Effect of combined addition of Ni and Sc on microstructure and high-temperature mechanical properties of an Al-Cu-Mn alloy

The effects of Ni and Sc modifications on the microstructure and mechanical properties of Al-Cu-Mn alloys at room and high temperatures were systematically studied. Results show that Ni and Sc modifications effectively refine the grains. Adding 0.3 % Ni leads to the formation of continuous network Al7Cu4Ni and spherical Al3CuNi phases at the grain boundaries, while the combined Ni and Sc addition causes the Ni-rich phase at the grain boundaries to become spheroidized and dispersed. At 300 degrees C, part of the Ni atoms in the modified alloy gets segregated at the Al20Cu2Mn3 phase interface, and the Mn atoms are distributed at the Al7Cu4Ni phase interface. It results in a solute drag effect and inhibition in the growth of the two. Sc forms Al3(Zr, Sc) phase with high temperature resistance in the alloy, which further improves the high-temperature stability of the alloy. The strong high-temperature resistance of the Ni-rich phase effectively pins the grain boundaries, preventing slip and enhancing grain boundary strength, thereby improving the alloy's high-temperature strength. The combined Ni and Sc modification results in the best high-temperature mechanical properties. At 300 degrees C and 350 degrees C, the alloy strength is 164.7 MPa and 116.2 MPa, respectively, representing increases of 28.4 % and 34.2 % over the base alloy, and 10.9 % and 21.3 % over the alloy with 0.3 % Ni. The main high-temperature strengthening mechanism is precipitation strengthening, with a minor contribution from solid solution strengthening. The yield strength contribution from stress transfer and eutectic strengthening due to Ni and Sc modification is lower than that of the base alloy and the 0.3 % Ni alloy. The fracture mode of the Ni and Sc composite-modified alloy is a mixed ductile-brittle fracture at room temperature, with brittle fracture becoming dominant at higher temperatures.