Objective was to study dual phase steels. Macroscopic effects of reversing the continuous phase from ferrite to martensite was examined. The next section looks at the role of carbide distribution and morphology precipitated within ferrite during thermal cycling. Finally, finite element modeling is used to assist in understanding the experimental results.

Objective was to study dual phase steels. Macroscopic effects of reversing the continuous phase from ferrite to martensite was examined. The next section looks at the role of carbide distribution and morphology precipitated within ferrite during thermal cycling. Finally, finite element modeling is used to assist in understanding the experimental results.

This review examines the various stages of fatigue damage on the basis of changes in slip character and dislocation sub-structures resulting from solid solution alloying, thermomechanical treatments or precipitation hardening. Cyclic hardening and softening are related to fatigue life of a wide variety of alloy systems, including pure metals, commercial alloys, intermetallic compounds and directionally solidified eutectics. The influence on fatigue behavior of variations in structure produced by processing (e.g., casting defects, inclusions, surface notches) also are considered. Finally, the effects of temperature and aggressive environments on crack nucleation and propagation are related to metallurgical structure.

The results of an investigation on the low cycle fatigue (LCF) behavior, at room temperature and 400°C for four conventional microstructures (Widmannstatten, basket-weave, equiaxed, and duplex) in a TC6 titanium alloy are presented. The fatigue crack nucleation and propagation in fatigue-tested specimens have been observed by scanning electron microscopy (SEM). The duplex microstructure is associated with the longest LCF life at room temperature and 400°C, while the Widmannstatten microstructure has the shortest. The crack initiation sites and propagation paths were examined and discussed. The cracks primarily initiated along slip bands on the specimen surface for all four microstructures. In addition, many voids appeared along slip bands for the equiaxed microstructure. By linking-up these voids, the formation of microcracks is realized. The propagation of interior cracks in specimens with Widmannstatten structure proceeded by cross-cutting W? platelets by way of a plastic blunting mechanism, whereas for the equiaxed microstructure interior cracks grew by the linking-up of voids by way of a renucleation mechanism.