Key Engineering Materials Vols. 592-593

Abstract: Service life of VVER-type nuclear reactor is limited by decrease in brittle fracture resistance of reactor pressure vessel produced of low-alloy low-carbon steel under effect of irradiation and/or elevated temperatures. In this work fracture surfaces were studied by Auger-electron spectroscopy in order to estimate the contribution of intergranular embrittlement to the degradation of reactor pressure vessel steels under the influence of operating conditions. It was demonstrated that irradiation induced segregation leads to an increase of P content in grain boundaries that promotes intergranular brittle fracture on fracture surfaces. The similar effect but to a lesser degree was shown in the case of long-term temperature exposure. The grain boundary structure was examined and an effect of carbides located on the grain boundaries is supposed due to increased phosphorus segregation on carbide/matrix interface boundaries.

Abstract: In this research work specimens of the metastable austenitic steels AISI 304 and AISI 347 with one phase (fully austenitic) and two phase (austenitic-α ́-martensitic) microstructure were monotonically loaded at ambient temperature. Using stress-strain and temperature measurements the deformation behavior was characterized in detail. To study the influence of morphology of deformation induced α ́-martensite on the stress-strain response a phase field model for α ́-martensite transformations was developed. With this approach it was possible to model the two phase austenite-α ́-martensite microstructure and investigate the deformation behavior on the micro level. With optical microscopy, magnetic and x-ray measurements the microstructure characterization of fully austenitic and austenitic-α ́-martensitic steels was realized.

Abstract: Instrumented scratch test was carried out to determine the scratch resistance of polyamide 6 (PA 6) nanocomposites, where two kinds of nanofillers were tested, both based on silicates: montmorillonite (MMT) and halloysite nanotubes (HNT). In this work the influence of the sliding velocity, normal applied load and time-dependent recovery on the penetration depth and scratch hardness was investigated. Optical microscopy was utilized to determine the width of the scratch grooves and scanning electron microscopy revealed the damage features of the scratched surfaces. Both MNT and HNT nanofillers improve the scratch resistance of PA 6 considerably. As a result of the microstructure of the polymer nanocomposites MNT gives PA 6 a better residual depth resistance while HNT raises its scratch hardness (i.e. reduces the scratch width). Furthermore, via different depth-sensing indentation techniques in the nano-, micro- and macro-range of loading the short-term performance (Martens hardness and indentation modulus) and the time-dependent creep behavior have been analyzed for PA 6 and the PA 6 nanocomposites as a function of applied load and temperature. Additionally, WAXS (wide-angle X-ray scattering) and DSC (differential scanning calorimetry) measurements to establish morphology–property relationships of the materials investigated considering the skin–core structure of the injection molded samples were made.

Abstract: Biomass power plants with high efficiency are desired as a renewable energy resource. High efficiency can be obtained by increasing temperature and pressure. An upgrade of the material performance to high temperature material is therefore required in order to meet the increased demands due to the higher temperature and the more corrosive environment. In this study, the materials high-temperature behaviours of AISI 304 and Alloy617 under slow deformation rate are evaluated using high-temperature long-term aged specimens subjected to slow strain rate tensile testing (SSRT) with strain rates down to 10^-6/s at 700°C. Both materials show decreasing stress levels and elongation to fracture when tensile deformed using low strain rate and elevated temperature. At high-temperature and low strain rates cracking in grain boundaries due to larger precipitates formed during deformation is the most common fracture mechanism.

Abstract: Structure of bimetallic composite obtained during explosive welding process exhibits strong heterogeneity in the vicinity of interface. The interface usually has a wavy shape with characteristic increase of hardness and largely deformed grains. Recently obtained fatigue tests for titanium-steel bimetal specimens under fully reversed push-pull loading show ratcheting phenomenon. In order to investigate this phenomenon mechanical testing and structural observations of titanium-steel bimetal and titanium before cladding have been performed. Fatigue characteristics in the form of relation between strain amplitude and fatigue life for titanium (Grade 1), steel (S355J2+N) and bimetal will be presented.

Abstract: This work has been done to research the physical nature of the mechanism of structural changes of the aluminum alloy 2024-T3 at the conditions of additional impulse load. There was revealed an anomaly growth in plasticity of the material after such type load realization at the room temperature. Based on fractographical research the impact of dynamic non-equilibrium processes on the structural changes of the material and its macro-mechanical properties was analyzed.

