The development of aluminium-zinc-magnesium alloys for superior stress corrosion resistance

Thesis (M.Sc.(Engineering))--University of the Witwatersrand, Faculty of Engineering, Department of Metallurgy, 1983.A thorough literature survey has been undertaken to provide the necessary understanding of: i) the general metallurgy and microstructure of Al-Zn-Mg alloys, ii) the stress corrosion cracking (SCC) of Al-Zn-Mg alloys (including accelerated SCC test methods), and iii) the influence of composition, microstructure and heat treatment on SCC properties. Three accelerated SCC test methods were evaluated using existing commercial alloys in different temper conditions. These were the notched rod load relaxation, the electrochemical acceleration and the slow strain rate SCC tests. The slow strain rate method gave the most reliable and reproducible results. This was therefore chosen for all subsequent testing. Baseline SCC test data was obtained from existing alloys in order to facilitate comparison of new alloy compositions * The microstructure of a representative Al-Zn-Mg alloy was examined using optical, scanning and transmission electron microscopy. The effects of quench rate from solution heat treatment, and ageing time and temperature on both the microstructure and SCC properties were investigated. Decreasing quench rate produced a moderate increase in resistance to SCC. The characteristic increase in resistance to SCC found by overageing was, however, associated with an unacceptable loss of mechanical properties, ABSTRACT A thorough literature survey has been undertaken to provide the necessary understanding of: i) the general metallurgy and microstructure of Al-Zn-Mg alloys, ii) the stress corrosion cracking (SCC) of Al-Zn-Mg alloys (including accelerated SCC test methods), and iii) the influence of composition, raicrostructure and heat treatment on SCC properties. Three accelerated SCC test methods were evaluated using existing commercial alloys in different temper conditions. These were the notched rod load relaxation, the electrochemical acceleration and the slow strain rate SCC tests. The slow strain rate method gave the most reliable and reproducible results. This was therefore chosen for all subsequent testing. Baseline SCC test data was obtained from existing alloys in order to facilitate comparison of new alloy compositions.. The raicrostructure of a representative Al-Zn-Mg alloy was examined using optical, scanning and transmission electron microscopy. The effects of quench rate from solution heat treatment, and ageing time and temperature on both the raicrostructure and SCC properties were investigated. Decreasing quench rate produced a moderate increase in resistance to SCC. The characteristic increase in resistance to SCC found by overageing was, however, associated with an unacceptable loss of mechanical properties. Melting, casting and hot working techniques were developed in order to fabricate defect-free small scale experimental alloy compositions. Seven experimental casts were made to cover a wide compositional variation {2n:4~6 wt*t, and Mg:0.8-2.5 wt.%). Slow strain rate SCC testing revealed the beneficial effects of having a zn:Mg ratio of 3:1 (wt.%)

Structural Reliability of Ceramics at High Temperature: Mechanisms of Fracture and Fatigue Crack Growth

Final report of our DOE funded research program. Aim of the research program was to provide a fundamental basis from which the mechanical reliability of layered structures may be understood, and to provide guidelines for the development of technologically relevant layered material structures with optimum resistance to fracture and subcritical debonding. Progress in the program to achieve these goals is described

Final Progress Report: FRACTURE AND SUBCRITICAL DEBONDING IN THIN LAYERED STRUCTURES: EXPERIMENTS AND MULTI-SCALE MODELING

Final technical report detailing unique experimental and multi-scale computational modeling capabilities developed to study fracture and subcritical cracking in thin-film structures. Our program to date at Stanford has studied the mechanisms of fracture and fatigue crack-growth in structural ceramics at high temperature, bulk and thin-film glasses in selected moist environments where we demonstrated the presence of a true mechanical fatigue effect in some glass compositions. We also reported on the effects of complex environments and fatigue loading on subcritical cracking that effects the reliability of MEMS and other micro-devices using novel micro-machined silicon specimens and nanomaterial layers

Role of Molecular Weight on the Mechanical Device Properties of Organic Polymer Solar Cells

