M. Song

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.

In Situ Study of Defect Migration Kinetics and Self-Healing of Twin Boundaries in Heavy Ion Irradiated Nanotwinned Metals

High energy particles introduce severe radiation damage in metallic materials, such as Ag. Here we report on the study on twin boundary (TB) affected zone in irradiated nanotwinned Ag wherein time accumulative defect density and defect diffusivity are substantially different from those in twin interior. In situ studies also reveal surprising resilience and self-healing of TBs in response to radiation. This study provides further support for the design of radiation-tolerant nanotwinned metallic materials.

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.

In situ observation of defect annihilation in Kr ion-irradiated bulk Fe/amorphous-Fe 2 Zr nanocomposite alloy

Enhanced irradiation tolerance in crystalline multilayers has received significant attention lately. However, little is known on the irradiation response of crystal/amorphous nanolayers. We report on in situ Kr ion irradiation studies of a bulk Fe96Zr4 nanocomposite alloy. Irradiation resulted in amorphization of Fe2Zr and formed crystal/amorphous nanolayers. α-Fe layers exhibited drastically lower defect density and size than those in large α-Fe grains. In situ video revealed that mobile dislocation loops in α-Fe layers were confined by the crystal/amorphous interfaces and kept migrating to annihilate other defects. This study provides new insights on the design of irradiation-tolerant crystal/amorphous nanocomposites.