Fracture toughness and fatigue-crack propagation in a Zr–Ti–Ni–Cu–Be bulk metallic glass

The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates (similar to 10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of K(lc)similar to 55 MPa root m, i.e., comparable with that of a high-strength steel or aluminum ahoy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to similar to 1 MPa root m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed

Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ≈2 μm, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these “designed” composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems

Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites

Nonlinear-elastic fracture mechanics methods are used to assess the fracture toughness of bulk metallic glass (BMG) composites; results are compared with similar measurements for other monolithic and composite BMG alloys. Mechanistically, plastic shielding gives rise to characteristic resistance-curve behavior where the fracture resistance increases with crack extension. Specifically, confinement of damage by second-phase dendrites is shown to result in enhancement of the toughness by nearly an order of magnitude relative to unreinforced glass

On the tear resistance of skin.

Tear resistance is of vital importance in the various functions of skin, especially protection from predatorial attack. Here, we mechanistically quantify the extreme tear resistance of skin and identify the underlying structural features, which lead to its sophisticated failure mechanisms. We explain why it is virtually impossible to propagate a tear in rabbit skin, chosen as a model material for the dermis of vertebrates. We express the deformation in terms of four mechanisms of collagen fibril activity in skin under tensile loading that virtually eliminate the possibility of tearing in pre-notched samples: fibril straightening, fibril reorientation towards the tensile direction, elastic stretching and interfibrillar sliding, all of which contribute to the redistribution of the stresses at the notch tip

Key to the Past: Community Perceptions of Yup’ik Youth Interaction with Culturally Relevant Education Inspired by the Nunalleq Archaeology Project

This study qualitatively describes a) the implementation of culturally relevant education (CRE) programs for Yup’ik youth in Quinhagak, Alaska that developed from the Nunalleq Project—a nearby archaeological excavation—and b) community members’ and program facilitators’ perceptions of associated youth social and psychological outcomes. Ten semi-structured interviews (seven community members, three program facilitators) were undertaken and analyzed using constant comparative analysis. Community members and program facilitators attributed numerous outcomes to the Nunalleq-related CRE, such as imparting practical skills (e.g., wilderness survival, artistic and technological skills), teaching young people to value their heritage (e.g., educating them about the struggles their ancestors overcame), and psychological outcomes (e.g., improving self-esteem). Interviewees also offered specific recommendations for planning future local CRE programs. These results provide guidance for local program planners and a framework for researchers to directly assess CRE outcomes in Quinhagak. This project is a step towards the development of a systematic approach to CRE outcome evaluation rooted in community members’ perspectives. Educators developing archaeology-inspired CRE programs in other Indigenous communities may also draw from this study’s results

Some Empirical Criteria for Attributing Creativity to a Computer Program

Peer reviewedPostprin

The effect of screening long-range Coulomb interactions on the metallic behavior in two-dimensional hole systems

We have developed a technique utilizing a double quantum well heterostructure that allows us to study the effect of a nearby ground-plane on the metallic behavior in a GaAs two-dimensional hole system (2DHS) in a single sample and measurement cool-down, thereby maintaining a constant disorder potential. In contrast to recent measurements of the effect of ground-plane screening of the long-range Coulomb interaction in the insulating regime, we find surprisingly little effect on the metallic behavior when we change the distance between the 2DHS and the nearby ground-plane.Comment: 5 pages, 4 figures, accepted for publication in PR

Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers

Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square