β relaxation and low-temperature aging in a Au-based bulk metallic glass: From elastic properties to atomic-scale structure

The slow β relaxation is understood to be a universal feature of glassy dynamics. Its presence in bulk metallic glasses (BMGs) is evidence of a broad relaxation time spectrum that extends to deep within the glassy state. Despite the breadth of research devoted to this phenomenon, its microscopic origin is still not fully understood. The low-temperature aging behavior and atomic structural rearrangements of a Au₄₉Cu₂₆.₉Si₁₆.₃Ag₅.₅Pd₂.₃ BMG are investigated in the regime of the slow β relaxation by employing an ensemble of experimental techniques such as high-intensity synchrotron x-ray scattering, modulated differential scanning calorimetry (MDSC), dynamic mechanical analysis (DMA), impulse excitation, and dilatometry. Evidence of a distinct slow β-relaxation regime is seen in the form of (1) an excess wing of the DMA loss modulus beginning at ∼50 °C, (2) a crossover effect of elastic modulus with isothermal aging at 50 °C, and (3) a broad, nonreversing and largely irreversible sub-Tg endotherm in theMDSC results. Atomic rearrangements occurring at the onset of the measured slow β-relaxation temperature regime were found to be confined mainly to the short-range order length scale while no significant atomic rearrangements occur on the length scale of the medium-range order. Furthermore, evidence is presented that suggests the crossover effect in Young’s modulus is due to the evolution of chemical short-range order. These results support the emergent picture of a dynamically heterogeneous glassy structure, in which low-temperature relaxation occurs through atomic rearrangements confined mostly to the short-range order length scale

Adhäsion zwischen Polymer und Metall oder Halbleiter. Beobachtungen mit spektroskopischen Methoden

Spectroscopic methods have been well proved as versatile tools for characterizing the bulk properties of materials. However, the successful exploitation of their potential in adhesion science depends on access to the interphase between polymer and substrate in the measurement. The paper describes several suitable special techniques of the electron and the infrared spectroscopies in general terms. Experimental results are presented for both model compounds and prepolymers containing triazin rings and/or cyanate groups. The data illustrate the scope of information that is obtainabie nowadays. With that stage of research in mind some aspects of a model for the interphase are discussed and open questions are deduced

Ermittlung von Schädigungsmechanismen in Klebungen nichtrostender Stähle als Beitrag zur Fertigungssicherheit und Zuverlässigkeit

Die Arbeit vergleicht die Auswirkungen unterschiedlicher technischer Alterungstests auf die mechanischen Eigenschaften der Klebverbindung mit den chemisch-strukturellen Veränderungen des Klebstoffpolymers unter Berücksichtigung von Grenzschichteffekten. Dazu werden die Bruchflächen von Zugscherproben ungealterter und gealterter Klebungen mit oberflächenanalytischen Methoden untersucht, wobei unterschiedliche Bereiche der Bruchflächen hinsichtlich Schädigung und Versagensmechanismen differenziert behandelt werden. Der VDA-Test führt zu klebstoffstimulierter Korrosion an der Grenzfläche Substrat-Klebstoff. Dieser Prozess dominiert die Festigkeitsabnahme des Klebverbundes. Das Klebstoffpolymer altert nur langsam mit annähernd linearer Progression. Die Verschlechterung der mechanischen Verbundeigenschaften korreliert nicht direkt mit chemisch-strukturellen Veränderungen des Klebstoffpolymers. Eine irreversible Änderung der elastischen Polymereigenschaften ist nicht nachzuweisen. Der VW-Test schädigt das Klebstoffpolymer in Bulk und Grenzschicht massiv. Diese Polymerdegradation dominiert die Festigkeitsabnahme. Die Verschlechterung der mechanischen Eigenschaften ist eng mit chemisch-strukturellen Veränderungen auf den Bruchflächen korreliert. Die Progression über den gesamten Alterungszeitraum ist nichtlinear und läuft bereits im ersten Alterungsmonat mit maximaler Geschwindigkeit ab. Das Polymer zeigt alterungsbedingte Versprödung

Functional nano fillers in epoxy-dicyandiamide adhesives for prolonged shelf life and efficient cure

Shelf life at room temperature and curing behaviour at elevated temperature are studied for hot-curing accelerated epoxies (EP, diglycidylether of bisphenol A plus dicyandiamide (Dicy)) by FTIR-spectroscopy and modulated DSC. The accelerator is added either directly or with nano-zeolite filler to the EP. Due to the immobilization of the accelerator in the pores of the nano-zeolite, the shelf life of this EP is 5 times longer than for the EP containing free accelerator. While the free accelerator acts during the whole heating step to curing temperature, the nano-zeolite does not release the accelerator before ca. 100 °C. As monitored by light microscopy, the released accelerator not only supports the curing but also stimulates the dissolution of the solid Dicy. As the result, network formation at 170 °C finishes within less than 25 minutes for the nano-filled EP

Experimental and Numerical Investigation of Size Effects in Polyurethane Adhesive Sealings

In our contribution we present an extended continuum-based material model which is able to capture size effects in adhesive bonds. The model is based on the introduction of a scalar-valued structure parameter and its related postulated balance equation. On the experimental side we perform shear tests on polyurethane bonds of anodised aluminium by using diferent shear rates and different thicknesses of the bonded joints. The results of these shear tests show a thickness-dependent stiffness behaviour of the specimens. Finally the obtained experimental data are fitted with the extended material model. The introduced model parameters are identified by using an optimisation tool based on biologic evolution strategies