Self healing of damage in Fe-based alloys

Steel components can exhibit premature and low-ductility creep fracture, when exposed to high temperatures for long times. The failure arises from the formation, growth and coalescence of ultra-fine cracks and cavities. Self healing of damage is a promising new approach to enhance the lifetime of the steel components, in particular for applications at high temperatures. This thesis aims to realize the self-healing of damage in Fe-based alloys and unravel the mechanism responsible for efficient self-healing in steels. In Chapter 3, the correlation of precipitation and deformation-induced defects is studied in high-purity Fe-Cu-B-N-C alloy samples at an elevated temperature of 550 oC. By comparing the aging behaviour of the samples with 0% and 8% prestrains using positron annihilation spectroscopy (PAS) and complementary small-angle neutron scattering (SANS) it is found that the dislocations induced by tensile deformation accelerate the Cu precipitation kinetics. The addition of carbon counteracts the effect of boron and nitrogen on the Cu precipitation, which may attribute to the formation of cementite with B and N incorporated in the structure. The PAS results indicate a sharp reduction in the contribution from open-volume defects accompanied by a strong Cu peak during the initial aging stage. This reflects the decoration of dislocations with Cu precipitates, demonstrating that the self-healing potential of Cu is not affected by the C in the Fe Cu B N C alloy. However, the tendency for Cu to precipitate at deformation-induced defects is relatively weak and spherical Cu precipitates are simultaneously formed in the matrix. Based on a detailed study of the atomic properties of potential elements and their high temperature solubility in a ferrous matrix, Au has been identified as an interesting alloying element to introduce self-healing of creep damage in Fe-based alloys. In Chapter 4, we have investigated the potential self healing of deformation-induced defects by Au precipitation during isothermal aging at 550 oC in Fe-Au and Fe-Au-B-N alloys using in-situ SANS and PAS. Two different samples with 0% and 24% pre-strain were used to study the influence of dislocations on the Au precipitation. It is found that dislocations induced by prior plastic deformation strongly facilitate the formation of Au precipitates, as no significant precipitation is observed for undeformed samples. Defect induced Au precipitates are formed both at dislocations and along grain boundaries. Transmission electron microscopy (TEM) observations confirm the heterogeneous nature of the Au precipitation. The addition of B and N does not alter the mechanism of the prestrained induced Au precipitation, but retards the Au precipitation. We have demonstrated that Au in Fe-based alloys shows the desired precipitation response to act as an efficient self healing agent for creep damage. Due to a high nucleation barrier for the Au precipitation in the iron matrix, precipitates are formed exclusively at defect sites. The defect-induced Au precipitation therefore provides a promising site-specific autonomous repair mechanism. In Chapter 5, the self-healing potential of damage in the Fe-Au system is further investigated by studying the spatial correlation of Au precipitates with defects. When damage was induced in Fe-Au-B-N alloy by a Knoop indenter at room temperature, the subsequent Au precipitation during high temperature aging (550 oC) is exclusively observed at the damage sites and along grain boundaries. For the cavities and cracks induced by prior room temperature overloading, preferential Au precipitation is observed at micro-cracks and cavities at grain boundary triple points in a wide region near the original fracture surface. A strong tendency of Au and hexagonal BN precipitation onto the free (external) surface is demonstrated during aging at high temperatures studied by X-ray photoelectron spectroscopy. In order to couple the defect formation and healing processes more directly, creep properties have been studied for the Fe-Au and Fe-Au-B-N alloys at high temperature (550 oC). Chapter 5 presents the site-specific Au precipitation on creep cavities induced during constant strain rate creep testing. The absence of grain boundary cavitation at some of the grain boundaries is coupled with a locally high density of relatively large Au particles with the same size as the expected pore sizes, which is most likely attributed to the healing of creep cavities by preferential Au precipitation. In Chapter 6, we have systematically studied the creep properties of the solutionized (as-quenched) Fe-Au and Fe-Au-B-N alloys during high temperature creep at constant stress. An improved creep lifetime was observed for solutionized Fe-Au and Fe-Au-B-N alloys compared to those obtained for solutionized and solution-depleted Fe-Cu samples. The mechanism responsible for the improved lifetime in the solutionized Fe-Au system is unravelled by studying the microstructures of the samples after creep (Chapter 7). The autonomous repair of creep damage by site-selective Au precipitation has been demonstrated. Combined electron-microscopy techniques demonstrate that the improved creep properties result from the selective Au precipitation at the early-stage creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The site-selective precipitation of gold atoms at the free surface of the creep cavities results in a pore filling, and thereby self healing of the creep damage. Grain boundaries and dislocations act as fast routes for solute gold transport from the matrix to the creep damage. The efficiency to heal creep damage is found to depend strongly on the applied stress. For lower stress levels filling fractions of up to 80% have been observed for the open-volume creep damage. Due to the limited availability, the application of Au solute atoms as healing agents in steel components is expected to be limited. Therefore, the efficiency of molybdenum as a potential self healing agent has been investigated in Chapter 8. Hardness tests on the aging behaviour of the Fe-Mo alloys with and without prestrain indicate that the dislocations induced prior to aging accelerate the precipitation kinetics. Creep properties are studied at constant stress at an elevated temperature of 550 oC. A filling of creep cavities by precipitates has indeed been observed in the microstructure of creep failed samples. The filling of creep cavities by precipitates is indicated by the irregular geometry of the relatively large precipitates (?1 µm) formed along grain boundaries and the close spatial correlation between the creep cavities and the precipitates