The 2xxx series Al-Cu alloys exhibit excellent strength at high temperatures and are widely used in high-speed rotating components in high-temperature environments, such as the compressor wheels for automotive. The refinement and thermal stability of intermediate θ' precipitates, which form during artificial aging in the temperature range of 150–200 °C,[1,2] are crucial for developing high-strength Al-Cu alloys for high-temperature applications.[2] In recent years, as the efficiency of internal combustion engines has increased, further improvements in heat resistance are required, thus attracting attention to Sc microalloying.[3,4] Sc microalloying fundamentally modifies θ' precipitation kinetics and thermal stability in Al-Cu systems through interfacial segregation mechanisms.[3,4] Sc partitioning to θ'/matrix interfaces reduces interfacial energy while enhancing precipitate nucleation density and coarsening resistance.[3,4] Apart from Sc, Mn is also usually added to Al-Cu alloys to stabilize θ' precipitates through solute segregation at the θ' precipitates semi-coherent interface.[5] However, the effect of Mn microalloying on 2xxx alloys with low Cu and Sc contents remains unclear.

In order to elucidate the effect of Mn addition on the thermal stability and creep resistance of Al-Cu-Sc alloy, Al-3.5Cu-0.1Sc (mass%) and Al-3.5Cu-0.3Mn-0.1Sc (mass%) alloys were fabricated by extrusion. Conventional heat-resistant alloy 2219 was also evaluated as a reference. After solution heat treatment, all alloys were subjected to T8 (3% strain, 72 h natural aging and artificial aging at 160 °C for 18 h), followed by exposure at 250 °C for 100 h. Tensile tests at 250 °C were conducted on T8 specimens before and after exposure, while creep tests were conducted on T8 specimens after exposure. Following exposure, the Al-3.5Cu-0.1Sc and Al-3.5Cu-0.3Mn-0.1Sc alloys demonstrated an increase in strength in contrast to the 2219 alloy, which exhibited a decrease in strength. Creep tests conducted at 250 °C under 130 MPa stressing showed a significant improvement in fracture time for the Al-3.5Cu-0.3Mn-0.1Sc alloy.

Transmission electron microscopy observation of the creep specimens revealed that Sc solute segregation occurred at θ'/matrix interfaces in both Sc-added alloys. However, Mn segregation on θ' precipitate semi-coherent surfaces could not be confirmed. In our study, there is a possibility that Mn contributes to solute strengthening, which hinders the movement of dislocations during creep deformations, since Mn has a high solubility of 1.8 mass% in Al compared to other transition elements.[5] These results suggest that Sc and Mn have different roles in improving the thermal stability and creep resistance properties of this alloy.

References
[1] S.K. Son, M. Takeda, M. Mitome, Y. Bando, Mater. Lett., 2005, 59 (6), 629–632.
[2] L. Jiang, B. Rouxel, T. Langan, T. Dorin, Acta Mater., 2021, 206, 116634.
[3] L. Jiang, J.K. Li, G. Liu, R.H. Wang, B.A. Chen, J.Y. Zhang, J. Sun, M.X. Yang, G. Yang, J. Yang, X.Z. Cao, Mater. Sci. Eng. A, 2015, 637, 139-154.
[4] C. Yang, P. Zhang, D. Shao, R.H. Wang, L.F. Cao, J.Y. Zhang, G. Liu, B.A. Chen, J. Sun, Acta Mater., 2016, 119, 68-79.
[5] A. Shyam, S. Roy, D. Shin, J.D. Poplawsky, L.F. Allard, Y. Yamamoto, J.R. Morris, B. Mazumder, J.C. Idrobo, A. Rodriguez, T.R. Watkins, J.A. Haynes, Mater. Sci. Eng. A, 2019, 138279.

Dr.-Ing. Kaharudin Amalina Aina Binti