Failure mode engineered high-energy-absorption metamaterials with biomimetic hierarchical microstructures and artificial grain boundaries

For numerous engineering applications, there is a high demand for protective lightweight structures with outstanding energy absorption performance and the ability to prevent catastrophic structural failures. In nature, most species have evolved with hierarchical biological structures that possess novel mechanical properties, including ultrahigh specific energy absorption, progressive laminated failure modes, and ability for crack arrestment, in order to defend themselves from hostile environments. In this study, a novel protective metamaterial having spherical hollow structures (SHSs) was developed with different hierarchical microstructures. An artificial failure mode engineering strategy was proposed by tailoring the microstructures of SHS unit cells. To demonstrate the effectiveness of the proposed method, a composite hierarchical SHS lattice structure was developed using a biomimetic laminated failure mode and through a hardening mechanism, mimicking crystal grain boundaries. The quasi-static compressive results indicated a significant improvement in the specific energy absorption, an enhanced plateau stress magnitude, and an obvious delay in the densification stage for the composite hierarchical SHS lattice owing to the constraining effect of its mesoscale grain boundaries and an increased number of intensively engineered laminated failure levels. This novel type of metamaterial was shown to be immensely beneficial in designing lightweight protective aerospace components such as turbine blade lattice infills.

Mechanisms of structural-phase transformations during crystallization of solder melt under conditions of magnetic-dynamic influences for carbide tools

The processes of melt formation were studied by methods of optical and electron scanning microscopy. These processes occur during induction brazing of a hard alloy to a steel holder and contact interaction of low-melting (copper-zinc system alloy) and refractory (iron-nickel) components of the solders. It is shown that the effect of a thermal and magnetic-dynamic high-frequency electromagnetic field on the components of the composite solder is how a high-strength solder joint is formed. The structure is forming by disperse hardening mechanism. The research of the contact interaction process for low-melting and high-melting components of solders during the soldering process of the tool showed that the formation of solder in brazed seams occurs through a number of stages and this does not lead to the formation of microstructures that are characteristic of alloys based on copper-iron-phosphorus, copper-zinc-nickel and copper-zinc-iron. Thus, the use of composite solders can reduce the soldering temperature by 40-50 K and increase the concentration of alloying species in the solder and change its structure. These advantages of composite solders reduce the thermal impact on contact materials, increase the strength of the weld and allow you to control the thickness of the brazed weld, and this is important when soldering hard alloys of WC-TiC (TaC) systems. High initial dissolution rates of nickel particles in the copper-zinc melt and the solubility of copper, zinc in nickel lead to the formation in the melt of quasi-liquid particles of the nickel alloy. When the melt is cooled, particles other than the surrounding alloy composition are formed. They are morphologically related to the grain structure of the solder. The formed alloy (solder) has the structure of a composite material in which the metal particles are enriched in nickel, and have the role of a reinforcing element.

Natural Aging Effect of Al-20Zn-3Cu Alloy on Mechanical Properties and Its Relation to Microstructural Change

We investigate the effect of the natural age-hardening response of the Al-20Zn-3Cu alloy with natural aging times up to 12 months. The ultimate tensile strength of the Al-20Zn-3Cu alloy is drastically enhanced from 308 to 320 MPa after 2 months and from 320 to 346 MPa after 9 months. Then, natural age hardening becomes saturated after 9 months. A microstructural investigation reveals that the natural age-hardening mechanism is mainly induced by the diffusion of the Zn element. First, a rapid decrease in the volume fraction of the eutectoid lamellae (α-Al+η-Zn) is observed at the early stage of natural aging, leading to an increase in the tensile strength. This originates from the relatively high diffusivity of Zn due to its low melting temperature. Then, the diffusion of Zn into the Al matrix induces clusters of solute atoms that enhance the growth rate of the nanoprecipitates formed in the Al matrix. As a consequence, the tensile strength of the natural-aged Al-20Zn-3Cu alloy increases drastically after 9 months, whereas the ductility is significantly degraded.

