C. Sun

In situ Evidence of Defect Cluster Absorption by Grain Boundaries in Kr Ion Irradiated Nanocrystalline Ni

Significant microstructural damage, in the form of defect clusters, typically occurs in metals subjected to heavy ion irradiation. High angle grain boundaries (GBs) have long been postulated as sinks for defect clusters, like dislocation loops. Here, we provide direct evidence, via in situ Kr ion irradiation within a transmission electron microscope, that high angle GBs in nanocrystalline (NC) Ni, with an average grain size of ~55xa0nm, can effectively absorb irradiation-induced dislocation loops and segments. These high angle GBs significantly reduce the density and size of irradiation-induced defect clusters in NC Ni compared to their bulk counterparts, and thus NC Ni achieves significant enhancement of irradiation tolerance.

Superior radiation-resistant nanoengineered austenitic 304L stainless steel for applications in extreme radiation environments

Nuclear energy provides more than 10% of electrical power internationally, and the increasing engagement of nuclear energy is essential to meet the rapid worldwide increase in energy demand. A paramount challenge in the development of advanced nuclear reactors is the discovery of advanced structural materials that can endure extreme environments, such as severe neutron irradiation damage at high temperatures. It has been known for decades that high dose radiation can introduce significant void swelling accompanied by precipitation in austenitic stainless steel (SS). Here we report, however, that through nanoengineering, ultra-fine grained (UFG) 304L SS with an average grain size of ~100u2005nm, can withstand Fe ion irradiation at 500°C to 80u2005displacements-per-atom (dpa) with moderate grain coarsening. Compared to coarse grained (CG) counterparts, swelling resistance of UFG SS is improved by nearly an order of magnitude and swelling rate is reduced by a factor of 5. M23C6 precipitates, abundant in irradiated CG SS, are largely absent in UFG SS. This study provides a nanoengineering approach to design and discover radiation tolerant metallic materials for applications in extreme radiation environments.

In situ study of defect migration kinetics in nanoporous Ag with enhanced radiation tolerance

Defect sinks, such as grain boundaries and phase boundaries, have been widely accepted to improve the irradiation resistance of metallic materials. However, free surface, an ideal defect sink, has received little attention in bulk materials as surface-to-volume ratio is typically low. Here by using in situ Kr ion irradiation technique in a transmission electron microscope, we show that nanoporous (NP) Ag has enhanced radiation tolerance. Besides direct evidence of free surface induced frequent removal of various types of defect clusters, we determined, for the first time, the global and instantaneous diffusivity of defect clusters in both coarse-grained (CG) and NP Ag. Opposite to conventional wisdom, both types of diffusivities are lower in NP Ag. Such a surprise is largely related to the reduced interaction energy between isolated defect clusters in NP Ag. Determination of kinetics of defect clusters is essential to understand and model their migration and clustering in irradiated materials.

Superior tolerance of Ag/Ni multilayers against Kr ion irradiation: an in situ study

Monolithic Ag and Ni films and Ag/Ni multilayers with individual layer thickness of 5 and 50u2009nm were subjected to in situ Kr ion irradiation at room temperature to 1 displacement-per-atom (a fluence of 2u2009×u20091014u2009ions/cm2). Monolithic Ag has high density of small loops (4u2009nm in diameter), whereas Ni has fewer but much greater loops (exceeding 20u2009nm). In comparison, dislocation loops, ∼4u2009nm in diameter, were the major defects in the irradiated Ag/Ni 50u2009nm film, while the loops were barely observed in the Ag/Ni 5u2009nm film. At 0.2u2009dpa (0.4u2009×u20091014u2009ions/cm), defect density in both monolithic Ag and Ni saturated at 1.6 and 0.2u2009×u20091023/m3, compared with 0.8u2009×u20091023/m3 in Ag/Ni 50u2009nm multilayer at a saturation fluence of ∼1u2009dpa (2u2009×u20091014u2009ions/cm2). Direct observations of frequent loop absorption by layer interfaces suggest that these interfaces are efficient defect sinks. Ag/Ni 5u2009nm multilayer showed a superior morphological stability against radiation compared to Ag/Ni 50u2009nm film.

Enhancement of strength and ductility in ultrafine-grained T91 steel through thermomechanical treatments

We report on the microstructure and mechanical properties of T91 alloy subjected to different thermomechanical treatments (TMTs). Equal channel angular extrusion (ECAE) was performed at 23, 300, 625, and 700xa0°C. Mechanical strength of T91 alloy is enhanced after cold-ECAE at the expense of ductility. In comparison, the hot-ECAE process leads to less pronounced hardening but retention of ductility and work hardening ability. Microstructure analyses reveal that cold-ECAE has significantly reduced the average grain size, while hot-ECAE is more effective to refine and uniformly redistribute carbide nanoprecipitates. Post-ECAE annealing (500xa0°C/10xa0h) leads to reprecipitation of large carbide particles in cold-ECAE processed alloy, whereas the hot-ECAEed specimens retain smaller nanoscale carbide precipitates and uniform microstructure. A mechanism is proposed to explain the influence of TMT on the evolution of precipitates. Strengthening of T91 alloy is discussed through a modified model that considers the effects of dislocation density, grain size, and precipitation hardening.