Materials Science Forum Vol. 555

Abstract: The Ni92.8Mo7.2 alloy powder was obtained by electrochemical codeposition from an ammonium solution of NiSO4 and (NH4)6 Mo7O24 at j = 100 mA m-2, on a titanium cathode. DSC measurements and determination of the dependence of electrical resistivity on temperature did not reveal any changes in powder structure in the temperature range from 293 to 460 K. Therefore, in this range, there was no significant change in magnetic susceptibility either. Structural relaxation took place in the temperature interval from 460 to 560 K causing an increase in magnetic susceptibility. At temperatures higher than 570 K, magnetic susceptibility rapidly decreased. The Curie temperature of the powder was 660 K. With the increasing powder pressing pressure the magnetic susceptibility increased while the electric resistivity decreased. With the pressing pressure increase, the pore size was decreased and a better contact between powder particles was established. This caused electrical resistivity decrease and magnetic susceptibility increase.

Abstract: Most of usual materials exhibit Poisson's ratio comprised between 0 and 0.5. But, for some kind of cellular materials, or for some stacking sequences of unidirectional plies, a composite material can exhibit negative or greater than 0.5 Poisson's ratios. In this paper, a study of different stacking sequences such as [±β/±θ]s plies made from highly anisotropic fibre pre-preg is presented. A special computer programme has been developed for this purpose. Eighteen stacking sequences, including the [±θ] ones, have been computed. The results show that at least one of Poisson's ratios varies between -0.8 to +0.4. Such kind of materials may find applications for particular cases, as their strength is significantly increased by this phenomenon.

Abstract: The aim of the present work is to determine the role of intermetallic (IM) phases in the fatigue crack propagation behavior of hot-forged Al-Zn-Mg-Cu alloys in T73 condition. To generate differences in the volume fraction and coarseness of various IM particles, the (Fe+Si) impurity level is varied from 0.23 to 0.37 mass%. The fatigue crack propagation tests are conducted in air at ambient temperature and a stress ratio R of 0.1. Characterization of the fatigue fracture surfaces is performed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Quantified IM particles data for each alloy are then related to the fatigue properties and fractographic analysis results. It was found that almost all particles of the Fecontaining phases (primarily (Cu,Fe,Mn)Al3 and Al7Cu2Fe) are broken and not effective in hindering fatigue crack propagation. On the other hand, the Mg2Si and soluble phase particles smaller than those of the Fe-containing phases contribute beneficially to fatigue life. These particles increase the tortuosity of the crack path and retard the crack growth rate. The crack growth rate decreases as the volume fraction of coarse Fe-containing particles increases, because more secondary cracks are produced decreasing the effective stress intensity at the main crack tip.

Abstract: The paper deals with the effects of thermo-mechanical processing parameters on the properties of two Al-Fe-Si foil alloys. Two twin roll cast alloys with high Fe/Si ratios (Fe/Si≈6) were processed under different combinations of homogenization, deformation and annealing treatments. The influence of small additions of manganese on the mechanical behavior and plastic anisotropy of sheets in hardening and annealing conditions are described. The corrosion resistance of tested sheets in chloride ambience is also presented, because this type of Al foils is dominantly used in the packaging (food) industry and heat exchangers.

Abstract: Structural integrity and service reliability depend on the fracture resistance of a material. Cracks in the material are the locations of stress concentration, and elastic-plastic deformation can occur causing the development of mixed-mode type of fracture ahead the crack tip. Crack behavior in the elastic-plastic region is analyzed applying numerical and analytical simulation based on fracture mechanics parameters, characterizing the response of the material at the crack tip. Numerical and analytical results are compared with the corresponding experimental results obtained in previously performed fracture mechanics tests with standard single-edge notch bending – SEN(B) specimens. The comparison shows an acceptable level of agreement, enabling application of the proposed numerical model of crack growth in the mixed-mode fracture analysis for structural integrity assessment.

Abstract: In this paper the micromechanical approach to ductile fracture was applied in a study of constraint effect on crack growth initiation in mismatched welded joints. The single-edged notched bend specimens (precrack length a0/W=0.32) were experimentally and numerically analyzed. The coupled micromechanical model proposed by Gurson, Tvergaard and Needleman was used. Constraint effect was tested by varying widths of the welded joints (6, 12 and 18mm). Highstrength low-alloyed (HSLA) steel was used as the base metal in a quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to obtain over- and undermatched weld metal. The micromechanical parameters used in prediction of the crack growth initiation on precracked specimen were calibrated on a round smooth specimen. The difference in fracture behavior between over- and undermatched welded joints obtained in experimental results was followed by numerical computations of void volume fraction in front of the crack tip.