Effect of Zr and Sc on microstructure and properties of 7136 aluminum alloy

We have linked experimental results to theoretical calculations and discussed the precipitation behavior of Zr and Sc in 7136 alloy and the influence of precipitation on the microstructure and mechanical properties of the alloy. The experimental results show that the added Sc atom can replace the Zr atom in the cubic Al3Zr phase with a lattice constant of a = 0.4417 nm, and form Al3(ScxZr1-x), which is also cubic and has a lattice constant of a = 0.4212 nm. A heterogeneous nucleation core promotes crystal-grain refinement, and the average size of the as-cast crystal grains is reduced from 82.4 μm to 51.7 μm. The valence electron structure and strongest bond energy of the Al3Zr phase with different structures was analyzed by the empirical electron theory. For Ll2-Al3Zr, the covalent electron number and bond energy of the strongest bonds are 0.3226 and 52.8 kJ/mol, respectively, and for DO23-Al3Zr, the numerical values are 0.4471 and 71.0 kJ/mol, respectively. Combined with the ab initio calculation, Ll2-Al3Zr has a good stability. Under suitable conditions, Ll2-Al3Zr will transform into the more stable DO23-Al3Zr. Quantitative analysis of the effect of strengthening mechanism such as fine grain strengthening and dispersion strengthening caused by the addition of Sc element on the yield strength of the alloy. The calculation results show that 0.2% elemental Sc addition can increase the alloy strength by 36.63 MPa, which is very similar to the measured result of 38.61 MPa.