For semiconducting polymers, such as regioregular poly­(3-hexylthiophene-2,5-diyl) (rr-P3HT), the molecular weight has been correlated to charge carrier field-effect mobilities, surface morphology, and gelation rates in solution and therefore has important implications for long-term reliability, manufacturing, and future applications of electronic organic thin films. In this work, we show that the molecular weight rr-P3HT in organic solar cells can also significantly change the internal cohesion of the photoactive layer using micromechanical testing techniques. Cohesive values ranged from ∼0.5 to ∼17 J m^–2, following the general trend of greater cohesion with increasing molecular weight. Using nanodynamic mechanical analysis, we attribute the increase in cohesion to increased plasticity which helps dissipate the applied energy. Finally, we correlate photovoltaic efficiency with cohesion to assess the device physics pertinent to optimizing device reliability. This research elucidates the fundamental parameters which affect both the mechanical stability and efficiency of polymer solar cells

The Effects of Terminal Groups on Elastic Asymmetries in Hybrid Molecular Materials

An asymmetric elastic modulus is a recently discovered and unexpected property of hybrid molecular materials that has significant implications for their underlying thermomechanical reliability. Elastic asymmetries are inherently related to terminal groups in the molecular structure, which limit network connectivity. Terminal groups sterically interact to stiffen the network in compression, while they disconnect the network and interact significantly less in tension. Here we study the importance of terminal group molecular weight and size (OH, methyl, vinyl, and phenyl) on the resulting elastic asymmetries and find that increasing the terminal group size actually leads to even larger degrees of asymmetry. As a result, we develop a molecular design criterion to predict how molecular structure affects the mechanical properties, a vital step toward integrating hybrid molecular materials into emerging nanotechnologies

Predicting encapsulant delamination in photovoltaic modules bridging photochemical reaction kinetics and fracture mechanics

Photovoltaic (PV) modules are subjected to environmental stressors (UV exposure, temperature, and humidity) that cause degradation within the encapsulant and its interfaces with adjacent glass and cell substrates. To save experimental time and to enable long-term assessment with intensive degradation only taking place after many years, the development of predictive models is indispensable. Previous works have modeled the delamination of the ethylene vinyl acetate (EVA) encapsulant/glass and encapsulant/cell interfaces under field aging conditions with fundamental photochemical degradation reactions that lead to molecular scission and loss of interfacial adhesion, characterized by the fracture resistance, Gc. However, these models were fundamentally limited in that the following aspects were not incorporated: (i) molecular crosslinking in the field, (ii) synergistic autocatalytic interactions of degradation mechanisms, (iii) connection between degraded encapsulant structure and its mechanical properties, and (iv) rigorous treatment of the plasticity contribution to Gc with finite element models. Here, we present a time-dependent multiscale model that addresses these limitations and is applicable to a wide range of encapsulants and interfaces. For the reference EVA encapsulant and its interfaces with the glass and cell, the presented model predicts an initial rise in Gc in the first 3 years of field aging from crosslinking, then a subsequent sharp decline from degradation mechanisms. We used nanoindentation to measure the changes in EVA mechanical properties over exposure time to tune the model parameters. The model predictions of Gc and mechanical properties match with experimental data and show an improvement compared to previous models. The model can even predict switches in failure interfaces, such as the observed EVA/cell to EVA/glass transition. We also conducted a sensitivity analysis study by varying the degradation and crosslinking kinetic parameters to demonstrate their effects on Gc. Model extensions to polyolefin elastomer- and silicone-encapsulants and their interfaces are also demonstrated. Degradation of module encapsulant mechanical characteristics that lead to embrittlement and delamination remains a leading cause of failure in solar modules. Extending module lifetimes beyond 30 years requires advanced predictive modeling that includes the fundamental materials degradation pathways and their dependence on operating temperature, UV, and moisture. We present a time-dependent multiscale mechanics model based on detailed molecular degradation reaction kinetics that connects the encapsulant bond density and interfacial bond density with its mechanical properties and adhesion energy.imag

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