HIGH STRAIN RATE BEHAVIOUR OF AN AZ31 + 0.5 Ca MAGNESIUM ALLOY

<em>The paper reports behaviour of magnesium alloy AZ31 (nominal composition 3 % Al - 1 % Zn – balance Mg) with an addition of 0.5 wt. % Ca at high strain rates. Samples were prepared by the squeeze cast technology. Dynamic compression Hopkinson tests were performed at room temperature with impact velocities ranging from 11.2 to 21.9 m.s^-1. A rapid increase of the flow stress and the strain rate sensitivity was observed at high strain rates. Transmission electron microscopy showed extremely high dislocation density and mechanical twins of two types. Adiabatic shear banding is discussed as the reason for the observed behaviour at high strain rates.</em><span> </span

Non-alloyed Ni3Al based alloys – preparation and evaluation of mechanical properties

The paper reports on the fabrication and mechanical properties of Ni3Al based alloy, which represents the most frequently used basic composition of nickel based intermetallic alloys for high temperature applications. The structure of the alloy was controlled through directional solidification. The samples had a multi-phase microstructure. The directionally solidified specimens were subjected to tensile tests with concurrent measurement of acoustic emission (AE). The specimens exhibited considerable room temperature ductility before fracture. During tensile testing an intensive AE was observed

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic areas. Here, we propose a concept of ‘Crack Propagation Unit (CPU)’ for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/γ′ channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters

Characterization of U-based thin films: the UFe₂₊ₓ case

We have characterized UFe₂₊ₓ films prepared by sputter deposition onto fused silica (SiO₂) and Si(111) substrates with the film thickness ranging from 75 nm to 900 nm. The X-ray diffraction results showed an amorphous character of the deposited material. Some of the films showed in addition a pattern of highly textured cubic Laves phase. Rutherford Backscattering Spectroscopy with 2 MeV He⁺ ions has been used to determine the composition, thickness and concentration depth profile of the films. A large ageing affect was observed within 1 month after that the films were exposed to air. Magnetic measurements revealed TC increasing with relative Fe concentration and reaching approx. 450 K in UFe₃.₀

Using the SAS System for Building Data Warehouses and Optimalization of ETL Processes

This diploma thesis deals with usability of the SAS system and its components for building and running data warehouse and complete solution of Business Intelligence. In the beginning it introduces the meaning and benefits of putting on Business Intelligence and its spot in an organization. It especially focuses on the running BI project in Kooperativa, a.s., insurance company. The main goal of this thesis is to aim on ETL processes of data warehouse, their specificity, characteristics and regular tasks solved across data layers, measuring their performance and feasibility of ETL optimalization. This optimalization can be considered from two different points of view – the first is a creation and maintenance of the ETL source code, the second is tuning for faster data processing. Log files, which are are the main source for performance monitoring, are processed by macroprogram specially tailored to this particular reason. Gained results are analyzed and on that basis I outline spots that need attention. The last part offers comparison of some alternatives to data transformation process typically solved by ETL tasks. Acquired results could be taken as hints used in designing and tweaking other akin ETL processes

Non-alloyed Ni3Al based alloys – preparation and evaluation of mechanical properties

The paper reports on the fabrication and mechanical properties of Ni3Al based alloy, which represents the most frequently used basic composition of nickel based intermetallic alloys for high temperature applications. The structure of the alloy was controlled through directional solidifi cation. The samples had a multi-phase microstructure. The directionally solidifi ed specimens were subjected to tensile tests with concurrent measurement of acoustic emission (AE). The specimens exhibited considerable room temperature ductility before fracture. During tensile testing an intensive AE was observed.Web of Science52331230

Evolution of mechanical properties of LAE442 magnesium alloy processed by extrusion and ECAP

The dependence of the mechanical properties on the increasing number of ECAP passes was investigated in the LAE442 magnesium alloy. The substantial grain refinement resulting from thermomechanical processing caused increase of the yield tensile strength and the ultimate tensile strength. Moreover, the fracture mechanism changed after thermomechanical processing, which led to a higher elongation to fracture

Long-term aging of Ag/a-C:H:O nanocomposite coatings in air and in aqueous environment

Nanocomposite coatings of silver particles embedded in a plasma polymer matrix possess interesting properties depending on their microstructure. The film microstructure is affected among others also by the RF power supplied during the deposition, as shown by transmission electron microscopy. The optical properties are characterized by UV–vis–NIR spectroscopy. An anomalous optical absorption peak from the Ag nanoparticles is observed and related to the microstructure of the nanocomposite films. Furthermore, a long-term aging of the coatings is studied in-depth in ambient air and in aqueous environments. It is shown that the studied films are not entirely stable. The deposition conditions and the microstructure of the films affect the processes taking place during their aging in both environments.ISSN:1468-6996ISSN:1878-551