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The load carrying capacity of the eutectic Si in AlSi alloys depends on its volume fraction, distribution, morphology and connectivity, namely its internal architecture. This can be modified by heat treatment at temperatures close to the eutectic point causing the spheroidisation and loss of connectivity of the eutectic Si. This decreases the load carrying capacity of the eutectic Si and, consequently, the room and high temperature strength of AlSi alloys is reduced. The presence of other phases such as ceramic short fibres and aluminides in AlSi alloys can result in the formation of hybrid three‐dimensional structures that strongly delay and/or suppress the morphological changes and the loss of connectivity of the eutectic Si. As a result, the thermo‐mechanical behaviour of these heterogeneous lightweight materials is improved and stable. These effects have been explored in the last years by the authors and the main results are compiled and presented in the present work with special emphasis in the relationship between the architecture and the strength of different AlSi‐based materials.

Short fiber reinforced metals (SFRMs) have shown promising mechanical properties at elevated temperature, [1] leading to applications for combustion engines where some components locally reinforced with short fibers (SFs) have been developed. [2][3][4] To ensure the reliability of such materials under long-term temperature/load exposure, the mechanisms controlling their thermomechanical behavior are to be known.In previous studies, [3,5] an SF reinforced AlSi12CuMgNi alloy showed a decrease in stationary creep rate with the creep exposure time extending several thousand hours at 300 8C. Microtomographic evaluations [5] revealed microstructural changes of the Si phase, specially of the interconnectivity between Si, SFs, and intermetallics.Models of creep behavior of SFRM have focused on the spatial arrangement of the fibers, [6] the influence of damage, [7] the work hardened zone between matrix and fibers, [8][9][10] and the constitutive creep law of the matrix material [11] but few have studied the influence of the interpenetrating architecture of the rigid phases (e.g., ref.[12]).In the present investigation a three-dimensional (3D) unit cell finite element (FE) model is proposed in order to study the influence of the 3D connectivity between Si and SFs on the creep resistance of AlSi-based SFRM. This simple model is based on 3D microstructural features and it is correlated with experimental results. Experimental

Das drei-dimensionale Gefüge einer AlSi12CuMgNi-Kolbenlegierung verstärkt mit 15 vol% Al 2 O 3 -Kurzfasern wird mittels Synchrotron Mikro-Tomographie dargestellt. Der lösungsgeglühte Ausgangszustand enthält ein zusammenhängendes Netzwerk aus Kurzfasern und Fe-und Ni-reichen Primärausscheidungen, das darüber hinaus durch eine Vielzahl von Si-Brücken des Eutektikums verbunden wird. Die Konnektivität nimmt mit der Dauer der Kriechbelastung bei 300 C zu, besonders durch die Reifung der Si-Teilchen, die zu einer in sich zusammenhängenden Struktur zusammenwachsen, die mit dem bereits vorhandenen Netzwerk mit den Fasern verbunden ist. Nach 6400 h Belastungsdauer ist die Konnektivität der steifen Phasen nahezu vollständig. Die Orientierung der Fasern wird mit drei-dimensionalen Fast Fourier Transformationen analysiert, was auf keine Umorientierung der Fasern in die Lastrichtung schließen lässt. Die früher beobachtete Verfestigung des Materials während der Kriechbelastung wird mit der zunehmenden Steifigkeit der hybriden Verstärkungsstruktur erklärt.Die Proben enthalten im Ausgangszustand bereits Poren an den Grenzflächen der Verstärkungsphasen. Die Porenanzahl ändert sich während der Kriechbelastung im sekundären Kriechstadium nicht, jedoch wachsen die Poren, sodass der Porenvolumenanteil verdoppelt wird. In der a-Aluminiummatrix wurden keine Kriechporen gefunden.Schlüsselworte: Mikro-Tomografie, kurzfaserverstärkte Metalle, Kriechverformung, Hybridverstärkung, Porenbildung;The evolution of the micro-structure during creep of an AlSi12-CuMgNi piston alloy with 15 vol% of Al 2 O 3 short fibres is investigated by means of synchrotron micro-tomography. The results reveal a 3D morphology of the rigid phases in the composite: the eutectic-Si, the short fibres and the Fe-and Ni-rich intermetallic particles, which form an interconnected hybrid reinforcement. The connectivity of these phases increases during creep exposure at 300 C due to the diffusion induced ripening of Si and of the intermetallic particles. The hybrid reinforcement reaches almost complete percolation after 6400 h of creep exposure. The fibre orientation analysed by three-dimensional Fast Fourier Transformation does not indicate any reorientation of the fibres along the load direction. The formerly observed strengthening effect during creep exposure is attributed to the increasing load carrying capacity of the interconnected hybrid reinforcement.The analysis of creep damage during secondary creep stage shows the increase of the void volume fraction by a factor of 2 with respect to the void content from processing, while the number of voids per volume remains practically constant. The voids are located at interfaces of the rigid phases and not within the a-aluminium matrix.