The present work is part of the comprehensive research of relations between the structure and mechanical properties of the AlZn4Mg1 alloy. Chemical composition of this alloy is Zn 4.97, Mg 1.21, Mn 0.25, Cr 0.13, Ti 0.05, Cu 0.015, Fe 0.28, Si 0.17, Al. The aim of the study of fatigue properties of the alloy is to provide information on the relations between fatigue properties, fracture structure and the heat treatment applied.
The structure of the AlZn4Mg1 alloy is comparatively complex. The matrix is formed by solid solution with f.c.c. lattice. Dissolved in the solid solution are accompanying elements (Zn, Mg, Si, Fe). Also found in the matrix is the secondary phase (precipitates), both on grain and subgrain boundaries and dispersed within the grain. The dispersion degree of the secondary phase depends on the manner of aging.
Due to high impurity content in raw materials (Fe, Si), large inclusions are formed in the structure with quite different mechanical properties.
On the basis of earlier experience two types of heat treatment were used in testing the fatigue properties.
Fatigue tests were made in alternating symmetrical cycle on a Wolpert‐Amsler 10 HFP 1478 pulsator. The stress amplitude for individual specimens was altered such that the Wöhler curve could be drawn up to 108 cycles. The cycling loading frequency did not differ considerably from the value of 125 Hz. Used in fatigue tests were cylindrical specimens that had been heat treated together whith reference specimens for tensile tests.
Subsequent to destruction, the specimens were examined metallographically and fractographically, the substructure in fracture vicinity was investigated by SEM.
Fractographic specimens were obtained by cutting away parts of the rod with fracture. For TEM, thin foils were taken from regions right under the fracture surface. The foils were cut with a diamond disk and subsequently thinned electrochemically. The JET method was used to prepare the foils.
The values of mechanical properties for all the material states studied are tabulated below.
Fibers fracture in tensile strained Mg and MgLi matrix composites strengthened with ~10% vol. short δ-Al2O3 fibers (Saffil) is investigated by „in-situ“ scanning electron microscopy and ex-situ“ determination of the length of fibers chemically recovered from tensile failed composites. Little interfacial reaction in Mg matrix composite results in poor interfacial bond so that composite failure proceeds via fiber pull-out with negligible fiber fragmentation. On the other hand, extensive fiber/matrix reaction in MgLi matrix composites promotes formation of strong interfaces which are linked with multiple fiber cross-breakage during tensile straining. These results are consistent with experimental tensile strengths of related composites.
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