Youxing Chen

In situ heavy ion irradiation studies of nanopore shrinkage and enhanced radiation tolerance of nanoporous Au

High energy particle radiations induce severe microstructural damage in metallic materials. Nanoporous materials with a giant surface-to-volume ratio may alleviate radiation damage in irradiated metallic materials as free surface are defect sinks. Here we show, by using in situ Kr ion irradiation in a transmission electron microscope at room temperature, that nanoporous Au indeed has significantly improved radiation tolerance comparing with coarse-grained, fully dense Au. In situ studies show that nanopores can absorb and eliminate a large number of radiation-induced defect clusters. Meanwhile, nanopores shrink (self-heal) during radiation, and their shrinkage rate is pore size dependent. Furthermore, the in situ studies show dose-rate-dependent diffusivity of defect clusters. This study sheds light on the design of radiation-tolerant nanoporous metallic materials for advanced nuclear reactor applications.

In Situ Studies on Twin-Thickness-Dependent Distribution of Defect Clusters in Heavy Ion-Irradiated Nanotwinned Ag

Recent studies have shown that twin boundaries are effective defect sinks in heavy ion irradiated nanotwinned (nt) metals. Prior in situ radiation studies on nt Ag at room temperature indicate that the accumulative defect concentration is higher in center areas in the 60-nm-thick twins, and twin boundaries are distorted and self-heal during the absorption of different types of defect clusters. In this follow-up study, we show that the spatial distribution of accumulative defect concentrations in nt metals has a clear dependence on twin thickness, and in certain cases, the trend of spatial distribution is reversed. Potential mechanisms for the counterintuitive findings are discussed.

In situ study on surface roughening in radiation-resistant Ag nanowires

Metallic materials subjected to heavy ion irradiation experience significant radiation damage. Free surface is a type of effective defect sinks to improve the radiation resistance in metallic materials. However, the radiation resistance of metallic nanowires (NWs) is largely unknown. Here we show, via in situ Kr ion irradiations in a transmission electron microscope, Ag NWs exhibited much better radiation resistance than coarse-grained Ag. Irradiation-induced prominent surface roughening in Ag NWs provides direct evidence for interaction between defect clusters and free surface. Diameter dependent variation of the surface roughness in irradiated Ag NWs has also been observed. This study provides insight on mechanisms of enhanced radiation resistance via free surfaces in metallic NWs.

Mechanically controlling the reversible phase transformation from zinc blende to wurtzite in AlN

ABSTRACT III–V and other binary octet semiconductors often take two phase forms—wurtzite (wz) and zinc blende (zb) crystal structures—with distinct functional performance at room temperature. Here, taking AlN as a representative III–V compound, we investigate how to control the synthesized phase structure to either wz or zb phase by tuning the interfacial strain. By applying in situ mechanical tests at atomic scale in a transmission electron microscope, we observed the reversible phase transformation from zb to wz, and characterized the transition path—the collective glide of Shockley partials on every two {111} planes of the zb AlN. GRAPHICAL ABSTRACT IMPACT STATEMENT The innovation of this paper is that a new type of plastic-deformation-driven reversible phase transition in AlN has been captured at atomic scale.

Atomistic modeling of Mg/Nb interfaces: shear strength and interaction with lattice glide dislocations

Using a newly developed embedded-atom-method potential for Mg–Nb, the semi-coherent Mg/Nb interface with the Kurdjumov–Sachs orientation relationship is studied. Atomistic simulations have been carried out to understand the shear strength of the interface, as well as the interaction between lattice glide dislocations and the interface. The interface shear mechanisms are dependent on the shear loading directions, through either interface sliding between Mg and Nb atomic layers or nucleation and gliding of Shockley partial dislocations in between the first two atomic planes in Mg at the interface. The shear strength for the Mg/Nb interface is found to be generally high, in the range of 0.9–1.3xa0GPa depending on the shear direction. As a consequence, the extents of dislocation core spread into the interface are considerably small, especially when compared to the case of other “weak” interfaces such as the Cu/Nb interface.

The Role of Bcc Mg/Nb Interfaces in Nanocomposite Deformation Observed via In-Situ Mechanical Testing in TEM

1. MPA-CINT, Los Alamos National Laboratory, Los Alamos, NM, USA 2. MST-8, Los Alamos National Laboratory, Los Alamos, NM, USA 3. Department of Metallurgical and Materials Engineering, Indian Institute of Technology (IIT) Madras, Chennai, India. 4. Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE , USA 5. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM, USA

Roles of strain and domain boundaries on the phase transition stability of VO2 thin films

The fundamental phase transition mechanism and the stability of the semiconductor-to-metal phase transition properties during multiple thermal cycles have been investigated on epitaxial vanadium dioxide (VO2) thin films via both ex situ heating and in situ heating by transmission electron microscopy (TEM). VO2 thin films were deposited on c-cut sapphire substrates by pulsed laser deposition. Ex situ studies show the broadening of transition sharpness (ΔT) and the width of thermal hysteresis (ΔH) after 60 cycles. In situ TEM heating studies reveal that during thermal cycles, large strain was accumulated around the domain boundaries, which was correlated with the phase transition induced lattice constant change and the thermal expansion. It suggests that the degradation of domain boundary structures in the VO2 films not only caused the transition property reduction (e.g., the decrease in ΔT and ΔH) but also played an important role in preventing the film from fracture during thermal cycles.