research is to identify the cause for the drastic difference in the stress-strain behavior between two potential manufacturer heat treatments that will serve as a simplified model case for high temperature cyclic fatigue.  Transmission electron microscopy (TEM) based microstructure analyses of samples which have been aged at 750"C for 100 hours and then hot-tensile tested at 750"C with a strain rate of   suggest continuous recrystallization as the mechanism responsible for the stable high temperature strength. The initial high temperature strength observed in the unaged sample was due to the precipitation of fine Laves phases which pinned down the motion of dislocations. As deformation progressed the strength increased until a critical precipitate size, volume fraction and dislocation density before Laves phases begun to rapidly coarsen and resulted in the abrupt decrease in strength. Microstructure evidence suggests the absence of precipitation strengthening effect in the aged samples could be a contributing factor to the decrease in peak strength between the aged samples and the unaged samples.ETDIn CopyrightHigh Temperature StrengthLaves Phase PrecipitationDislocation DensityContinuous RecrystallizationThesis