ORIENTATION DETERMINATIONS IN THE TRANSMISSION ELECTRON MICROSCOPE AND THE SUPERPLASTIC BEHAVIOR OF A DUPLEX STAINLESS STEEL (SHEAR BANDS)

The main aims of this thesis are to investigate the microstructures developed in a superplastically deformed duplex stainless steel and to develop techniques for the determination of crystal orientations.; The technique for the determination of crystal orientations was developed to enable rapid and easy analysis of the crystallography of grain and interphase boundaries. The accuracy and reproducibility of the technique was determined to be within +0.2 degrees and +0.05 degrees, respectively. The procedure adopted is unique in that it is applicable to situations wherein high angle Kikuchi lines are not present in the diffraction pattern. The method has also been applied to analyze accurate orientation relationships between ferrite and cementite in steels and is the only technique capable of such analyses.; The investigation on the microstructures formed during superplastic deformation in a duplex material is also the first of its kind. Specimens for microscopic examination were restricted to the peak superplastic region (denoted by region II). Grain boundary sliding is the accepted dominant deformation mechanism in region II. However, there exists a controversy as to whether the accommodation mechanism is due to diffusion creep or by dislocation movement. The results of this investigation are consistent with accommodation by dislocation motion in grains, grain and interphase boundaries. This is verified by the observation of dislocation pile-ups in the grains and twin boundaries, and by the presence of aperiodic dislocation structures in grain and interphase boundaries.; Active loop sources were also seen in both grain and interphase boundaries. These loop sources replace the dislocations which are lost to other boundaries. It is concluded that the rate controlling step for superplastic deformation in region II in duplex materials is a combination of dislocation climb from pile-ups in the matrix into boundaries and grain boundary dislocation climb through triple junctions. No evidence was observed for diffusional accommodation mechanisms.