Tzu-Yi Chang

This work reports a new methodology using indentation stress relaxation to characterize the dislocation velocity–stress exponent. Through the indentation stress relaxation process, the dislocation structure builds up at the rate governed by dislocation velocity, which is a function of the externally applied stress. The relationship between the dislocation velocity and stress can thus be derived from the indentation stress relaxation data of the stress as a function of time. In this study, instrumented nanoindentation stress relaxation experiments were performed on pure aluminum samples, following three different initial displacement rates of 100, 400, and 800 nm/s. Based on the scaling properties of dislocation kinetics, the data were interpreted to derive a dislocation velocity–stress exponent of 2.5 ± 0.5 for room-temperature aluminum. Crystal plasticity finite-element simulations were performed to illustrate the sensitivity of the proposed nanoindentation stress relaxation methodology to the dislocation velocity–stress exponent value.

Polymeric materials have multiple potential applications in nuclear power reactors including advanced and small modulus reactors, in addition to their extensive usage in safety electronic cables in the existing light water reactors. Through the qualification programs, knowledge accumulates about the polymer degradation kinetics under gamma-irradiation environment to facilitate the development of service-life predictive models. This paper aims to promote mechanistic-based predictability as a new approach to the qualification of polymeric components. Reviewed in this article are the current qualification standards and procedures, the current understandings of the degradation mechanisms, and kinetic models applicable to accelerated experiments for qualification.

Icosahedral (ICO) clusters are known to exist in many supercooled metallic liquids and believed to play an important role in stabilizing the liquid before it transitions into a glassy, crystalline or quasicrystalline solid. However, a detailed understanding of their formation energetics/dynamics is currently lacking and a set of key questions regarding these clusters remains to be answered. Here, we report our study on the formation energetics/dynamics of ICO clusters in liquid Cu₆₄Zr₃₆ and Ta by combining MD simulations with statistical and theoretical analysis. We present the formation Gibbs free energy, entropy, enthalpy of ICO clusters in the two liquids in the dynamic equilibrium regime (T > 0.75 T_m), determine the size of the spatial domain (number of coordination shells) surrounding the clusters from which the formation enthalpy is originated, and discuss the results in connection with liquid composition, degree of randomness, potential energy landscape, and glass transition.

Graphical abstract

Stacking fault tetrahedra (SFTs) are highly interesting three-dimensional vacancy defects in quenched, plastically deformed or irradiated face-centered-cubic metals and have a significant impact on the properties and subsequent microstructural evolution of the materials. Their formation mechanism and stability relative to two-dimensional vacancy loops are still debated. Equilateral hexagonal Frank vacancy loops (faulted, sessile) observed in microscopy have been considered unable to directly transform to SFTs due to separation of Shockley partial dislocations as well as embryonic stacking faults. Here using sufficiently long (up to tens of nanoseconds) molecular dynamic simulations, we demonstrate that such a transformation can in fact take place spontaneously at elevated temperatures under thermal fluctuation, reducing potential energy of defected atoms by <0.05 eV/atom. The transformation becomes easier with increasing temperature or decreasing loop size.

Pure metals so far have been obtained in quasicrystal (QC) forms only by templation – epitaxial growth on a QC substrate. Here, we report spontaneous formation of dodecagonal QC (DDQC) grains in pure tantalum (Ta), an early transition metal normally in a body-centered-cubic crystal structure. The DDQC grains comprise icosahedral clusters assembled in accordance with the Stampfli triangle–square tiling scheme and are formed directly from the supercooled liquid and the β-Ta phases during thermal devitrification of a Ta metallic glass in molecular dynamics simulations using a realistic quantum mechanically based interatomic potential. They co-exist with β-Ta and are retainable to and stable at room temperature, with a slightly lower configurational energy than β-Ta.