PSI - Issue 13

Lenka Kuchariková et al. / Procedia Structural Integrity 13 (2018) 1577–1582 Lenka Kuchariková/ Structural Integrity Procedia 00 (2018) 000 – 000

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The fracture surface was influenced significantly by microstructural components morphologies (eutectic Si and intermetallic phases) and their distribution in the cross section (Figs. 3-4). The fractography analysis confirmed that the hard and brittle intermetallic phases, based on Fe and hexagonal plates of eutectic Si particles, led to formation of the trans-crystalline cleavage fracture (Figs. 3a, b). The breaking mechanisms of the Fe-rich phases and eutectic Si particles were the same in both states (AS, T6), with differences only of the cleavage facets sizes – the larger in the as-cast state than after the T6 treatment, which relates to morphology changes of these particles (Fig. 1). The BSE analysis was used for better identification of the fracture surface microstructural features (Fig. 3). The trans-crystalline ductile fracture was related to the presence of the matrix and Cu-rich intermetallic phases in the microstructure (Fig. 3c). Due to the strong cohesion at the interfaces between the  -matrix and Si, the matrix is deformed under the local active stress. The cells are formed around the cracked Si particles by plastic deformation of the matrix. The traces of these phenomena are visible as the high tear ridges on the fracture surface (Fig. 3c). Cu intermetallic phases are also broken by the trans-crystalline ductile mechanism, Fig. 3c. The trans-crystalline ductile fracture was probably related to both the Cu-rich and the secondary phases, with changes only in size of pitting on the fracture surface. It is assumed, that the pitting was larger at Al 2 Cu phases than in Al-Al 2 Cu-Si ones.

Fig. 3 Trans-crystalline cleavage fracture surfaces in A226 cast alloy. (a) trans-crystalline cleavage fracture of the Si phase; (b) trans-crystalline cleavage fracture of the Fe-rich phases, (c) trans-crystalline ductile fracture of the Cu-rich phase and plastic deformation of the α phase on fracture surfaces .

4. Conclusions

The morphology of microstructural components impact on the fracture behaviour of the secondary A226 cast alloy was investigated and the following conclusions were drawn:  Mechanical properties of the secondary A226 cast alloy are comparable to those of the primary alloy. The heat treatment causes increase of mechanical properties compared to the as-cast state: about 47 % higher UTS and 42 % HBW 2.5/62.5/15. The absorbed energy was unchanged.  In the secondary A226 cast alloys the intermetallic phases were formed comparable to primary aluminium alloys. The presence of the higher amount of Fe did not lead to formation of the higher amount of the Fe-rich phases, due to the presence of Mn. The second phases observed were Fe-rich phases - Al 5 FeSi needles (in a very small volume of microstructure) and the skeleton-like Al 15 (FeMn) 3 Si 2 and the two Cu-rich intermetallic phases - Al 2 Cu and the ternary eutectic Al-Al 2 Cu-Si. The Al 15 (FeMn) 3 Si 2 phase was dominant from the Fe-rich phases thanks to the presence of Mn and from the Cu-rich phases the ternary eutectic was dominant.  The deep etching shows that phases in form of needles (Si particles and Fe-rich phases) on the scratch pattern are really large platelets and their morphology negatively affects mechanical properties and fracture behaviour of aluminium cast alloys in the as-cast state. Those phases belong to the harder particles in comparison to the Cu-rich phases and matrix. The Fe-rich phases in the skeleton-like form and both Cu-rich second phases have identical morphology on the deep etched samples as on the scratch pattern and it does not have the negative impact as the platelets. The heat treatment led to changes in morphology of microstructural parameters. The Si particles were fragmented into smaller particles and had rounded edges, Fe-rich phases were fragmented also into smaller particles and the Cu-rich phases were fragmented and dissolved and thus difficult to observe.