Abstract
Mechanism of fatigue crack initiation was investigated in ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP). Fatigue behaviour of UFG material was compared to the behaviour of material in an initial as-cast state. Focused ion beam technique (FIB) was applied to reveal the surface relief and early fatigue cracks.Two substantially different mechanisms of crack initiation were observed in UFG structure, which can be characterized as bimodal even after 6 ECAP passes by route Bc. The bimodality consists in a coexistence of very fine grained areas with higher content of Mg17Al12 particles and areas exhibiting somewhat larger grains and lower density of particles. The fatigue cracks which initiate in areas of larger grains are related to the cyclic slip bands; this initiation mechanism is similar to that observed in cast alloy. The second initiation mechanism is related to the grain boundary cracking which takes place predominantly in the fine grained areas.
Highlights
Magnesium alloys are used especially because of their low weight but still good mechanical properties
It can be seen that some amount of large primary Mg17Al12 particles and AlMn based intermetallic particles remained in the ECAPed structure
When the brittle constituents are present in the crack trail they fail by brittle manner or the crack grows through the interface of the solid solution and the hard particle, due to concentration of stress ahead of the crack tip. These observations are in agreement with Wolf et al [14] who observed the crack initiation on slip bands formed in the solid solution areas and on structural defects which were present in their material
Summary
Magnesium alloys are used especially because of their low weight but still good mechanical properties. They reported that the initiation of fatigue cracks takes place due to localization of the cyclic plastic deformation in slip bands which are formed in solid solution areas. The structure of specimens treated by 6 passes of ECAP consists of grains of solid solution of alloying elements in magnesium, Mg17Al12 intermetallic particles and AlMn based intermetallic particles.
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