Issue 35

A. Kowalski et alii, Frattura ed Integrità Strutturale, 35 (2015) 449-455; DOI: 10.3221/IGF-ESIS.35.51 452 of regression in the equation y = -14x + 252. The maximum bending moment and the amplitude of stresses σ a of about 190 MPa was recorded at the number of cycles N f about 6·10 4 , whereas minimum amplitude of stresses σ a of about 160 MPa was recorded with the number of cycles N f about 2·10 6 . The samples tested in the fatigue test are subjected to varying loads bending of a cyclic character. The stresses and deformations resulting from these loads lead to a fatigue of the material and are the main reason of the initiation of its cracking. It has also be found that the analyzed cracking caused by fatigue is of a stochastic character, unforeseeable, and therefore requiring a constant development of investigations concerning this branch of knowledge, both from the viewpoint of cognition and of practical use. Basing on the determined characteristics of fatigue ( σ a – N f ) we can determine the durability to fatigue of the investigated alloy. It is inevitable to estimate the fatigue life of the tested samples in order to prolong the possibility of exploitation and reliability and safety of the respective elements of the structure and mobile parts of machines consisting of the Al-Zn-Mg alloy. Figure 3 : Basquin’s fatigue characteristics concerning the AlZn6Mg0.8Zr alloy loaded due to cyclic bending. Metallographic observations revealed that in result of employing the variant of thermomechanical treatment, the transverse samples (LT) of the investigated alloy display a microstructure consisting of deformed grains of the matrix of the solution-α and numerous particles of intermetallic phases, differing in their size by about 3÷15 μm 2 , distributed in bands in the direction of crawling of the material during its plastic treatment, or occurring in local clusters in the grains of at their boundaries (Figs. 4÷6). An X-ray microanalysis (EDX) has shown that these may be, among others, larger phase particles of the type Mg 2 Si, Al 6 (FeMn) or Al 2 (FeMn) 3 Si, insoluble at the temperature of supersaturation (Fig. 6). These particles, probably of a primary character, do not contribute to an increase of the tensile strength, but may effect a decrease of the resistance to cracking and affect evidently the fatigue strength. Figure 4 : Precipitation of the particles of intermetallic phases in the matrix of the solution- α of the AlZn6Mg0.8Zr alloy Figure 5 : Particles of intermetallic phases distributed in bands and particles occurring in clusters Figure 6 : X-ray microanalysis of the chemical composition of precipitation at the bottom of the crater The results of qualitative microfractography of the tested samples of the AlZn6Mg0.8Zr alloy after the fatigue test have been presented in the microphotographs in Figs. 7 to 12. The fracture obtained in the range of low-cycling and restricted fatigue strength at the maximum value of the amplitude of stresses (σ a ) amounting to 190 MPa is of the transcrystalline