The Effect of Principal Elements on Defect Evolution in Single-Phase Solid Solution Ni Alloys

The objectives of this thesis are: firstly, to investigate the effect of increasing chemical complexity on radiation-induced defect evolution (both in voids and dislocation loops) in Ni based single-phased concentrated solid solution alloys (SP-CSAs). SP-CSAs typically contain two to five elements in high or equiatomic concentrations. The compositionally complex but structurally simple features of SP-CSAs make them great candidates in studying irradiation-induced defect interactions. This was done by first performing a study on SP-CSAs in a single elevated irradiation temperature, and followed by testing the dose and temperature dependence of radiation tolerance of these alloys. Secondly, to understand the role of increasing alloy concentrations on defect migration mechanisms with Ni-xFe (x=up to 35%) binary alloys. Thirdly, to understand the effect of alloying elements on radiation-induced microstructural evolution with pure Ni and Ni-20X (X=Fe, Cr, Mn and Pd) binary alloys. Most of the tested samples were irradiated with 3.0 MeV Ni2+ ions at 500℃ up to damage levels of 60 dpa at peak dose. A combination of Transmission electron microscopy (TEM) was used to characterize the microstructure evolution of irradiated CSA samples, emphasizing void swelling and dislocation loop formation. As a result, void swelling decreased while the growth of dislocation loops was suppressed with increasing chemical complexity of an alloy in general. The exceptions were attributed to the difference in CSAs melting temperature. Two interstitial migration mechanisms; 1D and 3D mode, were proposed to explain the qualitative observation of defect distributions throughout the irradiation depth. The suppression on void swelling and delay of dislocation loop growth were attributed to the relatively localized 3D migration mode, which enhanced defect recombination in the main irradiated region. The transition of 1D to 3D mode for interstitials was quantitatively analyzed in Ni-xFe binary alloys using the mean free path of interstitial clusters. In the study of varying irradiation temperatures, the equiatomic Ni-based high entropy alloys (HEAs) have demonstrated superior swelling resistance than Ni-based medium entropy alloys (MEAs) over three homologous temperatures of irradiation. In NiCoFeCrMn, an order of magnitude increase on swelling was observed with increasing temperature, but the swelling performance was still comparable to ferritic steels. Between the two HEAs studied, alloying with Pd was found to have a stronger suppression effect on void and dislocation loop growth than alloying with Mn. This was attributed to the higher lattice distortion observed in NiCoFeCrPd than in NiCoFeCrMn. No significant increase was observed on the equilibrium vacancy concentration as the number of alloying components increased. In the study of Ni-20X, it was found that the average size of defect clusters decreased as the solute atomic volume size factor increased. Oversized magnetic solutes can act as strong trapping sites for interstitials and suppress the growth of dislocation loops. The average dislocation loop size in Ni-20Fe was four times larger than Ni-20Pd (atomic volume factor is 10.6% and 41.3%) but an order of magnitude lower in density. Overall, the alloying effect on defects is more significant in concentrated binary alloys than in dilute binary alloys, due to the concentration difference of alloying atoms and the interstitial dominant migration mechanisms in the main irradiated region. Furthermore, this study demonstrated that similar results on swelling resistance can be achieved in HEAs and well-designed binary alloys with increased concentration or atomic volume factor of alloying elements.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149968/1/taiyang_1.pd