Materials Science Forum Vols. 567-568

Abstract: In this paper we will present how it is possible to couple a 3D experimental technique with a 3D numerical method in order to calculate the stress intensity factors along the crack front taking into account the real shape of the crack. This approach is used to characterize microstructurally short fatigue cracks that exhibit a rather complicated 3D shape. The values of the stress intensity factors are calculated along the crack front at different stages of crack propagation and it can be seen that the crack shape irregularities introduce rather important fluctuations of the values of KI, KII and KIII along the crack front. The values of KI obtained taking into account the real shape of the crack are significantly different from the ones calculated using an approach based on a shape assumption

Abstract: This paper deals with new approach of measurement of the concentration profiles, concerning the redistribution of substitution elements in close vicinity of welded joint of two different steels. These concentrations have been measured in three straight lines in a plane perpendicular to the weld interface. The concentration profiles have been evaluated from experimental data by modelling of diffusion based on 2nd Fick’s law. Original computation program for nonlinear regression was evolved based on Levenberg - Marquardt`s algorithm for evaluation of diffusivities and other quantities. Three groups of measured data enable variations in methodology of their processing. If each group is processed individually then the possible differences in mutual position of concentration profiles offers the view on chemical heterogeneity of welded joint.

Abstract: Diffusion Al-Si coatings are often used to protect rotor blades of aircraft engines against high-temperature corrosion in environments containing sulfur compounds. Besides other microstructural changes, the degradation of AlSi layers can be indicated by an increasing amount of surface oxide phases and changes in parameters of the layer geometry. In practice, the timetemperature area beyond a critical temperature of the outgoing gas is used as an empirical exploitation parameter D indicating a degradation level. The efficiency of such approach was investigated by analyzing degradation features in the surface layers of rotor blades after exploitations corresponding to different values of D. Determined simple relationships between the relative thickness of degraded layer and the parameter D verify the methodology and yield its clear geometrical interpretation. However, this method fails to provide reasonable information in case when the gas is burning outside the combustion chamber due to a sudden decrease of turbine revolutions.

Abstract: This paper deals with imaging by means of backscattered electrons in the high resolution scanning electron microscopy. Possible backscattered electrons detection systems are outlined and one of the most efficient, the high take of angle single crystal scintillation detector, is described in detail. Its advantages and disadvantages are discussed and the comparison with the secondary electron detection modes is shown. The high resolution micrographs taken by the backscattered electron detector as well as by the secondary electron detectors are displayed.

Abstract: The paper contains results of investigations of the crack growth in plane specimens made of Ti-6Al-4V titanium alloy and AlCu4Mg1 aluminium alloy under cyclic bending. The tests were done on specimens with the stress concentrators being one-sided sharp notch. On the fractures there have been observed first of all transcrystalline cracks through the α phase grains for both materials.

Abstract: The deformation behaviour of the ternary magnesium alloy AX41 (4%Al-1%Ca-balance Mg) were investigated in uniaxial tension tests at temperatures between 20 and 300 °C and at an initial strain rate ranging in the order 10-4 s-1. The yield stress of the alloy is very sensitive to the testing temperature. Stress relaxation tests were performed with the aim to reveal physical base of deformation processes.

Abstract: Fins in automotive heat exchangers are manufactured from Al-Mn-Fe-Si foils. Since continuous twin-roll casting (TRC) reduces energy and work consumption, it is the preferred manufacturing technology. The precipitation processes resulting from annealing of TRC Al-Mn-Fe- Si based alloys were studied by resistometric measurements during linear heating from room temperature up to 620 °C, hardness measurements and transmission electron microscopy examinations. Primary particles and precipitates of the cubic α-Al15(Mn,Fe)3Si2, hexagonal Al8Fe2Si and orthorhombic Al6(Fe,Mn) phases were identified. Phases of different types prevail in different alloys depending on composition. The increase of cold rolling prestrain prior to annealing induces a significant shift to lower temperatures of the start of precipitation. Prestrain accelerates precipitation kinetics by redistributing solute atoms and favoring their segregation on nucleation sites such as dislocations, subgrain and grain boundaries.

Abstract: In order to investigate the hydrogen effect on fatigue crack growth (FCG) behavior in a few kinds of practical alloys; austenitic stainless steels (solution-treated metastable type 304 and stable type 316L), an aluminum alloy (age-hardened 6061) and a low carbon steel (annealed 0.13%C-Fe), FCG tests were carried out in hydrogen gas and in nitrogen gas. The FCG rates of these materials are enhanced by hydrogen, though the acceleration degrees are different. A crack grows across grains by slip-off in 316L stainless steel and in age-hardened 6061 aluminum alloys even in hydrogen. Faceted area increases in 304 stainless steel and in low carbon steel in hydrogen. In 304 stainless steel, the ratio of facets to the entire fracture surface was not so large. Thus, the FCG rate is not significantly affected through the facets in 304 stainless steel. In low carbon steel, facets were increased considerably, though a crack grows step by step or after a large number of loading cycles even along grain boundaries. Anyhow hydrogen enhances the FCG rate of these materials through the influence on slip behavior. Based on above-mentioned results, the effect of loading frequency on FCG rate in hydrogen of the age-hardened 6061 aluminum alloy was also investigated. The FCG rate increases as the testing frequency decreases, though the FCG rate in hydrogen shows the tendency to saturate.

Abstract: Heat treatable AlMgSi alloys are being used increasingly for automotive skin sheet. AA6016 sheets produced from direct chill (DC) cast ingots are the material of choice in Europe. However, manufacturing of sheets using twin-roll casting (TRC) is a cost effective alternative to DC casting. The effect of processing parameters on the age hardening of TRC AA6016 sheets was assessed. The influence of quenching temperature after solution annealing and subsequent pre-aging at 100 °C was studied. Sheets in the artificially aged condition were prepared using a simulated car-body paint baking procedure (180 °C/30 min). The precipitation kinetics, resulting dispersion of the agehardening phase and their correlations with properties were investigated by differential scanning calorimetry, transmission electron microscopy and tensile tests. It is demonstrated that pre-aging results in increased age hardening effect, especially in the material quenched to 20 °C after solution annealing. The differences in strength are ascribed to the differences in the nucleation rate of β'' precipitates during stabilisation and the formation of stable GP-I zones in naturally aged specimen. The differences in precipitate microstructure inherited from the pre-treatment affect precipitation kinetics during paint baking and result in different precipitate dispersion and sheet strength.

Abstract: The structure of austempered ductile iron (ADI) matrix and consequently its mechanical properties are influenced by the heat treatment conditions, above all by the temperature and dwell length of isothermal transformation. The paper is focused on deeper understanding the interrelation between matrix mixture composition and static mechanical properties of ADI in dependence on the isothermal transformation dwell. Practical aim of the paper is to find the optimal isothermal transformation dwell range for ADI isothermally transformed at the temperature of 380 °C with emphasis on the level of static mechanical properties in tension. Microstructure and mechanical properties changes that proceed during isothermal transformation are observed and evaluated for the transformation dwells of 2, 5, 10, 25, 60, 120, 270, and 540 minutes.