Self-healing of deformation-induced defects in Fe-based alloys using positron annihilation techniques:

by M.E. Gramsma

Institution: Delft University of Technology
Year: 2015
Keywords: Self-healing materials
Record ID: 1250974
Full text PDF: http://resolver.tudelft.nl/uuid:1a8bbab8-bc9c-4331-935d-d278508755f7


This thesis investigates the self-healing mechanisms of deformation-induced defects in ferritic Fe-Au(-B-N) and Fe-Mo alloys, by segregation and precipitation of Au elements and Fe2Mo compounds. For this purpose, cyclic tensile tests on the Fe-Au(-B-N) alloys are conducted interrupted with isothermal ageing steps at 550 °C. Coincidence Doppler Broadening (CDB) measurements are performed to study the defect evolution and precipitation behaviour occurring at these defect sites, to clarify the correlation between precipitation and strain-induced defects. Using this technique, the Fe-Mo samples are studied for various ageing times up to 64 h. These measurements are conducted with 8% deformed and undeformed samples, to study the influence of dislocations on the Fe2Mo precipitation. Furthermore, Vickers hardness tests are conducted on undeformed and 8 % deformed Fe-Mo alloy samples for various ageing times, as well as creep tests for the Fe-Mo and Fe-Au(-B-N) alloys in combination with SEM observations. Next, diffusion-based finite element model calculations are performed on filling of a creep cavity for the Fe-Mo alloy. It is found from the model that the filling of a creep cavity on a grain boundary can be well compared with the growth model of a precipitate on a grain boundary. From the CDB measurements, it is found for the Fe-Au(-B-N) alloys that in a major part of the deformation-ageing cycles healing takes place after introducing defects. Being consistent with previous research [1], it is found that the addition of B and N to the Fe-Au alloy retards the Au precipitation. For the Fe-Mo alloy, it is found from CDB measurements that the segregation and precipitation of Fe2Mo is enhanced by induced dislocations and other defects. The creep lifetime of this alloy is improved compared with that of the Fe-Au alloy, together with a higher ductility. Based on the CDB measurements, supported by the creep measurements and SEM studies, it is shown that Mo solutes have a tendency to segregate selectively to dislocations and open-volume defects. The site-selective precipitation makes Mo a promising alternative for Cu and Au to act as an efficient self-healing agent for deformation-induced defects in Fe-based structural alloys for use at elevated temperatures.