Structure and Properties of Al-Ce-based eutectic alloys created by Laser Powder-Bed Fusion

Commercial precipitation-strengthened aluminum alloys are widely used in the automotive and aerospace industries for their low cost and high strength-to-weight ratio, but typically cannot be used in load-bearing applications for extended periods (months) above ~250 °C, because the nanoscale precipitates coarsen or dissolve into the Al matrix [1]. On the other hand, the most common aluminum casting alloys, which are based on the eutectic Al-Si system [2], contain large Si particles that are relatively thermally stable but provide little strengthening to the fast-creeping Al matrix [3–6]. Recent aluminum alloy development efforts have revealed that additions of rare-earth elements (REE) improve casting behavior through increased melt fluidity, and improve mechanical properties due to microstructural refinement and the formation of stable, high-melting intermetallic compounds [7,8]. A particularly inexpensive REE is cerium, which is often discarded during the refinement of more valuable REEs such as Nd and Dy, resulting in an excess Ce supply that makes it an economically feasible alloying element for aluminum even in high-volume production [9,10]. The binary Al-Ce system has a eutectic composition at 10 wt.% (2.1 at.%) Ce, with a fine “Chinese script” Al11Ce3 intermetallic phase forming upon solidification in the α-Al matrix [11]. The as-cast alloy has excellent mechanical properties without the need for heat treatments [12,13], and the micron-size Al-Al11Ce3 microstructure is highly resistant to coarsening and creep deformation up to 400 °C [14]. The Al11Ce3 phase also exists in magnesium-containing Al alloys [15–17], where it similarly remains stable up to 400 °C [18]. The addition of Mg as an alloying element improves casting characteristics while lowering density and providing solid-solution strengthening in the matrix [11,19], and prior investigations into ternary Al-8Ce-10Mg and Al-7Ce-9.Mg (wt.%) alloy showed a significantly higher elevated-temperature strength as compared to common aluminum piston alloys [9][20]. Laser line melting of a cast Al-12Ce specimen, simulating selective laser melting (SPM), showed a very fine microstructure shifting from eutectic to dendritic/cellular [21]. Very recently, an Al–3Ce–7Cu alloy was produced by SLM [22], and exhibited fine Al11Ce3 and Al6.5CeCu6.5 eutectic phases with good as-printed yield strength (274 MPa), ultimate tensile strength (456 MPa) and elongation (4.4%), as well as excellent high temperature tension resistance. Eutectic aluminum-nickel (Al-Ni) casting alloys are promising for high-temperature applications [23] due to the excellent chemical and thermal stability of the Al3Ni phase which resists coarsening up to ~400 ºC [23-26], the good fluidity of the melt, and the low tendency to hot tear [23, 24]. A high volume fraction of rod- or fiber-shaped Al3Ni can be created during eutectic solidification at 640 ºC [27] at a Ni concentration of 6.1 wt.% (2.9 at.%), ensuring a high volume fraction (~10 vol.%) of Al3Ni fibers, with orthorhombic crystal structure [28] and submicron diameters, even under standard solidification conditions. The Al3Ni microfibers are surrounded by the thin coherent layer of α-Al which have low interface energy between the layer and the Al3Ni fiber, enhancing their resistance to coarsening [29]. It has been suggested that the Orowan mechanism is the main strengthening mechanism in these alloys [30]. However, because eutectic Al-Ni alloys have microstructures typical of fiber-reinforced metal matrix composites, load transfer from matrix to fibers is another