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Title: Low cycle fatigue mechanisms in CMSX-4
Author: Charles, C. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This thesis investigates the low-cycle fatigue behaviour of CMS-4. The focus is on R=0 load controlled fatigue, with an emphasis on deformation at 750°C. A particular aim of this research is to identify how fatigue proceeds at stress concentrators. Here, high stresses in plain bars have been compared with similar maximum stresses around the known stress concentration of a Kt-2 notch. Fatigue tests were conducted on both plain and notched specimens, across a range of stresses and temperatures, and the deformation has been imaged using TEM. The plain bars show a range of deformation mechanisms, which have been described and analysed. Of particular interest is the presence of widely spaced dipoles of single dislocations, previously noted in tensile studies. These have been imaged on slip planes, and it is shown that they are not formed by the mutual attraction of two independent dislocations, as has been previously thought. Rather, they are two sides of an á110ñ loop, separated by a region of APB. These loops are expected to expand and contract within the precipitates, and it is shown that this could account for a significant degree of plastic strain within the plain bars. Although the fracture patterns and lifetimes are very similar to those seen in the plain bars, no microstructural evidence of significant deformation is seen from TEM examination of the notched bars. Visible dislocation density is consistently extremely low, and it is seen that there is no correlation between deformation in the bulk of the plain bars, the notched bars, and failure. Hence, a new mechanism of initiation is proposed for failure below 950°C. It is proposed that initiation below 950°C is related to the extrusion of g channels at the surface of internal porosity, and the concomitant formation of subsurface voids.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available