Issue 35

A. Kowalski et alii, Frattura ed Integrità Strutturale, 35 (2015) 449-455; DOI: 10.3221/IGF-ESIS.35.51 454 Figure 11 : Transcrystalline quasi-cleavage fracture with an increases share of plastic deformation on the surface of jogs of the investigated alloy. Figure 12 : Trace of plastic deformation on cleavage surface of AlZn6Mg0.8Zr alloy. D ISCUSSION he formation of new kinds of aluminium alloys with more favorable mechanical properties requires a penetrating scientific analysis actually aided by numerical methods, concerning both the chemical composition and technical processes. It has been found that the crucial feature affecting the widely understood properties of aluminium alloys is their microstructure, particularly their dimensions and shape of the grains, the dislocation structure, the size and morphology and distribution of the particles of the intercrystalline phases, mainly of phases with a large dispersion. Every one of these factors of the microstructure is essentially affected by the applied technological process of aluminium alloys assigned for plastic working. Cold rolling after supersaturation of the aluminium alloy belonging to the 7003 series permits to attain a higher strength thanks to the increased density of dislocations and higher share of nucleation of hardening phases, ensuring the formation of particles with a larger dispersion and restricting considerably the formation of zones without precipitations along the grain boundaries. The role of particles with a larger dispersion (submicroscopic dispersion) in the process of cracking of the aluminium alloy is, however, more complex. Their influence on the resistance of the alloys is both positive and negative. They affect positively the restriction of the growth of the grains. Small grains promote transcrystalline cracking, absorbing much energy. On such particles nucleate, however, also microvoids which may lead to an increased share of platforms between other voids appearing on big precipitations. This also affects considerably the process of destruction due to fatigue, resulting from the nucleation and increased cracking on the surface of elements exposed to varying loads. In the layer adjacent to the surface the stresses concentrate due, among others, to the presence of big particles of precipitations, bands of sliding, microintrusions or extrusions, as well as to zones deprived of precipitations along the grain boundaries adhesing to the surface. The resistance to cracking depends mainly on the inclination of the alloy to nucleation and the propagation of cracking. Thus, the microstructural factors restricting an easy nucleation of cracking caused by fatigue are undoubtedly precipitations of the intermetallic phases, hardening the aluminium alloys susceptible to shearing by dislocation or recrystallized grains retaining the minimum relative volume of big particles of primary precipitations. The rate of the increase of cracking due to fatigue depends also on the character of sliding. Sliding in big grains and the presence of precipitations crossed by dislocations promote the development of cracking caused by fatigue [7]. Plastic deformation in the course of thermomechanical treatment increases the density of dislocations, which again restricts the formation of broad bands of sliding during the deformation resulting from fatigue. Paradoxically, also the presence of numerous of intermetallic precipitations phases with large dimensions can prevent a localization of deformations in the slide bands, thus increasing the fatigue strength. The analysis of the results of many fatigue tests of aluminium alloys leads to the conclusion that the optimal mechanical properties of these alloys may condition the bimodal distribution of the size of precipitations of hardening phases, i.e. dispersive precipitations (from 1 nm to about 10 nm), increasing the yield point and tensile strength of the alloys and of particles with a diameter of 0.01 μm to about 0.2 μm, improving their fatigue strength. T