Mechanical Properties and Interface Microstructure of SAC305 Solder Joints Made to a Ag-Pd-Pt Thick Film Metallization: Part 2—Isothermal Aging Effects

The performance and reliability were documented for solder joints made between the 96.5Sn-3.0Ag-0.5Cu (wt.%, abbreviated SAC305) Pb-free solder and a Ag-Pd-Pt thick film conductor on an alumina substrate. The Sheppard’s hook pull test was used to assess the solder joint strength. The Part 1 study confirmed that the solder joint fabrication process had a wide process window. The current study determined that the SAC305 solder joints maintained that robustness after accelerated aging at temperatures of 70–205°C and time durations of 5–200 d. Short-term aging of 5–10 d caused a peak in the pull strength peak that resulted from precipitation hardening by Ag-Pd and (Pd, Pt)xSny intermetallic compound (IMC) particles. The pull strengths did not decrease significantly after longer aging times at 70°C and 100°C; those conditions were accelerations of typical service lifetimes. Longer aging times at temperatures of 135–205°C resulted in a gradual, albeit not catastrophic, strength decrease when the precipitation hardening mechanism was lost to dissolution of the particle phases and their reprecipitation at the solder/alumina interface. The failure modes were ductile fracture in the solder except for the most severe aging conditions. These findings confirmed that the SAC305 solder/Ag-Pd-Pt thick film interconnections have excellent long-term reliability for hybrid microcircuit and high-temperature electronics applications.

Study of beryllium hardening obtained by powder metallurgy

In the first part of the article the results of the study of powder hardening processes occurring during beryl powders consolidation by the hot pressing method are shown. The dependences of the content and morphology of the hardening phase depending on the content of low-melting impurities on the sintered Beryllium powder grains surface have been studied. A hypothesis is proposed that explains the transition of an oxide film from an amorphous to a crystalline state – devitrification, and the effect of low-melting impurities on the mechanism of the devitrification process and, as a consequence, on the effect of "dispersion-grain boundary" hardening. This hypothesis is based on theoretical confirmation with the provision of graphic material demonstrating the process of devitrification, accompanied by a dispersed-grain-boundary hardening mechanism. The final results of statistical processing carried out on industrial batches showing the dependence of the impurities content influence on the properties of hot-pressed beryllium are presented. In the second part of the article the results of studying the effect of hardening of beryllium obtained in the process of sintering by the method of hot isostatic pressing (HIP) are shown depending on the temperature of powders consolidation. Based on the results of electron microscopic studies, the dynamics of the reinforcing phase formation at the grain boundaries of sintered beryllium is shown. The quantitative dependence of the precision elastic limit and the conditional yield stress of gas-statically compressed beryllium on the size of the strengthening beryllium oxide particles and the consolidation temperature of the powders have been established. The resulting equation gives a description of the "dispersed-grain-boundary" hardening mechanism of isostatically pressed beryllium. All dependencies are also represented by graphic material reflecting the essence of the research.

A New Approach for Textual Password Hardening Using Keystroke Latency Times

Textual passwords are still widely used as an authentication mechanism. This paper addresses the problem of textual password hardening and proposes a mechanism to make textual passwords harder to be used by unauthorized persons. The mechanismintroduces time gaps between keystrokes (latency times) that would add a second protection line to the password. Latency times are converted into discrete representation (symbols) where the sequence of these symbols is added to the password. For accessing system, an authorized person needs to type his/her password with a certain rhythm. This rhythm is recorded at the sign-up time.This work is an extension to a previous work that elaborates more on the local approach of discretizing time gaps between every two consecutive keystrokes. In addition, more experimental settings and results are provided and analyzed. The local approach considers the keying pattern of each user to discretize latency times. The average, median and min-max are tested thoroughly.Two experimental settings are considered here: laboratory and real-world. The lab setting includes students studying information technology while the other group are not. On the other hand, information technology professional individuals participated in the real-world experiment. The results recommend using the local threshold approach over the global one. In addition, the average method performs better than the other methods. Finally, the experimental results of the real-world setting support using the proposed password hardening mechanism.