Microstructure and mechanical properties of cast, eutectic Al-Ce-Ni-Mn-Sc-Zr alloys with multiple strengthening mechanisms

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al11Ce3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al27Ce3Ni6 at higher Ni contents and slower solidification rates, and (ii) Al9(Ni,Mn, Fe)2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5-3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al11Ce3 and (ii) Al27Ce3Ni6 or Al9(Ni,Mn,Fe)2 platelets, all formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the alpha-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 degrees C.