Microstructure and mechanical properties of hypoeutectic Al-6Ce-3Ni-0.7Fe (wt.%) alloy

A hypoeutectic, Fe-modified Al-Ce-Ni alloy (Al-6Ce-3Ni-0.7Fe, wt.%) is studied in terms of microstructure, thermal stability, ambient temperature strengthening, and creep resistance. The as-cast microstructure consists of primary Al dendrites and interdendritic binary eutectic regions (Al-Al11Ce3 and/or Al-Al9(Fe,Ni)2), with micron/ submicron lamellar spacing, depending on the location along the height of the ingot. The cast alloy exhibits excellent coarsening resistance at 400 & DEG;C, with mostly unchanged microstructure and microhardness after 6 weeks of aging, indicating good thermal stability of Al11Ce3 and Al9(Fe,Ni)2. Orowan strengthening and load transfer are identified as strengthening mechanisms at ambient and elevated temperature. A high volume fraction of the intermetallic phases (providing load transfer) and relatively coarse eutectic spacing (for modest Orowan strengthening) result in a moderate as-cast microhardness of 566 & PLUSMN; 32 MPa. Creep resistance at 300 and 350 & DEG;C is similar to a binary Al-12.5Ce eutectic alloy (with twice the Ce content) because of two countering effects: Al-6Ce-3Ni-0.7Fe shows a higher volume fraction of strengthening intermetallic phases, but it also exhibits a large fraction of primary Al dendrites which weaken the alloy. By contrast, the alloy, when laserremelted at the surface, has a fully eutectic microstructure without primary aluminum dendrites achieved by high undercooling on solidification, with a refined network of eutectic phases that doubles the microhardness as compared to the cast alloy. Whereas coarsening is faster due to the shorter diffusion distances between the eutectic phases, hardness remains -30% higher than the as-cast alloy after -6 weeks aging at 400 & DEG;C.