Abstract: The objective of investigation was to determine the influence of chosen cooling rates after finish rolling on final microstructural and mechanical properties of the laboratory rolled products from steel 42CrMo4. Metallographic analysis showed that microstructure of rolled products, which were after finish rolling cooled down in accelerated manner by water sprays, was composed mostly by hardening phases and by smaller amount of ferrite. Microstructure of the rolled product cooled down in furnace in decelerated manner was formed by pearlitic blocks with minority occurrence of ferrite. Laboratory rolled products cooled down by more complicated modes, which consisted of combination of their cooling by water sprays with subsequent annealing in furnace, showed different phase morphology and distinct band structure. Results of tensile test have proved that the applied modes of cooling of rolled products by accelerated manner had only very small influence on their final mechanical properties. Considerably cooling the rolled product by decelerated manner in furnace resulted in drop yield strength, but on the other hand in an increase of ductility. More complicated modes of cooling the rolled products, which comprised also their annealing, showed the possibilities of significant influencing the strength, and particularly the plastic properties of the steel 42CrMo4.

Abstract: It has been widely accepted that the creep characteristics at high temperatures are mainly evaluated by a minimum creep rate and a time to fracture. Although, a shape of creep curve may vary depending on deformation conditions, the apparent minimum creep rates may become the same value. Thus, for detailed analysis and prediction of creep behavior, other values should be considered which reflects the shape of each creep curve. For the purpose, authors have proposed Satos Strain-Acceleration-Parameter (SAP) which reflects strain rate change during creep. Based on the concept of SAP, the whole creep curve can be represented by a set of small numbers of numerical parameters, and can be extrapolated from a part of creep curve [. It is also well accepted that the creep rates depend on microstructures, and microstructural changes cause strain rate change. The SAP would reflect stability and magnitude of microstructural change during deformation at high temperatures. In this paper, application of the concept of SAP to creep curves that show sigmoidal type primary creep is presented. The creep curve can be divided into two regime based on the SAP values. The sigmoidal creep curve is reasonably reproduced by the concept of Strain-Acceleration-Parameter, and reasonably agrees with experiment. Whole creep curve can be reasonably represented by a few numerical values which reflect shape of creep curve in each regime. The concept of SAP is applicable for quantitative evaluation of both normal and sigmoidal type of creep curves.

Abstract: Different approaches have been proposed in order to determine the material behavior of ductile materials. Since, the mechanical properties of a mechanical component are modified during manufacturing process due to plastic deformation, heat treatment and etc, a non-destructive indentation experimental procedure addressed to predict the elastic-plastic properties of material after manufacturing process is of interest. This is especially true for small size components where it is complex to extract specimens to test on standard test system. Based on dimensionless analysis and the concept of a representative strain, different approaches have been proposed to determine the material properties of power law materials by using indentation process. In this work, the Johnson-Cook (JC) constitutive model of the aluminum alloy Al6061-T6 is characterized by means of a well-defined optimization procedure based on micro-indentation testing and high fidelity finite element models and an optimization procedure but without the concept of dimensionless analysis and a representative strain. This methodology allows determining a set of JC constants for Al6061-T6. The obtained results have good agreement with parameters calibrated by means of universal standard tests and reverse engineering approach.

Abstract: It is well known that the addition of ceramic phases in an alloy e.g. aluminum, in form of fibers or particles influences its mechanical properties. This leads to a new generation of materials, which are called metal matrix composites (MMCs). They have found a lot of application during the last twenty-five years due to their low density, high strength and toughness, good fatigue and wear resistance. Aluminum matrix composites reinforced by ceramic particles are well known for their good thermophysical and mechanical properties. As a result, during the last years, there has been a considerable interest in using aluminum metal matrix composites in the automobile industry. Automobile industry use aluminum alloy matrix composites reinforced with SiC or Al₂O₃ particles for the production of pistons, brake rotors, calipers and liners. However, no reference could be cited in the international literature concerning aluminum reinforced with TiB particles and Fe and Cr, although these composites are very promising for improving the mechanical properties of this metal without significantly alter its corrosion behavior. Several processing techniques have been developed for the production of reinforced aluminum alloys. This paper is concerned with the study of TiB, Fe and Cr reinforced aluminum produced by the stir-casting method.