Microstructure design of cast Al alloys for high temperature

Al-alloys are very attractive materials due to their low density, excellent mechanical properties and relative low price compared to Ti and Mg alloys. Particularly, cast Al-Si alloys are widely used in the automotive and aircraft industries. Cast Al-Si alloys with 7wt.% of Si can form a three-dimensional (3D) eutectic-Si network embedded in the ductile a-Al matrix. This contributes to the strength of these alloys by load transfer from the Al-matrix to the stiffer highly interconnected eutectic Si. The addition of transition elements such as Ni and Cu are considered to be effective for increasing the high temperature strength of cast Al-Si alloys by forming stable aluminides. Recently, our group has shown that adding ~ 1wt% Ni to a eutectic Al-Si alloy can form a highly interconnected 3D structure of aluminides together with the eutectic Si. Furthermore, the aluminides and the eutectic Si present a large degree of contiguity resulting in a long-range hybrid 3D structure. Contrarily to the case of Ni-free Al-Si alloys, the interface between the eutectic Si and the aluminides is retained during heat treatments. Thus, spheroidisation and disintegration of the eutectic Si is suppressed. As a consequence, both the aluminides and the eutectic Si maintain their complex interconnected structures. The thermomechanical properties of multiphase materials, as it is the case of Al-Si based cast alloys, are strongly influenced by the geometrical arrangement of their constituents. Furthermore, highly interconnected rigid structures embedded in a metallic matrix can result in an improvement of the elevated temperature strength. The description of the 3D architectures resulting from the addition of aluminide forming elements, such as Ni and Cu, to eutectic Al-Si alloys as well as the determination of their load carrying capacity is an open scientific challenge. The objective of the proposed project is to elaborate knowledge-based methods which improve the mechanical properties at elevated temperatures by investigating: a) the interaction between eutectic Si and different aluminides in cast Al-Si based alloys. For this, the 3D architectures formed by the eutectic Si and the aluminides in different thermal conditions will be characterized by advanced tomographic methods. b) the reinforcement capability at high temperature of the structures mentioned in a). This will be done by studying specific thermomechanical properties of the alloys and performing in situ bulk diffraction experiments to determine the load carrying capability of the phases` architecture.

Owing to their light weight, Al alloys are attractive and well established in transportation. Particularly, cast alloys based on the Al-Si system are used to produce cylinder heads, pistons and valve lifters. The gas temperature within a combustion chamber of a diesel engine can reach up to 300400 C with thermal cycles of ?T = 220 C occurring in each combustion cycle. The temperature amplitude during start-up may reach 300 C, and therefore high-temperature strength and thermal fatigue resistance are important requirements for the design of piston alloys. Previous studies on AlSi12 and AlSi12Ni alloys had shown that cast Al-Si alloys are formed by a soft ? -Al matrix and three-dimensional (3D) networks of rigid phases such as eutectic Si and, eventually, Ni-rich aluminides. The morphology and interconnectivity of these rigid networks can be modified by heat treatments close to the eutectic temperature giving them an important versatility to tailor their thermomechanical properties. Similarly, cast Al-Mg-Si alloys are formed by an ?-Al matrix and, at least, the intermetallic phase Mg2Si which influences the mechanical properties of the alloys. Based on this, the objectives of the project were to determine:the interaction between eutectic Si or Mg2Si and different types of Cu-, Ni- and Fe-based aluminides in cast Al-Si and Al-Mg-Si alloys. The architectures formed by the eutectic phases and the aluminides during casting and their evolution after different thermal treatments were characterized and quantified using advanced 3D tomographic methods.the reinforcement capability at high temperature of the 3D structures studied in a). This was performed investigating the thermomechanical behavior of the multiphase cast Al alloys.The investigations showed that the addition of Cu, Ni and Fe results in the formation of several complex rigid aluminides whose thermal stability is influenced by the casting conditions and subsequent solution heat treatments. Furthermore, these aluminides present a high degree of contiguity with each other and with the eutectic Si resulting in a long range hybrid 3D structure embedded in the Al-matrix that determines the room and high temperature strength and thermal fatigue resistance of the alloys. The high contiguity between the aluminides and the eutectic Si strongly reduces the well-known spheroidisation of eutectic Si and conserves the strength of the Al-Si alloys even after long (> 4 h) solution heat treatments. The addition of 2 wt% of Ni and 5 wt% of Cu to AlSi12 showed the best performance in terms of strength and thermal fatigue resistance. Regarding the cast Al-Mg-Si alloys, the results obtained in the project showed that the eutectic Mg2Si also builds rigid 3D networks whose morphology and connectivity can be modified by solution heat treatments. The strength at 300C decreases after 1 h at 540C and remains practically constant during subsequent solution treatment. The correlation with morphological analysis shows that the dominant process in the period where strength decreases is the partial loss of interconnectivity of Mg2Si, while the shape remains practically constant. This indicates that the elevated temperature strength is more sensitive to the interconnectivity of the Mg2Si architecture than to the shape of the individual particles.